-
XIV National Conference on Structural Engineering, Acapulco
2004
Offshore Structures A new challenge
How can the experience from the marine concrete industry be
utilized
Knut Sandvik, Rolf Eie and Jan-Diederik Advocaat, of Aker
Kvaerner Engineering & Technology AS
Arnstein Godejord, Kre O.Hreid, Kolbjrn Hyland and Tor Ole
Olsen, of Dr.techn.Olav Olsen a.s
Norway
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
TABLE OF CONTENT Summary 1 Marine Concrete Structures
............................................................................................
4
1.1 General
....................................................................................................................................
4 1.2 Floating Concrete
Sea-structures.............................................................................................
4 1.3 Performance in the marine environment
.................................................................................
6 1.4 Considerations for new field
development..............................................................................
8 1.5 Design aspects
.........................................................................................................................
8
3 The Norwegian Experience and
Know-how.................................................................
13 3.1 The structures
........................................................................................................................
13 3.2 Research and development
....................................................................................................
16 3.3 Decommissioning of offshore concrete
platforms.................................................................
17
4 Project Execution Typical
...........................................................................................
18 5 Ongoing Projects
.............................................................................................................
26
5.1 The Sakhalin II
Project..........................................................................................................
26 5.2 The Adriatic LNG Terminal
Project......................................................................................
29
6 Novel Concepts
................................................................................................................
32 6.1 Floating LNG Terminals
.......................................................................................................
32 6.2 Floating airport or navy
base.................................................................................................
33 6.3 MPU Heavy
Lifter.................................................................................................................
34 6.4 MPU Semo
............................................................................................................................
35 6.5 Other novel concepts
.............................................................................................................
36
7 Project Execution in Mexico
..........................................................................................
39 7.1 Engineering and Design
........................................................................................................
39
7.1.1 Conceptual design
.........................................................................................................
39 7.1.2 Detail
design..................................................................................................................
39 7.1.3 Rules and Regulations proposed for concrete projects in
Mexico ................................ 40
7.2 Fabrication
Site......................................................................................................................
41 7.3 Options for fabrication
..........................................................................................................
42 7.4 Construction and
Methods.....................................................................................................
43 7.5 Material Qualities
..................................................................................................................
43
7.5.1
Concrete.........................................................................................................................
44 7.5.2 Ordinary reinforcement
.................................................................................................
44 7.5.3 Prestressed
reinforcement..............................................................................................
45
7.6 Cathodic Protection
...............................................................................................................
45
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
7.7 Steel/Concrete Connection Methods
.....................................................................................
46 7.7.1 Riser
support..................................................................................................................
46 7.7.2 Mooring brackets / Towing brackets / Fairleads
........................................................... 46
7.7.3 Embedment plates
.........................................................................................................
47
References
...............................................................................................................................
48
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
3
Summary The concrete construction industry is widely spread and
every country has its own. Concrete structures have been used in
the marine environment for a very long time. Examples are bridges,
docks and lighthouses. Particularly in war times, when steel is
scarce, concrete has also been used for barges and ships. The long
history of marine concrete structures is interesting and represents
valuable experience. Concrete structures have proven especially
well suited to develop offshore oil and gas fields. More than 40
major offshore concrete structures make a good job at supporting
the processing facilities of hydrocarbon plants offshore. They are
constructed over the past 30 years, and perform well in all the
different environments from the arctic to tropical waters, and from
sandy stiff seabed to very soft clays. A number of the platforms
are permanently floating, and they also show good and efficient
behaviour. How may all this experience be utilized to further
develop the offshore oil and gas industry? The authors of the
present paper, being representatives of the Norwegian offshore
concrete industry, have experience from all levels of design and
construction of small and large offshore concrete structures.
Examples are described in this paper, including the important
elements of project execution, the experiences from design and
construction, the durability of offshore concrete structures and
the associated required maintenance, as well as the issues of
removal and recycling of such structures. Ongoing projects
(Sakhalin II in Russia and Adriatic LNG Terminal in the Adriatic
Ocean) are briefly presented. Recent trends and some novel concepts
for further development are discussed. The paper concludes with
some thoughts on project execution in Mexico, including
engineering, construction and construction methods, materials and
labour. It is the hope of the authors that the paper will represent
a fairly comprehensive description of offshore concrete structures,
or at least a place to start in the search for improved oil and gas
field development.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
4
1 Marine Concrete Structures
1.1 General Fig. 1-1 shows the still floating caissons for a
roll on/roll off facility in Port dAutonome Abidjan, built by
Selmer Skanska.
Figure 1-1. Floating caissons
The picture shows that having access to water and an innovative
concrete construction industry may be a good, and possibly
sufficient, starting point for building offshore structures. Some
situations may call for more complicated structures, and deep
sheltered waters may be a requirement for the construction. Many of
the offshore structures described in this paper are from Norway
which has deep fjords, protected from the ocean. Although many of
the examples in this paper describe complex offshore concrete
structures, it is important to recognize the value of simplicity.
Ingenuity and standard means of construction will bring the best
results.
1.2 Floating Concrete Sea-structures The history of floating
concrete sea structures goes back to the 19th century. In 1848
Lambot for the first time used reinforced concrete to build a boat.
During World War I, 14 concrete ships were built due to the steel
shortage - including the 130 m long U.S.S. Selma. At that time
reinforced concrete had already been used in shipbuilding (small
ships) in the Scandinavian countries. World War II concrete ships
saw widespread wartime service in battle zones. Twenty-four of
these ships were large sea-going vessels and 80 were sea-going
barges of large size. The cargo capacities ranged from 3.200 to
140.250 tons. Ref. 1, by Morgan, gives a good description of the
early development of the concrete hull.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
5
A number of notable pontoon bridges have been built of concrete.
Ref. 2 gives an overview of the long traditions within this area.
The first floating concrete bridge was built across Lake Washington
in 1940. In the late 1950s, a number of pre-stressed concrete
ocean-going barges were constructed in the Philippines
(additionally 19 barges from 1964 to 1966), and concrete
lighthouses were constructed as caissons in the 1960s. Concrete
lighthouses are installed in the Irish Sea, in Eastern Canada and
in the Gulf of Bothnia. Many pontoons, barges and other crafts have
been successfully built in the former USSR, Australia, New Zealand
and the UK. Over the years, starting back in mid 1920s, some 70
temporary floating immersed concrete tunnels have been built in the
following countries: USA, Canada, Argentina, Cuba, UK, Denmark,
Sweden, Holland, Belgium, Germany, France, Hong Kong, Taiwan
(Republic of China), Japan and Australia. During the 1970s concrete
gained recognition as a well-suited material for construction of
offshore platforms for the exploration of oil in the North Sea.
Permanently floating offshore vessels related to the petroleum
industry are now installed in the Java Sea, in the North Sea and
outside the coast of Congo in West Africa. From 1950 to 1982 it was
registered that approximately 1.130 concrete hulls had been built.
Most of them are small with overall length less than 50 m. Among
the bigger ones, two groups of sizes are dominant, - approximately
250 hulls with length ranging from 58 to 67 m, and 40 hulls with a
length of 110 m. Concrete hulls and barges - examples from practice
The ARCO barge (ref. 3) The Ardjuna Sakti is a floating
pre-stressed concrete LPG storage facility with overall dimensions
140.5 x 41.5 x 17.2 m (length x beam x depth). Fully loaded, the
vessel displaces 66.000 tons. The ARCO barge was built and
completely outfitted in Tacoma (Washington) and towed 16.000 km
(10.000 miles) across the Pacific Ocean to the Java Sea in 1976,
where it is permanently moored. Concrete barge C-Boat 500. The
prototype barge, of 37 m length, 9 m beam and 3.1 m depth and of
500 dwt loading capacity was built in Japan in 1982. Heidrun TLP
(ref. 4) Conocos Heidrun platform is the worlds first TLP with a
concrete hull and the largest permanently floating concrete
structure ever with a concrete volume of 67.000 m3. The topside
related weight is 89.000 tons (net 65.000 t topside) and the
displacement 285.000 tons. The platform was installed on location
in the North Sea in 1995, at a water depth of 345 m. Troll Oil Semi
(ref. 5) Norsk Hydros Troll Oil FPS platform is the worlds first
concrete catenary anchored floater. The Troll Oil semi submersible
hull has a concrete volume of 46.000 m3 and supports a topside
weight of 32.500 tons. The displacement is 190.000 tons. The
platform was installed on location in the North Sea in 1995, at a
water depth of 335 m. Nkossa barge (ref. 6 & 7) Elf Congos
Nkossa barge is the worlds largest pre-stressed concrete barge. The
floating production vessel of which the dimensions are 220 x 46 x
16 m was built in Marseille, France, and towed 4500 nautical miles
to the west coast of Congo in West Africa where it was permanently
anchored in 170 m
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
6
water depth in 1996. The total displacement fully loaded is
107.000 tons, and the concrete volume of the barge is 27.000 m3.
The hull supports six topside modules with a total weight of 33.000
tons.
1.3 Performance in the marine environment Considering the wide
use of concrete for marine applications there is surprisingly
little documentation to be found on in-service performance. The
apparent cause for this is that provided satisfactory design and
execution, concrete is an optimal material for harbour, coastal and
offshore construction as it combines durability, strength and
economy. This fact is supported by studies of floating concrete
docks back in the 1970s, showing dramatic savings, requiring less
than 10% the maintenance of similar all-steel docks, ref. 1. Other
structures also utilize the water-tightness properties of concrete;
storage tanks, nuclear containment structures and submarine
tunnels. Sare and Yee, ref. 8, report negligible repair and
maintenance costs for the 19 pre-stressed concrete barges
constructed in the Philippines during 1964-66 for Lusteveco, with
no need for dry-docking. After many years in service, average
annual maintenance cost of the concrete barges are found to be
about 1/3 compared to steel barges. The fabrication cost of Yees
barges showed a saving of 16 percent compared to that of steel. In
the period 1974 to 1975, the total downtime per floating barge per
year for maintenance work was six days for the concrete structures.
The similar steel barges had an average downtime of 24 days. The
Refiner I barge, checked by Bureau Veritas for issuing necessary
certificates for the towed voyage, was designed for 4.2m wave
height. It is worth noting that the vessel in fact endured a storm
in the Bay of Biscay during which time the conditions were
undoubtedly more severe than those contemplated in the calculations
(the pontoon drifted in winds of force 10-11 and angles of roll and
pitch of 14 and 10 respectively were observed). The unit behaved
perfectly well through this unexpectedly severe environment. It
seems to be general consensus that concrete vessels and barges have
proved to have good seagoing qualities, to be safe and strong, and
suffer much less from vibration than steel ships - to the crews
satisfaction. The 1970s and 1980s saw the spectacular development
of offshore bottom fixed concrete structures, installed in up to
300 meters (1000 ft) of water depth in the midst of one of the
worlds stormiest oceans, the North Sea. It is remarkable how well
these structures have performed in the hostile marine environment,
successfully withstanding the extreme loads from waves approaching
30 meters in height as well as the dynamic cyclic forces.
Experience has shown that the offshore concrete structures
currently in use are virtually maintenance-free. It is generally
recognized that the first concrete platforms in the North Sea were
over-inspected and that the need for extensive instrumentation of
platforms of common types should be reconsidered. A comprehensive
list of references to information pertaining to the performance of
North Sea concrete structures is presented in ref. 9 and 19. No
significant sign of material deterioration, corrosion of
reinforcement or other material-related deficiencies have been
observed. Falling objects or ramming ships mainly cause observed
damages. Platforms designed for 20 years operation have now passed
the end of their prescribed design life. Inspections and
investigations confirm that their lifetime in general can be
extended. Various codes give well-established rules for assessing
fire resistance. Two hydrocarbon fires inside North Sea concrete
platform shafts in the late seventies are reported. The consequence
was a surface
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
7
scaling about 10-20 mm deep over a height of 5-10 m. This
marginal impact is attributed to the large heat capacity and low
thermal conductivity of concrete. No repair was found necessary -
clearly demonstrating the excellent fire resistance of concrete.
Concrete is normally considered to be one of the best fire proofing
materials available, a factor of unquestionable importance for an
offshore oil or gas platform/storage. There are many instances,
both ashore and afloat, of fire causing no more than local to no
damage to concrete structures. As an example constituting the most
impressive testimonial that could possible be called for,
Derrington (ref. 11) reports how two concrete barges survived the
Bikini Atoll nuclear bomb tests in good shape when their cargo of
fuel oil was set alight - moored only 100 yards from the test
centre. Wartime brought the additional hazards of bombs and mines.
Morgan ref. 12 reports that in 1944 a 1000-tonne German concrete
barge hit a mine, which exploded under the stern - the vessel was
able to reach shore by being repaired while afloat with underwater
concreting. Lusteveco, operator of Yees concrete barges, was quite
pleased with their performance and say one of the most endearing
aspects of pre-stressed concrete hulls is their ease of repair. The
barges serviced the Vietnam War area for a period of nine years and
a number of barges were rocketed or damaged by plastic bombs. The
damage was usually confined to severely cracking of concrete within
a limited area of 1m x 2m on the surface of the hull - a damage
consequence limitation credited to the rigid pre-stressed concrete
hull. In March 1973 one of Lustevecos 2000 dwt dry cargo barges,
L-1960, hit a mine at the starboard side transiting the Mekong
River fully loaded with rice intended for Phnom Penh. After some
temporary repairs, the barge was towed safely to the Philippines
for permanent repairs. The cost of this 10 days repair job was US$
4381. Pre-stressed concrete was chosen as the hull material for the
ARCO barge because of its seaworthiness, competitive cost, fire
resistance, durability and speed of construction. After almost
twenty years of continuous service, various tests were carried out
for the concrete barge. Due to its excellent condition, ARCO has
given its barge an indefinite lifespan - a solid proof of the
excellent performance of concrete in a marine environment as well
as its good fatigue resistance. There are also examples of
premature failures for concrete structures in coastal areas (e.g.
bridge piers and quay structures) - suggesting that the marine
environment is demanding and imposes special requirements on
materials and workmanship. It is in this context important to
distinguish between the onshore and offshore concrete industry. The
problems experienced in coastal areas for the onshore concrete
industry is caused by, for example ref. 13: improper cover,
misplaced reinforcement, improper handling, placing of concrete or
poor quality of concrete (e.g., seawater contaminated aggregates,
improper concrete mix proportions). In Norway, the experiences with
coastal bridges have shown that the principal causes of failure are
the same as reported above. In general, however, marine structures
built by the onshore concrete industry have suffered very limited
from degradation - refer for example list of surveys presented in
FIPs state of the art report on inspection, maintenance and repair
of concrete sea structures, ref. 14. In 1999 a large Norwegian
research program, ref. 10, investigated the durability of concrete
structures, covering bridges, industrial structures, quays and
offshore platforms. Main emphasis was set on chloride penetration
and reinforcement corrosion. Six offshore concrete structures were
investigated:
Statfjord A (16 years in operation at time of inspection)
Gullfaks A (7) Gullfaks C (4) Oseberg A (8/9) Troll B (2)
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
8
Ekofisk Tank (17/22) Calculations based on the chloride profiles
showed that the investigated offshore platforms were in excellent
condition and that there would be no risk for corrosion within
their expected lifetime. Two of them would theoretically not reach
chloride concentrations representing risk for corrosion at the
position of the reinforcement bars, until having serviced for more
than 200 years. The good performance of the offshore concrete
structures is attributed mainly to high quality concrete (i.e. high
strength / low permeability concrete in situ), proper design
including sufficient cover to reinforcement, good workmanship &
construction techniques and thorough quality assurance.
1.4 Considerations for new field development The general
conclusion drawn from service performance of the offshore concrete
structures is that they have proved excellent behaviour and require
significantly lower expenditure for inspection, maintenance and
repair than steel structures. Experience has shown that offshore
concrete structures currently in use are virtually
maintenance-free. Over the years wide experience has been gained in
the field of post-tensioned concrete offshore structures. The
successful construction of the Heidrun TLP, the Troll Oil Semi and
the Nkossa barge has opened for interesting potentials to the
offshore industry as to the suitability and economics of concrete
floating structures. Recent studies also conclude that floating
concrete structures are well suited for floating LNG plants. The
increasing importance of local content in the development projects
is favouring concrete as the building material in countries with
limited number of offshore steel yards. Concrete structures can be
built in greenfield areas with very little infrastructure. The
majority of the workforce does not need special education and can
be recruited locally. Hence, choosing concrete may significantly
increase the local content of a project.
1.5 Design aspects Design Life and Reuse: The offshore concrete
structures installed to date have been designed for 25-70 years.
The Troll GBS was designed for a 70-year life and the Heidrun TLP
is designed for 50 years. There is not a significant additional
cost related to extension of design life from for instance 30 years
to 50 years or 70 years. One reason is the fact that reinforced and
pre-stressed concrete is not sensitive to fatigue. With the
extensive design life possible for a concrete platform there is
obviously a very good possibility for reuse. The investigations
carried out on durability and conditions of existing concrete
platforms clearly prove a great potential for reuse. Stiffness:
Concrete structures generally have large stiffness. The result is
less flexibility and less deformation applied onto outfitting steel
etc. Robustness: Under maximum credible accidents, such as major
leakage, collision or fire, a properly designed and constructed
pre-stressed concrete vessel has better inherent safety than a
comparable steel vessel. This is one of the conclusions from a
technical feasibility and safety study of a 297 m (974 ft) long
storage/processing vessel carrying LPG in free-standing tanks
performed by Gerwick et. al., ref. 15. Here a pre-stressed concrete
vessel was designed and compared with an existing steel vessel
designed according to the ABS requirements. It was also found that
the concrete hull, being stiffer, developed
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
9
significantly lower dynamic amplification and had a lower risk
of failure than the steel vessel. The concrete hull was found to
have adequate safety to justify its use for vessels in hazardous
cargo service without limitation as to length. Impact Resistance:
The concrete material has excellent resistance to impact loads.
This has been proven through history, and the result is that
concrete is widely used in military installations, shelters, in
buildings which need to be failsafe and which are regarded as
exposed to terror attacks etc. The concrete hull of a concrete
floater will typically be designed for impact loads from any
possible dropped object. Still there will be a design requirement
to design for accidental filling of any compartment adjacent to
sea, or adjacent to piping, which is connected to sea. Fire
Resistance: As mentioned above concrete is normally considered to
be one of the best fire proofing materials available. Two fires
inside shafts of North Sea concrete structures have been reported,
and damages have been too small to decide any repair work to be
done. The combination of excellent fire- and impact-resistance is
of course very important for units producing hydrocarbons.
Maintenance Free: Appropriately designed and constructed concrete
hulls in the marine environment are almost free of maintenance.
Regulatory inspection of the concrete structure is mainly limited
to visual inspection and entails no significant cost. Recently
constructed concrete platforms have been designed for an
operational life of 50-70 years. The maintenance/OPEX aspect was
decisive in Elf Congos decision to chose a concrete barge on the
Nkossa project, ref. 6 and 7, as the concrete hull offered
significant savings in expected maintenance cost. The barge will
fulfil its functions on site without interruptions for 30 years and
there is expected virtually no maintenance. To quote Campbell,
American Bureau of Shipping Surveyor ref. 16: The history of
concrete for marine construction is very favourable. There is
little doubt that a well designed, well built, concrete structure
will have a longer service life than a comparable steel structure.
Motion Characteristics: The motion characteristics of a concrete
hull are typically better than for a steel floater designed for the
same purpose. This conclusion can be drawn based on reports from
ship captains (World War II ships and Yees barges), several studies
and recently confirmed by both analyses and model testing for very
large FPSOs (BP Atlantic Frontier Stage 2 / Schiehallion, hull
length 280 m). The generally somewhat larger mass and draught,
result in improved motion characteristics. For the very harsh
environmental conditions West of Shetland (the Schiehallion FPSO),
the mooring size and cost was reported nearly identical for steel
and concrete hulls (ballasted condition governing). The general
picture, however, is that the mooring costs for concrete
vessels/semis are approximately 10 per cent more expensive than for
a steel hull - illustrating that the mooring costs must be included
in cost comparison steel versus concrete. Material properties;
strength and weight: It is obvious that the weight of the platform
is of importance. A vessel must carry its own weight plus a
payload. For the concrete platform the payload is the topside and
equipment, as well as any ballast required for hydrostatic and/or
geotechnical stability. In ref. 17 Jan Moksnes presents some of the
results of Norwegian research on concrete over the past 20 years.
The benefit of this research has been and is significant for the
concrete construction industry. In brief relevant design aspects of
importance for the type of structures under consideration are:
o High stiffness, providing a stable foundation for tanks and
other attachements o Good resistance to environmental loading
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
10
o Excellent behavior at low temperatures o Favorable in
ice-infested waters o Robust with respect to accidental loading
such as ship impact, dropped objects or terrorist
attacks
o Good resistance to oil and gas process hazards o Functional
and safety features common to a land based plant. o Good resistance
to cold spot incidents o Enhanced material properties with
decreasing temperature o Excellent fatigue resistance o Good
durability, and basically maintenance-free o Standard offshore
concrete quality applied o No need for skilled labor for the bulk
of the construction work, enabling local execution o Good
resistance to seismic loading o May be decommissioned and removed,
possibly reused
Important also is the cost of the structure. Key qualitative
parameters for cost of concrete structures are summarised
below:
o Low complexity structures are both faster and more cost
effective to construct. o Close integration of engineering and
construction as well as main operations o Local availability of
labour and materials o Design basis requirements (waves, soils,
functional requirements) o Generally cost effective to complete as
much as possible in the graving dock. Unnecessary
stops in slip forming should be avoided
o Preparation of the graving dock may add cost to the project.
However, it may prove economically sensible to extend and/or deepen
the dock to increase the completion grade.
o Construction schedule. Construction time is related to
simplicity and concrete volume. For LNG terminals, assembling the
tanks from pre-cast elements may decrease the length of the
schedule.
For small structures the initial construction may be performed
on barge(s). After float off from the barge the remaining
construction is performed while afloat. However, in most cases a
dry-dock has been used for the initial and also sometimes the
complete construction phase. All concepts developed are designed to
be removed from the offshore location in a controlled way. Followin
g the removal the structure may be reused or deconstructed and
recycled.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
11
2 Offshore Concrete Structures for the Oil and Gas Industry
Since the Ekofisk Tank was installed in 1973, 41 major offshore
concrete structures have been built, see ref. 18. Fig.2-1 shows the
tow of Beryl A in 1975, from its construction site in a sheltered
Norwegian fjord, on its way to the harsh environment of the North
Sea, fig. 2-2. Figures 2-1 and 2-2 illustrate some of the design
criteria for offshore structures, and at the same time indicate why
concrete may be the best choice of construction material. Many
offshore locations are calm and friendly, but not all. Fig. 1-3
shows an example of an ocean where it is not straightforward to
build. The platforms may then be prefabricated elsewhere, installed
and possibly completed with respect to the foundation (piling,
ballasting, grouting) and topside installation. The degree of
inshore completion influences the cost and safety of the field
development. The typical offshore concrete structure is of the
caisson type, often termed Concrete Gravity Structure, CGS. The
caisson provides buoyancy in the construction and towing phases,
and acts as a foundation structure in the operation phase. The
caisson may also provide storage volume for oil or other liquids.
This multiple usage of the structure may prove very economic,
particularly when storage is required. Steel structures may of
course also be built to provide buoyancy and storage, but buoyancy
at large water depth is complicated and expensive for steel
structures as they are exposed to significant pressure loading.
Figure 2-1. Beryl A during tow to installation site, in 1973
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
12
Figure 2-2. The environment of the sea.
The offshore concrete structures for the oil and gas industry
are located at various and very different parts of the world. There
are structures in ice-infested waters, in seismic zones and in very
harsh marine environments, but also in relatively calm areas. Some
are located at large water depth, others in shallow areas. The
foundation conditions vary from very stiff sand to very soft clays,
and some of the structures float permanently. Some of the
structures have storage facilities, and all have a hydrocarbon
processing plant facility of some kind. Such various conditions for
the offshore concrete structure call for different designs. As
stated previously the offshore concrete structures behave very
well, and perform their task of supporting the oil and gas
processing facility. The oil companies are frequently evaluating
extension of operational life, and modifications to enable further
facilities as the offshore field may contain additional
hydrocarbons.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
13
3 The Norwegian Experience and Know-how
3.1 The structures The inshore construction of concrete offshore
structures provides good conditions for quality construction. The
construction site of Aker Kvrner at Hinna, near Stavanger in
Norway, was likely the best and most professional and effective
construction site in the world. The construction of 17 large
offshore concrete structures, created a large amount of expertise,
see Fig. 3-1. Multidisciplinary groups of specialist companies
participated in design of these platforms and their various units
and outfitting. Some of the companies contributing considerably
were Aker Kvaerner with their designs of topside, mechanical
outfitting and marine operations, Multiconsult, Aas-Jakobsen, SWECO
Grner and Dr.techn.Olav Olsen designed the concrete substructures,
both concept and detail designs, and NGI designed the
foundation.
Figure 3-1. Concrete Structures Constructed at Hinna, Norway
The structures built by Aker Kvaerner represents, in terms of
concrete volume, approximately half of the total volume of the
offshore concrete structures of the world.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
14
Many of the concrete platforms built by Aker Kvaerner are
rewarded, in Norway and internationally. In 1976 and 1995 the
Condeep platform was awarded the Norwegian Betongtavlen, in 1990
and 1998 FIPs prize for outstanding structures. In 2000 the readers
of Teknisk Ukeblad (a Norwegian engineering magazine) elected Troll
A the engineering achievement of the century. Olav Olsen was the
first Norwegian recipient of the prestigious FIP medal and the
Gustave Magnel Golden Medal, mainly due to the pioneering work of
designing offshore concrete structures. Of the more impressive
structures is the Troll A platform, shown in Fig. 3-2. The Troll A
platform, a gas wellhead and processing platform, was complete with
skirt piles and topside when towed out and installed in the North
Sea, see fig. 3-3.
Figure 3-2. The Troll A Condeep, with other structures.
The lower part of the Troll A structure was subjected to a water
pressure of 350 m (1150 ft) during construction, and carried the
topside weight of 22 000 t 150 m (490 ft) above the sea level
during tow from construction site to the offshore installation
site.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
15
Figure 3-3. The Troll A Condeep during tow-out.
The soil condition at the site of the Troll A platform is very
soft, popularly termed yoghurt. For this reason the base of the
platform (more than 16000 square meters in area) is equipped with
36 m long skirts to enable the safe foundation of the structure.
The 100-year design mud line moment is 100 000 MNm. Most of the
existing CGS s are resting on dense sand with short steel skirts
penetrating the sand for protection against scour. For soft soils,
deeper skirts are required. This skirt pile principle was developed
for Gullfaks C in the mid 1980s. The soil conditions with very soft
clay, required skirts penetrating down to stiffer layers of soil.
Gullfaks C has skirts penetrating 22 meters into the soil. Troll A
has skirts as mentioned, so does Draugen (fig. 3-4), with concrete
skirts penetrating 9 meters into the soil. The GBS platforms
Gullfaks C, Draugen and Troll A are all located in the North
Sea.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
16
Fig. 3-4. The Draugen CGS-MONO
In recent years the principle has been used also for TLP
foundations (Snorre, Heidrun), jackets (Europipe 16/11, Sleipner T)
and suction anchors for floaters as an alternative to ordinary
piles. Provided feasible soil conditions, skirt foundations have
many advantages both economically and technically compared to other
solutions:
o Lower material and fabrication cost o Reduction of foundation
area o No piling or need for heavy subsea hammers o High position
accuracy o Short installation time o Reduced need for solid
ballast
3.2 Research and development Many of the offshore concrete
structures of the world are located in moderate waterdepths. As is
seen in fig.3-1 most of the Norwegian built platforms are located
in medium to large waterdepths, with the foundation structures for
the Heidrun tension leg platform being deepest, at 345 m of
waterdepth. For installation by bouyancy it is very important to
use high strength concrete. For this reason a considerable amount
of research has been performed in Norway, as described by Moksnes
in ref. 17.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
17
This research has been very successful, and the results benefit
not only the oil and gas industry, but the entire concrete
industry. One inherent benefit is that high strength concrete is
also very durable in the marine environment. In the general sense,
pushing the limits for the application of offshore concrete
structures required a vast amount of development in many areas,
such as construction techniques (slipforming etc.), marine
operations, analyses for environmental loads and structural
response, soil testing and instrumentation, etc.
3.3 Decommissioning of offshore concrete platforms Even though
the structures may be fit for many years, international regulations
will put constraints on the use of the oceans. Particularly
important here is the OSPAR (OSlo PARis) Convention. In July 1998
it was decided that all platforms in the North Sea should be
removed after completing their duties. An exemption was made for
concrete platforms, because of the believed complexity of the
operation. Extensive work has been performed on the subject,
particularly JIP work that has proprietary rights. The
international fdration internationale du bton, fib, initialised
work on the subject, in their Task Group 3.2 Recycling of Offshore
Concrete Structures. The conclusions were:
It is feasible to remove the offshore concrete structures. Due
respect is required; we shall be humble to the task. Removing the
entire installation is most likely the safest and most cost
efficient way to
remove the topside. The Task Group 3.2 realises the political
and economic aspects of the issue. Several joint industry projects
have addressed the subject, but their conclusions are not, as
of
now, publicly available. The OSPAR Convention requires that the
topside of the concrete platforms must be removed. The work of TG
3.2 is described in Ref. 18 and 20. The OSPAR applies to the North
East Atlantic Ocean, including the North Sea. Other parts of the
world will have different rules to relate to, but the essence is
likely to be similar.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
18
4 Project Execution Typical Typically the execution of offshore
concrete structures consists of several main project phases. Some
projects may constitute all the below phases whereas others may
just include a few of these. After completion of front end
engineering design and decision to develop the project and award of
contract(s) the projects may go through the following main
phases:
- Detail design of dry dock and construction site - Detail
design of concrete structure prior to concrete structure
construction start - Dry dock and construction site development -
Construction of lower part of concrete structure inside the dry
dock - Float out of dry dock and mooring at inshore wet
construction - Construction of upper part of the concrete structure
at wet construction site - Installation of topsides facilities
and/or other type of outfitting - Tow to installation site,
positioning and installation at location
The main typical construction phases are shown in the below
figure.
Construction in dry dock (Draugen) Float out from dry dock
(Troll)
Construction at wet site (Draugen) Installation of topsides
(Draugen , deck mating)
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
19
Tow to offshore installation site (Draugen) Installation at
offshore location (Troll)
Figure 4-1: Typical main construction phases
Detail design of dry dock and construction site During this
phase detail design of the dry dock and the construction site
facilities are performed. The geotechnical design of the dry dock
should be given special attention, as this is the key to ensure a
water tight and safe dry dock. The floor in the dry dock is
normally located 10 to 15 meters below sea level and hence the dock
is exposed to this differential pressure. This phase is normally on
critical path of the project execution schedule. Detail design of
concrete structure prior to concrete structure construction start
Prior to start construction a global design of the entire structure
is performed to define all wall thicknesses and prestressing
layouts etc. as well as overall quantities and layout of
reinforcement. This phase also includes an overall design of all
the mechanical systems that should go inside the concrete
structure. Further local design and detailing of rebar arrangements
and the mechanical systems for the lower parts (the first casting
sequences) has to be completed before start of any construction
work. If the project includes development of a construction site
this design work is normally conducted concurrent to the completion
of the construction site and dependant on the duration of this
work, this phase may be on critical path of the execution schedule.
If the project does not include construction site development this
phase will be on critical path. The remaining part of the concrete
and mechanical systems detail design will be performed with an
overlap to the construction work. This part of detail design is not
normally on the critical path of the execution schedule. Dry dock
and construction site development During this phase the dock will
be excavated or digged out. Further all the facilities needed to
support the concrete structure construction work will be
established. This phase will normally be on critical path of the
execution. In some cases it is possible to use an existing dry dock
and only minor adjustments and mobilisation activities will be
required.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
20
Construction of lower part of concrete structure inside the dry
dock (See Figure 4-1) When the dock is established construction of
the lower sections of the concrete structure can commence. Normally
as much as possible would be built in the dry dock as the access
and logistics is better that at a wet construction outside the dock
or any other nearby location. However, in some cases it may be more
economic to complete more of the structure at the wet site rather
than to establish a deeper dock. Normally the concrete construction
work will be on the critical path for most of execution schedule
following start concrete structure construction. Float out of dry
dock and mooring at inshore wet construction (See Figure 4-1)
Following completion of the construction work inside the dry dock
the dock will be flooded with water to the same level as the
external sea. Then the dock berm or gate will be removed and tugs/
winches attached to move the structure out of dock and tow the
structure to a wet construction site or in some cases directly to
the installation location. To get the structure afloat internal
water ballast will be removed and the lift off carefully monitored.
To ensure a sufficient under base clearance and a controlled lift
off a comprehensive weight control system is used. Construction of
upper part of the concrete structure at wet construction site (See
Figure 4-1) When moored at the wet construction site the
construction work will continue to complete the structure. For
structures to be installed in deeper waters, say 50 metres or more,
a wet construction site would be required. During this phase a
floating rig will be established to support the construction work,
this normally consists of a number of storage and other type
barges. The construction crew will normally be shuttled from shore
during this phase. A well organised and planned logistics is of
utmost importance to ensure an efficient construction during this
phase. Installation of topsides facilities and/or other type of
outfitting (See Figure 4-1) Upon completion of the concrete
structure the topsides facilities (deck) may be installed at the
wet site. If installed at the wet site this is normally performed
as a float over (deck mating) operation. The deck will arrive to
the wet site supported on one or two barges. To transfer the deck
onto the concrete structure the structure is ballasted down with
just a few meters remaining above sea level. Then the deck is
floated over the structure, which will be gradually de-ballasted to
transfer the weight of the deck from the barges onto the concrete
structure. Alternatively the topsides facilities can be installed
at the offshore site. This may either be performed as a high deck
float over or by lifting of modules. Tow to installation site,
positioning and installation at location (See Figure 4-1) Following
completion of the concrete structure end possibly installation of
the topsides facilities the structure will be towed to the offshore
installation location where it will be positioned and installed.
The towing operation is normally conducted by utilising 3-5 ocean
going tug boats and average tow speed is in most cases around 3
knots. At the offshore location the tugs will be reconfigured to
some kind of a star formation to control the positioning of the
structure. When in position the structure is ballasted down to the
seabed by sluzing in of water. The in place foundation stability
will either be ensured by sufficient weight and friction or by
penetrating the lower parts of the structure (skirts) into the soil
skirt piling. Project execution methodology and strategy The
complexity of this type of projects requires a clear execution
strategy and a well developed and systematic methodology. Aker
Kvaerner and the Norwegian know-how cluster have over the years
developed a systematic and efficient way to execute this type of
projects.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
21
The main elements of the project execution strategy are:
- Develop and maintain one single integrated project team. -
Provide management focus and emphasis on the final result through
Improvement Leadership. - Empower and align team members through
team buildings. - Organise the project in distinct phases to
continuously control schedule and divide the product
into manageable parts with clear objectives and delegated
responsibility for the economical result.
- Focus on improved standard of HSE and quality in work from day
1. - Systematic technology transfer and training of local
personnel.
Each of these bullet points are further outlined below. Develop
and maintain one single integrated project team. The project shall
develop a competent organisation supported by effective systems and
specialists for key performance areas such as Cost, Schedule,
Quality and Safety. The organisation will be structured for project
execution through effective control and co-ordination of the main
process from Engineering, Procurement and Construction through to
Marine Operations. The organisation should be staffed by highly
qualified personnel, expatriates and locals based on the principle
of best man on the job. The integrated project management team
should be responsible for and have the experience to manage the
total EPCI process, see Figure 4-2 below.
Figure 4-2: Integrated Project Management
An overall project risk assessment shall form the basis for
prevention based programs to ensure that project milestones and key
targets are met. Identification and management of the project
schedule critical path will hold the key to a timely delivery of
the project. Extensive work will therefore be performed to identify
the critical path and review measures to reduce its length and
consequential effects.
Integrated
Project
Management
CivilConstruction
Engineering&
D i
Finance&Accounting
Facilities&Maintenance
Materials&Logistics
ProjectControl
Procurement
MechaniclInstallation
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
22
Improvement Leadership In general, an Improvement Leadership
process should be established. This process should focus continuous
improvement effort on all key result areas such as cost, schedule,
HSE, quality and regulatory compliance. Early in the project a plan
for Improvement Leadership shall be established to support the key
result areas given by the project objectives. This plan shall
contain the Management Improvement Policy, the goals and the
approach for each key result area as well as how the achievement of
the goals will be measured. The Improvement Leadership is a
continuous process that will support prevention-based programs such
as risk assessment, technology transfer, value engineering,
constructability reviews, potential problem analysis and quality
improvement. These leadership and management principles are
currently applied with great success for the ongoing Sakhalin II
project under construction in Far East Russia. Team Building and
Alignment to Project Goals Team building sessions shall be arranged
to develop the project organisation into a single integrated team
focused on the project processes and goals and to establish a
common understanding and acceptance of cultural differences,
organisational goals, responsibilities and good working
relationships. Team strength means empowered team members promoting
personal quality and having the confidence to openly discuss
problems. Local Content The Construction of a offshore concrete
structure will provide many jobs to the local society. In addition
to direct employment, a project will give many indirect benefits to
people and businesses. Individuals will develop new skills
improving their job prospects. For business, this type of work will
translate into new hiring and additional training of staff,
enhancing new skill levels, capabilities and competitiveness of
many companies. This will enable them to attract additional oil and
non-oil related work on a local, national and international scale.
Such projects should lead to a major transfer of technology to the
local society. Based on previous experience from Norway,
Newfoundland and Southeast Asia the value of the local content will
typically be in the range of 50 to 90 % of the total project value
The local content will normally be related to staff and labour
man-hours expenditures, local material supply, rented services and
equipment and comes in the following categories: Personnel
resources
- Management. - Supervision. - Labour force.
Materials
- Civil. - Mechanical. - Materials for dock and site
construction.
Equipment for site development and operation
- Camp.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
23
- Transport. - Stores. - Offices. - Security.
Labour for such projects will be recruited from the local area
and supplemented with handpicked skilled labour. An extensive
training and induction program will be put in place to prepare the
labour and local supervision for the task and to secure a proper
transfer of technology. Local supervisors will be employed to
directly supervise the local workforce under the control of
specialist expatriate supervision. A number of specialist personnel
with extensive construction experience will be included in the
management and supervision of the project. An important role for
the experts is the transfer of knowledge to the local supervising
staff and to the work force. Training sessions and mock-ups of some
parts of the structures will be used in this context. The learning
curve is reasonably short if a proper information and training
system is established. In general, therefore, it is not necessary
to bring in especially skilled labourer for the construction work.
It will, however be required to use a limited number of hands-on
foremen or lead hands in the start up phases of new construction
operations, for example for rebar installation in different parts,
slipforming operations, pre-stressing and other special work
operations. All commodity bulk materials such as Cement, Fly Ash,
Concrete Aggregates and Reinforcing Steel, Post -Tensioning tendons
and equipment, Wood and Plywood in addition to structural steel and
piping related to mechanical outfitting should to the extent
possible be procured locally. Materials for concrete production and
the properties for fresh and hardened concrete shall be qualified
in due time before start of construction. The qualification is a
relatively comprehensive exercise that could be combined with
mock-ups for training purposes. All supplies are to be sourced on a
competitive bidding basis. Materials for temporary facilities
including the construction camp, site office, workshops and stores
and associated services will be procured locally. Specialised
materials and equipment will be tendered internationally if not
available locally. Execution Methodology Aker Kvaerner has together
with partners over several years developed a compressive project
execution model that is focused on the quality of the end delivery
to the clients. To achieve this attention has to be given to the
following items: Predictability! More effective work processes
Building commercial awareness Effective communication and
utilisation Enhanced risk management & control Zero mindset HSE
philosophy Common approach provides the basis for continuous
improvement The below figure gives a high level overview of the
execution model. The core of the model is to control quality of
information at all levels. Further stringent gates are introduced
to check out that
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
24
sufficient quality of the information is reached to move forward
into the next phase or sub-phase of the project. A high degree of
automatisation of this process is obtained by using state of the
art IT/IS tools.
Figure 4-3: Project execution model, phases and levels.
STRATEGIC
EXECUTION
CONTROL
SPECIFIC WORKPROCESSES, SYSTEMS AND PROCEDURES
Corporate
Business Areas
Project
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
25
The philosophy of the model is to ensure end product quality for
all parties. This is best obtained by focusing on the system
definition and maintaining a system focus all the way through to
final completion of the product. This philosophy is illustrated on
the below figure.
Figure 4-4: Project execution philosophy.
Each of the phases as shown in Figure 5-3 is further divided
into sub-phases and detailed work process for each discipline is
developed. The end product is divided into a large number of
quality objects on which the completion (or quality level) is
constantly monitored by detailed checklists. Utilisation of this
systematic and comprehensive model has proven to give predictable
and reliable project execution.
System Design Completion
OperationProcurementFabricationConstruction
Con
trac
t Aw
ard
Take
-ove
r
Client requirements:- operation- maintenance- HSE
Client specs & rules Client experience
SYSTEM FOCUS
Internal knowledge &lessons learned
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
26
5 Ongoing Projects Offshore concrete structures has to be
considered niche products that are especially well suited for some
special applications such as: Structures for harsh environments
such as ice and iceberg infested waters Structures for LNG
facilities due to the good behaviour of concrete when subjected to
cryogenic
temperatures Structures that require local construction Over the
last 10-year period there has typically been 1-2 project ongoing at
any time. The last structure completed was the Malampaya project
that was installed in 2001. Presently there are 2 construction
projects ongoing constituting a total of 5 concrete structures. An
additional 2-4 projects are in the front-end engineering phase and
most of them may hopefully be approved by the end of next year,
some of these are outlined in the next section. The two projects
currently under construction is the Sakhalin II project for
Sakhalin Energy Investment Company (SEIC, with Shell being the lead
party) and the Adriatic LNG Terminal (ALT) project with ExxonMobil
as the lead party. Both these projects are executed using the
Norwegian know-how cluster with Aker Kvaerner being in a lead
role.
5.1 The Sakhalin II Project SEIC is currently developing phase
II of the Sakhalin II block outside the Sakhalin island in far east
Russia. The development comprises eight main items; The PA-B
Platform, PA-A Platform, an Onshore Processing Facility, the
Lunskoye Platform, an Infrastructure Upgrade Project, Onshore
Pipelines, an LNG Plant and an Oil Export Terminal. Concrete
structures will be utilised for the PA-B Platform and Lunskoye
Platform as also shown below.
Figure 5-1: The Sakhalin II Phase 2 development. Concrete
structures for the PA-B and Lunskoye platforms
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
27
Construction work is currently ongoing at the construction site
in Vostochny Port about 4 hours car ride from Vladivostok. The
construction site has been developed and construction of the two
concrete structures is well underway and the project is on
schedule. The picture in Figure 5-2 below shows the site prior to
site development (April 2003).
Figure 5-2: Concrete structures construction site in Vostochny
Port prior to start site development
The pictures in Figure 5-3 below shows the status in April 2004
(one year after start site development) and September 2004. As can
be seen the lower parts of the two structures are completed and
preparations are ongoing to start construction of the concrete
towers (shafts).
Status April 2004: Dock completed and construction of both
structures started.
Construction Site Location
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
28
Status Sept. 2004: Lower parts of both structures completed.
Preparations for construction ongoing
Figure 5-3: Sakhalin II construction work
The below Figure 5-4 identifies the locations where the main
portions of the work are performed. As can be seen the majority of
the work is performed locally and in Russia. However, design,
intial procurement and work preparation is performed outside Russia
(Norway and Finland).
Figure5-4: Main work locations
Offshore FieldLUN-A and PA-B
Tow 18 days
CGBS WorkSiteVostochny Port
CGBS / Mechanical Outfitting Detail engineering
Work preparation/Procurement
Mechanical Prefabrication
Mechanical Prefabrication
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
29
Major achievements on the project so far includes: One lost time
injury approaching 4.000.000 hours since lost time injury The
project is on schedule More than 90% local labour at site More than
95% (weight) of material purchased in Russia Technology transfer
program is working well and the need for expatriates is gradually
reduced
5.2 The Adriatic LNG Terminal Project ExxonMobil, Qatar
Petroleum and Edison Gas is currently developing a facility to
receive, store and regasify LNG in the Adriatic Sea about 15 km off
the coast of Italy. The gas will be piped to shore and sold in the
Italian market. The LNG is shipped from Qatar through the Sues
channel as shown on the below figure.
Figure 5-5: LNG shipping rout and terminal location
The facility will be developed based on a bottom fixed concrete
structure with internal LNG storage tanks and the regasification
plant on the top deck, see Figure 5-6 below. Aker Kvaerner is
executing the project under a FEED & EPCI contract.
LNGTerminalLocation
LNG Plant
LNGTransportRout
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
30
Figure 5-6: The Adriatic LNG Terminal general configuration
The FEED has been completed and detail design has commenced. The
concrete structure as well as installation of the LNG storage tanks
and the regasification plant will take place in Algeciras, Spain.
Deepening of the dock is completed and establishment of the
facilities to support the construction work is due to start.
Figure 5-7: The Algeciras construction site modification work
ongoing
Work is performed at several locations as shown on the map in
Figure 5-8 below.
Footprint Concrete Structure
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
31
Figure 5-8: Main work locations
Following the work in at the construction site the completed
structure will be towed to the installation location in the
Adriatic Sea where it will be installed and made ready for
receiving LNG. The LNG carriers will be moored directly to the side
of the structure. The mooring and berthing facilities are displayed
in Figure 5-6 above.
Client HQ Project Management Topside Engineering
LNG Tanks EngineeringProject Management
Technical Co-ordinationGBS Engineering
GBS Construction andTerminal Assembly
Pipeline Engineering
Topside modules - Europe,Korea etc.Tank fabrication - Europe
orJapan
Installation, Commissioningand Start-up
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
32
6 Novel Concepts Over the last couple of years we have seen a
growing interest for offshore concrete structures. This has mainly
been driven by the oil companies desire to start developing oil and
gas fields in more harsh environmental regions such as outside the
Sakhalin island (Russia), in the Barents (Russia) and at Grand
Banks outside Newfoundland (Canada). Further it has also been
driven by the need to develop more LNG export and import terminals,
especially we see a number of terminals being considered for the
North American market. In addition there is a growing desire to
increase the local content for these types of developments.
Concrete structures are well suited to meet all of these three
demands. Aker Kvaerner and our partners are constantly monitoring
the market to be in front with regards to developing the concepts
and technology required to respond to the market demand. Three
areas are considered to be of special interest: Further
developments outside the Sakhalin island, where we may see a need
for more that 15 new
platforms over the next 10-15 years. If so it should be possible
to develop a long-term sustainable business in the region.
On Grand Banks outside Newfoundland it is very likely that we
will see another large oil field being developed on the basis of
using a large ice berg resistant concrete structure.
LNG terminals may represent the most promising market for this
type of structures over the next 5-10 year period. It is likely
that we will see a number of these terminals being developed for
North America, Mediterranean region and also for far east countries
such as Japan and Korea. Just now we are working on a number of
possible prospects where the two most advanced are:
The Port Pelican terminal (ChevronTexaco) in the Gulf of Mexico,
and The Baja California terminal in Mexico. Both these terminals
are currently in the front end engineering design phase. In
addition to the near term project opportunities briefly outlined
above we are constantly striving to develop new and novel solution
targeted for the somewhat more distant future. Some examples are
given below.
6.1 Floating LNG Terminals As the gas fields to be developed is
situated at deeper and deeper waters the industry is looking for
solutions to liquify the gas (make LNG) at the field rather than
build long and expensive pipelines to shore. Together with some key
technology partners we are trying to develop solutions for this.
The main challenges are: LNG transfer in the open sea sufficient
regularity Sloshing of LNG inside the tanks for partly filled tanks
Vessel motions and in impact on large rotating equipment on
topsides Significant topside facilities large weights
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
33
An example of such a concept is shown in Figure 6-1 below:
Figure 6-1: Floating LNG export terminal with concrete hull
6.2 Floating airport or navy base A few years ago we worked with
US Navy to develop a concept for a relocatable US Navy base. The
concept is based on four large self-propelled semisubmersible hulls
that could be connect together to create a complete navy base. The
idea is that is will be much more economic and faster way to
mobilise large military forces. This would also reduce the need for
regional presence. The concept is shown in the below Figure 6-2.
The same basic principle may be applied for regular airports.
Figure 6-2: Mobile Offshore Navy Base.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
34
6.3 MPU Heavy Lifter As mentioned previously there are
requirements to remove offshore steel platforms (OSPAR Convention).
For the North Sea alone this represents a market value of some 10
billion $. This market potential initiated the development of a
robust, inexpensive Heavy Lifter that utilises the simple principle
of Archimedes in order to lift straight up. One solution is a
concrete U-shaped semi-submersible Heavy Lifter, designed based on
the principles Simple, Safe, Robust and Cost-effective.
Dr.techn.Olav Olsen developed the design; the ownership is by MPU
Enterprise (ref. 21, 22 and 23.)
Figure 6-3. The MPU Heavy Lifter for removing and installing
offshore structures.
Figure 6-4. The MPU Heavy Lifter in the tank-test.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
35
6.4 MPU Semo The experience from the hydrodynamic tests of the
Heavy Lifter in fig. 6-4 gave strengthened confidence in the MPU
SEMO, shown in fig. 6-5. The SEMO is a floating mono-hull
structure, built of concrete. The incentive of the design was the
complexity of the moored ship solution, currently a popular option
for offshore field development. Such ships are swivelling around a
turret that is moored to the seabed. The design philosophy of the
SEMO is that it may be round, and not ship-shaped, as it is not
going anywhere. It may also be considered an enlarged turret,
without a complicating body attached to it. Local content will be
more important in the future, and the MPU-SEMO creates interesting
opportunities with regard to local fabrication and assembly. For
many countries it is important to build new industry and to further
develop the economy. The fabrication of an MPU-SEMO can give
significant amount of work locally, which could give such a concept
a political advantage compared to solutions built in SE Asia or
Europe.
Figure 6-5. The MPU SEMO.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
36
6.5 Other novel concepts
Figure 6-6. Fixed /floating LNG storage
Figure 6-7. GBS Nnwah-Bilah LNG Project
Figure 6-8.
A proposal for an offshore terminal, containing storage for oil,
condensate and gas.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
37
Figure 6-9. Submerged Floating Tunnel
A solution for future infrastructure,by the Norwegian Submerged
Floating Tunnel Company
(ref.25)
Figure 6-10. LNG Terminals
Figure 6-11. Suction anchors
Built for the Snorre TLP.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
38
Figure 6-12.Urban development on floating concrete structures,
developed by Marfloat
Figure 6-13. Urban development. High rise on prefabricated
stranded cellar/parking structure.
Yogamid developed by Finn Sandml
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
39
7 Project Execution in Mexico ChevronTexaco is currently
developing a solution for an offshore LNG receiving terminal to be
installed inside the Coronado island outside Rosarito in Mexico,
Figure 9-1 below shows the overall configuration of the proposed
terminal. Aker Kvaerner and our partners are supporting
ChevronTexaco in this effort and one of our tasks is to assist in
assessing project execution in Mexico.
Figure 7-1: The proposed Baja California terminal
configuration
7.1 Engineering and Design
7.1.1 Conceptual design The success of conceptual design
requires an overall understanding of the key elements that is
governing for the offshore concrete structure. Most of these
elements are described in this paper, such as functional
requirements and close relation with the construction methods. In
this respect conceptual design of offshore concrete structures is
similar to any other conceptual design of structures. The main
difference is the required understanding of the water, both as an
acting load and as a medium for floating.
7.1.2 Detail design A typical construction project may require
somewhere in the range of 1000 drawings, for the concrete structure
only. These drawings do not only describe the concrete geometry and
reinforcement, but all
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
40
details of connections, anchors, pipes, openings etc. Such
projects are multidisciplinary, and it is a considerable task to
organize and manage them. For the structural design specialized
tools have been developed for analyses and design. These tools are
generally commercially available.
7.1.3 Rules and Regulations proposed for concrete projects in
Mexico There are several recognized international rules and
regulations pertaining to the design and execution of offshore
concrete structures. The overall common requirement is that the
structure shall be designed, executed, transported and installed in
such a way that:
The reliability level of the installed platform meets the
intended reliability level. All functional and structural
requirements are met.
The present draft ISO/CD 19903 Petroleum and natural gas
industries - Offshore structures - Fixed concrete structures, lists
all those areas of design that are particular to offshore concrete
structures, and acknowledges that design may be performed according
to national standards provided it is supplemented with additional
rules for all those areas not properly covered by the national
standard. It then in a note states that the Norwegian Standard NS
3473, ref. 24, is recognized to meet all those requirements
relevant for the design of offshore concrete structures. This
Norwegian Standard is available in the English language. To the
best of our knowledge, there should not be any special difficulties
with respect to rules and regulations for a concrete offshore
structure constructed in Mexico. National regulations and standards
applicable in the place of use of the structure can be different
from those given in international standards, such as the ISO suit
of standards. In such cases it must be ensured that the
requirements of safety and durability are met. This applies to all
phases of planning, design, execution, transportation, installation
and possible removal. In Norway the following main class notation
is used for ship-shaped floating structures complying with Det
Norske Veritas (DNV) class requirements and the Norwegian rules and
regulations as issued by the Norwegian Petroleum Directorate: +1A1
Oil Production and Storage Vessel (N) The analyses and design will
then follow the extended calculation procedures for hull structures
- additional class notation CSA-2. The wave loading experienced by
a permanently moored barge is quite different compared to that of a
sailing merchant ship (may in general vary some 25 to 35% above
that of the response calculated from DNV rules for merchant ships).
This calls for a separate hydrodynamic analysis, independently of
the fact that concrete barges are unusual and cannot easily be
fitted into existing standard steel categories. The analyses and
design of a concrete hull will therefore follow the traditional
approach for offshore concrete floaters, as demonstrated and
approved in detail engineering of the ARCO barge, Heidrun TLP,
Troll Oil Semi and the Nkossa barge:
Hydrostatic analyses (drafts, internal/external water levels,
still water moments and shear forces, afloat stability - intact and
damage)
Hydrodynamic analyses (global responses and hydrodynamic wave
pressures)
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
41
Mooring analyses and design Structural response analyses (finite
element analyses) Structural design verification (code checking
according to Norwegian standard
NS3473 or DNV concrete design rules harmonized with the rules in
NS3473) We are not aware of any offshore concrete structure that
has been built to other national standards than the Norwegian NS
3473, without the need of extensive supplements. Areas normally not
adequately covered for offshore structures are such as; fatigue,
tightness, design provisions for shell type members, design for
durability and cracking etc. Elf Congos Nkossa barge has been
designed to Bureau Veritas shipbuilding specifications, using the
NS3473 for the concrete design verification, where it is registered
as a certified hull. The Canadian Hibernia platform and the
Australian West Tuna, Bream B and Wandoo platforms have all been
designed according to NS 3473. The detail design of the Sakhalin
GBS is carried out in accordance with the approach set out by Det
Norske Veritas Rules for Classification of Fixed Offshore
Installations (DNV Rules). The reference standard for concrete
design is British Standard BS8110, but according to what stated
above, specific interpretations and additions have been necessary
in order to make this standard applicable for offshore structures.
For this purpose the Norwegian standard NS 3473 is used as
supplement. BS8110 is most likely used because the initial
conceptual phases were performed by a UK consultancy. The reference
standard to be used should be agreed at an early stage in the
project, as the choice of standard might strongly influence the
platform geometry and dimensions, while standards not intended for
offshore use might be unnecessarily conservative on certain aspects
relevant to offshore conditions. The reference standard shall give
the design parameters required for the type of concrete, e.g.
normal weight or lightweight concrete, and strength class used. For
high strength concretes and lightweight concrete, the effect of
reduced ductility shall be considered. This in particular applies
to the stress/strain diagram in compression, and the design
parameter used for the tensile strength in calculation of bond
strength, and transverse shear resistance.
7.2 Fabrication Site A general concrete substructure
construction facility is described previously. It should have
reasonable supporting infrastructure facilities and labour
resources, to support the scale of construction required. Detailed
evaluation of specific sites would allow cost and land availability
issues to be determined.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
42
Figure 7-2. A typical construction dock for a GBS facility.
By way of example, the green-field construction facility
prepared for the Malampaya concrete gravity substructure is
illustrated in Figure 7-2. The construction facility was located in
a remote part of Subic Bay in the Philippines.
7.3 Options for fabrication A medium size concrete platform
built in concrete will require a fabrication dock with an area of
some 140 x 140 m, and a water depth of 10-12 m. The concrete hull
can be completed in the dock if a sufficient water depth is
available, or it can be completed in a floating condition at a
place outside the dry dock, with sufficient water depth. The cost
of a fabrication dock will depend on the soil, size, depth etc. In
Australia and in the Philippines a gravel dock was prepared for the
Wandoo and the Malampaya platforms at the cost of US$ 8 mill.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
43
There is also a possibility that the concrete can be built on
flat land and skidded into the sea. The lower hull may be built to
a level when it can float, and the remaining of the hull will be
built while floating.
7.4 Construction and Methods The small concrete platform is
ideal for slip forming of the vertical walls. Slip forming is a
very effective fabrication method for high concrete structures.
Even the lower hull with an assumed height of some 10-12 m will
most likely be slip formed. There are advanced slip-forming systems
allowing for changes in slip forming geometry over the height of
the structure. I.e. the upper hull can be built with a sloping
outer and/or inner wall. In Figure 7-3 below one slip forming
system is presented. The mechanical outfitting will be placed
during construction, in sequence with other construction
activities. All steel will be attached to the concrete at
preinstalled embedment plates. Such plates are placed during the
slip forming, and after slip forming steel pipes etc will be welded
onto embedded steel plates. Penetrations through walls or slabs
will also be placed during cast/slip forming.
Figure 7-3. Interform slipforming system
7.5 Material Qualities Any concrete platform and all
appurtenances, piping and fitting shall be fabricated from
materials suitable for the service and life of the facilities. The
concrete contractor shall develop a concrete mix that satisfies
strength, durability requirements in their operating environment
and construction methodology. Steel elements shall be selected to
resist the factors of design stresses, fatigue, corrosion and
brittle fracture. Additional factors shall be considered with
respect to hot and cold working and weld ability including
resistance to lamellar tearing.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
44
The concrete mix designs shall be in accordance with the
concrete specification given by the civil contractor. As a minimum,
the concrete specification shall cover the following items:
mechanical and chemical strength, mix design, batching,
workability, durability, testing, reinforcement corrosion, warm
weather concreting and quality control. For small concrete
platforms there are 2 types of concrete, which can be used; normal
density (ND) and light weight aggregate (LWA) concrete. The
LWA-concrete has about 20% less density than the ND-concrete, which
is an advantage for floaters. However, small concrete platforms are
partly weight stable platforms and needs weight in the base. Hence
it may be beneficial to use ND-concrete in the lower part and LWA
in the upper part. The advantage must be compared to the possible
disadvantage of dealing with 2 different materials at the site and
in engineering. In a feasibility study the design of the concrete
hull is recommended to be based on Normal Density (ND) concrete
grade C60 and lightweight aggregate concrete (LWA) concrete grade
LC55, both as defined by NS3473. These concrete qualities are well
proven in many countries, and are regarded as fairly
straightforward to produce. For ordinary and pre-stressed
reinforcement the grades KT500TE (NS 3570) and St 1570/1770
(EURONORM) are recommended.
7.5.1 Concrete Concrete grade : ND C60 LC55 Structural material
strength fcn (MPa) : 36.4 33.6 Design compressive strength, ULS fcd
(MPa) : 29.1 26.9 Nominal structural tensile strength ftn (MPa) :
2.375 2.25 Design tensile strength, ULS ftd (MPa) : 1.90 1.8
Modulus of Elasticity Plain concrete, ULS Ecn (MPa) : 29 399 Plain
concrete, SLS Eck (MPa) : 30 534 23 226 Plain concrete, FLS
(0.8Eck) Eck (MPa) : 24 427 18 581 Static, short-term loads (incl.
reinf.) Ec (MPa) : 35 000 25 000 Dynamic Ecdyn (MPa) : 40 000
Poisson's Ratio, Coefficient of Thermal Expansion Poisson's ratio =
0.20 The coefficient of thermal expansion T = 1010-6 oC-1
Density ND C60 LWA C60
Plain concrete: 24.0 kN/m3 (2.45 t/m3) 19.3 kN/m3 (1.95 t/m3)
Reinf. concrete: 150kg/m3 300kg/m3 500kg/m3
25.0 kN/m3 (2.55 t/m3) 26.0 kN/m3 (2.65 t/m3) 27.5 kN/m3 (2.80
t/m3)
20.4 kN/m3 (2.08 t/m3) 21.1kN/m3 (2.15 t/m3) 22.6kN/m3 (2.30
t/m3)
7.5.2 Ordinary reinforcement Design Strength, Grade K500TE Yield
stress :fy = 500 MPa Design strength, ULS :fs = 435 MPa The yield
strain is y = 2.5 x 10-3
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
45
Nominal diameters are 8, 10, 12, 16, 20, 25 and 32 mm. Modulus
of Elasticity Es = 2.0105 Mpa
7.5.3 Prestressed reinforcement General The pre-stressing steel
to be used is Grade 270 according to ASTM A 416-85. Strand
Properties The properties of pre-stressing strands are tabulated
below.
Strand type 15 mm (0.6") Nominal diameter 15.2 Nominal area 140
Nominal mass 1.10 Yield strength 1670 Young's Modulus 1.95105
Tendon units: 6-7, 6-12, 6-19, 6-20, 6-27 and 6-37 may be used.
For this study a pre-stressing force of 150 KN/strand will be
used.
7.6 Cathodic Protection In a concrete structure there is a
considerable amount of reinforcement steel. For a concrete offshore
structure the reinforcement steel is covered by minimum 50 mm of
concrete to protect against seawater penetrating into the
reinforcement and subsequent corrosion. In addition to a
significant concrete cover the reinforcement steel is connected to
a cathodic protection system together with all other steel
structures attached to the concrete structure. The reinforcement
steel is acting as wires between the different anodes, usually made
of aluminium. The steel outfitting will, in addition to being
connected to anodes, be surface protected according to regulations
and design life requirements. The aluminium anodes are mounted onto
steel rods, which are welded to embedment plates. Such anodes can
be spotted on the upper part of the Snorre TLP anchors in Figure
7-4 below.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
46
Figure 7-4. Snorre TLP anchors with anodes mounted in upper
part
The design of the corrosion protection system is usually based
on the following assumptions:
there is no electrical insulation between topside structural
steel and mechanical outfitting (MMO and CMO), risers, moorings
etc., attached to the hull
all steel in flooded tanks, cells/shafts is electrically
continuous with the reinforcement via the embedment plates
7.7 Steel/Concrete Connection Methods Steel/concrete connections
are generally based on the principle of embedding steel into the
concrete behind reinforcement steel layers so that the embedded
item becomes a part of the structure. In order to reach behind the
layers of reinforcement typically dowels with heads, steel rods
with shear keys or similar can be used. Examples of steel/concrete
connections are connection between topside and hull, connection
between concrete lower hull and steel upper hull, attachment of
risers, fairleads, towing and mooring brackets, piping support,
support of external and internal steel decks etc.
7.7.1 Riser support The riser support will be attached to the
concrete hull via embedment plates described below. The size and
capacity of the embedment plates will be selected to resist the
loads from the riser system. In case of steel catenary risers the
flexi-joints may be attached to the upper edge of the lower hull.
This will give a good load-bearing capacity.
7.7.2 Mooring brackets / Towing brackets / Fairleads
Pre-stressing tendons are used to attach the steel plate to the
concrete. In addition the rear steel surface may be fitted with
shear keys etc.
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
47
7.7.3 Embedment plates Figure 7-5. The plate is connected to the
concrete via dowels with forged head. Behind each plate there will
be an additional amount of ordinary reinforcement. Typically there
are many types of standard plates designed covering the full
spectrum of loads. Some embedment plates have shear profiles
embedded into the concrete, which make them able to resist large
shear forces. Towing and mooring brackets are also a type of
embedment plates usually connected to the concrete with post
tensioning cables instead of, or in addition to, dowels.
Figure 7-5. Typical embedment plate
-
XIV National Conference on Structural Engineering, Acapulco 2004
Offshore Structures A new challenge
48
References [1] Morgan, R. G. Development of the concrete hull.
Concrete Afloat, Proceedings of the
conference on concrete ships and floating structur