SHIP TYPES BY PHYSICAL SUPPORT
Ships can also be classified by the means of physical support
while in operation. Three broad classifications that are
frequently used by naval architects are shown at the following
slide reproduced from an “Introduction to Naval Architecture”
by Gillmer and Johnson.
• Aerostatic Support
• Hydrodynamic Support
• Hydrostatic Support
Another type of classification of ships
AEROSTATIC SUPPORT
Aerostatic support is achieved when the vessel rides on a cushion of air
generated by lift fans. These vessels tend to be lighter weight and
higher speed vessels. The two basic types of vessels supported
aerostatically are
• air cushion vehicle (ACV) hovercraft
• surface effect ships (SES)
Air Cushion Vehicles (ACV)
Air Cushion Vehicles (ACVs) or hovercraft continuously force air
under the vessel allowing some of the air to escape around the
perimeter as new air is forced downwards. They are usually
propelled forward by airplane propeller type devices above the
surface of the water with rudders behind the air flow to control
the vessel.
Hovercrafts are very expensive for their size, but have the unique
property of being amphibious. The Navy utilizes some hovercraft
as LCACs (Landing Craft Air Cushion vehicles) because of this
ability. Their use has opened over 75% of the world's coastline to
amphibious assault compared with 5% with conventional landing
craft.
Surface Effect Ship (SES)
The Surface Effect Ship (SES) or Captured Air Bubble (CAB) craft,
are similar to ACV’s in that they use a cushion of air to lift the vessel.
However, the SES has rigid side walls that extend into the water. This
prevents the SES from being amphibious but reduces the air pumping
requirements and makes them more directionally stable. The side
walls also contribute to the hydrostatic or hydrodynamic support of
the craft allowing the SES to carry more payload. They are usually
propelled by water jets or super cavitating propellers.
A supercavitating propeller is a kind of screw propeller so shaped as
to create a steady cavitation space and prevents the cavitation vapor
bubbles from collapsing on the blades where it might damage the
blade.
The vessel utilises a cushion of air trapped between the sidewalls and flexible bow and stern
seals to lift a large part of the hull clear of the water to reduce drag
HYDRODYNAMIC SUPPORT
Hydro is the prefix for water and dynamic indicates movement.
The two basic types of vessels supported hydrodynamically are
• planing vessels
• hydrofoils.
Planing Hulls
Planing hulls are designed to run on top of the water at high
speeds. To achieve this they typically have a very flat stern. The
hull design (shape) does not limit the maximum attainable
speed but does affect the power required for it to get top of the
water.
Planing hulls use the hydrodynamic pressures developed on the hull at
high speeds to support the ship. They are very fast, some capable over
50 knots. In smooth water they ride very comfortably. When moving
through waves, planing vessels ride very roughly, heavily stressing
both the vessel structure and passengers. This was particularly true of
older types which used relatively flat bottom hulls. Modifications to
the basic hull form, such as deep V-shaped sections, have helped to
alleviate this problem somewhat.
Planing hulls require much larger engines for their size than
displacement hulls. These factors above serve to limit the size of
planing vessels. However, these ships are used in a variety of roles
such as pleasure boats, patrol boats, missile boats, and racing boats.
At slow speeds the planing craft acts like a displacement ship and is
supported hydrostatically.
Hydrofoils Hydrofoil craft are supported by underwater foils, like the wings of
an aircraft. At high speeds these underwater surfaces develop lift and
raise the hull out of the water. Bernoulli’s Principle is often used to
explain how a wing develops lift. These vessels are very fast, reaching
speeds of 40 - 60 knots and compared to planing boats, hydrofoils
experience much lower vertical accelerations in moderate sea states
making them more comfortable to ride.
The hydrofoil can become uncomfortable or even dangerous in heavy
sea states due to the foils breaking clear of the water and the hull
impacting the waves. If the seaway becomes too rough the dynamic
support is not used, and the ship becomes a displacement vessel.
The need for the hydrofoils to produce enough upward force to lift
the ship out of the water places practical constraints on the vessel's
size. Therefore, the potential crew and cargo carrying capacity of
these boats is limited. Hydrofoils are also very expensive for their
size in comparison to conventional displacement vessels.
The U.S. Navy formerly used hydrofoils as patrol craft and to carry
anti-ship missiles (Pegasus Class), but does not use them anymore due
to their high acquisition and maintenance costs.
HMCS Bras d'Or (FHE 400) was a hydrofoil that served in the Canadian
Forces from 1968 to 1971 the vessel exceeded 63 knots (117 km/h;
72 mph), making her possibly the fastest warship in the world
HYDROSTATIC SUPPORT
Hydrostatically supported vessels are by far the most common
type of water borne craft. They describe any vessel that is
supported by “Archimedes Principle”.
Word definition of Archimedes Principle
“An object partially or fully submerged in a fluid will experience a
resultant vertical force equal in magnitude to the weight of the volume of
fluid displaced by the object.”
This force is called the “buoyant force” or the “force of buoyancy”.
Archimedes Principle
Archimedes Principle can be written in mathematical format as
follows.
F=ρg ∇
F is the magnitude of the resultant buoyant force
(kg.m/s2 = N)
ρ is the density of the fluid (kg/m3)
g is the acceleration due to gravity (m/s2)
∇ is the volume of fluid displaced by the object in (m 3)
Displacement Ships
Hydrostatically supported ships are referred to as “displacement ships”, since they
float by displacing their own weight in water, according to Archimedes Principle.
These are the oldest form of ships coming in all sizes and being used for such
varied purposes as carrying cargo, launching and recovering aircraft,
transporting people, fishing, and war fighting.
Displacement hulls have the advantage of being a very old and common
type of ship. Therefore, many aspects of their performance and cost
have been well studied. In comparison to other types of vessels the cost
of displacement ships is fairly low with respect to the amount of payload
they can carry. Disadvantages of displacement vessels include their
limited speed and at times, their seakeeping ability (how they respond
to ocean waves).
SWATH
A special displacement ship is the Small Waterplane Area Twin Hull
(SWATH). Most of the underwater volume in the SWATH ship is
concentrated well below the water's surface as shown in the
following figure. This gives them very good seakeeping
characteristics.
They also have a large open deck and are therefore useful in a variety
of applications requiring stable platforms and a large expanse of deck
space. SWATH vessels are currently utilized as cruise ships, ferries,
research vessels, and towed array platforms.
Two major disadvantages of SWATH ships are deep draft and cost.
Additionally, these vessels present the designer with structural
problems differing from other ships, particularly with respect to
transverse bending moments.
Submarines
Submarines are hydrostatically supported but above 3 to 5 knots
depth control can be achieved hydrodynamically due to the lift
created by the submarines planes and body of the hull.
Submarines have typically been used as weapons of war, but lately
have also seen some non-military application. Some submarines
are being designed for the purpose of viewing underwater life
and reefs. Unmanned submersibles have been used for scientific
purposes, such as finding the Titanic, as well as a wide variety of
oceanographic research.
As our world continues to expand in population and energy
consuming products are ever growing, we are dependent on
“energy” more than ever. Oil and gas are still our most
important source of energy.
Within the world of oil and gas, crude oil is called “petroleum”.
Petroleum is a combination of the Greek word PETRA and the
Latin word OLEUM which means “Rock oil”. Crude oil actually
comes from the rocks which is entrapped within the rock
formations. According to scientists, oil and gas come from the
remains of the plants and miniscule animals that lived and died in
the sea millions of years ago. As time passed large amounts of
sediment covered the organic material. The pressure and heat on
the organic material transforms this material into oil and gas
during millions of years.
Early Days
In the early years of 1800 whale oil was used for illumination purposes. As
a consequence whales have been hunted to extinction and people were
anxious to find new alternatives.
Around these times an oil well near Pennsylvania came to the surface of
the land. It literally leaked out of the rocks which inspired a man named
Colonel Drake to recover this rock oil and sell it as an inexpensive
substitute for whale oil. After some years of trial and error resulted in the
technique of drilling to collect the oil from its point of origin, initially at a
depth of 21 metres. In 1897 this drilling is extended to depth of 90 metres
in the ocean, the first steps to offshore activities.
The first real offshore oil was found 9 seamiles offshore in a water depth of
as little as 6 metres. From then on over the last 50 years progress has been
revolutionary. Offshore oil and gas developments are now taking place in
over 40 countries hundreds of kilometres from the offshore in ever
increasing depths.
Definition of Offshore
Offshore refers to industrial activities in open sea, starting from the
search of oil and gas to production and transporting them to shore.
In this section a brief overview of fixed and mobile offshore units, such
as dredgers, pipe laying vessels, drilling vessels, oil production, storage
and offloading units and several types of support and transportation
vessels are going to be given.
Seismic Survey Vessel
The decision to drill at a given location is based on the result of seismic and
geological surveys of the structure underlying the sea bed. The purpose of
this vessel is to produce detailed information for oil companies as a basis for
actual production drilling. This information is the result of the evaluated
reflected sound waves in the sea floor. To obtain these results sound waves are
initiated by the vessel by means of air guns, the reflections are collected by a
number of detectors within long cables towed by the vessel
Jack-ups
A jack-up rig is a type of mobile platform that is able to stand still on the
sea floor, resting on a number of supporting legs. The most popular designs
use 3 independent legs, although some jackups have 4 legs or more.
The jack-up drilling rig is used for exploration drilling approximately 10
metres to maximum 150 metres water depth. The jack-up barge is a
triangularly or rectangular shaped barge that is towed to the work location.
At the location the barge raises its deck alongside the legs with the lower ends
of the legs resting on the seabed.
Usually three legged ones are used for exploration drilling and 4 legged ones
as work barge for construction work.
Long distance transport of jack-ups is by towing with a tug or by heavy lift
transport ships.
Drilling operations take place in the
elevated condition with the platform
standing on the sea bed. This type of
platform is used for drilling operations
in water depths up to about 100 m. Jack-
ups spend part of their life as floating
structures. This is when such platforms
are towed to a new location
by means of ocean-going tugs. In this
mode, the legs are lifted up and extend
upwards over the platform.
Drilling vessels
When geological predictions based on seismic surveys have been established
that a particular offshore area offers promising prospects for finding oil, a
well is drilled to examine these predictions. These drilling operations are
carried out from barges, ships, semi-submersibles or jack-up rigs in medium
to deep water.(150 to 3000 metres water depth)
To maintain position during drilling operations the ships are either anchor
moored in an anchor pattern or rely on dynamic positioning (DP) depending
on the water depth.
A dynamically positioned (DP) vessel uses its propellers, rudders, tunnel
thrusters to stay in position. A control system continuously determines the
required vector based on information from a position reference system,
like radio or hydro-acoustic beacons or GPS.
Semi-submersible drilling unit
A semi-submersible drilling unit is used for
drilling the exploration and production
wells in 150 to 2500 metres water depth
An important advantage of the semi
submersible type in comparison with the
ship shaped typed drilling vessel is the better
motion behaviour of the unit in harsh
environments which can give an extended
working window.
A crane vessel, crane ship or floating crane is a
ship with a crane specialized in lifting heavy loads. The
largest crane vessels are used for offshore construction.
Conventional monohulls are used, but the largest crane
vessels are often catamaran or semi-submersible types as
they have increased stability. On a sheerleg crane, the
crane is fixed and cannot rotate, and the vessel therefore
is maneuvered to place loads.
Crane vessels
Pipelaying barges/semisubs/vessels
For the installation of oil and gas pipelines,
• anchor moored or DP flat bottomed barges,
• semi-submersibles or
• ship shaped vessels
are used.
One of the problems of laying pipes on the sea bed lies in the
limited capacity of the pipe to accept bending stresses. As a
result, it is necessary to keep a pipe line under considerable
tension during the laying operation otherwise the pipe might
buckle and collapse. The tension in the pipe line is maintained
using the anchor lines of the pipe laying vessel.
Pipe laying vessels can consist of semi-submersibles or ship-
shaped hulls. Semi-submersibles have the advantage of better
motion characteristics in waves which is beneficial for the
pipe laying operation. On the other hand, ship-shaped vessels
have a higher variable load capacity and a much higher transit
speed.
Pipe laying vessels are usually moored by means of anchor
systems which are continually being relocated as the laying
operation progresses. A new development is a pipe laying vessels
kept in position and deriving the pipe line tension by using a
dynamic positioning (DP) system instead of anchor lines; for
instance pipe laying vessel ’Solitaire’, operated since 1998 by
Allseas Marine Contractors. This figure shows a comparison of
this vessel with the much smaller ’Lorelay’ of this company,
operated since 1986. The considerable increase of size is obvious
here.
After laying the pipe, it has - in many cases - to be buried in
the sea bed or to be covered by gravel stone. Another
possibility is to tow a trencher along the pipe, which acts as a
huge plow. The trencher lifts the pipe, plows a trench and
lowers the pipe trench behind it. The sea current takes care of
filling the trench to cover the pipe.