1)
By Nagaraj.S.Anavekar
INTRODUCTION TO OFFSHORE STRUCTURES & DESIGN
(FOR OIL & GAS - EXPLORATION & PRODUCTION)
1.0 TYPES OF OFFSHORE STUCTURS
FIXED OFFSHORE STRUCTURES
a) Jacket or Template structures ( up to 300m)
b) Compliant Towers ( up to 600m)
c) Concrete gravity Platforms ( up to 100m)
d) Jack-up Rigs ( float during towing) ( up to 80m )
FLOATING OFFSHORE STRUCTURES (up to 2500m)
a) Tension Leg Platforms (TLP)
b) Semi-submersibles rigs (FPU)
c) Spar platforms (SPAR)
d) Ship shaped (FPSO)
2.0 OIL & GAS PROD. PROCESS OVERVIEW
3.0 DESIGN OF FIXED OFFSORE STRUCTURS
3.1 DESIGN CODES (Related to all types of offshore structures
)
a) AMERICAN PETROLEUM INSTITUTE
1. API RP2A-WSD (21st edition) Recommended practice for
Planning, Designing and constructing Fixed Offshore
Platforms-Working Stress Design.
2. API RP2A-LRFD (21st edition) Recommended practice for
Planning, Designing and constructing Fixed Offshore Platforms-Load
&Resistance Factor Design.
3. API RP2L (4th edition) - Recommended practice for Planning,
Designing and constructing Heliports for Fixed Offshore
Platforms
b) AISC WSD 9th Edition.
Manual of steel construction Allowable stress design (AISC
American Institute of steel construction, Inc. Specification for
Structural steel buildings, Chicago, Illinois-USA)
c) CAP 437 (Civil Aviation Publication 437)
Offshore Helicopter Landing Areas- Guidance on Standards -
Published by Safety Regulation group of CIVIL AVIATION AUTHORITY,
United Kingdom (UK).
d) DNV Det Norske Veritas (Norway)
Following are the DNV offshore codes:
Recommended Practices
1DNV RP C2012002Buckling Strength of Plates
2DNV RP C2022002Buckling Strength of Shells
3DNV RP C2032001Fatigue Strength Analysis of Offshore Steel
Structures
4DNV RP C2042004Design Against Accidental Loads
5DNV RP B4012005Cathodic Protection Design
Offshore Service Specifications
1DNV OSS 1012001 Rules for Classification of Offshore Drilling
and Support units
2DNV OSS 1022001 Rules for Classification of Floating Production
and Storage Units
3DNV OSS 103 Draft 2001 Rules for Classification of LNG/LPG
Floating Production
and Storage Units
Offshore Standards
1DNV OS C1012000Design of Offshore Structures, General
2DNV OS C1022000Structural Design of Offshore Ships
3DNV OS C1032000Structural Design of Column-stabilised Units
(LRFD Method)
4DNV OS C1042001Structural design of self-elevating Units
(LRFD-method)
5DNV OS C1052001Structural Design of Tension-leg units (LRFD
Method)
6DNV OS C1062001Structural Design of Deep-draught Units (LRFD
Method)
7DNV OS C2012002Structural Design of Offshore Units (WSD
method)
8DNV OS C4012001Fabrication of Offshore Structures
9DNV OS E4012001Helicopter Decks
Classification Notes
1Note No. 30-1July 1995Buckling Strength Analysis
2Note No. 30-2Aug 1984Fatigue Strength Analysis for Mobile
Offshore Units
3Note No. 30-3Oct 1987Spherical Shells subjected to Compressive
Stresses
4Note No. 30-4Feb 1992Foundations
5Note No. 30-5Mar 2000Environmental Conditions and Environmental
Loads
7Note No. 30-7June 2001Fatigue Assessment of Ship Structures
9Note No. 31-1June 1999Strength Analysis of Hull Structures in
Bulk carriers
10Note No. 31-2May 1980Strength Analysis of Hull Structures in
Roll on / Roll off Ships
11Note No. 31-3Jan 1999 Strength Analysis of Hull Structures in
Tankers
12Note No. 31-4Sep 1987Column stabilised Units (Semi submersible
Platforms)
13Note No. 31-5Feb 1992Strength Analysis of Main Structures of
Self Elevating Units
14Note No. 20-2Feb 1990Ships - Inclining Test & Lightweight
Survey
e) British codes (BSI) ( Mainly for British offshore
sector).
1. BSI BS EN ISO 19900 01/01/2002 - Petroleum and natural gas
industries General requirements for offshore structures-
2. BSI BS EN ISO 19901-1 01/25/2006 Petroleum and natural gas
industries Specific requirements for offshore structures Part 1:
Metocean design and operating considerations.
3. BSI BS EN ISO 19901-2 04/07/2005 Petroleum and natural gas
industries - Specific requirements for offshore structures - Part
2: Seismic design procedures and criteria.
4. BSI BS EN ISO 19901-4 08/26/2003 Petroleum and natural gas
industries Specific requirements for offshore structures Part 4:
Geotechnical and foundation design considerations.
5. BSI BS EN ISO 19901-5 08/11/2003 Petroleum and natural gas
industries Specific requirements for offshore structures Part 5:
Weight control during engineering and construction.
6. BSI BS EN ISO 19901-7 01/23/2006 Petroleum and natural gas
industries Specific requirements for offshore structures Part 7:
Station keeping systems for floating offshore structures and mobile
offshore units-AMD 16988: March 30, 2007.
7. BSI BS EN ISO 19902 01/31/2008 Petroleum and natural gas
industries Fixed steel offshore structures ACTV-CURR
8. BSI BS EN ISO 19903 01/31/2007 Petroleum and natural gas
industries Fixed concrete offshore structures-AMD 16926: March 30,
2007.
9. BSI BS EN ISO 19904-1 12/29/2006 Petroleum and natural gas
industries Floating offshore structures Part 1: Monohulls,
semi-submersibles and spars-AMD 16927: March 30, 2007.
3.2 TYPES OF ANALYSIS & DESIGN REQIURED
INPLACE CONDITION
a) Global In-place analysis (Operating & Extreme storm
case). (3.2.1)
b) Corrosion protection (anode design for jacket
structure/piles) (3.2.2)
c) Miscellaneous member local analysis. (3.2.3)
d) Fatigue analysis. (3.2.4)
e) Pushover or collapse analysis (Intact & Damage).
(3.2.5)
f) Earthquake (Seismic) analysis. ( 3.2.6 )
INSTALLATION CONDITION
a) Lift analysis
b) Load-out analysis (including set-down)
c) Jacket launching & upending analysis
d) Barge transportation analysis.
3.2.1 GLOBAL INPLACE ANALYSIS (OPERATING & EXTREME
STORM)
The purpose of this analysis is to check jacket & Deck
structure strength adequacy for operating and extreme storm waves.
The SACS analysis & design includes following steps.
a) Structural modelling of jacket & Deck members.
b) Specifying LDOPT, OPTIONS, LCSEL, Section name, Group name
(Ky,Kz) etc.
c) Specifying support conditions at mud-line (Pile head , fixed,
pinned etc)
d) Specifying wind areas / drag areas, wind shielding etc.
e) Specifying Drag coefficient (Cd) & Inertia coefficient
(Cm).
f) Specifying Marine growth
g) Specifying Group/Member overrides
h) Specifying Dummy members ( Non structural members - Risers,
Riser guards, Boat landing, Fenders etc)
i) Gravity Loading (Basic or primary ):
i) Generated self weight with buoyancy
ii) Generated self weight no buoyancy
iii) Jacket non Generated loads (pile sleeves, Anodes, Boat
landing /Fender walkway loads, Pile grout etc.)
iv) Non modelled dead load of Deck ( Stair, ladder, handrail,
non modelled members, joint/node stiffeners, Safety net &
plating of helideck ,
v) Riser content weight
vi) Conductor content weight.
vii) Bridge loads (on Deck)
viii) Bridge Friction loads (on Deck)
ix) Equipment Operating loads (on Deck)
x) Piping operating loads (on Deck)
xi) Wireline rig operating loads (on Deck)
xii) Open area live loads (define separate load case for each
deck).
xiii) Open area loads on Helideck (Storage).
j) Environmental loads :
i) Wave loads. (act from direction as per environmental
survey)
ii) Current loads. (act to direction as per environmental
survey)
iii) Wind loads. (act from direction as per environmental
survey)
Operating & Extreme environmental loads to be applied in 8
or 12 directions depending on shape of the Jacket.
k) Load combinations (sample case , rectangular jacket ) :
201) Total Gravity load with contingency
202) Total Deck load
203) Total Live load on Decks
204) Total Bridge load
301) 201+203+204+ (Operating Wave + Current + Wind) 0 deg , Min
W.D
to
308) 201+203+204+ (Operating Wave + Current + Wind) 330 deg ,
Min W.D
311) 201+203+204+ (Operating Wave + Current + Wind) 0 deg , Max
W.D
to
318) 201+203+204+ (Operating Wave + Current + Wind) 330 deg ,
Max W.D
401) 201+203+204+ (Extreme Wave + Current + Wind) 0 deg , Min
W.D
to
408) 201+203+204+ (Extreme Wave + Current + Wind) 330 deg , Min
W.D
411) 201+203+204+ (Extreme Wave + Current + Wind) 0 deg , Max
W.D
to
418) 201+203+204+ (Extreme Wave + Current + Wind) 330 deg , Max
W.D
l) Platform pile foundation properties: Pile structural &
soil properties are specified in a separate file and is called PSI
input file in SACS. Pile Structure Interaction (PSI) module of SACS
analyze the behaviour of a pile supported structure subject to one
or more static load conditions. Finite deflection of the pile
(P-Delta effect) and non-linear soil behaviour both along and
transverse to the pile axis are accounted for. The program uses a
finite difference solution to solve the pile model which is
represented by a beam column on non-linear elastic foundation. The
structure resting on the pile is represented as a linear elastic
model.
PSI first obtains the pile axial solution, then uses the
resulting internal axial forces to obtain the lateral solution of
the piles. In general, soil exhibits non-linear behaviour for both
axial and transverse loads, therefore an iterative procedure is
used to find the pile influence on the deflection of the
structure.
Features of PSI module are
i) Tubular & H pile cross sections with variable
properties.
ii) Soil axial behaviour may be represented by adhesion data,
non-linear T-Z data, or as a linear spring.
iii) End bearing effects may be accounted for. Represented by
Q-z curves.
iv) Soil lateral behaviour represented by non-linear P-Y
curves.
v) Basic soil properties may be used to generate the soil axial
properties in the form of T-Z curves or adhesion data, end bearing
T-Z data and/ or lateral soil properties in the form of P-Y curves,
based on API-RP2A recommendations.
vi) Creates up to two equivalent linearized foundation
super-elements to be used by dynamic analysis in lieu of pile stub
(equivalent pile stub that yields the same deflections and
rotations as the soil/pile system).
vii) Creates foundation solution file containing pile stresses
to be used for fatigue analysis.
3.2.1.1 UNDERSTANDING CALCULATION OF WAVE & CURRENT
FORCES
Wave load on structure is computed based on following
methods;
1) Morrison equation if L/D >5 ( L- wave length , D-member
dia.)
2) Diffraction theory if L/D 0.10, earthquake analysis shall be
performed as per API RP 2A Cl. 2.3.6d.
Accordingly, the response spectrum analysis is carried out using
API RP 2A-WSD response spectrum based on soil type. This response
spectrum is applied equally along both principal horizontal axes of
the platform and 50% of horizontal ground accelerations applied in
the vertical direction simultaneously. An overall damping ratio of
5% is generally considered.
Response spectrum analysis is carried out using SACS-DYNAMIC
RESPONSE module. The CQC (Combined Quadratic Combination) method
has been used for combining the responses from the different modes
in each direction while SRSS (Square Root of the Sum of the
Squares) method has been used for combining the responses from
different directions as per API RP 2A-WSD.
Combination of static & Earthquake results:
The results of response spectrum analysis needs to be combined
with the results of static analysis assuming the earthquake axial
stresses are either tensile or compressive using COMBINE module. In
either case, all other stresses are assumed to have the same sign
as the corresponding static stresses.
EMBED Excel.Sheet.8
Pile Foundation
PILE
SEASTATE
single load case
Super element
ANALYSIS
ANALYSIS
generation
generation
(zk95-sac-dyn.inp)
(For center of damage wave)
DYNAMIC
vibration modes
SEASTATE
transfer functions
ANALYSIS
calculation
ANALYSIS
calculation
(dyninp.inp)
(wave frequencies
dynmas
to be investigated)
dynmod
(zk95-sac-stf.inp)
WAVE
RESPONSE
ANALYSES
FATIGUE
( zk95-ftginp )
ANALYSES
ftglst
( saccsf.tf1 to saccsf.tf8 )
zk95-wrs-315.inp)
8 analyses for
8 directions
wvrinp.csp
(zk95-wrs-0.inp to
_1178625719.xls
Sheet1Pile FoundationPILESEASTATEsingle load caseSuper
elementANALYSISANALYSISgenerationgeneration(zk95-sac-dyn.inp)(For
center of damage wave)DYNAMICvibration modesSEASTATEtransfer
functionsANALYSIScalculationANALYSIScalculation(dyninp.inp)(wave
frequenciesdynmasto be
investigated)dynmod(zk95-sac-stf.inp)wvrinp.cspWAVE(zk95-wrs-0.inp
toRESPONSE8 analyses forzk95-wrs-315.inp)ANALYSES8 directions(
saccsf.tf1 to saccsf.tf8 )FATIGUE( zk95-ftginp
)ANALYSESftglst
Sheet2
Sheet3