PHEONWJ-S-PRC-0010~0 (Guidance on Fixed Offshore Jacket Platform Design)
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PT. PHE ONWJ
Guidance on Fixed Offshore Jacket Platform Design
Revision Log Register
Document Number : PHEONWJ–S–PRC-0010
Document Title Revision
: Guidance on Fixed Offshore Jacket Platform Design
: 0
Page
Date
Revision
PHE ONWJ
Reviewer
PHEONWJ–S–PRC–0010 Rev. 0
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Guidance on Fixed Offshore Jacket Platform Design
Table of Contents
Revision Log Register......................................................................................................................... 2
Table of Contents ............................................................................................................................... 3
1. Scope ............................................................................................................................................. 5
2. References ..................................................................................................................................... 5
2.1 Indonesian National Regulations ......................................................................................... 5
2.2 Company Specifications ...................................................................................................... 5
2.3 Standards and Codes .......................................................................................................... 6
2.4 Order of Precedence ........................................................................................................... 6
3. Definition of Terms.......................................................................................................................... 6
4. Abbreviations .................................................................................................................................. 7
5. General Design Considerations ...................................................................................................... 7
5.1 Design Philosophy............................................................................................................... 7
5.2 Criteria of Loadings.............................................................................................................. 7
5.2.1 General ................................................................................................................... 7
5.2.2 Dead Loads............................................................................................................. 7
5.2.3 Live Loads............................................................................................................... 8
5.2.4 Environmental Loads .............................................................................................. 8
5.2.5 Construction Loads ................................................................................................. 8
5.2.6 Removal and Reinstallation Loads.......................................................................... 8
5.2.7 Dynamic Loads ....................................................................................................... 8
5.3 Importance of Structural Members....................................................................................... 8
5.4 Chart Datum ........................................................................................................................ 8
5.5 Air-gap................................................................................................................................. 9
5.6 Splash-zone......................................................................................................................... 9
5.7 Marine Growth ..................................................................................................................... 9
5.8 Corrosion............................................................................................................................. 9
5.8.1 Corrosion Protection ............................................................................................... 9
5.8.2 Corrosion Allowance ............................................................................................. 10
6. Jacket Design Considerations ...................................................................................................... 10
6.1 Computer Model ................................................................................................................ 10
6.2 Foundation Design ............................................................................................................ 11
6.2.1 Foundation Simulation .......................................................................................... 11
6.2.2 Pile Stress............................................................................................................. 11
6.2.3 Pile Safety Factor.................................................................................................. 11
6.2.4 Pile Installation...................................................................................................... 11
6.2.4.1 Underdrive Allowance ........................................................................................... 11
6.2.4.2 Overdrive Allowance ............................................................................................. 11
6.2.4.3 Pile Driveability ..................................................................................................... 12
6.2.4.4 Pile Lengths .......................................................................................................... 12
6.2.5 Skirt Pile Connection to Jacket.............................................................................. 13
6.2.6 Pile Group Effects ................................................................................................. 13
6.3 Member Design ................................................................................................................. 13
6.3.1 Slenderness Ratios............................................................................................................ 13
6.3.2 D/t Ratios........................................................................................................................... 14
6.3.3 Conical transitions ............................................................................................................. 14
6.3.4 Allowable displacements ................................................................................................... 14
6.4 Connections....................................................................................................................... 15
6.4.1 Welded Connections.......................................................................................................... 15
6.4.2 Bolted Connections............................................................................................................ 15
6.4.3 Doubler Plate..................................................................................................................... 15
6.5 Tubular Joint Design.......................................................................................................... 16
6.6 Environmental Loading ...................................................................................................... 17
6.6.1 Wave and Current Forces ..................................................................................... 17
6.6.2 Wave Force Coefficients ....................................................................................... 18 PHEONWJ–S–PRC–0010 Rev. 0
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Guidance on Fixed Offshore Jacket Platform Design
6.6.3 Wave Shielding ..................................................................................................... 18
6.6.4 Current Blockage Factors ..................................................................................... 18
6.6.5 Wind Forces.......................................................................................................... 18
6.6.6 Marine Growth ...................................................................................................... 19
6.7 Accidental Loading ............................................................................................................ 19
6.8 In-Service Wave Analysis .................................................................................................. 19
6.9 In-Service Fatigue Analysis ............................................................................................... 19
6.9.1 General ................................................................................................................. 19
6.9.2 Computer Model ................................................................................................... 20
6.9.3 Stress Concentration Factors................................................................................ 20
6.9.4 Wave Slam............................................................................................................ 20
6.9.5 S-N Curves ........................................................................................................... 21
6.9.6 Fatigue Design...................................................................................................... 21
6.10 Transportation Strength Analysis....................................................................................... 21
6.11 Launch Analysis ................................................................................................................ 22
6.11.1 Launch Trajectory Analysis ................................................................................... 22
6.11.2 Launch Stress Analysis......................................................................................... 23
6.11.3 Member Slam........................................................................................................ 23
6.11.4 Hydrostatic Collapse ............................................................................................. 23
6.12 Jackets Lifted from Transportation Barges ........................................................................ 23
6.13 Floatation and Upending.................................................................................................... 24
6.14 Jackets Installed over pre-drilled wells............................................................................... 25
6.15 Loadout Analysis ............................................................................................................... 25
6.16 On-Bottom Stability Analysis.............................................................................................. 25
6.17 Barge Stability ................................................................................................................... 26
6.18 Seismic Analysis................................................................................................................ 26
6.19 Vortex Shedding ................................................................................................................ 26
6.20 Wave Slam ........................................................................................................................ 27
6.21 Hydrostatic Design............................................................................................................. 27
7. Appurtenance Design Considerations........................................................................................... 27
7.1 Boat Landings.................................................................................................................... 27
7.2 Riser Guards ..................................................................................................................... 27
7.3 Riser Clamps ..................................................................................................................... 28
7.4 Sumps and Pump Casings ................................................................................................ 28
7.5 Conductor Guide ............................................................................................................... 28
7.6 Skirt Pile Guide Framing.................................................................................................... 29
7.6.1 Battered Skirt Piles................................................................................................ 29
7.6.2 Vertical Skirt Piles ................................................................................................. 29
7.7 Flooding and Grout System ............................................................................................... 29
7.8 Mudmats............................................................................................................................ 30
7.9 Walkway, Stairway, and Landings ..................................................................................... 30
7.10 Handrail and Ladders ........................................................................................................ 30
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Guidance on Fixed Offshore Jacket Platform Design
1. Scope
This guidance establishes the guidelines for the design of new fixed offshore jacket platform.
This specification is also applicable for modification design of existing facilities. Any exceptions
or changes to the guidelines of this specification shall be submitted in writing for resolution by
the COMPANY.
This guidance shall be read in conjunction with the other COMPANY specifications and other
relevant documentations to obtain a complete definition of scope of work and associated
requirements.
This guidance covers jacket, pile, conductor, appurtenances and personnel safety structure.
Where an item of work not specifically covered herein, the relevant codes and standards as
well as guidelines of any designated or appointed representatives of the COMPANY (i.e. a
Certifying Agency (CA), Marine Warranty Surveyor (MWS), etc) should be applied.
2. References
The following references shall be interpreted as the minimum requirements applicable to the
subject work, and no statement contained in this guidance shall be construed as limiting the
work to such minimum requirements. Any requirements stated herein which contravene these
guidances, standards and codes shall immediately be brought to the attention of COMPANY
for resolution. The latest editions of the following standards, codes and regulations and other
documents referenced therein shall govern all work.
Where relevant national regulations exist, the recommendations of national regulation shall be
applied together with the standards and codes to which they refer. These regulations may thus
complement or amend the provisions of the present document.
If there are no national regulations covering all or part of the subject of this document, the
reference documents shall be strictly applied, as supplemented by the provisions of this
specification and the project documents.
In all cases, CONTRACTOR shall avoid to mix the requirements of several different codes or
regulations when designing a particular item of a structure.
2.1 Indonesian National Regulations
•
•
Regulation of the Minister of Mining No. 05/P/M/Pertamb/1977 dated 22 October
1977 concerning Obligation to have Certificate of Worthiness of Construction for Oil
and Gas Offshore Platform.
Directorate General of Oil and Natural Gas Decree, SK 21K/38/DJM/1999
2.2 Company Specifications
The following COMPANY specifications shall be applied to the subject work.
Reference
Title
on Provision of Cathodic Protection System of
PHEONWJ-S-PRC-0003 Guidance Offshore Structures by Sacrificial Anode
PHEONWJ-S-PRC-0005 Guidance on Platform Fabrication
PHEONWJ-S-PRC-0006 Guidance on Boat Landing Design
PHEONWJ-S-PRC-0007 Guidance on Riser Guard Design
PHEONWJ-S-SPE-0101 Specification for Structural Steel and Miscellaneous
Metal PHEONWJ–S–PRC–0010 Rev. 0
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PHEONWJ-S-SPE-0102 PHEONWJ-S-SPE-0103 PHEONWJ-S-SPE-0106 PHEONWJ-S-SPE-0107 PHEONWJ-S-SPE-0108
Specification for Structural Welding
Specification for Jacket Leg Closure Diaphragms
Pile and Conductor Installation Specification
Platform Installation Specification
Specification for Barge Bumper
2.3 Standards and Codes
The Jacket of offshore platform shall be designed and supplied in accordance with the
latest edition of the following international codes and standards at the effective date of
the contract.
Reference API RP 2A WSD
API RP 2A LRFD
(ISO 19902 : 2007)
API Spec 2B API RP 2L
AISC AWS D 1.1 AISC LRFD
Title
Recommended Practice for Planning, Designing and
Constructing Fixed Offshore platforms, Working Stress
Design
Recommended Practice for Planning and Constructing Fixed
Offshore Platforms Load and Resistance Factor Design
Specification for Fabricated Structural Steel Pipe
Recommended Practice for Planning, Designing and
Constructing Heliports for Fixed Offshore Platforms
Manual of Steel Construction – ASD
Structural Welding Code – Steel
Manual of Steel Construction – Load and Resistance Factor
Design
2.4 Order of Precedence
For the purpose of executing the design activity, the following document hierarchy must
be adhered to in the event of any conflicting requirements:
Priority:
• First The Project Specification
• Second International Standard & Codes
In cases of conflict between COMPANY Specification and any of the applicable
Standards and Codes, CONTRACTOR shall immediately submit the matter in writing to
COMPANY who will provide a written clarification or resolution before proceeding on
any analysis or design.
3. Definition of Terms
For purpose of this document, the following definitions apply:
COMPANY PT. Pertamina Hulu Energi (PHE) Offshore North West Java
CONTRACTOR The party which is awarded the contract for engineering
services of fixed offshore platform and fabricator.
Throughout this document, the word ‘structure’ is used to cover all decks, bridges, jackets,
platforms, equipment support etc.
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4. Abbreviations
AISC API ASD AWS CA MWS PHE ONWJ WSD LRFD
American Institute of Steel Consruction
American Petroleum Institute
Allowable Stress Design
American Welding Society
Certifying Agency
Marine Warranty Surveyor
Pertamina Hulu Energi Offshore North West Java
Working Stress Design
Load and Resistance Factor Design
5. General Design Considerations
5.1 Design Philosophy
•
•
• •
•
•
Principles for load-out, transportation, installation, lifting, launching or for any
particular phase or condition, which has a major impact on certain component of the
structures, shall be integrated in the design at the very beginning of the design
phase.
Offshore hook-up works shall be reduced to a minimum and shall be made as easy
as possible. Consequently, each element, even if of secondary importance, shall be
analyzed in order to maximize the amount of work to be carried out on the fabrication
site.
Structure shall be detailed to avoid corrosion and wear and minimize fatigue (such as
indication of weld profile control on jacket drawings).
Inspection and maintenance works of installed structures shall be easily
accomplished. Maximum access to all elements and at all phases of platform life
should be provided.
Safety shall be considered as priority and shall be taken into consideration from the
beginning of the design, including for the temporary phases (load-out, transport and
installation). Particular attention shall be paid to accessibility, circulation routes,
escape routes, muster areas, partitions, fire resistance, finish materials, etc.
Future extensions and additional loads (if any such as COMPANY reserve) shall be
considered and the consequent design arrangements shall be made.
5.2 Criteria of Loadings
5.2.1 General
The following loads and any dynamic effects resulting from them should be
considered in the development of the design loading conditions. Design
environmental load conditions are those forces imposed on the platforms by
the selected design event; whereas, operating environmental load conditions
are those forces imposed on the structure by a lesser event which is not severe
enough to restrict normal operations, as specified by the operator.
5.2.2 Dead Loads
Dead loads are the weights of the platform structure and any permanent
equipment and appurtenant structures which do not change with the mode of
operation. Dead loads should include the following:
1. Weight of the platform structure in air, including where appropriate the
weight of piles, grout and ballast.
2. Weight of equipment and appurtenant structures permanently mounted on
the platform.
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Guidance on Fixed Offshore Jacket Platform Design
3. Hydrostatic forces acting on the structure below the waterline including
external pressure and buoyancy.
5.2.3 Live Loads
Live loads are the loads imposed on the platform during its use and which may
change either during a mode of operation or from one mode of operation to
another. Live loads should include the following:
1. The weight of drilling and production equipment which can be added or
removed from the platform.
2. The weight of living quarters, heliport and other life support equipment, life
saving equipment, diving equipment and utilities equipment which can be
added or removed from the platform.
3. The weight of consumable supplies and liquids in storage tanks.
4. The forces exerted on the structure from operations such as drilling,
material handling, vessel mooring and helicopter loadings.
5. The forces exerted on the structure from deck crane usage.
These forces are derived from consideration of the suspended load and its
movement as well as dead load.
5.2.4 Environmental Loads
Environmental loads are loads imposed on the platform by natural phenomena
including wind, current, wave, earthquake, snow, ice and earth movement.
Environmental loads also include the variation in hydrostatic pressure and
buoyancy on members caused by changes in the water level due to waves and
tides. Environmental loads should be anticipated from any direction unless
knowledge of specific conditions makes a different assumption more
reasonable.
5.2.5 Construction Loads
Loads resulting from fabrication, loadout, transportation and installation should
be considered in design.
5.2.6 Removal and Reinstallation Loads
For platforms which are to be relocated to new sites, loads resulting from
removal, onloading, transportation, upgrading and reinstallation should be
considered in addition to the above construction loads.
5.2.7 Dynamic Loads
Dynamic loads are the loads imposed on the platform due to response to an
excitation of a cyclic nature or due to reacting to impact. Excitation of a platform
may be caused by waves, wind, earthquake or machinery. Impact may be
caused by a barge or boat berthing against the platform or by drilling
operations.
5.3 Importance of Structural Members
The structural members of the resisting structures are distributed into categories of
importance, mainly in regard of the consequences of their possible failure during
platform production or during installation of constituting parts of the platform (lifting of
packages, etc). This distribution of structural members into categories of importance
shall be consistent with COMPANY material specification
5.4 Reference Datum
All elevations shall refer to Mean Sea Level (MSL).
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Guidance on Fixed Offshore Jacket Platform Design
5.5 Air-gap
The bottom of the lowest deck or piece of equipment should be located at an elevation
which will clear the calculated crest of the design wave including HAT and maximum
surge height with adequate allowance for safety.
Omni-directional guideline wave heights with a nominal return period of 100 years or
unless noted otherwise by COMPANY, together with the applicable wave theories and
wave steepness should be used to compute wave crest elevations above storm water
level, including guideline storm tide.
Unless stipulated otherwise, the air-gap shall be taken as 5 feet (1.50 m) minimum from
the lowest part of the structure or piece of equipment, and shall also take into account
any known or predicted long term seafloor subsidence and installation tolerance.
Wave loads shall be taken into consideration on jacket structure. However they will not
be accumulated to the global wave loads acting onto the jacket and the foundations
unless noted otherwise in the project specification.
These wave forces affecting the items located in the air-gap may be evaluated as per
API RP 2A.
In case of large equipment located in the air-gap, COMPANY may request to consider
global wave load effect (i.e. effects on deck, jacket and pile design).
5.6 Splash-zone
Splash zone is that part of a platform, which is intermittently exposed to the air and
immersed in the sea. Unless specify otherwise on the project specification, the splash-
zone to be considered for structural design shall extend between:
• The upper limit of the splash zone (SZU) shall be calculated by:
SZU = U1 + U2 + U3
where:
U1 = wave crest calculated from 100 year max. wave height
U2 = highest astronomical tide level (HAT)
U3 = upper storm surge
• The lower limit of the splash zone (SZL) shall be calculated by:
SZL = L1 – 10 ft. (or 3 m)
Whichever is greater
Where:
L1 = lowest astronomical tide level (LAT)
5.7 Marine Growth
All structural members, conductors, risers, and appurtenances should be increased in
cross sectional area to account for marine growth thickness. Also, elements with
circular cross-sections should be classified as either “smooth” or “rough” depending on
the amount of marine growth expected to have accumulated on them at the time of the
loading event.
5.8 Corrosion
5.8.1 Corrosion Protection
The platform structural components shall be protected against corrosion based
on the location on the platform and project requirements. The primary protection
for the jacket structure in the submerged zone shall be with a sacrificial anode
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Guidance on Fixed Offshore Jacket Platform Design
cathodic protection system. The system shall be designed in accordance with
the COMPANY specification.
Jacket members and appurtenances in the splash zone shall be protected by an
appropriate coating system in addition to the corrosion allowances specified
below. Jacket members above the splash zone and the topsides shall be
protected in accordance with the relevant project specification.
5.8.2 Corrosion Allowance
Unless otherwise specified in the particular specifications, the following
corrosion allowance shall be applied to the platform structural members in the
splash zone:
Jacket legs, Deck Legs, pile above jacket, primary vertical and diagonal jacket
braces and caissons in the splash zone shall be protected by a ¼" (6 mm) thick
corrosion allowance.
All other major appurtenances i.e. boat landings, barge bumpers and riser
guards in the splash zone shall also be designed with corrosion allowance by
1/8” thickness.
For fatigue analyses, the corrosion allowance may be considered equal to one
half of the above values 1/8" (3 mm).
6. Jacket Design Considerations
6.1 Computer Model
1. Structural models shall represent all effects contributing to loads, actions, stiffness,
etc. of a structure and shall contain sufficient detail to accurately represent the
weight, buoyancy, stability, stiffness, environmental force characteristics of the
structure, dynamic/hydrodynamic behavior and boundary condition of the structure to
be analyzed.
2. An integrated model of the jacket, piling, major appurtenances and deck contributing
to the stiffness of the structural system shall be used for in-service analyses.
• For substructure design, the deck shall be modeled in sufficient detail to
accurately represent stiffness, load distribution and interaction with the jacket.
• The effect of the added thickness of jacket members for corrosion allowance shall
not be used in the in-service analyses (i.e. operating storm, design event,
seismic, etc.).
3. Pre-service analysis models shall include any preinstalled sections of piling and/or
appurtenances.
4. The additional strength of the increased wall thickness for corrosion allowance may
be used in the pre-service analyses.
5. Computer models shall include all primary structural members.
• Joint cans and brace stubs shall be modeled for primary members.
• Unless required for specific analysis, secondary members such as pump casing
supports, J-tube supports, mudmat knee braces and minor conductor guide
framing shall not be modeled as structural elements.
• Dead loads of, and environmental loads on, secondary members and
appurtenances shall be included in models.
6. If greater than one quarter of the chord diameter, primary brace centerline offsets at
joints shall be modeled.
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7. Conductors shall be included in the model to collect loads from wave and current for
in-service conditions, but shall not contribute to structural or foundation stiffness
unless these members are framed such that they contribute to the structural and/or
foundation stiffness.
8. Conductor modeling shall properly model the spatial-behavior relationship of the
conductors within the jacket.
9. The effects of marine growth on members in and below the splash zone shall be
modeled.
6.2 Foundation Design
6.2.1 Foundation Simulation
The foundation model will be based on main piles or skirt piles at each corner
leg. The characteristics foundation of the jacket will be modelled using the pile
member stiffness and the relevant soil curves of P-Y, T-Z and Q-Z from Soil
Report.
If the foundations are modeled by stiffness matrix, they shall be representative
of soil behavior for the applied load case.
For the storm and normal operating load cases a full soil-structure interaction
analysis shall be performed using appropriate computer program. The effect of
scour will be included as applicable.
6.2.2 Pile Stress
The pile wall thickness in the vicinity of the mudline, and possibly at other points,
is normally controlled by the combined axial load and bending moment which
results from the design loading conditions for the platform. The moment curve
for the pile may be computed with soil reactions determined due consideration to
possible soil removal by scour.
6.2.3 Pile Safety Factor
Foundations should have an adequate margin of safety against failure under
the design loading conditions. The following factors of safety should be used for
the specific conditions indicated:
Condition
Operating Storm Seismic
Safety Factor
2.0
1.5
1.0
6.2.4 Pile Installation
6.2.4.1 Underdrive Allowance
For piles having thickened sections at the mudline, the pile wall thickness
make-up shall be designed to allow for the possibility of pile driving refusal
before reaching the design penetration.
An underdrive allowance shall be provided by extending the length of heavy-
wall material in the vicinity of the mudline based on design requirement. Ten
feet (3 m) allowance will be considered unless particular calculation stated
otherwise.
6.2.4.2
Overdrive Allowance
For piles having thickened sections at the mudline, the pile wall thickness
make-up shall be designed to allow for the possibility of pile driving beyond
design penetration.
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Guidance on Fixed Offshore Jacket Platform Design
In instances where the pile is expected to encounter a specific bearing stratum
or when there are uncertainties in the soil data, an overdrive allowance should
be provided and the length of heavy wall material in the vicinity of the mudline
shall be extended
The final value of the overdrive shall be determined by the designer, taking into
account items such as the level of uncertainty in the soil information (number
and quality of soil samples and type of soil tests, etc.), total length of piles, and
percentage of contribution from the specific bearing stratum to the total pile
bearing capacity, etc. 10 feet (3 m) allowance will be considered unless
particular calculation stated otherwise.
6.2.4.3 •
•
• •
•
Pile Driveability
A pile driveability analysis shall be performed to determine the required pile
wall thickness and hammer requirements for both continuous and
interrupted driving caused by the installation of add-on sections, the
changing of hammers, etc.
A pile driveability analysis using a wave-equation program shall be
performed in accordance with the recommendations made in the
geotechnical reports.
The pile shall be checked with its tip both plugged and unplugged for a
range of hammer sizes.
For assessing driveability with steam hammers, the assumed global hammer
efficiency shall not exceed 70% without written verification of higher
efficiencies from the hammer manufacturer.
For assessing driveability with hydraulic hammers, the assumed global
efficiency shall not exceed 90% without written verification of higher
efficiencies from the hammer manufacturer.
6.2.4.4
Pile Lengths
1. Main Piles
a. Pile section lengths shall be designed in accordance with API RP 2A
b. Main piles shall have pile section lengths with stick-up lengths such that
slenderness ratio (kl/r) is less than 200.
c. For determining the slenderness ratio, the pile length under consideration
shall start at the last set of pile centralizers and an effective length factor
(k) for cantilever condition of 2.0 shall be assumed.
d. The pile make up shall be planned to avoid pile add-ons when the pile tip
is at a stratum where hard driving is expected, such as granular or
cemented soils.
2. Vertical Skirt Piles
a. The stick-up height of single-piece, vertical skirt pile shall have a
slenderness ratio (kl/r) less than 200 in which the effective length factor
(k) for cantilever condition of 2.0 shall be assumed.
b. The pile length (l) shall be equal to the fabricated pile length minus the
self penetration length and the length from mudline up to the last set of
centralizers in the skirt pile sleeve.
c. The self-penetration of the pile shall be based on the static ultimate
capacity of the soil with no end bearing and with consideration given to
the buoyancy of the pile and the hammer weight.
d. The lateral design load on the pile shall be the maximum of the following:
The maximum wave and current load corresponding to sea conditions
representative of the area during pile installation.
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Guidance on Fixed Offshore Jacket Platform Design
The lateral component of gravity resulting from the largest possible
deviation from the true vertical of the pile due to the existing gap
between the pile and pile sleeve or 2% of the total hammer weight
applied at the hammer center of gravity, whichever is greater.
e. Pile stress checks shall be performed as follows:
Combined axial compression and bending using the static axial (fa)
and bending (fb) stresses with no increase in allowable stresses.
Combined static stress and driving stresses shall be governed by API
RP 2A.
6.2.5 Skirt Pile Connection to Jacket
The design for connection of the skirt piles to the structure shall assume
minimum grout strength of 2,500 psi.
6.2.6 Pile Group Effects
Pile group effects shall be investigated for pile spacings less than four pile
diameters.
Design of pile groups or clusters shall include group effects.
A reduction in lateral and axial capacity of the soil may be required
depending on pile spacing, type of soil, structure geometry and pile axial
load.
6.3 Member Design
Member stresses shall be checked at the ends of members and throughout their lengths
in accordance with API RP 2A.
Brace-member end forces may be calculated at the face of the chord instead of at the
centerline of the chord, when appropriate.
Member stresses due to aspects which are not specifically covered in the computer
structural analysis shall be investigated by manual calculation and results combined
with computer results to ensure that the stress and deflection limitations are not
exceeded.
In addition, all structural framing shall meet the following guidelines:
6.3.1 Slenderness Ratios
The maximum slenderness ratio for jacket tubular members shall be limited to
120, subject to fulfillment of vortex shedding design requirements. For sensitive
non-redundant members COMPANY may limit the maximum slenderness ratio
to 90. For jacket located in particular seismically active areas the maximum
slenderness ratio shall be limited to 80 (API RP 2A Earthquake design
requirements).
The buckling coefficient (k) shall be chosen for each member in accordance
with API RP 2A recommendations as follows.
Situation
Superstructure Legs • Braced
• Portal (unbraced)
Jacket Legs and Piling • Grouted Composite Section
• Ungrouted Jacket Legs
• Ungrouted piling between shim points
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Effective Length
Factor k
1.0
K (1)
1.0
1.0
1.0
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Guidance on Fixed Offshore Jacket Platform Design
Deck Truss web members • In-plane action
• Out-of-plane action
Jacket Braces • Face to face length of main diagonals
• Face of leg to certerline of joint length
(2)
of K braces
0.8
1.0
0.8
0.8
0.9
0.7
Deck truss Chord Members 1.0
1) Use effective length alignment in commentary of AISC. This may be modified
to account for conditions different from those assumed in developing the
chart.
2) At least one pair of members framing into the joint must be in tension if the
joint is not braced out of plane.
Longer Segment Length of (2)
• X Braces
• Secondary Horizontals
6.3.2 D/t Ratios
Maximum D/t ratios shall be in accordance with API RP 2A, with associated
member capacity reductions to account for local buckling or subject to the
provision of ring stiffeners.
The minimum D/t ratios shall be following below numbers:
Members
Braces (508 mm diameter or less) Braces (greater than 508 mm diameter) Joint Cans *Use of joint-can D/t ratios of less than 20 must COMPANY
Min. D/t Ratios
30
20
20*
be approved in writing by
The D/t ratio of entire pile should be small enough to preclude local buckling at
stresses up to the yield strength of pile material. Consideration should be given
to the different loading situations during the installation and the service life of
piling. For in-service conditions, and for those installation situations where
normal pile driving is anticipated or where piling installation will be by means
other than driving, the limitation of API RP 2A Section 3.2 should be considered
to be minimum requirement. For pile that are to be installed by driving where
sustained hard driving (820 blows per meter with the largest size hammer to be
used) is anticipated, the minimum piling wall thickness used shall not be less
than
Metric formula: t = 6.35 + D/100, D in mm.
Minimum wall thickness for normally used pile sizes should be as per API RP
2A Section 6.10.6 API RP 2A.
6.3.3 Conical transitions
Conical transitions shall be concentric and shall be designed in accordance with
API RP 2A.
6.3.4
Allowable displacements
Relative horizontal displacements main deck shall be limited to 1/200 of the
platform height between the main deck level and the mudline jacket horizontally
braced level under extreme environmental conditions.
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6.4 Connections
6.4.1 Welded Connections
The connections shall be designed in such a way that:
• They shall allow for a direct, straightforward and simple load transfer from
one structural member to another, avoiding stress concentration and induced
bending stresses as far as possible
• Their fabrication shall be as simple as possible; constructability and welding
access shall be particularly verified during design
• Notching effects are eliminated
• Corrosion is reduced to a minimum. This means avoiding shapes or
arrangements that induce retention or ponding of water or hydrocarbons, in
order to avoid hazards and ease inspection and maintenance/painting.
Tubular joints shall be designed in accordance with API RP 2A requirements.
The joints shall be configured to provide the minimum gaps as shown on the
drawings. Minimum separation between braces as per API RP 2A is 2 inches or
51 mm. Eccentricities of the brace centerlines greater than D/4 shall be
appropriately modeled and accounted for in the design.
Ring stiffened joints and node barrels shall be designed in accordance with API
RP 2A. Appropriate closed ring solutions shall be used to evaluate stresses and
deformations.
Primary non-tubular joints shall be designed to develop the full strength of the
incoming members.
6.4.2 Bolted Connections
In case bolted connections have to be implemented, they shall be designed
according to AISC Allowable Stress Design (latest edition):
• Part 4: Connections.
• Part 5: Specifications and Codes:
− Chapter J: connections, joints and fasteners
− Specification for structural joints using ASTM A 325 or A 490 bolts.
For the splash zone and above, the bolting shall be zinc plated or galvanized
and then painted.
Reference shall also be made to COMPANY material specifications PHEONWJ-
S-SPE-0101.
6.4.3 Doubler Plate
Connection of all jacket appurtenances such as, but not limited to anodes,
bumpers, boat landing, any type of protective frame, mooring bollard, sea-
fastening, riser guard etc. to jacket main structure shall be achieved via doubler
plates with rounded corners and fillet welded to the supporting member.
Doubler plates shall be of the same steel properties as the supporting member.
Doubler plate thickness shall not be less than the incoming bracing wall
thickness.
When necessary for high forces, doubler plates shall be annular plates with
inner and outer welds.
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Weld
Weld
Supporting member
Doubler Plate
For punching shear checking, the equivalent diameter of the incoming brace
with a doubler plate shall be the brace diameter plus 2.5 times doubler plate
thickness for both sides. This assumption based on AISC bearing distribution
for steel. No increase of the wall thickness of the chord or of the brace shall be
considered due to the doubler plate without more detailed analysis.
brace
Doubler
plate
1
2.5
Doubler plates thickness and size shall be designed and justified against
tension, punching and bending for all load conditions, and for fatigue condition.
For those verifications, the thickness of the doubler plate shall not be cumulated
with the thickness of the supporting member.
Similarly to the above, the strength of the fillet weld between the doubler plate
and the supporting member shall be fully justified for all load conditions and for
fatigue condition.
6.5 Tubular Joint Design
Tubular joints shall be designed in accordance with API RP 2A.
In addition, all tubular joints shall meet the following guidelines:
a. Wherever possible, nominally concentric jacket leg and brace joints shall be detailed
with working points offset in either direction to obtain a minimum clearance of 2
inches between adjacent braces, minimizing joint-can length.
b. When detailing brace joint cans, if simple joint detailing is not practical, balloon joints
shall be used with conical transitions back to nominal brace diameter to provide a
minimum of 2 inches clear, whenever possible.
c. Where overlapping joints can not practically be avoided, a minimum overlap equal to
one quarter of the overlapping member diameter shall be detailed.
d. If an increased wall thickness in the chord at a joint is required, the thickened chord
shall extend past the outside edge of the bracing a minimum of one quarter of the
chord diameter or 12 inches, whichever is greater.
e. The length required for tapering the thickened chord down to the nominal member
thickness shall not be included in the previously stated can length.
f. Joint cans which occur at the end of a chord member as a stub end shall extend past
the edge of the bracing a minimum of one quarter of the chord diameter or 12
inches, whichever is greater.
g. The total length of all joint cans shall be detailed in even increments of 2’-6", unless
specified otherwise in writing by the Fabrication CONTRACTOR.
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h. If an increased wall thickness or special steel is required for a brace end, the
thickened can or special material shall extend a minimum of one brace diameter or
24 inches, whichever is greater.
i. In the case of a thickened brace end, the length required for tapering the thickened
brace end down to the nominal brace thickness shall not be included in the
previously stated length.
j. A 1-on-4 taper shall be detailed for reducing a joint can or brace end thickness to the
nominal member thickness.
k. Grouted joint composite action may be accounted for using Lloyd’s Register
procedures.
• The procedure allows an effective chord wall thickness to be calculated equal to
the thickness which gives a moment of inertia equal to that of the chord-pile or
chord sleeve cross section, but not to exceed 1.75 times the actual chord
thickness.
• The equivalent chord thickness is then used, instead of the actual chord
thickness, in calculation of the joint geometric parameter gamma (γ) to calculate
the allowable punching shear.
• The actual chord thickness is used to calculate the acting punching shear.
l. Launch-leg joints and other ring-stiffened joints shall be analyzed using appropriate
closed-ring solutions.
• The stress calculated in the stiffener outer fiber may be allowed to approach the
material yield stress during transportation and launch.
• For all other cases, this stress shall be limited to basic allowable stresses with an
appropriate stress ratio increase.
• Similarly, the stress in the stiffened chord shall be limited to the basic allowable
stress with the appropriate stress-increase ratio.
• The effective length of the stiffened chord wall used to compute the effective
section for ring-stress analysis calculations shall not exceed 1.25 times the chord
diameter on either side of the brace centerline.
m. Joint flexibility may be modeled using appropriate empirical formulae or results from
local joint analysis.
n. The API requirement stating tubular-joint connections shall be designed for a
minimum of 50% of the effective strength of the member and shall apply to primary
members only (i.e., excluding conductor guide framing, caisson and riser supports
and other appurtenance connections).
6.6 Environmental Loading
6.6.1 Wave and Current Forces
1. Wave and current forces for in-service analyses shall be computed using the
Morison equation and an appropriate wave theory and shall assume that the
current acts simultaneously and co-linearly with the wave propagation.
a. Current and wave particle velocities shall be added prior to force
determination in Morison’s equation.
b. A wave kinematics factor should be taken from the site-specific
metocean study or based on the guidance in API RP 2A if the site
specific report does not contain a value.
2. For in-service spectral fatigue analyses, a wave kinematics factor of 1.0 shall
be used.
CONTRACTOR shall develop a wave pressure regime using an appropriate
wave theory, if the data is not provided
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6.6.2 Wave Force Coefficients
Hydrodynamic wave coefficients for use in Morison’s equation for extreme and
operating conditions shall be determined in accordance with API RP 2A.
In-place:
Surface
Cd
Cm
Smooth 0.65 1.6
Rough 1.05 1.2
Fatigue:
Surface
Cd
Cm
Smooth 0.70 2.0
Rough 0.70 2.0
6.6.3 Wave Shielding
1. For extreme and operating conditions, wave shielding factors that account
for wave force reductions on closely spaced conductors shall be applied in
accordance with API RP 2A.
2. For fatigue conditions, no benefit from wave shielding shall be applied.
6.6.4 Current Blockage Factors
Current blockage factors as a function of the number of platform legs and wave
direction are given below. For other conditions, factors should be computed
using API RP 2A guidance.
No. of Legs Heading
Factor
3
all
0.90
4
4
4
6
6
6
8
8
8
end-on
diagonal
broadside
end-on
diagonal
broadside
end-on
diagonal
broadside
0.80
0.85
0.80
0.75
0.85
0.80
0.70
0.85
0.80
For either freestanding conductors or braced caissons the blockage factor shall
be 1.0.
6.6.5 Wind Forces
1. Wind forces shall be assumed to act omni-directionally, simultaneously and
co-linearly with the wave and current forces.
2. The one-hour sustained wind speed associated with the wave condition shall
be used for the substructure analysis.
3. Wind loads on the flare boom shall be determined by considering all
members and pipes individually (i.e. without using projected area to calculate
total wind force).
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6.6.6 Marine Growth
Allowance shall be made available for marine growth in wave force
computation. Use 2 inches from mean higher high to -150 feet unless a smaller
or larger value of thickness is appropriate from site specific studies.
6.7 Fire, Blast and Accidental Loading
Fixed offshore platforms are subject to possible damage from accidental loading as
follows:
• Vessel collision
• Dropped object
• Fire accident
• Blast
If the platform safety study identifies a significant risk from this type of loading, the effect
on structural integrity of the platform should be assessed in accordance with API RP 2A
requirements.
6.8 In-Service Wave Analysis
Minimum eight wave and wind attack directions are required for symmetrical,
rectangular and square platform and minimum of 12 directions required for tripod
jackets.
• Wave directionality (i.e., different wave heights from different directions) shall be
taken into account in the analysis.
• For each wave approach direction, waves shall be stepped through the structure to
determine the wave positions that cause maximum base shear and maximum
overturning moment.
The in-service wave analysis shall include inertial loads due to wave-platform dynamic
response, if appropriate.
6.9 In-Service Fatigue Analysis
6.9.1 General
1. Deterministic fatigue shall be performed on the jacket, if required by
code/standard spectral fatigue shall also performed.
2. The in-service fatigue design life of the platform components shall be at least
two times the service life of the platform.
Fatigue Safety factor
Failure Critical
3.
4. 5. 6.
Can be inspected
Can not be inspected
No 2 5
Yes 5 10
In-service cumulative-damage ratios (CDRs) shall be developed in a manner
that will permit easy combination with the transportation CDRs (in
accordance with the criteria stipulated below) to determine final combined
damage ratios.
The jacket designer shall provide the in-service deck CDRs to the deck
designer and to the transportation CONTRACTOR.
For each brace end at which fatigue life is to be determined, eight points
around the circumference shall be checked.
Fatigue design of caisson attachments and conductor framing shall be
performed.
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7. Fatigue design for the attachment of jacket appurtenances such as boat
landings, barge bumpers and riser guards is not required where doubler
plate is installed.
8. The intended service life of platform for oil field development is 20 years
while for gas field development is 30 years unless specified otherwise in
project basis of design.
6.9.2 Computer Model
The structural computer model used for the in-service fatigue analysis will be
identical to that used for the operating loading with the following exceptions:
1. The conductor-guide bracing in jacket framing levels shall be modeled in full
detail to permit fatigue calculations in these areas.
2. The foundation model shall be linearized using a representative fatigue
wave.
6.9.3 Stress Concentration Factors
1. Stress concentration factors (SCFs) for simple joints shall be determined
using Efthymiou equations.
a. For joints and/or connections outside the parameters of the Efthymiou
equations, applicable SCF formulations shall be presented to the
COMPANY for approval.
b. The load path shall be considered in developing the SCFs.
c. Joint geometry shall be proportioned such that the SCFs do not exceed
6.0.
d. COMPANY approval is required for exceptions to these criteria.
2. A minimum SCF of 2.5 shall be used for simple TKY joints.
3. SCFs at ring-stiffened joints, such as launch-leg joints, shall account for the
presence of the ring stiffeners.
4. A minimum SCF of 2.5 shall be used for ring-stiffened joints.
5. SCFs at jacket leg joints with grouted main piles or internally-grouted
sleeves may account for the reduction in chord ovalization and consequently
a reduction in hot spot stress.
a. Grouted-joint composite action may be accounted for using Lloyd’s
Register procedures (6.5.k).
b. The procedure allows an effective chord wall thickness to be calculated
equal to the thickness which gives a moment of inertia equal to that of the
chord-pile or chord-sleeve cross section; but it shall not exceed 1.75
times the actual chord thickness.
c. The equivalent chord thickness, instead of the actual chord thickness, is
then used in calculating the joint geometric parameter gamma.
6.9.4 Wave Slam
1. Wave slam shall be included in the determination of brace-end fatigue life in
the top framing level of the jacket.
2. A calculation procedure should be adopted which accounts for the actual
behavior of the structural element subjected to slamming.
3. Wave slam shall also be considered during in-place operating condition.
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6.9.5 S-N Curves
1. The API X’ curve shall be used in the calculation of fatigue lives at TKY
connections in accordance with API RP 2A-WSD and/or API RP 2A-LRFD,
as applicable.
2. The API X curve may be used in the calculation of greater fatigue lives of
TKY connections in which the appropriate weld profile control is specified in
accordance with API RP 2A-WSD and/or API RP 2A-LRFD, as applicable.
3. The endurance limit associated with the API S-N curves shall be used.
6.9.6 Fatigue Design
1. The preferred methods of improving fatigue life, in order of preference, at a
member end shall be one of the following:
a. Increase chord wall thickness
b. Increase brace end diameter
c. Weld profile control
d. Toe grinding or peening (Use of this technique shall be approved by
COMPANY on a case-by-case basis and should be avoided unless there
are no other feasible alternatives.)
2. Alternative methods of improving fatigue life, such as adding ring stiffeners
or grouting an internal sleeve, shall not be used unless specially approved in
writing by the COMPANY.
6.10 Transportation Strength Analysis
A three-dimensional computer model of the jacket and barge under tow shall be used to
analyze the stresses in the jacket and barge.
a. The jacket model shall include sufficient detail to evaluate the stress in all framing
members and joints that experience transportation-motion-induced loads.
b. When a rigid barge model is not sufficient—generally for barges in excess of 300
feet in length—the barge model shall contain sufficient detail to capture the
overall flexibility characteristics of the vessel and to permit the distribution of
hydrodynamic loads to model hogging and racking effects.
The jacket-barge model shall use spacers and tie-downs to simulate actual connection
conditions.
a. Vertically-oriented spacer members shall be attached at all hard points on the jacket
launch runners to support and elevate the jacket to its proper height above the
launch skidways.
b. Vertically-oriented members shall support vertical load only.
c. Tie-down members shall be modeled at each major horizontal elevation.
d. The barge end of the tie-down members shall be attached to nearby barge joints
using a series of statically-determinate rigid links.
The analysis of the jacket shall include all forces from the barge/cargo motion imposed
during transportation, which includes but is not limited to the masses due to the
following:
• The transport structural weight, including mill tolerance
• The load contingency factors
• The weight of all preinstalled rigging and shipped-loose items
A minimum of eight headings shall be considered, including all combinations of beam
seas, head seas and quartering seas.
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For the specified significant-wave height, the mean spectral period shall be varied
between the upper and lower bounds in order to find and analyze the maximum inertial
forces acting on the jacket.
The extreme tow inertial forces shall represent the 1-in-1000 highest values (RMS x
3.34) of the motions using a spectral sea model.
The lateral component of gravity due to roll caused by the design wave and wind shall
be accounted for in the jacket stress analysis.
a. Conservatively, the phase differences of the six components of inertial force may be
neglected.
b. Linear load combinations shall be formed for each wave direction to combine the
extreme global force components using all possible signs.
6.11 Launch Analysis
6.11.1 Launch Trajectory Analysis
1. Three-dimensional launch simulation analyses shall be performed to
determine the barge-jacket behavior during launching operations and the
jacket’s stability and bottom clearance after launch.
2. The launch-trajectory analysis shall be continued until final jacket equilibrium
is attained.
3. The time step in the launch simulation shall not exceed the following values:
a. Before rocker-arm tipping: 0.50 second.
b. After rocker-arm tipping: 0.25 second.
c. After jacket-barge separation: 1.00 second up to end of movement.
4. The velocity of the jacket in relation to the barge shall be greater than 3.0
ft/sec at the time of jacket tipping.
a. A minimum coefficient of friction of 0.05 may be used when the launch
skidway system consists of greased timber on PTFE (Teflon) plates.
b. For other skidway systems, CONTRACTOR shall submit (with
appropriate documentation) a coefficient of friction for COMPANY
approval.
5. The launch weight shall include mill and weld tolerance, load contingency
and all preinstalled rigging.
6. As detail design progresses, reductions in the load contingency can be
proposed by CONTRACTOR for approval by COMPANY.
7. The launch trajectory analyses shall be performed considering the following
variation in basic parameters:
No 1
Parameter
Launch weight 2 Transverse
center of gravity
3 Longitudinal
center of gravity
Description
-1% to +4%
±1% of average jacket width at nominal elevation
of center of gravity
±1% of average jacket length at nominal
elevation of
center of gravity
center ±1% of average jacket height
4 Vertical of gravity
±40% of estimated value
5 Coefficient
of friction
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Cd = 0.65 to produce the deepest penetration of
the jacket in the water
6 Hydrodynamic
Cd = 0.85 to produce the worst condition for
coefficients
jacket velocity at the time of jacket-barge
separation
Multiple cases with one ballast compartment
7 Damaged
condition
flooded in a main leg or buoyancy tank
8. Sufficient combinations of the above basic parameters and damaged
conditions shall be analyzed to produce the worst-case launch scenario.
9. For the intact condition, the jacket shall be designed such that minimum
bottom clearance during launch shall be the greater of 10% of the water
depth at the launch site or 30 feet.
a. For the damaged condition, bottom clearance shall be the greater of 2%
of the water depth at the launch site or 10 feet.
b. All clearances shall be relative to the lowest astronomical tide (LAT).
6.11.2 Launch Stress Analysis
1. Launch stress analysis shall be performed for each of the following:
• As each jacket launch-truss hard point passes over the rocker-arm pin
before tipping.
• At rocker-arm tipping.
• As each jacket launch-truss hard point passes over the rocker-arm pin
after tipping.
2. The applied loading to the jacket shall include the jacket weight, buoyancy,
inertial forces, hydrodynamic forces and reactions from the barge.
3. The rocker-arm beam load distribution shall account for the relative stiffness
of the rocker-arm beam and launch leg and shall satisfy moment equilibrium
constraints on the rocker-arm beam.
6.11.3 Member Slam
1. Jacket members that enter the water within 10 degrees of horizontal shall be
checked for slam effects using the predicted velocities from the launch-
trajectory analysis, a Cd = 2.0 and Morison’s equation.
2. Dynamic amplification shall not be considered.
6.11.4 Hydrostatic Collapse
1. The susceptibility of tubular members to hydrostatic collapse during launch
shall be determined in accordance with API RP 2A-WSD and/or API RP 2A-
LRFD, as applicable.
2. The minimum factor of safety for the axial compression case shall be 2.0.
6.12 Jackets Lifted from Transportation Barges
Jackets lifted from transportation barges shall be designed as per API RP 2A
requirement and considering the following items:
1. A lift analysis shall be performed to determine stresses imposed on the jacket.
2. A three-dimensional jacket structural model shall be used.
3. Sling arrangement shall be modeled on the basis of a statically determinate lift
condition.
4. Design shall include the effects of sling-length tolerances.
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5. Lift points and sling configuration shall be modeled to provide suitable distribution of
load to the crane hook.
6. Other lifting criteria such as COG shift, DAF, etc shall be addressed on relevant
COMPANY specification.
6.13 Floatation and Upending
Floatation and upending analyses shall be performed to investigate trim angles,
stability, bottom clearance and, for hook-assisted upending, crane-vessel hook heights
and loads.
The upending jacket weight shall include mill and weld tolerance, load contingency and
the weight of all preinstalled rigging and equipment installed after upending.
As detail design progresses, reductions in the load contingency can be proposed by
CONTRACTOR for approval by COMPANY.
Upending and floatation analyses that consider the following variance in basic
parameters shall be performed:
No 1
Paramater
Description
3
-1% to +4%
of average jacket width at nominal
Transverse center of gravity ±1% elevation of center of gravity
of average jacket length at nominal
Longitudinal center of gravity ±1% elevation of center of gravity
4
Vertical center of gravity
±1% of average jacket height
5
Coefficient of friction
±15% of estimated value
6
Damaged condition
Multiple cases with one ballast compartment
flooded in a main leg or buoyancy tank
2
Launch weight
No variance in the center of buoyancy shall be considered.
Sufficient combinations of the above basic parameters and damage conditions shall be
analyzed to produce the worst-case floatation and upending scenario.
During floatation and upending, the following minimum metacentric heights shall be
maintained for intact as well as damaged conditions:
a. Transverse (GMT): 3.0 feet.
b. Longitudinal (GML): 0.0 feet (during rotation).
c. Longitudinal (GML): 3.0 feet (vertical position).
Only ballasting shall be permitted during the uprighting phase of the upending
procedure.
a. Dewatering of flooded compartments shall only be used when correcting jacket
attitude for damaged conditions.
b. Dewatering shall take place away from the installation site but within the anchor
pattern of the installation vessel.
c. The dewatering process can be carried out by injecting compressed air or nitrogen
into one or more compartments depending on the location and type of damage to the
jacket.
For a hook-assisted upending, the jacket shall have a minimum undamaged reserve
buoyancy of 12% of jacket weight. PHEONWJ–S–PRC–0010 Rev. 0
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For a self-upending, the jacket shall have an undamaged reserve buoyancy of 5 - 9% of
jacket weight.
For damaged conditions, the minimum reserve buoyancy shall be 5% of jacket weight.
During upending, a minimum bottom clearance of 10 feet shall be maintained above the
highest seafloor obstruction throughout the floatation and upending procedures.
After the jacket is in the vertical position, the minimum clearance above seafloor
obstructions, such as wellheads or docking piles, may be reduced to 5 feet.
All clearances shall be relative to lowest astronomical tide (LAT).
The jacket shall be designed such that, in the post-launch condition, the jacket shall
float in a stable equilibrium position within 10 degrees of vertical for the worst-case
intact and worst case damaged conditions.
The top of jacket shall be accessible to personnel by access ladders from a zodiac
(inflatable) boat.
6.14 Jackets Installed over pre-drilled wells
Jackets may be installed over subsea pre-drilled wells template, in such case the Jacket
shall be designed as follows:
1. A docking sequence shall be developed.
2. A jacket-template docking analysis shall be performed to determine the
configuration, size and length of docking piles.
3. The docking analysis shall conform to current industry practice and shall be
developed with the concurrence of the Installation CONTRACTOR.
4. The analysis shall account for the mass of the jacket during installation, fabrication
tolerances, current forces, hook load, momentum and other dynamic effects.
5. Maximum environmental criteria to perform the docking operation shall be
determined.
6. The docking system shall be designed to prevent any docking forces from being
transmitted through the template to existing wells.
6.15 Loadout Analysis
The jacket shall be designed for stresses occurring during loadout.
The analysis shall fully consider the method of loadout, behavior of the foundation and
characteristics of the barge.
A minimum of four jacket support conditions shall be investigated during loadout.
The maximum allowable deflection of the barge supporting the jacket shall be
determined for each stage of loadout.
6.16 On-Bottom Stability Analysis
An on-bottom stability analysis shall be performed to assure the pre-piled integrity and
stability of the jacket after it is set on the sea floor.
a. The mudmats shall be designed globally to avoid overstressing the soils and causing
jacket instability due to soil failure.
b. The local design of the mudmats and associated support framing shall be governed
by the stresses imposed on them in developing the ultimate bearing capacity of the
soil.
The load conditions to be considered in the on-bottom stability analysis shall include still
water and the one-year-storm wave with the following conditions:
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a. Jacket on bottom with all legs flooded to MLW.
b. Jacket on bottom with all legs flooded to MLW with the weight of various
combinations of pile strings supported by the jacket, as appropriate.
The weight and locations of the pile strings shall consider the pile installation sequence.
6.17 Barge Stability
All barges shall satisfy the intact and damaged stability requirements set forth in the
United States Coast Guard (USCG), "Requirements for Mobile Offshore Drilling Units".
In addition to meeting the USCG requirements, the following stability requirements shall
be met:
a. The range of static stability of barge and cargo about an axis shall exceed 40
degrees.
b. The angle of down flooding (the point where non-watertight opening immersion
starts) shall be greater than 20 degrees.
c. The wind shall correspond to the 1-in-100 tow for the specific route planned.
d. The analysis shall take into consideration the time of the year that the tow will take
place.
e. For the intact dynamic stability analysis, factors of safety against overturning by a
100-knot wind shall not be less than 1.4 calculated as the ratio of areas under the
righting and overturning moment curves up to the second intercept of those curves
or the angle of down flooding, if less.
f. For the damaged dynamic stability analysis, factors of safety against overturning by
a 50-knot wind shall not be less than 1.4 calculated as the ratio of areas under the
righting and overturning moment curves up to the second intercept of those curves
or the angle of down flooding, if less.
6.18 Seismic Analysis
In areas where it is seismically active, jacket shall be designed against seismic loads to
perform a seismic analysis. The detailed methods and procedures for performing the
analysis shall be submitted to COMPANY for approval. API RP 2A requirement shall be
followed.
Geo-hazard study such as Probabilistic Seismic Hazard Assessment (PSHA) for the
particular site shall be conducted to determine horizontal Peak Ground Acceleration and
its spectrum for Strength Level Earthquake (SLE) and Ductility Level Earthquake (DLE)
as per API RP 2A requirement.
The above accelerations will be imposed to the platform in-place computer model to
analyze its impact to the structural integrity of platform.
PGA for particular area will be included on project particular structural specification or
design basis.
6.19 Vortex Shedding
Vibration problems associated with vortex shedding during fabrication, pre-service and
in-service conditions shall be considered in platform design.
a. Dynamic responses and the associated forces shall be checked for grout lines, flood
lines, casings, J-tubes, I-tubes, conductors and slender structural members.
b. The maximum limiting damping value shall not exceed 1% for members in water.
Member susceptibility to wind-induced vortex shedding shall be assessed in accordance
to the following:
a. The one-minute sustained wind speed of the 1-year storm shall be used for the site
under consideration.
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b. The maximum limiting damping value shall not exceed 0.2% for members in air.
Members shall be designed to withstand vortex-induced loading.
As required, vortex suppression devices may be used.
a. Temporary suppression devices shall be designed for easy removal prior to platform
installation.
b. Permanent suppression devices shall be designed, fabricated and inspected to meet
long term service requirements.
6.20 Wave Slam
All members, including walkways, stairways, conductor guides, riser clamps and pump
casings in the wave zone shall be designed for wave slam forces.
Bending stresses due to both horizontal and vertical slam forces in combination with
other global stresses shall be investigated.
Current velocity components shall not be included in the wave kinematics when
calculating the slam loading.
For X-braces, members shall be assumed simply supported out of the plane of the
framing.
Member lengths may be reduced to the face of the jacket leg.
Wave slam shall be calculated using Morison’s equation with a drag coefficient of 3.0.
To account for dynamic amplification, member mid-span moments shall be amplified by
2.0 and end moments amplified by 1.5.
6.21 Hydrostatic Design
Design for hydrostatic loadings shall be in accordance with API RP 2A-WSD.
For members with D/t ratios greater than 120 or a yield stress greater than 60 ksi,
hydrostatic design shall be completed in accordance with API Bul 2U.
For the design of large-diameter members, out-of-roundness shall be considered in
accordance with API Spec 2B.
The actual member length between the faces of the chords or between ring stiffeners
may be used.
7. Appurtenance Design Considerations
7.1 Boat Landings
Boat landings, associated connections and local framing shall be designed for boat
impact loads, environmental loads, uniform live loads and dead loads.
Vessel size, velocity and load cases shall be as specified in COMPANY specification
no. PHEONWJ-S-PRC-0006-0 Guidance on Boat Landing Design.
7.2 Riser Guards
Riser guards shall be provided on all external risers of a new platform and shall be
designed for the vessel size, velocity and load cases specified in COMPANY
specification no. PHEONWJ-S-PRC-0007 Guidance on Riser Guard Design.
Riser guards shall be designed to swing from one end to permit installation of future
risers without having to remove the riser guard.
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Guidance on Fixed Offshore Jacket Platform Design
7.3 Riser Clamps
Riser clamps shall be designed to withstand all environmental and operating loadings
such as pipeline thermal expansion and surge.
Riser clamps shall include a neoprene liner.
7.4 Sumps and Pump Casings
Sumps and pump casings and their supports shall be designed to withstand all
environmental and operating loadings.
The bottom elevation shall be determined to satisfy process design.
7.5 Conductor Guide
The criteria in the table below govern the design of the conductor guide and arm when
the conductors are installed using the drilling rig.
a. These criteria shall be considered for the design of the conductor guide and arm.
b. These criteria are in addition to the consideration of loads imposed by pre-installed
conductors.
Elevation
Description
1.5 times the weight of the string that will initially pass this
level.
times the weight of the string that will initially pass the
Second Level 1.5 second level.
Subsequent 0.5 times the weight of the string that will initially pass
Levels
these levels.
Top Level
The criteria in the table below govern the design of conductor guide and arm when
conductors are supported off the top jacket elevation during installation.
a. As a minimum, these criteria shall be considered for the design of conductor guide
and arm.
b. The design shall not practically restrict the number of locations that may be worked
at one time.
c. The criteria are in addition to the consideration of loads imposed by preinstalled
conductors.
Elevation Description
times the weight of the string that will initially pass this
Top Level 1.5 level.
times the weight of the string that will initially pass the
Second Level 1.5 second level.
Subsequent 0.5 times the weight of the string that will initially pass
Levels
these levels.
The top level of conductor guide and arm shall be designed to minimize surface area
and maintainability.
The conductor guides and arm at the top of jacket level shall have no cavities where
water may be trapped. PHEONWJ–S–PRC–0010 Rev. 0
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Guidance on Fixed Offshore Jacket Platform Design
The design shall include conductor guides and arm at the mudline elevation unless
predrilled wells are present.
7.6 Skirt Pile Guide Framing
7.6.1 Battered Skirt Piles
The design of skirt pile framing for battered skirt piles shall consider the loads
imposed during the installation of the piles.
As a minimum, the criteria in the table below shall be considered for the design
of the skirt pile guide framing:
Elevation
Description
A vertical load equal to 1.1 times the weight of the
heaviest string to be supported from this level, or 1.5 times
Top Level the weight of the string that will initially pass this level,
whichever is greater.
A vertical load equal to 1.5 times the weight of the string
Second Level that will initially pass the second level.
Subsequent A vertical load equal to 0.5 times the weight of the string
that will initially pass these levels.
Levels
7.6.2 Vertical Skirt Piles
The design of skirt pile guides for a one-piece, vertical skirt pile shall consider
the loads imposed during the installation of the piles.
As a minimum, the criteria in the table below shall be considered for the design
of the skirt pile guide framing:
Type
Description
lateral load equal to 0.2 times the weight of one skirt
Skirt Pile Guides A pile shall be supported by the skirt pile guide.
A vertical load equal to 1.1 times the weight of one skirt
pile shall be supported on the top of a skirt pile sleeve
Skirt Pile Sleeve for the worst skirt pile location to assess the jacket
stability and structural integrity of the jacket, including
the skirt pile sleeve framing, jacket leg and mudmats.
7.7 Flooding and Grout System
The flooding and grouting system shall be designed to have primary and secondary
systems which shall be separated to preserve utility in case of damage to the primary
system.
The system shall be fully detailed to allow fabrication from the design drawings.
When underwater hammers will be used, flooding and grout piping systems shall be
designed to resist severe impact forces.
Piping supports in the splash zone shall be minimized.
When no longer needed, the flood and grout lines and supports will be removed down to
El. (-) 25’-0".
The design of the flood and grout line supports shall consider this requirement to permit
easy removal of the lines.
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Guidance on Fixed Offshore Jacket Platform Design
7.8 Mudmats
Steel mudmats shall be provided, unless COMPANY has approved an alternative.
7.9 Walkway, Stairway, and Landings
Stairs, walkways and landings shall be designed for a 100 psf live load.
For process and quarters platforms, two independent stairways shall be provided
between the main deck and the top of jacket. The stairs shall be located to provide
direct routes for normal use as well as emergency escape.
Minimum stair slope shall be 30°. Maximum stair slopes shall be as follows:
(a) Above cellar deck : 38°
(b) Below cellar deck : 42°
Stair treads shall be proportioned such that the sum of the rise and run is approximately
18". Stair treads shall be prefabricated of the same material as the deck grating,
generally serrated bar grating using Galvanized Corten Steel. Plain galvanized steel
may not be used. The treads shall have non slip nosing and shall be bolted directly to
the stair stringer angle clips. Treads shall be 2’ 6" long.
Landings shall be provided to limit the maximum rise of individual stairways to 20 ft.
Preferred landings are either 90° or 180° type, which shall be 2’ 9" minimum length. If
straight landings are used, the minimum length shall be 4’ 0".
Stairs and landings shall be fitted with double handrails.
An intermediate landing shall be provided below the cellar deck to serve as access to
any below cellar deck levels, equipment, or sump tank.
7.10 Handrail and Ladders
Stairways, platforms and walkways are to be provided with handrails consisting of steel
top rails, mid rails, post and toe plates. Hand-railing shall enclose all platform areas and
stairways.
Handrails shall be shop fabricated in sections suitable for shipping and erection. Shop
connections may be welded or bolted.
Contact surfaces on handrails shall be smooth and free from burrs and sharp
projections. Corner of handrail shall be made from elbow piece.
Ladders shall be set perpendicular to deck platform to provide access.
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APPENDIX I
ASSESSMENT OF WIND-INDUCED VORTEX SHEDDING
Step 1 Adjust the extreme one-minute wind speed using the wind variation with elevation law:
Vy = (y/H) 1/n VH
where
Vy = wind velocity at height y
VH = wind velocity at reference height H
y = elevation of member at its centroid
n = 7 for the 1 minute sustained wind
Step 2
Compute the natural frequency (f) of the member in air:
0.5
2
f = (22.4 x (E x l/m) ) / 2 x π x LL )
where
I = moment of inertia of member (in4)
E = modulus of elasticity of steel (lb/in2)
m = mass per unit length (slugs/in)
LL = joint-to-joint member length (in)
Note: Member is assumed fixed at both ends.
Step 3 Compute the critical velocity for the member:
vcritical = 4.7 • f • d
where
vcritical = critical velocity (in/sec)
-1
f = natural frequency of member (sec
)
d = outside diameter of member (in)
Step 4 Adjust the critical velocity to account for member relative orientation with respect to
the wind direction.
Step 5 If a member critical velocity is less than its corresponding design wind speed, the
following vibration suppression methods may be considered:
• Change member size/span to increase the critical speed over the design speed.
• Use cables to shift the member frequency and thus its critical speed.
Other methods of vibration suppression shall not be used unless specifically
approved by COMPANY in writing.
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