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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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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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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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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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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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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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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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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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


Effective Length

Factor k

1.0

K (1)

1.0

1.0

1.0

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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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PT. PHE ONWJ


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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PT. PHE ONWJ


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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PT. PHE ONWJ


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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PT. PHE ONWJ


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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PHEONWJ-S-PRC-0010~0 (Guidance on Fixed Offshore Jacket Platform Design)

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