Chevron Vessel Impact Design Basis_Rev 2

8/20/2019 Chevron Vessel Impact Design Basis_Rev 2

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VESSEL IMPACT DESIGNBASIS OF FIXED

OFFSHORE PLATFORMS

FOR

CHEVRON


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Chevron Accidental Loading Design Premise Project Page 2Vessel Impact Design Basis of Fixed Offshore Platforms

Report No: OG-365-09-DB-001 Rev. 2Issue Date: September 2008

VESSEL IMPACT DESIGN BASIS OF FIXEDOFFSHORE PLATFORMS

Prepared byWS Atkins

On Behalf ofChevon Engineering Technology Company

COMMERCIAL IN CONFIDENCE

Prepared by: ................................... Checked by: ...................................L. Wang S. Simoni, J. Bucknell

Authorized by: ..............................…..S. Simoni

WS Atkins Inc Chevron Engineering Technology Company12121 Wickchester Lane, Suite 550 6001 Bollinger Canyon RdHouston, TX 77079 San Ramon, CA 94583USA USATel.: +(1) 713 463 6180 Tel.: +(1) 925 842 8734Fax.: +(1) 713 589 7381 Fax.: +(1) 925 842 8626


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DOCUMENT ISSUE CONTROL SHEET

Chevron Accidental Loading Design Premise Project

Vessel Impact Design Basis of Fixed Offshore Platforms

Issue Date Purpose Prepared Checked Approved

A 07/15/2008 For Internal Review LW SS, FM

0 07/18/2008Draft for Client

Review LW SS, FM SS

1 08/14/2008

Draft Incorporating

Client InitialComments LW SS, JB SS

2 09/25/2008Final Incorporating

Client FinalComments

LW SS, JB SS

NOTE:

This document has been specifically produced for the purposes of the VESSEL IMPACT

DESIGN BASIS OF FIXED OFFSHORE PLATFORMS and is only suitable for use inconnection therewith. Any liability arising out of use of this document by Chevron EngineeringTechnology Company or a third party for purposes not wholly connected with the above projectshall be the responsibility of the clients, who shall indemnify Atkins against all claims, costs,damages and losses arising from such use.


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TABLE OF CONTENTS

1. INTRODUCTION .......................................................................................................... 6

1.1 Background..................................................................................................... 6 1.2 Purpose .......................................................................................................... 6

2. REFERENCE DOCUMENTS .......................................................................................... 7

2.1 Chevron Specifications................................................................................... 7 2.2 Codes and Standards ..................................................................................... 7 2.3 Publications .................................................................................................... 7

3. DEFINITIONS .............................................................................................................. 8

3.1 Design Impact Event....................................................................................... 8 3.2 Impact Zone.................................................................................................... 8 3.3 Attendant Vessels .......... .......... ........... .......... ........... ........... .......... ........... ....... 9 3.4 Others............................................................................................................. 9

4. MECHANICS OF VESSEL IMPACT .............................................................................. 11

4.1 General......................................................................................................... 11 4.2 Vessel Impact Absorption Mechanism.......................................................... 11 4.3 Design Impact Energy .................................................................................. 12 4.4 Calculation of Vessel Impact Loads.............................................................. 12 4.5 Impact Load Application ............................................................................... 15 4.6 Operational Vessel Impact.............................................................................. 8

5. DESIGN OF NEW P LATFORMS TO WITHSTAND VESSEL IMPACT ................................. 18

5.1 General......................................................................................................... 18 5.2 Design of Jacket Legs and Braces in Impact Zone ...................................... 18

5.3 Design of Barge Bumpers............................................................................. 19 5.4 Design of Boat Landings............................................................................... 19 5.5 Design of Riser Guards ................................................................................ 20 5.6 Good Practice in Detailing of Designs .......................................................... 21 5.7 Impact Survival Acceptance Criteria............................................................. 21

6. A SSESSMENT OF EXISTING P LATFORMS TO WITHSTAND VESSEL IMPACT .................. 24

6.1 General......................................................................................................... 24 6.2 Condition Assessment .................................................................................. 24 6.3 Impact Loads ................................................................................................ 24 6.4 Estimation of Damage .................................................................................. 25 6.5 Impact Survival Acceptance Criteria............................................................. 25

7. VESSEL IMPACT A NALYSIS ...................................................................................... 26

7.1 General......................................................................................................... 26 7.2 Analysis Methods............ .......... ........... ........... .......... ........... .......... ........... .... 26 7.3 Available Software .......... .......... ........... .......... ........... .......... ........... ........... .... 29 7.4 Modeling of Structure.................................................................................... 30

8. P OST -IMPACT S URVIVAL A SSESSMENT .................................................................... 38


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1. INTRODUCTION

The principal objective of this document is to define generic design criteria to be usedon Chevron projects to assess fixed steel offshore structures against accidental vesselimpact.

The criteria are a compilation of acceptable industry practices and should be combinedwith regional specific criteria for the platforms to be assessed.

1.1 BACKGROUND

During the operational life of a fixed offshore platform, there is the possibility that thestructure could be accidentally impacted by a vessel. Vessel impact is a major hazardto offshore structures. Due consideration shall be given to the design of substructure toprovide robustness against such events. Vessel impact analysis now forms animportant and essential design case for fixed offshore platforms. The platform shouldbe designed to survive the initial vessel impact and the post impact criteria.

1.2 PURPOSE

The objective of this document is to explain the standard methods of checking theintegrity of a fixed steel offshore platform subject to vessel impact, and to describe thedesign recommendations adopted by Chevron. The design basis has been developedfor general worldwide application. The procedure is applicable to new and existingplatforms.

This document was prepared for consideration to be included as part of the ChevronEngineering Standards (CES) developed and maintained by the Floating and FixedSystems Unit of the Facility Engineering Department.


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2. REFERENCE DOCUMENTS

The following documents form a part of this Design Basis. Unless otherwise specifiedherein, use the latest edition.

2.1 CHEVRON SPECIFICATIONS

CIV-SU-1.19A Design of Platform Structures – Application: Fixed OffshorePlatforms

CIV-EN-100 Ultimate Limit Strength (ULS) of Fixed Offshore Platforms

2.2 CODES AND STANDARDS

Several worldwide offshore codes and standards offer guidance on vessel impact fornew and existing platforms (summarized below). Code requirements and additionaldescriptions of vessel impact analysis approaches and guidance can be found in these

documents.

API RP 2A

API RP 2A, Section 18, Fire, Blast, and Accidental Loading

ISO 19902

ISO 19902, Clause 10, Accidental Situations

UK HSE Guidance Notes

HSE Guidance Notes, Section 15, Loads

DNV RP C204

DNV-RP-C204, Design Against Accident Loads

NORSOK standard N-004

NORSOK standard N-004, Design of Steel Structures, Annex A, Design Against Accidental Actions

2.3 PUBLICATIONS

UK Health Safety Executive, Loads, OTR 13/2001, 2002

DNV Technical Note, Impact Loads from Boats, TNA202, 1981

Veritec, Design Against Accidental Loads, Report No. 88-3127, 1988

Health and Safety Executive, Technical Policy Relating to Structural Response to ShipImpact, December 2006.


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3. DEFINITIONS

3.1 DESIGN IMPACT EVENT

Design Impact Event represents an event selected from impact scenarios that require

explicit design considerations. Design impact events are primarily based on accidentscenarios involving vessels that are expected to operate in the vicinity of the platform.Vessel impact scenarios should be developed by a risk assessment process, involvinga multi-discipline team of experienced engineers. The most likely impact scenarios arethe broadside impact of one of the legs of the platform and the bow/stern impact of oneof the braces in the splash zone [11]. Practices that account for accidental scenariosare provided in Section 18 of API RP 2A [1].

For purpose of a rigorous impact analysis, design impact events shall be establishedrepresenting bow, stern, and broadside impacts on exposed platform elements. Vesselorientation and velocity shall further define the impact event. Operational restrictions onvessel approach sectors may limit the exposure to impacts in some areas of the

structure.

Design impact events shall consider two energy levels of vessel impacts, i.e. accidentalvessel impact, representing a rare condition with high energy level, and operationalvessel impact, representing a frequent condition with low energy level.

3.2 ACCIDENTAL VESSEL IMPACT

Accidental vessel impact represents an ultimate condition based on the vessel driftingout of control in the worst sea-sate in which it may operate close to the platform. Foraccidental vessel impact, the impact loads should be resisted or impact energy shouldbe absorbed without complete loss of the structural integrity.

3.3 OPERATIONAL VESSEL IMPACT

Operational vessl impact represents a serviceability condition based on the type ofvessel which would routinely approach alongside the platform with a velocityrepresenting normal manouvering of the vessel approaching, leaving, or standingalongside the platform. For operational vessel impacts, a vessel speed of 0.5 m/s iscommonly used. During operational vessel impacts, the impact energy shall beabsorbed by localised denting of brace or leg and elastic deformation of the structureonly, and the structure should only suffer minor damage without impairing thefunctionality of the platform.

3.4 IMPACT ZONE

The impact zone is defined by the portion of a platform vulnerable to impact by supplyvessel. The impact zone is a function of the vessel freeboard, tidal range and operatingsea states. The following conditions should be considered in determining the range ofpossible impact zones:


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• Vessel maximum and minimum draft• Mean low and high water spring tides• Operating sea states when the vessel may be in use• Associated surge with the operating wave height• Platform settlement• Water depth tolerance• Vessel geometry for bow, stern and broadside impacts

The greatest frequency of impact will be near the mean still water level. All exposedelements at risk in the impact zone should be assessed for vessel impact during normaloperations.

3.5 ATTENDANT VESSELS

It is not practical or economical to design a platform for a major collision, hence thestructure should be designed to absorb the impact energy from vessels regularly visitingthe platform, i.e., the supply vessels. These vessels vary in size from 2,000 to 5,000tons. The vessel size in specific region should be confirmed prior to assessment. Byway of example, for the northern North Sea, a vessel can be 5,000 ton, whereas in thesouthern North Sea a mass of around 2,500 ton is more normal. For Gulf of Mexicostructures in mild environments and reasonably close to their base of supply, a 1,000ton vessel represents a typical 55 m to 60 m supply vessel. For deeper and moreremote locations in the Gulf of Mexico the vessel mass can be different.

The attendant vessel details should include vessel velocity, displacement, added mass,flexibility, maximum and minimum draft and vessel shape.

3.6 OTHERS

Accident scenario — Accidents result from the occurrence of a series of one or moreevents that combine to cause an undesirable and unplanned outcome. Such a series ofevents constitutes an accident scenario. The events may result from mechanical faultor human and organizational error.

Ductility — Ductility is a generic term that characterizes the ability of a component orsystem to deform without experiencing collapse due to brittle fracture or buckling. Aductile component or system may experience some diminishing strength as it deformsand still be considered ductile.

Linear analysis — Linear analysis assumes all components and system respondlinearly to loading.

Non-linear analysis — Non-linear analysis takes into consideration the non-lineareffects of individual component behaviour, including non-linear material behaviour aswell as the non-linear deflection of the structural components and system.


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4. MECHANICS OF VESSEL IMPACT

4.1 GENERAL

A vessel impact is characterized by a rapid dissipation of kinetic energy by the impacted

structure and the vessel as strain energy. In some instance, the vessel will strike aglancing blow and a portion of the impact energy will remain as kinetic energy followingthe impact.

4.2 VESSEL IMPA CT AB SORPTION MECHANISM

During an impact between a supply vessel and a steel structure, a number ofmechanisms are available to absorb the strain energy:

• Local denting of the impacted member

• Platform structural deformation, including local bending of the impacted memberand platform global deformation

• Vessel local indentation

Local Denting of Impacted Member

Under lateral impact, circular tubular sections are susceptible to localized denting. Thisenergy absorption can be determined either from load-deformation curves or bydetailed modeling of the impacted member.

The contribution to energy dissipation from local denting is normally of significance for jacket legs only. For braces in typical jackets the denting energy dissipation is smallcompared to the total impact energy and may be neglected.

Platform Structural Deformation

Apart from local denting of the impacted member, energy will be absorbed by elasticand plastic deformation of the impacted member, the platform and foundation. Thisenergy will be calculated using the area under the platform load-displacement curve atthe point of impact obtained from the ship impact analysis.

In general, resistance to vessel impact is dependent upon the interaction of memberdenting and member bending. Platform global deformation may be conservativelyignored. For platforms of a compliant nature, it may be advantageous to include the

effects of global deformation.

Vessel Indentation

The deformation of the vessel can be a significant energy absorption component whenvessel impacts on jacket leg. Energy absorption by local deformation of the ship maybe based on the force-indentation curves provided in DNV RP C204 [4] if no specific


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data is available. It is noted that these curves were developed based on North Seasupply vessel with a displacement of 5,000 tons.

For vessel impacts on jacket braces, it is typically assumed that all energy is dissipatedby braces.

4.3 DESIGN IMPACT ENERGY

Several offshore codes offer the guidance on the determination of design impact energyfor accidental vessel impact. They however have difference in approach.

The default vessel impact energy recommended in API RP 2A is based on the vesselsize (from operations) and a minimum vessel speed of 0.5 m/sec. Guidance is given inthe commentary section of APR RP 2A, C18.9.2, “Vessel Collision” [1]. This approachis tailored to the Gulf of Mexico (GOM) environment and operating practices.

Norwegian codes [4, 9] specify a vessel size of 5,000 tonne displacement drifting at 2.0m/second yielding a kinetic energy of 14 MJ for broadside impact and 11 MJ for bow orstern impact. These design kinetic energies are to be shared by the platform and thevessel.

The UK HSE Guidance Notes [3] define the accidental broadside impact energy of a5,000 ton vessel traveling at 2 m/s as 14 MJ, which is same as the Norwegian coderequirement. Based on studies of observed platform damage from actual vesselimpacts, however, HSE have modified the theoretical impact energy from an accidentalvessel impact to account for known deficiencies in the theoretical method. HSE requirethat the platform’s contribution to energy dissipation should be minimum 4 MJ. This isdifferent from the Norwegian code requirement, where the share of energy is notprescribed, but depends on the relative stiffness of the vessel and platform.

The recent ISO 19902 Standard [2] reinforces the HSE approach but highlights theneed to establish accidental design conditions taking account of known site specificvessel operations.

The HSE approach is adopted here to derive the design impact energy for an accidentalvessel impact.

4.4 CALCULATION OF VESSEL IMPACT LOADS

Vessel impact loads are typically characterized in terms of impact energy. The totalkinetic energy involved in a vessel impact can be calculated using Equation (4.1).

2

2

1amv E = (4.1)

where

E = kinetic energy of the vessel (KJ)

m = vessel mass (tonne)


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a = added mass factor

= 1.4 for broadside impact

= 1.1 for bow or stern impact

v = vessel velocity (m/s)

The key factors in determining the vessel kinetic energy are mass and velocity.

4.4.1 Acci dental Vessel Impact Design Energy

The velocity at which a drifting vessel may impact a facility depends on the actual seastate in which the impact occurs. The vessel drifting velocity is related to the expectedenvironmental conditions under which the vessels will be operating [3]:

s H v

2

1= (4.2)

Where

v = vessel drifting velocity (m/s)

Hs = maximum permissible significant wave height for vessel operations nearthe platform (m)

The deficiencies with this approach are:

1. The added mass factor is dependent on impact duration and is notstraightforward to estimate.

2. The vessel is unlikely to come to a complete stop (in sway, yaw and roll) andhence not all the energy will go into the impact.

3. The platform will not see all the energy – some will be absorbed by the vesselitself.

In recognition of these deficiencies the UK HSE carried out studies of observed platformdamage to determine the amount of energy actually absorbed by the platform. On thebasis of the study results, the HSE Guidance Notes define the accidental broadsideimpact energy of a 5,000 tonne vessel traveling at 2 m/s as 14 MJ, but only require 4MJto be absorbed by the jacket structure without collapse. The vessel velocity of 2 m/srepresents a vessel drifting out of control in a sea state with significant wave height ofapproximately 4 m.

On the basis of the above it is possible to define, in simple terms, a design impactenergy seen by the platform structure only as:

3500/22

12

⎟

⎠ ⎞

⎜

⎝ ⎛ = s

H amKE (4.3)

where KE = design impact energy to be absorbed by the platform structure only (MJ).


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For a broadside impact of a vessel of 5,000 tonne operating in a sea state withsignificant wave height of approximately 4 m, Equation (4.3) produces design impactenergy of 4 MJ, which is to be absorbed by structure alone.

This formula takes no account explicitly of current velocity and may therefore be seenas appropriate for non-tidal or open water operational conditions where diurnal

velocities are low. Where current velocities are significant (typically in littoral andestuarine areas), it is proposed that the formula is adapted as follows:

3500/2

1

2

12

⎟ ⎠ ⎞

⎜⎝ ⎛ +=

cs U H amKE (4.4)

where U c = operational current velocity (m/s).

The operational current velocity must be set according to the circumstances, and beconsistent with the operational sea state, H s . It will include only a small wind-inducedcomponent, comprising mainly of tidal effects. It may in fact be argued that the wind-induced component is already included in the wave-induced velocity computation(Hs /2), and thus only tidal current should be added.

The vessel mass should be the mass for the size of supply vessel expected to servicethe platform and the velocity should be the drifting velocity that would be reached bythat vessel in the maximum operating storm condition. The vessel size and themaximum operating storm condition could be determined from a site-specific riskassessment. Design engineers should attempt to obtain the vessel size and themaximum operating storm condition from the project team. Once those are established,the vessel drifting velocity can be determined. Table 4-1 provides various vessel sizesand design impact energy criteria for six different geographical regions including Gulf ofMexico (GOM), northern North Sea (NNS), southern North Sea (SNS), offshore eastcoast Trinidad, offshore northern Angola and shallow water offshore Nigeria. Forpreliminary engineering, or in areas where a risk assessment is not carried out, thesevalues can be used to estimate design impact energy prior to obtaining operations inputfor site-specific analysis.

It should be noted that the values in Table 4-1 represent the impact energy criteriarequired to be dissipated by the structure alone, and these values may not beconservative and should be used with caution. The lowest criteria of the Gulf of Mexicoreflect the smaller vessel sizes and the lower operational sea states. The most onerouscriterion is in the northern North Sea where vessel sizes are larger and operating seastates are more severe.

The minimum impact energies with the current velocity ignored are also presented in

Table 4-1. For the northern North Sea, a minimum impact energy of 4 MJ is computedwhen current is ignored. It is noted that the more conservative approach is that thecurrent velocity is taken into account.

4.4.2 Operational Vessel Impact Design Energy

For operational vessel impacts, a vessel speed of 0.5 m/s is commonly used. Table 4-2provides various vessel sizes and design energy criteria for operational vessel impact


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for six different geographical regions including Gulf of Mexico (GOM), northern NorthSea (NNS), southern North Sea (SNS), offshore east coast Trinidad, offshore northern

Angola and shallow water offshore Nigeria. For preliminary engineering, or in areaswhere a risk assessment is not carried out, these values can be used to estimatedesign impact energy prior to obtaining operations input for site-specific analysis.

It should be noted that the values in Table 4-2 represent the impact energy criteriarequired to be dissipated by the structure alone, and these values may not beconservative and should be used with caution.

4.5 IMPACT LOAD APPLICATION

The width of contact area during impact is in theory equal to the height of the vertical,plane section of the ship side that is assumed to be in contact with the tubular member.For large widths, and depending on the relative rigidity of the cross section and the shipside, it may be unrealistic to assume that the tube is subjected to flattening over theentire contact area. In lieu of more accurate calculations it is proposed that the width ofcontact area be taken equal to the diameter of the hit cross section [4].

In the global analysis of the impacted member and the structure the impact load is oftenmodeled as a concentrated load applied at the point of impact. This is a reasonableassumption as far as bow/stern impact is concerned. It also yields a lower bound withrespect to beam resistance.


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Chevron Accidental Loading Design Premise Project Vessel Impact Design Basis of Fixed Offshore Platforms


Region

Criteria

GOM NNS SNS TrinidadNor An

Vessel mass, m (tonne) 1500 5000 2500 2500 2500

Added mass factor, a 1.4 1.4 1.4 1.4 1.4

Significant wave height, H s (m) 1.87 4.0 2.0 2.0 2.0

Operational current speed, U c (m/s) 0.261 0.33 2 0.68 2 0.44 3 0.

Including current speed 0.4 5.4 1.4 1.0 0.7 Impact energyKE (MJ) 4

Ignoring current speed 0.3 4.0 0.5 0.5 0.5 1 Assumed the same tidal current as NNS2 70% peak spring current (HSE Guidance Notes – Figure 11.6)3 Approximate operational current speed associated with 2m H s 4 Impact energy required to be dissipated by the structure itself

Table 4-1: Regional Design Criteria for Accidental Vessel Impact


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Region

CriteriaGOM NNS SNS Trinidad Nor An

Vessel mass, m (tonne) 1500 5000 2500 2500 2500

Vessel speed, v (m/s) 1 0.5 0.5 0.5 0.5 0.5

Impact energy KE (MJ) 2 0.3 0.9 0.4 0.4 0.4

1 Vessel speed of 0.5 m/s is assumed2 Impact energy required to be dissipated by the structure itself

Table 4-2: Regional Design Cri teria for Operational Vessel Impact


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5. DESIGN OF NEW PLATFORMS TO WITHSTAND VESSEL IMPACT

5.1 GENERAL

The adequacy of the structure design shall be verified by demonstrating adequate

strength and ductility against accidental vessel impact events that represent vesselimpact scenarios.

Most structural framing systems that meet the general design practices as stated inChevron “Design of Platform Structures – Application: Fixed Offshore Platforms”(CIV-SU-1.19A) are capable of resisting supply vessel bumping without causing animmediate threat to the structural system integrity.

Conductors and risers should be located within the main structural framing, ifpossible. Additional local protection may be required to prevent supply vesselsfrom penetrating the main framing perimeter and impacting conductors and risers.If located outside the framing (installed after platform installation), risers should be

protected from damage by riser guards or by operational procedures preventingsupply vessel access.

5.2 DESIGN OF JACKET LEGS AND BRACES IN IMPACT ZONE

Design of jacket legs and braces shall follow Section 5 within Chevron “Design ofPlatform Structures – Application: Fixed Offshore Platforms” (CIV-SU-1.19A).Specific design considerations for vessel impact shall include:

1. The accidental vessel impact design energy criteria calculated based on themethodology within Section 4.4 should be used for the design of jacket legsand braces in the impact zone. The substructure should be checked for

vessel impact and its post impact strength to ensure that the jacket andfoundation can absorb the impact energy without causing progressivecollapse of the structure.

2. Main jacket column rows (elevations) shall have X-bracing in areas ofvessel impact, unless provided direct protection by riser guards or boatlandings in the impact zone.

3. Braces should be designed to fail away from the joint (e.g. at the brace/stubconnection) in order to facilitate any repairs.

4. Design should ensure that any element failure occurs before joint failure

and that elements remote from the impacted member remain elastic.

5. Where exposed to vessel impact, knee-braces or other members thatsupport gravity loads should have the ability to absorb the energy of avessel impact. The ability of the brace to absorb the impact energy shouldaccount for the potential of significant tension being developed in the braceduring impact. The structural subsystem to which the brace is attachedshould be checked including this tension.


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5.3 DESIGN OF BARGE BUMPERS

Barge bumpers shall be removable and mounted on shock cells. Barge bumpersusing shock-cells shall be designed to meet the following requirements:

• The accidental vessel impact design energy criteria calculated based on themethodology within Section 4.4 should be used for the design of bargebumpers. If from a risk evaluation perspective the likelihood of the designaccidental vessel impact to the barge bumpers is acceptable low, the bargebumpers might be required to withstand the operational vessel impactdesign energy also calculated based on the methodology within Section4.4.

• The barge bumper should extend a certain distance beyond the bottomsupport to prevent supply vessel from hooking underneath the bumper.

• Barge bumper assemblies shall be designed such that the bumper face isat the minimum practical distance from the jacket leg. However, the

bumper face shall extend beyond the face of the boat landings and riserguards.

• The bumper shall fail under extreme loads in a manner that the platform legconnections are not damaged.

• Load shall be applied halfway between the post supports for sizing thesupporting shock cells.

• Load shall be applied at one-third points for sizing the supporting shockcells.

• Shock cells shall be checked assuming that the applied load is applied inthe plane of the barge bumper and at an angle of 30 degrees to the plane ofthe barge bumper.

• The energy absorbing units and flanged connections shall be designed tofail in such a fashion that the jacket and the impacting vessel are notsubject to hazard caused by system element collapse or detachment.

• Design of barge bumpers shall provide for a field elevation adjustment of +/-0.91 m (3 ft).

5.4 DESIGN OF BOAT LA NDINGS

Design of boat landings shall follow Section 7.1 within Chevron “Design of PlatformStructures – Application: Fixed Offshore Platforms” (CIV-SU-1.19A). Specificdesign considerations for vessel impact shall include:

• The accidental vessel impact design energy criteria calculated based on themethodology within Section 4.4 should be used for the design of boat


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landings, associated connections and local framing. If from a riskevaluation perspective the likelihood of the design accidental vessel impactto the boat landings is acceptable low, the boat landings might be requiredto withstand the operational vessel impact design energy also calculatedbased on the methodology within Section 4.4.

• Connections of the boat landings to the platform should be designed tominimize damage to primary members during vessel impact.

• The bottom elevation of the boat landing shall be located at an elevation topreclude boats in the wave trough from contacting the underside of the boatlanding.

• The boat landing design shall consider provision for a field elevationadjustment of 1.2 m (4 ft).

• Shock cells can be used to meet the impact criteria.

5.5 DESIGN OF RISER GUARDS

The function of riser guards is to prevent damage to risers due to accidental vesselimpact. Specific instructions for riser guard design shall follow:

• The accidental vessel impact design energy criteria calculated based on themethodology within Section 4.4 should be used for the design of riserguards. If from a risk evaluation perspective the likelihood of the designaccidental vessel impact to the riser guards is acceptable low, the riserguards might be required to withstand the operational vessel impact designenergy also calculated based on the methodology within Section 4.4.

• Riser guard layout shall consider not only initial installation but futureremoval and reinstallation of the riser guard for future riser installation.Small riser guards with plan for future riser installation shall be designed toswing from one end to enhance the installation of future riser. Large riserguard should be designed with stabbing guides so that it can be removedand reinstalled or replaced if necessary.

• Impact area on riser guard shall be assumed at MLW (mean low water) plusor minus 1 m.

• Connections to the jacket shall be preferably on jacket legs where possible.

• Mild grade steel should ordinarily be used for riser guard fabrication,although connection details and king posts are likely candidates for highstrength steel.

• Connections shall be designed to minimize damage to primary structuralmembers and joints in the jacket.


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members should be designed with sufficient ductility to allow mobilization ofmembrane tension in the members prior to joint failure.

5.7.1.1 Ducility Limits

The maximum energy that the impacted member can absorb will be limited by local

buckling or fracture.

Local Buckling

Depending on the slenderness of the cross section, local buckling may take place.For higher slenderness members, the bending moment capacity and hence theenergy dissipation capacity would be degraded once local buckling occurs. Thecritical deformation for local buckling occuring is specified in Section 3.10.2 withinDNV RP C204 [4].

Critical Strain

If local buckling does not take place, the maximum energy that the impactedmember can dissipate will be limited by fracture. Fracture is assumed to occurwhen the tensile strain due to the combined effect of rotation and membraneelongation exceeds the critical strain ε cr . Table 5-1 lists the critical strain ε cr andstrain hardening parameters H proposed in DNV RP C204 [4].

Steel Grade

EN ASTM EquivalentCritical Strain, ε cr

Strain HardingParameter, H

S235 A 36 20% 0.0022

S355 A 572 Gr 50 A 992 Gr 50 A 913 Gr 50

15% 0.0034

S460 A 913 Gr 65 10% 0.0034

Table 5-1: Crit ical Strain and Hardening Parameters

5.7.1.2 Deflection Limits

In the splash zone risers are often located on the inside of the platform near one ofthe legs, receiving structural protection from the strongest members in the platform.However, for the sake of convenience some risers are tied to the braces of theplatforms. Brace impact and subsequent severance can have unacceptableconsequence to risers, and hence riser integrity may well be one of the governingcriteria. It is recommended in Reference [11] that the maximum brace deflection of1.0 m is taken as the impact criteria for the brace with risers tied to.


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The function of riser guards is to prevent damage to external risers due toaccidental vessel impact. The riser guards should be checked to avoid anypotential for vessel ingress that could lead to the external risers being struck.

5.7.2 Operational Vessel Impact

For operational vessel impacts, the substructure and piles shall be capable ofabsorbing design energy without impairing the functionality of the platform. Theimpact energy shall be absorbed by localised denting of impacted leg or primarybrace members and elastic deformation of the remaining structure only.

Member deflections shall be limited in order to protect risers and conductors. Riserguards, boat landings, barge bumpers or other vessel impact protection systemsshould be designed to facilitate component replacement and/or repair followingoperational vessel impact.


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6. ASSESSMENT OF EXISTING PLATFORMS TO WITHSTAND VESSELIMPACT

6.1 GENERAL

A fixed offshore platform is subject to possible damage from vessel impact duringnormal operations. If assessment process identifies a significant risk from vesselimpact loading, the effect on structural integrity of the existing platform should beassessed. The purpose of assessment is to determine the capacity of an existingstructure to withstand an accidental vessel impact and to identify and optimize theextent of any required strengthening, repair or other mitigation and the associatedurgency.

In contrast with design, assessment is concerned with the platform in-placecondition. The assessment process may involve detailed review, analysis, testing,or calculation of the aspects of the design that are non-complinat with the standard.State-of-art scientific and technical knowledge and the best available data may be

used in this process.

6.2 CONDITION ASSESSMENT

The structural input data for assessment can be gathered over the structure’slifetime and used to better represent its state and condition at the time ofassessment. For assessment it is therefore important that a reliable and up-to-datedatabase is assembled.

The input data should be both accurate and representative of actual conditions atthe time of the assessment. Any changes in use, modifications to deck payload,platform design drawings, repairs, inspection history and other pertinent

information should be obtainable from the platform inspection records.

Any damage, repairs and modifications outlined in the routine annual inspection orspecial inspection reports should be modeled to best represent the as-is conditionof the platform.

Where drawings are not available, or are inaccurate, additional inspection of thestructure and facilities may be required to collect the necessary information. Insome instances additional detailed inspection, using appropriate techniques, toverify suspected damage or deterioration or major modifications might benecessary.

6.3 IMPACT LOADS

The process of assessment is intended to determine the “best estimate” of both theloading and response of the structure. This will require a high degree of familiaritywith relevant in-service performance data. The design impact loads for existingplatforms should be developed taking into account site specific data concerning:

• Vessel sizes


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• Operations area traffic• Prevailing local weather/seas• Marine operations practices.

The impact energies may be calculated using the methodology presented in

Section 4.4. Impact loads will be applied at the impact locations.

6.4 ESTIMATION OF DAMAGE

6.4.1 Damaged Members

One of the major difficulties of assessing existing platforms with damaged orcorroded members is to accurately model their load carrying capacity, andespecially their ductility, after such capacity is reached. If alternative load pathsare available to bypass a damaged member, the member may be removed fromthe model.

The severity of the damage should be reviewed on a case-by-case basis todetermine if the damaged member has lost its load carrying capacity and whetherthe member should be removed from the model, if necessary.

Less severe dents, caused by vessel impacts or dropped objects, should bechecked individually in accordance with the procedures given in ISO Draft [2]. Ifdamaged members are found by the procedure to be stressed to an acceptablelevel, the post-damage stiffness properties should be specified and included in theglobal structural model to represent the damaged members. Damaged memberproperties may be determined by reference to published data (e.g. Smith et al [22]and Moan et al [23]), or through finite element analysis or experimentation.

6.4.2 Damaged Joints

Cracked joints should be modeled in sufficient detail to assess the impact of thedamage on the global behavior of the structure. A lower bound estimate of thestructure's strength will be obtained by removing the affected joint(s)/member(s)from the model. If the structure cannot maintain integrity with the member removeda less conservative estimate will be obtained by reducing the strength of theaffected joint by some factor.

6.5 IMPACT SURVIVAL ACCEPTANCE CRITERIA

The impact survival acceptance criteria for new platforms documented in Section

5.7 will be applicable to the assessment of existing platforms.


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7. VESSEL IMPACT ANALYSIS

7.1 GENERAL

The mechanical response to vessel impact loads is generally concerned with

energy dissipation, involving large deformations and strains far beyond the elasticrange. Hence, plastic methods of analysis should be used. The structural integritycan either be verified by non-linear finite element analysis or by means of simplifiedplastic analysis.

The design against vessel impact is often based on application of simplified handcalculation method in combination with linear elastic frame program. Rigorousnon-linear finite element methods are being used in vessel impact analysis to anever increasing extent. Non-linear finite element analysis takes into considerationthe non-linear effects of individual component behavior, including non-linearmaterial behavior as well as the non-linear deflection of the structural componentsand system.

7.2 ANA LYSIS METHODS

7.2.1 Simpli fied Plastic Analys is

If simplified plastic analysis methods are used, the part of the impact energy thatneeds to be dissipated as strain energy can be calculated by means ofconservation of momentum and conservation of energy. Simple formulasamenable for hand calculations may be found in the commentary to Section 18(C18.9.2) of API RP 2A [1], DNV-RP-C204 [4], and the Annex A of NORSOKstandard N-004 [9].

7.2.2 Non-Linear Finite Element Analys is

Non-linear finite element methods include quasi-static impact analysis and dynamicimpact analysis. The majority of vessel impact analyses performed to date haveused quasi-static methods. However, when the duration of impact is short,dynamic effects can be significant.

The impact duration depends on the size and configuration of both the structureand the vessel, and on the nature of the impact. Dynamic effects can be significantwhen the duration is of the same order or less than the structure's natural period.In such cases an assessment of the dynamic behavior during the impact should beconsidered.

Quasi-Static Impact Analysis

The quasi-static impact analysis may be used when the platform movement andhence kinetic energy of the structure is small. This applies to stiff structures whosenatural periods are short and where the global displacement during the impact isrelatively small.


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A quasi-static ship impact analysis is performed by applying functional loads (deadand live loads) to the jacket first and then introducing impact loads sequentially.The energy absorbed by structure is calculated by summing the work done due tostructural deformation, local member denting at the point(s) of impact and (ifincluded) local denting of the vessel.

The energy absorbed through structural deformation will be the area below theforce-displacement curve, provided that the displacement is measured at the pointof load application. Local denting may be calculated either automatically bysoftware (if applicable) or manually by referring to published formulae. Someprograms such as USFOS will automatically insert a growing dent, arising duringan impact, in the structural model for a quasi-static analysis, and automaticallyupdating the member properties of the impacted member to account for denting.

USFOS can automatically account for the energy absorption due to the denting,bending and elongation of the impacted member and due to the global deformationof the jacket. USFOS can also automatically calculate the energy absorbed byvessel by inserting a spring between the impact load and the structure representing

the stiffness of the vessel.

The following quality assurance should be performed during the quasi-static impactanalysis:

• The applied gravity loads agree with expected values.

• The impact load-displacement curve and displaced shape appearreasonable.

• The calculated dent depths and energies are reasonable and compare wellwith expected values.

• The pile head loads do not exceed calculated capacities. Piles should bechecked for vertical slippage not exceeding a limiting value.

• The maximum impact load should be reasonable. The energy absorbed inthe vessel (if modeled) should be checked by finding the energy under thevessel force-displacement curve.

Dynamic Impact Analysis

Dynamic impact analysis should be used when the kinetic energy of the structureabsorbs significant impact energy. This type of analysis is recommended forrelatively compliant structures. Dynamic impact analyses require much moreanalysis experience than quasi-static analyses. Dynamic impact analyses shouldonly be performed where necessary, and with care.

Dynamic impact analyses can be carried out as time domain dynamic calculationsin which the impact actions represent both the direct impact and the inertia of thestructure. Because the two excitations do not attain their maximum value at thesame time, the duration of the time simulation should be long enough to cover all


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relevant phases of the collision. Typically, the direct impact due to the collisionattains its maximum value early during the collision, while the effects of inertiareach their maximum values later during the collision. Energy is absorbed in boththe structure and the vessel, but additional absorption sources such as energyimparted to platform vibration and energy dissipating from radiating wavesgenerated as a result of the collision can also be represented.

Initially, the functional loads (dead and live loads) are statically applied to thestructure. A mass representing the vessel and associated added mass is given aninitial velocity corresponding to the design vessel velocity, and hence the mass hasthe required total kinetic energy of the vessel. The flexibility of the vessel isnormally represented by a spring between the mass and the structure at the pointof impact. The vessel impact analysis is then performed in the time domain, withthe transfer of momentum from the vessel to the structure solved at each time step.If the program is unable to include local member denting within dynamic analyses,a dent has to be explicitly modeled.

The following quality assurance should be performed during dynamic analysis:

• Mode shapes are generated and checked for the first 3 natural periods.

• A sensitivity study should be performed on the time-stepping interval toensure adequate resolution is used.

• Energy time-histories of kinetic energy, structural energy, member dentingenergy and vessel denting energy should be checked.

• The impact force time-history should be checked against the vessel dentingcurve.

7.2.3 Analys is Cases

Jacket Legs and Braces

Potential impact from vessel to jacket legs and braces in the impact zone shall beconsidered. Analysis cases depend on the project specifications, but shouldpreferably include as a minimum:

• Impact at nodes, which will tend to maximize the impact load and the loadson the piles.

• Impact between framing levels, which will tend to maximize local denting

and member bending.• Orthogonal and diagonal cases.

• The full range of impact elevations should be considered.


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Other software can perform vessel impact analyses that are not listed here;however, they may not have the special features of the software listed here. Ifusing another code, be sure that it uses a documented approach to perform thevessel impact. The code should be tested or calibrated to perform the specific typeof vessel impact analysis for fixed platforms as described in this document. Theuser should ask if there has been prior vessel impact work that can be reviewed or

whether there are any benchmark problems to demonstrate the software’s vesselimpact analysis capability.

7.4 MODELING OF STRUCTURE

7.4.1 Data Requirements

7.4.1.1 Drawings

The primary structural framing drawings, including main jacket framing, deckframing and pile drawings, and the drawings for the appurtenances which are partof analysis focus (e.g. conductors, risers, boat landings, conductor/riser guards,barge bumpers) should be made available.

For existing platforms, as built drawings should be used, if possible. The drawingsshould also reflect the current configuration of the platform, since structuralchanges may have occurred since the platform was installed.

7.4.1.2 Weight Report

Jacket weight, and topsides dead and operating loads should be made availablefor the vessel impact analysis.

7.4.1.3 Geotechnical Report

Site specific data, including shear strength profile and pile axial compression andtension capacity curves, should be developed based on modern APIrecommendations. Pile driving records may be available to determine actual pilepenetration.

7.4.1.4 Appurtenance Schedule

The actual number and location of conductors, risers, boat landings,conductor/riser guards, bumpers, and other appurtenances are usually found onthe drawings but are best confirmed via the inspection reports and photos. Thenumber of conductors actually installed on the platform tends to routinely vary from

the number of slots and should be independently verified.7.4.1.5 Inspection Data

Inspection reports provide information about the current state of the platform,including damage, if any, such as dents, cracks, holes, or corrosion. Theinspection report should also be used to establish actual marine growth (versuscode based marine growth that may be used for new design). In most cases, but


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not all, the inspection report also contains information, such as the number andlocation of risers and caissons, location of boat landings, platform orientation,verification of the platform underwater elevations, and other useful information.

Above-water photos of the platform are critical for the engineer to provide a “feel”for the platform, such as overall configuration and size, but they also provide visual

confirmation of the amount of deck equipment, orientation, number of boatlandings, number of risers and conductors, and deck elevation and overall platformcondition such as corrosion. These and other items that can be seen in the photosshould match what is in the drawings. If there is no match, these items need to befield verified.

7.4.1.6 Metocean Data

The contents of metocean data should contain the following:

• Wave height and associated period by direction;

• Coexistent current velocity and profile by direction;

• Coexistent wind speed by direction;

• Tidal data;

• Wave kinematics factors;

• Water depth.

7.4.2 Modeling Requirements

The structural model should include the three dimensional distribution of platformstiffness. Reference should be made to the structural drawings for the definition ofgeometry, member sizes and steel grades, etc. For the as-is condition of anexisting structure reference should be made to the inspection records and repairrecords, if available.

7.4.2.1 Frame Modeling

Primary Framework

The primary framework of the structure comprises those members, which providethe global stiffness and strength of the structure. They are the legs, the piles, thevertical diagonal members, and the main plan bracing members. The primaryframework should be included in the model of the structure.

Secondary Framework

The secondary framework consists of members, which only marginally contribute toglobal stiffness and strength of the framework. Unless they are part of the vesselimpact analysis focus (e.g. boat landings, conductor/riser guards and bargebumpers), the structural contribution of these members may be neglected andneed not be included in the model as structural members.


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When neglecting the structural contribution of secondary members, their loadattracting properties (self-weight and hydrodynamic) should be accounted for andincluded in the appropriate loading conditions.

Deck Structure

The stiffness of the deck structure should be modeled in sufficient detail toadequately represent the deck/jacket interface such that the applied topsideloading and the structural self-weight are appropriately distributed to thesubstructure framework. The deck, and if appropriate, the modules, should be fullymodeled as in most cases redistribution of load through the deck can occur.

Pile Connectivi ty

The sliding action of piles within the legs should be modeled with the appropriateconstraint conditions, which allow unrestrained differential axial displacement butcouple the lateral displacements of piles and legs.

Care should be taken in modeling the pile/leg interface at the lowest horizontal planelevation. The top of pile deflection should be checked to ensure that there isenough top-of-pile displacement to transfer the pile to leg load laterally. If this isnot the case the lateral displacement of the piles and legs should be uncoupled atthis elevation.

Grouted Piles

Grouted piles should be included in the model either as a composite member or asa steel member with thickness adjusted to give equivalent member properties.Since grouting will increase the stiffness of the member, any additional stiffnesscaused by the grouting should be included in the model.

For existing platforms, in instances where grouted legs cannot be readily confirmedon the basis of as-built drawings it may be necessary to inspect the legs, possiblywith ultrasonic testing (UT) methods. If the grouting information is not available forassessment, it will be conservative to assume piles ungrouted.

Conductors

Conductors can contribute significantly to the lateral foundation stiffness andstrength of a structure. In that case, the conductor should be modeled andanalyzed as a structural element in the structure and included in the integratedstructure-soil model using non-linear soils p-y curves generated from site specificdata, when available. Consideration should be given to the group effect of theconductors, if necessary.

Conductors, when contributing significantly to the platform's foundation stiffnessand strength and modeled as structural elements, generate a load redistributionthrough the mudline plan bracing. The conductor guide framing at the mudline maybe heavily loaded and needs more detailed inclusion in the structural model asprimary framework. This requires special care, as the assessment is likely to show


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overstressing of the mudline braces, and modeling may not accurately capture thetransfer of loads to the legs.

Conductor Connectivity

The sliding action of straight conductors within their guide frames should be

modeled with the appropriate constraint conditions, which allow unrestraineddifferential axial/vertical displacement but couple the lateral displacements ofconductors and guide frames. Annular gap effects should be considered.

Conductor Guide Framing

It is essential to correctly model the stiffness of the mudline conductor guide framesuch that the overall simulation accurately represents the behavior and henceshear is correctly proportioned between conductors and piles. The use of non-linear gap elements at the conductor/conductor frame interface is particularlyvaluable. This attention to correct simulation is particularly important when theconductors are idealized, i.e. when say, twelve conductors are simulated by four.

This generally requires some plane frame analysis studies to determine a realisticmodel. It is however recommended conductors are modeled individually to betterrepresent global stiffness and load distribution to jacket.

Leg Stubs

If there are centralizers at the level of the bottom bay framing, leg stubs should notbe modeled. If there are no centralizers, the pile will contact at the bottom of thestub causing moments in the leg and hence the stub should be modeled.

Corrosion Allowance

Allowance for corrosion by reducing as-built wall thickness will only be included forassessment purposes if the annual inspection survey indicates that there is actualcorrosion loss.

Grouted Members

Grouting is a simple repair method used to eliminate inelastic buckling and provideadditional stiffness and strength for members with bows, dents and holes. Anyadditional stiffness as a result of grouting should be included in the model sinceadditional stiffness attracts additional forces. The grouted member should beincluded in the model either as a composite member or as a steel member withthickness adjusted to give equivalent member properties.

Strengthened Elements

Friction, grouted and long-bolted clamps may have all been used for strengtheningdeficient jacket members in the Chevron fleet. If sufficient detail is available of thestrengthening, appropriate techniques should be used to accurately representthese strengthened elements within the structural model. Where insufficient


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information is available it should be assumed that the strengthening provides fullreinstatement or additional capacity within the structure.

7.4.2.2 Joint Modeling

Joint Eccentricity

In joints with two or more braces in one plane, the intersection points of bracemember/chord centerlines should be explicitly modeled. This is particularlyimportant in structures with large diameter legs and stocky member design andbracing at skirt piles, where the braces could be short. Joint eccentricities willintroduce additional end moments in the connected members.

Joint Flexibility

The use of joint flexibility may be used in certain circumstances to decrease thecalculated stresses in members and joints, however, the introduction of jointflexibility can increase as well as decrease the calculated stresses. Face-to-face

modeling with flexible joints give some benefit in cases where secondary momentsare high, but the benefit must be weighed against the increased complexity inmodeling.

Grouted Joints

The strength of a grouted joint is likely to exceed that of the connected members.The grouting will also significantly increase the rotational restraint imposed by the

joint, and thereby increase the buckling capacity of the connected member(s).

One modeling technique to represent the increased joint stiffness is to model the joint by rigid links from the chord center to the face of the chord.

Doubler Plated Joint s

Doubler plating is used on joints to improve the static strength and in mostinstances no detailed modeling is required. In cases where the platform hasdoubler plates, the impact on joint strength should be assessed during theassessment post-processing.

Ground Joints

Grinding is commonly used on cracks to improve fatigue life by reducing stressconcentration and removing hairline cracks. In most instances the impact on jointstrength should be negligible and detailed modeling should not be required.

7.4.2.3 Foundation Modeling

Sections 6.7, 6.8 and 6.9 of API RP 2A-WSD 21st Edition [ 1 ] recommendationsshould be used to simulate soil reaction for axially loaded piles. Laterally loadedpiles and pile group action (except that p-y modifiers for pile group in soft clay)should be derived by the method given in the paper, “Procedures for Analysis of


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Laterally Load Pile Groups in Soft Clay” [24]. The application procedures areclearly described, but they require reference to the paper, “Correlation for Design ofLaterally Load Piles in Soft Clay” [25].

Structural/Soil Interaction

The foundation should be modeled and analyzed as a fully integrated part of thestructure using non-linear p-y and t-z curves representing the soil stiffness andcapacity. Particular care should be taken when modeling thin layers near the mud-line, where p-y curves change rapidly, and to accurately model the soil layers in thedepth which the piles are expected to terminate.

To properly assess the pile penetrations Chevron pile driving records will bereviewed to determine actual penetrations. If it is apparent that the platforms havesustained seafloor scouring, it is necessary to account for any loss of soil-pilecontact in the models.

The non-linear response curves for lateral resistance (p-y), skin friction (t-z) and

end bearing (q-z), should be modeled into the analysis and the soil structureinteraction automatically solved by an iterative technique. In this way individualpiles never carry more than their ultimate load because excess load isautomatically shed to other piles, and the effects of the redistribution of thefoundation loads on the structure is also automatically determined.

For platforms with pile groups, the non-linear soil p-y and t-z curves of individualpiles should be adjusted to account for pile group effects. The influence of a pilegroup on global structural behavior may be modeled by simpler means, such as theuse of an equivalent single member with the equivalent structural and foundationproperties.

Pile/Structure InteractionThe modeling of the pile/leg connection can significantly affect the distribution ofshears and moments into the jacket and can significantly alter the stresses in thepiles. The use of gap elements is particularly valuable. However, each caseshould be treated on its merits as the cost of introducing a non-linear (NL) link tomodel the gap between leg and pile can increase the computing cost for a typicalanalysis by 100%. But where there are bottom bay extensions, or when pile headmoments are high, and bottom bay shear is critical the introduction of NL links, by‘softening’ the jacket/pile connection and hence reducing pile head moments, cangive stress reductions of the order of 10% and more to the jacket. Equally, if thepile maximum stresses, which occur 20’ - 30’ below mudline are critical, NL links,which should lead to increased maximum bending stresses, should be used to givean upper bound on pile stresses.

P-Y Modifiers for Conductor s

Where the conductors as capable of carrying a proportion of the base shear,consideration should be given to including load deflection (p-y) curves in thestructural model. Since such curves are specific to the diameter of the conductors,


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8. POST-IMPACT SURVIVAL ASSESSMENT

8.1 GENERAL

The platform should be designed to meet the post-impact criteria. The damaged

platform should retain sufficient residual strength after vessel impact to safely resistplatform normal operating loads and environmental loads with a specified returnperiod. This will ensure that there will be adequate time for carrying out offshorerepairs.

Post-impact assessment shall be performed, taking into account the extent of likelydamage estimated from vessel impact analysis. Component distortion, loss ofstiffness and induced eccentric loading caused by dented geometries shall beaccounted for in the analyses.

The post-impact assessment may be achieved using either linear strength analysisor non-linear pushover analysis. Criteria for post-impact assessment require a

good knowledge of the mechanical properties of the structural steel, includingcritical strain at rupture, dynamic yield stress, and strain hardening characteristics.

8.2 MODELING OF DAMA GED STRUCTURE

Vessel impact would cause denting and bowing of members. The relativemagnitude of denting and bowing depends on the energy of input and the D/t ratio,the overall slenderness of the member and the degree of restraint (rotational andaxial) afforded to the ends of the member by the rest of the structure. In addition tobowing and denting, deep scratches and gouges can be formed.

Special attention should be given to defensible representation of actual stiffness ofdamaged members or joints in the post-impact assessment. Present codes do notaddress all damage scenarios and engineering expertise must be brought to bearon the assessment of damaged member and the consequence to the platform’soverall structural integrity.

The severity of the damage should be reviewed on a case-by-case basis todetermine the load bearing capacity of the damaged member. The residualstrength of dented members should be estimated using the provisions of the ISOdraft [2]. The effect of a bow on a member’s axial carrying capacity has beenevaluated by several researchers. Recommended practices are given in “AnIntegrated Approach for Underwater Survey and Damage Assessment of OffshorePlatforms” [27]. Damaged members may be considered totally ineffective providedthat their wave areas are modeled in the analyses.

8.3 POST-IMPACT SURVIVAL LOADS

A platform should be capable of withstanding a suitable subsequent stormconditions in addition to normal operating loads after an impact has occurred. APIRP 2A [1] and UK HSE Guidance Notes [3] recommend that after a design vesselimpact case the structure should be able to withstand environmental loads with a


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return period of at least 1 year. In the post-impact condition the platform normaloperating loads and environmental loads are to be combined.

8.3.1 Platform Operating Loads

Platform operating loads to be considered in the post-impact analysis include dead

and live loads for platform operating condition.

8.3.2 Environmental Loads

Environmental loads to be considered in the post-impact analysis include wave,current and wind loads. Wave directions shall be chosen to maximize the loads inthe damaged components.

8.4 ANA LYSIS OPTIONS

A lower bound of the system residual strength may be developed using linearanalysis methods, where all component resistance factor of safety are set equal to1.0. The resistance should be characterized by the environmental load that willcause the first component to exceed its capacity. This bound will be conservative.If the lower bound resistance is insufficient to demonstrate adequate strength, theneither a non-linear pushover analysis should be performed or remedial actionsshould be undertaken.

A best estimate of the system residual strength can be developed using non-linearpushover analysis methods, where post-yield, non-linear component behavior isaccounted for explicitly in the analysis. Non-linear pushover analysis is performedby incrementally increasing the environmental loading until the global structuralsystem becomes unstable, i.e., an incremental increase in the load cannot beresisted. A non-linear pushover analysis will identify a mechanism of failure

corresponding to the residual strength level achieved. Non-linear pushoveranalysis should follow the approach and procedure provided in Chevron “UltimateLimit Strength (ULS) of Fixed Offshore Platforms” (CIV-EN-100).

8.5 POST-IMPACT SURVIVAL ACCEPTANCE CRITERIA

The platform should retain sufficient residual strength after vessel impact. Residualstrength targets should be based on the following for specific applications:

• Criticality of components• Exposure to damage

• Condition of any damaged components, and• Tolerance for interruption of operations.

Critical components that support quarters or facilities, such as knee braces, shallhave sufficient residual strength to survive possible damage without causingsubsystem collapse.


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REFERENCES

1. API RP2A – WSD, “Recommended Practice for the Planning, Designing andConstruction of Fixed Offshore Platforms – Working Stress Design”, 21 st Edition, Errata and Supplement, October 2005.

2. ISO 19902 Fixed Steel Offshore Structures.

3. Health and Safety Executive, “Offshore Installations: Guidance on Design,Construction and Certification”, Fourth Edition, 1993.

4. Det Norske Veritas, “Design Against Accidental Loads”, DNV-RP-C204,November 2004.

5. Det Norske Veritas, “Impact Loads from Boats”, TNA202, 1981.

6. EN 1991-1-7:2006, Eurocode 1, “Actions on Structures”, 2006.

7. Health and Safety Executive, “Technical Policy Relating to StructuralResponse to Ship Impact”, December 2006.

8. Veritec, JIP-Design Against Accidental Loads, Report No. 88-3127, 1988.

9. Norwegian Technology Standards Institute, “Design of Steel Structures”,Norsok standard N-004, October 2004.

10. HSE, “Loads”, Health Safety Executive OTR 013/2001, 2002.

11. Visser, V., “Ship Collision and Capacity of Brace Members of Fixed SteelOffshore Platforms”, Health Safety Executive RR 220, 2004.

12. Skallerud, B. and Amdahl, J., “Non-linear Analysis of Offshore Structures”,Research Studies Press Ltd., 2002.

13. MSL, “Joint Industry Project Report: Effect of Vessel Impact on Intact andDamaged Structures”, DOC REF C209R007 Rev 1, July 1999.

14. Kenny, J. P., “Protection of Offshore Installations against Impact”, HealthSafety Executive OTI 88 535, 1988.

15. Robson, J. K., “Ship/Platform Collision Incident Database (2001)”, HealthSafety Executive RR053, 2003.

16. Ronalds, B. F., “Vessel Impact Design for Steel Jackets”, OTC 6384, 1990.

17. Norwegian Technology Standards Institute, Norsok Standard N-003, “Actionand Action Effects”, February 1999.


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18. American Institute of Steel Construction, “Specification for the Design,Fabrication and Erection of Structural Steel for Buildings – Allowable StressDesign and Plastic Design”.

19. Visser, V., “Resistance of Jack-up Conductors to Boat Impact”, Health SafetyExecutive OTO 98 029, 1998.

20. Zeioddini, M., Harding, J. E. and Parke, G. A. R., “Effect of Impact Damageon the Capacity of Tubular Steel Members of Offshore Structures”, MarineStructures, Vol 11 141-157, 1998.

21. Allan, J. D. and Marshall, J., “The Effect of Ship Impact on the Load CarryingCapacity of Steel Tubes”, Health Safety Executive OTH 90 317, 1992.

22. Smith, C. S., et al., “Buckling and Post-Collapse Behavior of Tubular BracingMembers Including Damage Effects”, Behavior of Offshore Structures, BOSS,Cranfield, 1979.

23. Moan, T., and Taby, T., “Collapse and Residual Strength of DamagedTubular Members”, Proceedings of the Fourth International Conference onBehavior of Offshore Structures, Delft, July 1985.

24. Bogard, D. and Matlock, H., “Procedures for Analysis of Laterally Loaded PileGroups in Soft Clay”, Proceedings of the Conference on GeotechnicalPractice in Offshore Engineering Practice, ASCE, 499-535, 1983.

25. Matlock, H., “Correlation for Design of Laterally Loaded Piles in Soft Clay”,Offshore Technology Conference, OTC 1204, Houston, Texas, May 1970.

26. Petro-Marine Engineering, "Platform Assessment Analyses - Significance ofGroup Y-Modifiers”, Report No. 1167/81/83.

27. Kallaby, J., and O'Connor, P., "An Integrated Approach for UnderwaterSurvey and Damage Assessment of Offshore Platforms”, OTC 7487, OffshoreTechnology Conference Proceedings, May 1994.

28. Chevron ETC, "Metocean and Hydrodynamic Criteria for Shallow FixedStructures and Pipelines off West Africa”, Revision 11, June 2004.


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Appendix A Sketches of Typical Riser Guards and Barge Bumpers


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Chevron Vessel Impact Design Basis_Rev 2

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