8/13/2019 31401010 Dec 99

1/47

PETRONAS TECHNICAL STANDARDS

DESIGN AND ENGINEERING PRACTICE

MANUAL

RISER DESIGN

PTS 31.40.10.10

DECEMBER 1999

2010 PETROLIAM NASIONAL BERHAD (PETRONAS)All rights reserved. No part of this document may be reproduced, stored in a retrieval system or transmitted in any form or by any means (electronic,

mechanical, photocopying, recording or otherwise) without the permission of the copyright owner.

8/13/2019 31401010 Dec 99

2/47

PREFACE

PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication,of PETRONAS OPUs/Divisions.

They are based on the experience acquired during the involvement with the design, construction,operation and maintenance of processing units and facilities. Where appropriate they are basedon, or reference is made to, national and international standards and codes of practice.

The objective is to set the recommended standard for good technical practice to be applied byPETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemicalplants, marketing facilities or any other such facility, and thereby to achieve maximum technicaland economic benefit from standardisation.

The information set forth in these publications is provided to users for their consideration anddecision to implement. This is of particular importance where PTS may not cover everyrequirement or diversity of condition at each locality. The system of PTS is expected to besufficiently flexible to allow individual operating units to adapt the information set forth in PTS totheir own environment and requirements.

When Contractors or Manufacturers/Suppliers use PTS they shall be solely responsible for thequality of work and the attainment of the required design and engineering standards. Inparticular, for those requirements not specifically covered, the Principal will expect them to followthose design and engineering practices which will achieve the same level of integrity as reflectedin the PTS. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting from hisown responsibility, consult the Principal or its technical advisor.

The right to use PTS rests with three categories of users :

1) PETRONAS and its affiliates.2) Other parties who are authorised to use PTS subject to appropriate contractual

arrangements.

3) Contractors/subcontractors and Manufacturers/Suppliers under a contract withusers referred to under 1) and 2) which requires that tenders for projects,materials supplied or - generally - work performed on behalf of the said userscomply with the relevant standards.

Subject to any particular terms and conditions as may be set forth in specific agreements withusers, PETRONAS disclaims any liability of whatsoever nature for any damage (including injuryor death) suffered by any company or person whomsoever as a result of or in connection with theuse, application or implementation of any PTS, combination of PTS or any part thereof. Thebenefit of this disclaimer shall inure in all respects to PETRONAS and/or any company affiliatedto PETRONAS that may issue PTS or require the use of PTS.

Without prejudice to any specific terms in respect of confidentiality under relevant contractual

arrangements, PTS shall not, without the prior written consent of PETRONAS, be disclosed byusers to any company or person whomsoever and the PTS shall be used exclusively for thepurpose they have been provided to the user. They shall be returned after use, including anycopies which shall only be made by users with the express prior written consent of PETRONAS.The copyright of PTS vests in PETRONAS. Users shall arrange for PTS to be held in safecustody and PETRONAS may at any time require information satisfactory to PETRONAS in orderto ascertain how users implement this requirement.

8/13/2019 31401010 Dec 99

3/47

TABLE OF CONTENTS

1. INTRODUCTION1.1 SCOPE1.2 DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS

1.3 DEFINITION

2. DESIGN INTERFACES2.1 GENERAL2.2 TOPSIDE INTERFACE2.3 JACKET INTERFACE2.4 PIPELINE/EXPANSION SPOOL INTERFACE

3. RISER/TIE-IN CONCEPTS3.1 SUMMARY OF MAIN RISER TYPES3.2 RISER INSTALLATION METHODS3.3 SUBSEA TIE-IN METHODS3.4 FLEXIBLE SPOOLS3.5 SELECTION OF RISER/PIPELINE TIE-IN METHOD

3.6 AVAILABLE CONSTRUCTION METHODS

4. RISER ROUTING AND LOCATION4.1 BASIC ROUTING REQUIREMENTS4.2 APPROACH TO PLATFORMS4.3 SAFETY

5. DESIGN DATA5.1 RISER SYSTEM/PLATFORM DATA5.2 SOIL DATA5.3 METOCEAN DATA5.4 ATMOSPHERIC CONDITIONS5.5 EARTHQUAKE5.6 RETURN PERIODS

5.7 DIRECTIONALITY

6. RISER AND TIE-IN SPOOL ANALYSIS6.1 FAILURE MODES6.2 DESIGN LOADS6.3 LOAD CASES6.4 WALL THICKNESS DETERMINATION6.5 PIPELINE EXPANSION6.6 EXPANSION LOOP6.7 RISER STRUCTURAL ANALYSIS6.8 ALLOWABLE STRESSES6.9 ALLOWABLE STRAINS6.10 OVALISATION

6.11 COLLAPSE6.12 VORTEX SHEDDING6.13 FATIGUE

7. RISER SUPPORT DESIGN7.1 RISER SUPPORT TYPES7.2 DESIGN CONSIDERATIONS7.3 LOADING CONDITIONS7.4 CORROSION PROTECTION

8. J-TUBE DESIGN8.1 DESIGN DATA8.2 J-TUBE ROUTING8.3 J-TUBE SIZING AND RADIUS OF CURVATURE

8.4 PULL-IN LOADS8.5 STRUCTURAL DESIGN OF J-TUBE AND SUPPORTS

8/13/2019 31401010 Dec 99

4/47

8.6 APPURTENANCES8.7 CORROSION PROTECTION

9. FITTINGS9.1 FLANGES9.2 GASKETS9.3 BOLTING9.4 VALVES9.5 BENDS

10. RISER MATERIALS AND CORROSION PROTECTION10.1 GENERAL10.2 LINEPIPE10.3 EXTERNAL COATING10.4 CATHODIC PROTECTION

11. MECHANICAL PROTECTION11.1 PROTECTION FROM BOAT IMPACT11.2 PROTECTION FROM DROPPED OBJECTS

11.3 PROTECTION FROM SNAGGING LOADS12. INSTALLATION REQUIREMENTS12.1 RISER INSTALLATION TOLERANCES12.2 INSTALLATION FEASIBILITY12.3 CLEARANCE FOR HYPERBARIC WELDING12.4 CONSTRUCTION AIDS12.5 TEMPORARY CONSIDERATIONS

13. REQUIREMENTS FOR OPERATIONS AND MAINTENANCE

14. DESIGN OUTPUT14.1 GENERAL14.2 DESIGN DOCUMENTATION14.3 AS-BUILT DOCUMENTATION

15. REFERENCES

APPENDICES

APPENDIX 1 FIGURES

8/13/2019 31401010 Dec 99

5/47

1. INTRODUCTION

1.1 SCOPE

This new PTS specifies requirements and gives recommendations for the design of

offshore pipeline riser systems, which include the piping, riser clamp supports and anyexpansion spool or anchoring system at the base of the platform. This PTS identifies abroad approach to the design including:

definition of riser system and interfaces;

potential riser concepts;

riser routing;

analysis requirements;

support design;

J-tube design;

fittings and materials.

This PTS does not present a methodology, but is intended to act as a checklist of designactivities for consideration by an experienced engineer.

For the purpose of this PTS, it is assumed that the riser design is based onPTS 31.40.00.10

The scope of this PTS includes only rigid metallic risers; flexible risers and non-metallicrisers are excluded from the scope.

1.2 DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS

Unless otherwise authorised by PETRONAS, the distribution of this PTS is confined tocompanies forming part of the PETRONAS or managed by a Group company, and toContractors and Manufacturers/Suppliers nominated by them.

This PTS is intended for use in offshore exploration and production facilities.

If national and/or local regulations exist in which some of the requirements may be morestringent than in this PTS, the Contractor shall determine by careful scrutiny which of therequirements are the more stringent and which combination of requirements will beacceptable as regards safety, environmental, economic and legal aspects. In all cases, theContractor shall inform the Principal of any deviation from the requirements of this PTSwhich is considered to be necessary in order to comply with national and/or localregulations. The Principal may then negotiate with the Authorities concerned with the objectof obtaining agreement to follow this PTS as closely as possible.

1.3 DEFINITION

1.3.1 General definitions

The Contractor is the party which carries out all or part of the design, engineering,procurement, construction, commissioning or management of a project, or operation ormaintenance of a facility. The Principal may undertake all or part of the duties of theContractor.

The Manufacturer/Supplier is the party which manufactures or supplies equipment andservices to perform the duties specified by the Contractor.

The Principal is the party which initiates the project and ultimately pays for its design andconstruction. The Principal will generally specify the technical requirements. The Principalmay also include an agent or consultant authorised to act for, and on behalf of, thePrincipal.

The word shallindicates a requirement.

The word shouldindicates a recommendation.

8/13/2019 31401010 Dec 99

6/47

1.3.2 Specific definitions

J-Tube J-shaped tube installed on a platform, through which apipeline can be pulled to form a riser.

Piping components items integrated in the pipeline/riser such as flanges, tees,

bends, reducers and valves.

Riser support structure intended for fixing the riser to the platform or forlocal or continuous guidance of the riser assembly.

Riser system riser pipe, supports, integrated piping components andcorrosion prevention system.

1.4 ABBREVIATIONS

ASCE American Society of Civil Engineers

EPDM Ethylene Propylene Diene Monomer

ESD Emergency Shut Down

HAT Highest Astronomical Tide

QRA Quantitative Risk Assessment

LAT Lowest Astronomical Tide

RTJ Ring Type Joint

SMYS Specified Minimum Yield Stress

1.5 CROSS-REFERENCESWhere cross-references to other parts of this PTS are made, the referenced sectionnumber is shown in brackets. Other documents referenced in this PTS are listed in (15).

8/13/2019 31401010 Dec 99

7/47

2. DESIGN INTERFACES

2.1 GENERAL

The riser system shall be designed as a part of the total offshore pipeline system. For

design purposes, it is necessary to define the extent of the riser assembly and to establishthe interfaces between the riser system and adjacent systems. The interfaces provide apoint where loading and/or displacement, and the requirements of the various systems canbe defined and reconciled.

The riser interface points can be summarised as follows:

topside and supports;

jacket and supports;

pipeline/tie-in spool.

The riser analysis model shall take into account the effects of the interface points as furtherdetailed below.

2.2 TOPSIDE INTERFACE

The design of the riser system requires detailed interfacing with the platform topsides. Thecode break for the riser system shall extend up to and include the pig trap (includingassociated pipework and valves) or, if no pig trap is fitted, to the first isolation valve off theriser. The riser supports fall outside the code break. The following design issues shall beaddressed:

design responsibility;

exact location of code breaks marked on process engineering flow schemes (PEFS);

piping layouts;

structural layouts;

instrument connections;

electrical isolation;

overlap of riser and piping analyses;

support locations;

access for pigging operations;

access for valve overhaul.

2.3 JACKET INTERFACE

The design of the riser system requires detailed interfacing with the jacket structure. Theriser supports fall outside the code break. The following design issues shall be addressed:

design responsibility;

code breaks; details of the structural layout and dimensions of the jacket members;

platform deflections;

riser routing;

riser support locations and type;

electrical isolation;

riser loadings on riser supports;

ESD valve location;

structural protection;

hook-up to top section of riser.

8/13/2019 31401010 Dec 99

8/47

2.4 PIPELINE/EXPANSION SPOOL INTERFACE

The design of the riser system requires detailed interfacing with the pipeline/expansionspool. The interface between the riser and the submarine pipeline depends on the method

of connection, geometry and type of riser and should be agreed in each case. The followingdesign issues shall be addressed:

design responsibility;

location of code breaks;

riser routing;

pipeline approach;

expansion spool layout;

overlap of riser and expansion spool structural analysis (often performed in one analysisfrom pipeline to pig trap);

the tie-in method.

8/13/2019 31401010 Dec 99

9/47

3. RISER/TIE-IN CONCEPTS

3.1 SUMMARY OF MAIN RISER TYPES

Risers for platforms may be broadly grouped into the following categories:

riser for steel jacket platform; riser for gravity base structure (concrete);

J-tube riser - this category is further discussed in (8);

caisson riser system; consists of a caisson which forms a structural encasement and anumber of riser pipes which are installed in the caisson. Caissons are protective devicesto eliminate environmental loading on the riser pipes. Caisson riser systems may reducethe number of riser supports which otherwise would be required for conventional risers;

flexible riser for a floating facility (outside the scope of this PTS).

3.2 RISER INSTALLATION METHODS

3.2.1 General

Risers are usually pre-installed with the jacket structure. Otherwise they can be retrofittedonto existing platforms. This may be by the conventional method of lift, set and subsea tie-in. Alternatively, one of the following methods may be used without the need for subsea tie-ins:

stalk-on method;

bending shoe riser method;

barefoot riser;

J-tube installation (8).

3.2.2 Conventional method

Retrofitted risers are fabricated in sections, lifted from a barge and lowered into suitable

riser supports which may also be retrofitted onto the jacket. The number of riser sectionsdepends on the water depth and the length of the barge. The riser normally consists of anupper section behind the jacket bracing (to provide safety against boat impact) which isconnected to lower sections positioned on the outside of the jacket. After installation, asubsea tie-in is made to the pipeline.

A form of retrofit riser clamp may be installed after a jacket has been in service for sometime. In this case, provision shall be made for aligning the clamps/guides. This is achievedby connecting the riser clamp/guide which is also clamped to a structural jacket member orstub, depending on the size of riser. Retrofitting of these clamps/guides involvesconsiderable diver time. Alternatively, a riser ladder, or more simply riser support stubs,may have been installed on the jacket in the fabrication yard for future retrofitting of risers.

Retrofitting methods without the need for subsea tie-ins are described below.

3.2.3 Stalk-on riser method

For shallow water, this is the most commonly used riser installation method. After thepipeline has been laid with its end on the sea bottom and close to the platform, the laybarge is moored in position. The riser bend which will eventually connect the horizontalpipeline to the platform deck is measured and the location at which the pipeline will be cutfor connection to the bend is marked. The pipeline is then lifted from the seabed byapplying tension to the pipe. In very shallow water with small diameter pipelines, this is nota problem; however, larger lines in deeper water require a substantial length of pipe to besupported off the bottom to avoid overstressing the pipe. The pipe is then cut at the mark,the bend is welded onto the free end of the pipe and the pipe and bend are lowered down.This process of adding pipe is continued until the pipe reaches the bottom. The riser is then

secured to the platform using diver-operated clamps.Expansion spools can be set simultaneously with this method.

8/13/2019 31401010 Dec 99

10/47

In deeper waters, the handling of the pipe and riser becomes increasingly difficult and evenhazardous to both pipe, equipment and personnel.

The main advantages of this technique are:

weld connections are made above surface and can be fully inspected, ensuring weld

quality; diver activities are relatively simple, requiring only a normally-skilled team using

standard tools. The expense and time delay involved in mobilising specialised contractorpersonnel are avoided;

there is no requirement for underwater welding.

The disadvantages of this technique are:

the lifting, welding and lowering operation is vulnerable to environmental conditions;

careful planning and strict compliance with the predetermined lifting and loweringprocedures are vital to avoid overstressing the pipeline and riser;

greater adjustability in the riser clamps is required because the riser cannot be movedfore and aft once it is welded to the pipeline.

3.2.4 Bending shoe riser method

This method, which was developed by Shell Oil Company, consists of installing a curvaturelimiting shoe on the platform during onshore fabrication. The pipeline is then laid to thestructure and positioned under the bending shoe either by manoeuvring the barge orattaching cables to the line as it is laid and pulling it under the shoe. Once the line is in thecorrect orientation with respect to the centreline of the bending shoe, specially designedhydraulic clamps on the platform capture and secure the riser. These clamps may beinstalled either during onshore fabrication or immediately before the riser is installedoffshore. In very deep water or for pipe with low stiffness, it may be necessary to installauxiliary cables on the riser to assist with the installation. Other than inspection, thismethod of riser installation requires a minimal amount of underwater work.

3.2.5 Barefoot riser method

This method has been used successfully in the Gulf of Mexico. The method is simple andshould find many applications, especially for deepwater platforms. The pipe weight and wallthickness are selected such that the pipe can be lifted vertically at the water surface withoutexceeding a specified, non-buckling, bend in the sag portion of the line. The methodconsists of approaching the platform with the pipe suspended vertically at the watersurface. The pipe is then positioned at a tangent to and in contact with the upper end of aseries of pipeline clamps on the platform. The lifting load is decreased according to aprescribed schedule which forces the riser pipe into each successive clamp and puts thebottom span into compression. The riser is then clamped to the platform once the desiredcurvature in the sag-bend is achieved. The necessary hydraulic or electrically operatedriser clamps can be installed offshore using a rail guidance system to land each clamp at a

predetermined elevation. Diver time, other than for inspection, would be minimal for thismethod of riser installation.

One other version of this approach to riser installation is called the Guide Rail method. Thistype varies from the previously described method primarily in the clamp used to attach theriser to the guide rail. The guide rail is a continuous T section or H beam welded to theplatform side or jacket leg during shore fabrication. The installation sequence requires thelay barge to lay away from the platform while a riser barge attaches the riser clamp to therail and continues to add pipe as the riser is lowered. The lay barge continues to moveaway from the platform during this operation. After the riser is in position, it is secured to theplatform by welding the clamps located above the water line to the rail and by having diversattach the submersed clamps with set screws.

8/13/2019 31401010 Dec 99

11/47

3.3 SUBSEA TIE-IN METHODS

3.3.1 General

In most cases, tie-in of the pipeline to the offshore facility is achieved by inserting an

expansion spoolpiece. The purpose of the spoolpiece is to absorb expansion loadings, andaccommodate the installation tolerance on the pipeline.

The spoolpiece connections may be made up using one or a combination of the followingmethods:

mechanical connectors;

flanged tie-in using RTJ swivel ring flanges; or

hyperbaric welding.

These tie-in methods are further described below.

3.3.2 Mechanical connectors

A variety of mechanical connectors are available and they generally consist of two

components:

a gripping system, to anchor the connector onto the pipe;

a sealing system, using either metallic or elastomeric seals.

Mechanical connectors are alternatives or supplements to flanges and can offer certainadvantages depending on their design, e.g.:

some are easier to install (boltless flanges);

some can accommodate a degree of misalignment (ball joints);

some can be installed directly onto the bare pipe end;

some are suitable for diverless application.

Mechanical connector systems are not yet as reliable as welded or flanged connections,hence they are mainly used for emergency repairs to pipelines where speed of repair isessential and the equipment for other repair methods is not available.

Mechanical connectors have been developed that can be activated from the surface byhydraulics and without direct diver intervention. To achieve this type of connection,accurate positioning of the end of the pipeline is essential. Once positioned, the pipeline ispulled into the connector which is then activated and clamps around a special hub fitted tothe end of the pipeline.

3.3.3 Flanged tie-ins

Flanged tie-ins performed by divers on the seabed are effected by installing a flangedmake-up spool between the flanged ends of the lines to be connected. The spool isfabricated at the surface to the exact dimensions required, using a template which has

been made up on the seabed and retrieved at the surface.Due consideration should be given to the location of the flanges and, where possible, theyshould be located to minimise bending loads in the flanged joint. The integrity of flangeswhen subjected to high bending loads shall be confirmed by analysis.

The following recommendations apply to flanged tie-ins:

the flange shall be of the ring joint type;

one of the flanges shall be of the swivel ring type, to facilitate alignment of bolt holes;

the specified internal bore of the flange shall be the same as that of the pipeline;

the gaskets shall be made of an alloy which is softer than the flange material;

wall thickness differences between the flange body and the pipeline shall beaccommodated by specifying a tapered slope of not less than 1:5;

all bolts shall be tightened using hydraulic tensioning equipment.

8/13/2019 31401010 Dec 99

12/47

Subsea flanges and fittings should be bolted together using hydraulic tensioningequipment. Hydraulic bolt tensioning equipment is used on either side of a flange to stretchthe bolts to a predetermined tension. With the tension maintained on the bolt, the nuts areturned down onto the flange, to bar tight, prior to relaxation of the equipment. In this waythe flange can be tensioned to meet the service load. Washers are not used on subsea

pipe-to-pipe joints as these are prone to contact corrosion, which causes the bolts toslacken with time.

3.3.4 Hyperbaric welding

Sub-surface or hyperbaric welding is performed with the pipeline on the seabed. Specialframes are required to align the pipeline ends to be welded, and the welding itself isperformed in a special habitat. The systems presently available are operated from a bargeor a diving support vessel. This method requires extensive diving capability and specialwelding procedures.

As an alternative to hyperbaric welding, the weld can be performed inside an atmosphericchamber into which the pipeline is pulled. However, this method requires furtherdevelopment to be fully operational and is not presently recommended.

3.4 FLEXIBLE SPOOLS

Flexible spools can be installed directly without the necessity of preparing a template, andcan considerably speed up the tie-in work. Flexible spools also have the ability toaccommodate thermal expansion/contraction. The extra cost of the flexible spools shouldbe weighed against the diving time savings on a project-by-project basis.

3.5 SELECTION OF RISER/PIPELINE TIE-IN METHOD

In general, welding is the preferred method for permanent tie-ins as far as this is practicaland economic. The welding may be performed at the surface or on the seabed. Thedisadvantage of the hyperbaric welding technique is that it is a specialised activity, requiringdedicated spreads and a high level of training of the operational personnel. Alternativesshall be subjected to a cost/risk justification.

3.6 AVAILABLE CONSTRUCTION METHODS

Depending on the conditions at the intended location, such as weather, current velocity,wave heights, tidal effects, seabed conditions, water depth etc., there may be a preferencefor one of the possible construction methods. This in turn could put certain limitations on theselection of line sizes. The preferred construction method will also be dependent on theavailable construction equipment and on the cost of mobilising the required spreads withdedicated equipment and handling capability.

The type of riser to suit a particular application depends largely on the pipe size, the

platform type, the direction of approach of the pipeline and whether the riser is to beinstalled during platform fabrication or at some time after the platform is placed.

8/13/2019 31401010 Dec 99

13/47

4. RISER ROUTING AND LOCATION

4.1 BASIC ROUTING REQUIREMENTS

Selection of riser routing and location on a platform shall meet the following requirements

as far as practical: the riser shall have the minimum exposure to damage;

the riser shall not be located below the accommodation or the helideck, or close toescape routes from the accommodation or the temporary safe refuge;

the riser shall be accessible for inspection and maintenance;

in meeting these requirements, gas risers shall have precedence over oil risers.

The selection of riser routing and location shall also consider the following general factors:

safe and economical installation;

supply boat mooring area locations;

location of future risers;

minimisation of risk of damage by vessel collisions, by positioning risers within the

structure above a depth of 20 m below LAT; minimisation of risk of damage by dropped objects;

location of ESD valves, and their maintenance and inspection;

minimisation of risk of interference with future construction, drilling, workover or platformmaintenance or repair operations;

access for subsea and topsides hook-up.

Consideration should be given to environmental loading conditions, particularly in thesplash zone (5.3.3),where riser lengths and horizontal routing should be minimised.

Jacket bracing layout should be considered as this will determine the possible supportlocations and thus influence riser span lengths.

4.2 APPROACH TO PLATFORMSDetailed consideration shall be given to the approach routes to the platform.

This will include consideration of:

potential crossings;

seabed obstruction;

existing platforms/seabed facilities in close proximity;

angle of pipeline approach;

pipeline expansion requirements;

routing to minimise risk of damage by dropped objects;

accessibility for future positioning of jack-up rig.

When pipelines have to approach the jacket with angles greater than 30 from the

perpendicular to the jacket face, the spacing between the risers should be increased toallow more space between the lines on the sea bottom.

If the direct approach of a pipeline would be hampered by the future position of a jack-uprig, doglegs can be installed. Doglegs should also be used in preference to tight curvedapproaches to jackets and provide a means of allowing for pipeline expansion.Consideration should be given to the routing from the bottom riser clamp to the seabed asthis section is particularly susceptible to riser expansion, platform movement and scour-induced spans.

Where several platforms together form a complex, they should have a staggered layoutalong a straight line (spine) in order to:

free as much of the jacket faces as possible for risers;

allow easy barge access;

8/13/2019 31401010 Dec 99

14/47

position different production functions along the spine, so that future extension of anyfunction is perpendicular to the spine;

allow for new functions to be installed along the spine.

A dedicated riser platform may be installed to supply additional riser capacity (with scraper

barrels and manifolds) and/or to improve safety and reduce the overall risk levels onproduction facilities.

For new developments and extensions of existing complexes a careful study of the newlayout should be made in conjunction with anchor patterns (especially the drilling rigs),pipeline approaches, approach path for jack-up rigs and supply boat mooring.

4.3 SAFETY

The design shall include a safety assessment which shall quantify the effect of the risers onplatform safety and may include the use of risk analysis to determine the need for additionalprotective measures. Consideration should be given to the use of cost-benefit analysis toassess the relative merits of different protective measures.

The requirement for and location of ESD valves should be addressed as part of thedevelopment of the platform specific safety case.

Any risk analysis performed shall take into account analysis of the risk from both naturaland man-made hazards. Natural hazards shall include but not be limited to corrosionattack, marine life attack, extremes of temperature and environmental conditions. Man-made hazards shall include but not be limited to platform loading and off-loadingoperations, vessel activities, dragged anchors, trawl gear, abrasion by cables and chains,impact by vessels and dropped objects.

8/13/2019 31401010 Dec 99

15/47

5. DESIGN DATA

5.1 RISER SYSTEM/PLATFORM DATA

5.1.1 Process data

The following data are common to all elements of the riser:

minimum bore requirement to meet throughput requirements;

fluid type and density - maximum and minimum;

design life;

design pressure;

maximum allowable operating pressure;

hydrostatic test pressure for testing in fabrication yard and for system test;

design temperature - maximum and minimum;

normal operating temperature - maximum and minimum;

internal corrosion allowance (if appropriate).

5.1.2 Riser dataThe following data are required, as a minimum, for the riser design:

riser type - whether a conventional riser for steel platform, for gravity base structure,J-tube riser or caisson riser;

installation philosophy - whether pre-installed or retrofit;

method of tie-in;

steel grade;

outside diameter;

wall thickness;

internal/external coating - type, thickness and density;

insulation - type, thickness and density;

field joint material - type, thickness and density; valve, fitting and pig trap weight, rating and location;

bend radii, and thinning;

mechanical protection requirements.

5.1.3 Pipeline data

steel grade;

outside diameter;

wall thickness;

internal/external coating - type, thickness and density;

insulation - type, thickness and density;

field joint material - type, thickness and density;

expansion movements at free end; degree of trenching, self-burial and/or rock dump.

5.1.4 Expansion tie-in spool data

steel grade;

outside diameter;

wall thickness;

internal/external coating - type, thickness and density;

riser/spool connection type;

geometry of expansion spool;

mechanical protection requirements;

bend radii and thinning.

8/13/2019 31401010 Dec 99

16/47

5.1.5 Platform data

substructure type and dimensions;

details of other risers, caissons and/or J-tubes;

possible support locations and load restrictions;

immediate substructure settlement into seabed; long-term substructure settlement into seabed;

anode locations and details;

platform displacements under 100-year design condition.

5.2 SOIL DATA

Soil data provide information regarding resistance of the soil to pipeline movement (lateraland longitudinal friction coefficients), soil strength deterioration due to cyclic wave loading,load bearing capacity of the soil and susceptibility of soil to scour.

ASCE classification of soils and grain-size;

specific gravity of the soils;

soil friction angle for sands; undisturbed shear strength of clay soils;

remoulded (disturbed) shear strength or sensitivity.

5.3 METOCEAN DATA

5.3.1 Seawater

water density;

water kinematic viscosity;

marine growth elevations, thickness and density.

5.3.2 Water depth and tides

water depth, referred to a consistent datum (e.g. LAT)

lowest astronomical tide (LAT);

highest astronomical tide (HAT);

storm surge, i.e. maximum tide level for a specified average return period.

5.3.3 Splash zone

The splash zone range is defined as the astronomical tidal range plus the wave heighthaving a probability of exceedance of 0.01. The upper limit of the splash zone isdetermined by assuming 65% of this wave height above HAT and lower limit by assuming35% below LAT.

5.3.4 Currents

maximum current velocity for a range of current directions (usually 8), heights aboveseabed (usually every 10 m) and return period (usually 1 and 100 years);

relationship between the occurrence of wave-induced currents and the steady currents;

the number of hours of occurrence per year for the ranges of steady current from zero tothe maximum steady current. These data are used for riser span fatigue calculations.

5.3.5 Waves

maximum wave height for a range of directions (usually 8) and a range of return periods(usually 1 and 100 years);

the most probable wave period associated with each maximum wave height;

the number of waves per year for ranges of wave height from zero to the maximumwave height.

8/13/2019 31401010 Dec 99

17/47

5.4 ATMOSPHERIC CONDITIONS

5.4.1 Wind

maximum wind velocity for return periods of 1 and 100 years;

maximum and minimum ambient air temperatures.

5.4.2 Ice

the maximum thickness of icing on risers;

the maximum thickness and occurrence of permanent ice;

the maximum velocity and occurrence of pack ice.

5.5 EARTHQUAKE

In regions of the world prone to earthquakes, the response of the platform under the100-year seismic event is required.

5.6 RETURN PERIODS

The riser system should be designed to withstand loadings resulting from the 100-yearreturn period storm conditions during the operating design condition. The one-year returnperiod storm condition should be used for analysis during the installation and hydrostatictesting design conditions.

Where the design life of the pipeline is very short (typically less than 10 years),consideration may be given to reducing the design storm return period to less than 100years, based on a suitable risk evaluation.

If suitable seasonal data are available, seasonal one-year return period storm conditionsmay be used for the installation and hydrostatic testing design conditions. Such data shouldnot be used if there is a possibility of the relevant construction activity being performed

outside the season to which the data relate.

5.7 DIRECTIONALITY

Given sufficient hydrographic data, it is acceptable to account for the incident angle of waveand current attack on the pipeline/riser system. Tidal currents are strongly directional. If thewave and current data can be represented as a rosette, giving variation of wave height (orcurrent value) with direction for a given return period, then the resulting flow velocities maybe resolved perpendicular to the pipeline axis to give the (most critical) design loadingcondition.

8/13/2019 31401010 Dec 99

18/47

6. RISER AND TIE-IN SPOOL ANALYSIS

6.1 FAILURE MODES

The riser analysis shall consider the following failure modes:

excessive yielding; buckling;

fatigue.

6.2 DESIGN LOADS

The riser analysis shall consider the following design loads:

6.2.1 Weight loads

Static loads due to weight shall include the following:

pipeline/riser material;

coatings; attachments such as anodes, flanges, buckle arrestors, couplings etc.;

transported fluids;

marine growth;

buoyancy.

The weight loads shall be determined based on the nominal dimensions of the pipelinesystem components, except for fluid where maximum values shall be used.

Concrete weight coatings may absorb water, and this shall be considered.

6.2.2 Pressure loads

The riser pressure design shall be based on the internal design pressure.

Cyclic variations in pressure may induce fatigue, and this shall be considered.

6.2.3 Thermal loads

Thermal expansion or contraction loads induced in the pipeline/riser system by virtue of fullor partial restraint of pipeline/riser movement shall be considered during the analysis.

6.2.4 Residual loads

Residual loads are loads left in the pipeline system after installation, and include:

residual axial loads (such as lay tension);

loads due to curvature at direction changes in the pipeline route; and

loads induced by vertical curvature due to the seabed undulations along the pipeline

route.

Any permanent curvature or elongation produced during installation that results in residualloads should be taken into account.

6.2.5 Dynamic loads

Dynamic loads induced as a direct result of the operation of the pipeline system may havean effect on the structural strength of the pipe and its supports.

The riser analysis shall include dynamic loads resulting from slugging and piggingoperations.

Surge pressures occur when liquid flow is suddenly stopped or slowed, for example by thesudden closure of a valve.

8/13/2019 31401010 Dec 99

19/47

6.2.6 Support reaction loads

Shear forces, axial forces and bending moments will be introduced into the pipeline systemby supports which displace or constrain the riser, and shall be included in the riser analysis.

Substructure displacements as the result of storm loading and settlement fall into this

category of loading.

Possible scouring underneath the bottom riser bend and tie-in spool should also beconsidered as well as any ESD valves and associated pipework when determining thedeadweight support loading.

6.2.7 Hydrodynamic loads

Hydrodynamic loads are caused by the movement of water particles past and around asubmerged object. The water particle movement is caused by currents and wave action.

Consideration should be taken of the following factors when determining the hydrodynamicloads:

selection and applicability of wave theories with regard to water depth;

selection of the appropriate steady current profile for combination with the wave currentprofile;

breaking waves in shallow water;

storm surges in steady currents;

selection of appropriate drag, lift and inertia coefficients;

determination of combined drag, lift and inertia forces with regard to phase angle;

velocity amplification around jacket members;

the use of maximum wave data, not significant wave data;

the use of irregular sea-state data.

6.2.8 Wind loads

Wind loading on sections of a riser above sea level shall be considered.

The effects on wind load due to the proximity of other risers or structural members shall beconsidered.

Vortex shedding excitation of the riser from wind loading and disturbances to the flow fieldfrom change in wind speed or dynamic excitation of members adjacent to the riser shallalso be considered.

6.2.9 Seismic load

If seismic loads are taken into account in the platform design, they should be taken intoaccount in the riser design.

6.2.10 Ice loads

The loads associated with the formation of ice on the riser or the passage of pack ice pastthe riser shall be considered, if appropriate.

8/13/2019 31401010 Dec 99

20/47

6.3 LOAD CASES

The riser analysis shall consider at least three load cases, as follows:

6.3.1 Load case 1 - Installation loadsThis load case shall consider the entire installation sequence, namely:

onshore riser handling;

load-out;

jacket upending, stalking-on or retrofitting.

Loads considered shall be combined as appropriate and include:

weight and buoyancy loads;

hydrodynamic loads appropriate to the phase of work;

dynamic loads due to vessel motions.

6.3.2 Load case 2 - Hydrotest

This load case covers the onshore and offshore hydrotests and includes loads due to:

weight;

pressure;

thermal effects (if any);

residual loads (if any);

support reactions, and hydrodynamic loads appropriate to the period of the test.

6.3.3 Load case 3 - Operational

This load case covers the operation of the riser and includes the following types of load:

weight and buoyancy;

pressure; thermal;

residual;

dynamic;

support reaction;

hydrodynamic;

wind;

seismic;

ice.

NOTE: The combination of loads that produces the highest stresses at one point may not be the samecombination that produces the highest stresses at another point (e.g. different wave directions andphase).

8/13/2019 31401010 Dec 99

21/47

6.4 WALL THICKNESS DETERMINATION

The riser wall thickness required for pressure containment shall be determined inaccordance with PTS 31.40.00.10

An internal and external corrosion allowance shall be determined and added to the wallthickness required for pressure containment.

An allowance for thinning during the bending process shall be added to the riser bend wallthickness.

The wall thickness may not be governed by pressure containment and consideration shallbe given to the following:

adoption of a single wall thickness for riser bends and straights;

the use of a non-standard outside diameter, in order to achieve a constant internaldiameter along the pipeline system;

the addition of an allowance for mechanical damage to the riser such as gouging bycables;

the increase of the wall thickness for ease of installation and to increase the spacingbetween supports.

6.5 PIPELINE EXPANSION

6.5.1 General

The design of the pipeline and riser system shall consider the pipeline expansion due to theeffects of temperature and pressure. If pipeline expansion results in loads and stresses thatexceed acceptable limits, an expansion loop or other method of reducing the expansioneffects shall be provided.

6.5.2 Expansion analysis considerations

The pipeline expansion due to temperature and pressure shall be determined for thefollowing phases:

operation;

hydrotest.

The pipeline expansion analysis shall consider both the functional loading and the resultingloads due to restraint. The functional loading should consider the loads due to the following:

temperature;

pressure;

self weight (including weight of steel, coating, attachments, components, contents andmarine growth);

configuration.The restraining loads should consider the reactions due to the following:

pipe seabed friction;

trenching and backfilling;

riser or platform tie-in spoolpiece;

subsea facilities such as subsea safety valves;

anchors (such as rock dumping).

The expansion analysis should consider the maximum expansion mechanism resultingfrom the minimum friction coefficient.

Pipeline expansions derived for both maximum operational conditions and hydrotestconditions shall be based upon an appropriate pipe soil friction coefficient to determine the

critical design loading. Where a thin layer of soil with a high friction coefficient overlays one

8/13/2019 31401010 Dec 99

22/47

with a much lower coefficient, consideration should be given to possible pipeline settlementinto the seabed from repeated expansion and contraction movements.

For a buried pipeline, the frictional restraint of the soil overburden may be included as partof the restraining seabed resistance. The design should give consideration of theuncertainties inherent in this method of placement.

The pipe soil friction coefficients normally include a range of coefficients for various piperoughness and soil properties.

The design shall include the effects of potential seabed scour on pipeline expansion.

Changes in pipe wall thickness and/or weight coating thickness, and any discontinuities inpressure or temperature such as may be found at a valve station, shall be taken intoconsideration together with the pipeline length when determining the pipeline expansion.

6.5.3 Expansion control methods

Pipelines at platforms have the potential to expand towards the platform. When the amountof pipeline expansion and the corresponding load on the riser exceeds the allowable riser

loading, then some form of pipeline expansion control shall be incorporated in the design.In general, the control of pipeline expansion on the riser is achieved by either restrainingthe pipeline and forcing expansion away from the platform or by incorporating anexpansion-absorbing mechanism.

The restraining of pipelines near the riser may be achieved by the following methods:

rock dumping;

trenching and backfilling;

increasing the pipeline submerged weight;

axial anchoring;

apply high-friction coating to low-friction-coated flowlines.

Pipeline expansion-absorbing mechanisms may include the following:

provision for riser flexibility;

expansion loop;

flexible pipe.

6.6 EXPANSION LOOP

Pipeline expansion should be accommodated by flexibility in the bottom of the riser. If thepipeline expansion is such that the pipe and riser termination cannot accommodate theexpansion load, an expansion loop shall be provided.

The spool shall be made as compact as possible for ease of installation.

The expansion loop shall be designed to accommodate the maximum pipeline expansion

from either operation or hydrotest conditions, without applying unacceptable loads orstresses to the pipeline, riser or subsea structure. Flanges shall avoid locations subject tohigh bending loads.

The maximum stress in the expansion loop and the maximum loads on the riser or subseastructure shall be determined using conservative values of lateral friction coefficients at theexpansion loop.

The environmental loads shall be applied to the expansion loop design in combination withmaximum operational and hydrotest functional loading conditions. The wave crest shall bepositioned to give maximum loading on the expansion loop and four wave directions shallbe considered.

Considerations shall be given to potential scour around a platform or subsea structure and

the effect on the expansion loop design.

8/13/2019 31401010 Dec 99

23/47

Consideration shall also be given to vortex shedding criteria for any pipe span between thebottom riser support and the pipe touchdown point on the seabed.

As it is necessary for the spool to move relatively freely, local lateral stability underenvironmental loading may not be achieved. If the spool is unstable, the maximum lift forceacting on the spool should be less than the submerged weight of the spool.

A spool which is either trenched or partially buried will experience a lower hydrodynamicforce than when exposed on the seabed. The design shall consider the effect of thisshielding.

The stability of a subsea pipeline is dependent on the stability of the soil on which it isplaced. If the seabed is unstable then the pipeline will become unstable with it. The stabilityof the seabed shall therefore be considered in addition to the stability of the pipeline.

If there is evidence that the seabed becomes mobile in storm conditions, the depth of theunstable soil below the seabed should be determined. The stability design should assumethat the pipeline is only buried to the depth by which the pipeline embeds in stable soil, andnot the embedment depth of the original undisturbed seabed.

A more extreme possibility in sandy soils of low density is that the soil near the seabed mayliquefy under extreme storm conditions. The excess pore pressures within the soil maybecome equal to the confining pressures on the soil, resulting in zero effective stress andzero soil strength. If this occurs, there is the possibility of severe instability coupled withsettlement of the pipeline. The possibility of soil liquefaction shall be assessed whereappropriate, or where there is evidence of the phenomenon occurring.

6.7 RISER STRUCTURAL ANALYSIS

Detailed strength analysis of risers shall be carried out using a validated finite-elementcomputer program. The computer model shall include the expansion offset, the riser up thejacket and riser-associated piping on the deck, up to and including the pig traps.

The pipe system shall be modelled using pipe and elbow elements. Node spacing shall becarefully selected to provide adequate stress output summaries of critical locations (i.e.pipeline elbow).

The riser guides and supports shall be modelled by applying restraints to the model with therequired degrees of freedom.

The thermal offset/soil friction interaction is complex and will be modelled by springs forsmall movements of the spool, or forces for larger movements of the spool.

If overstressing due to hydrodynamic loads is predicted, then one or more of the followingshould be adopted:

relocation of clamps;

use of additional clamps;

increase in riser pipe material grade and/or wall thickness; use of anti-fouling coating and/or cleaning systems to reduce marine growth.

6.8 ALLOWABLE STRESSES

Stresses shall be evaluated in accordance with PTS 31.40.00.10

6.9 ALLOWABLE STRAINS

A riser shall be so designed that it remains elastic under any combination of functional andenvironmental loads. Allowable strain design is notallowed for risers, except for allowablebending strain during the installation of a J-tube riser.

8/13/2019 31401010 Dec 99

24/47

6.10 OVALISATION

The riser design shall ensure that pipe ovalisation, F, does not exceed 2.5%.

where:

( )( )

FD D

D Dx=

+

max min

max min

100

and:

F = Ovalisation

Dmax = maximum OD

Dmin = minimum OD

The design shall consider ovalisation that results from pipe manufacture, external pressureand pipe bending.

6.11 COLLAPSE

The riser design shall ensure the pipe is not subject to collapse/local buckling under any ofthe load cases. Collapse results from excessive external pressure and/or pipe bending.Appropriate safety factors against collapse are given in DnV Rules for Submarine Pipelines.

Note: Specialist advice should be sought when using cold-expanded linepipe as the DnV Rules underestimatethe effect of residual stresses.

6.12 VORTEX SHEDDING

The riser and clamping/support arrangement shall be designed so that significant cross-flow vortex-induced vibrations do not occur. Analysis of vortex-induced vibration shall be

based on natural frequencies calculated in the course of the structural analysis of the riseras a whole. The analysis shall take account of interaction with nearby structural elementsand other risers.

If it is not possible to eliminate in-line vortex-induced vibration by design, then a fatigueanalysis shall be performed to demonstrate an acceptable fatigue life.

6.13 FATIGUE

The fatigue analysis shall consider fatigue damage from cyclic loadings due to pressure,temperature, waves and vortex-induced vibration.

The riser pipe shall have a fatigue life of at least 10 (ten) times the intended service life.

Conservatively, six shutdown and start-up cycles per year shall be assumed whenassessing the fatigue life of risers.

8/13/2019 31401010 Dec 99

25/47

7. RISER SUPPORT DESIGN

7.1 RISER SUPPORT TYPES

Riser supports are normally one of the following types:

Guide clamp

This type of riser clamp restrains the riser from movements perpendicular to its axis whilstallowing rotation and axial movement (see Figure 1).

Deadweight support clamp

This type of clamp supports the deadweight of the riser whilst allowing rotational and axialmovement depending on its detailed design. Axial movement of the riser is restricteddownwards only (see Figure 2).

Anchor clamp

An anchor clamp fixes the riser at the location of the support in all directions and preventsrotation, including torque (see Figure 3).

Anchor clamps can either be fabricated from steel plate and welded to the riser by meansof a doubler plate and circumferential fillet welds, or they can be manufactured as a fittingsimilar to a flange and welded into the riser string by means of full penetration welds. Theformer type of anchor clamp is most common due to its ease of fabrication. The latterintegral type of anchor clamp is used where riser loads are particularly high.

Topsides support

The topsides supports are designed by others and fall outside the scope of this PTS.

Special support

Other types of support are used in cases where the required riser restraints differ fromthose indicated above. A riser guide permitting the movement of the riser in the direction ofthe pipeline expansion is an example of a special clamp.

7.2 DESIGN CONSIDERATIONS

Riser support types shall be selected and the supports designed to provide the riserrestraint and movement requirements determined from the riser strength analysis. Wherepossible, riser supports shall not be located in the splash zone.

Except for the integral riser anchor flange, riser supports shall be designed and fabricatedin accordance with the structural design rules for the structure. The integral anchor flangeshall be in accordance with ASME VIII. The fabricated anchor flange shall make use ofdoubler plates welded to the riser with circumferential fillet welds.

Riser supports shall be designed without large stress concentrations particularly when

subjected to fluctuating loading. The possibility of fatigue damage of supports shall beexamined and, if necessary, a fatigue analysis carried out to confirm adequate fatigue lifeand possible requirements for inspection for fatigue damage. Combined stresses shouldnot exceed 0.6 SMYS.

Bolts shall be designed for pre-tensioning to give a maximum allowable stress of 50% ofSMYS.

Access shall be provided for the use of hydraulic bolt-tensioning equipment. Bolts shall beof sufficient length for the use of hydraulic bolt-tensioning equipment and nuts shall beprovided with pre-drilled holes for the use of a Tommy bar for bolt rotation. All bolts of asupport should have the same diameter. Correctly tensioned bolts minimise fluctuatingstresses under cyclic loading and therefore improve fatigue performance and reduce thede-stressing tendency of the bolt.

Supports shall be designed to facilitate their installation and that of the risers.

8/13/2019 31401010 Dec 99

26/47

For retrofit risers, specific attention shall be given to the requirement for position adjustmentof supports to enable a stress-free riser installation. The required adjustment shall bedetermined taking into account the following tolerances and accuracies:

dimensional accuracy of as-built drawings;

dimensional accuracy of measurements by divers; alignment accuracy of installed clamps;

clamp closure tolerance;

misalignment adjustment tolerance;

riser fabrication dimensional control accuracy;

riser transport and handling effect on dimensional variation.

At least 250 mm of adjustment should be provided in the riser clamp design in order toaccommodate the stack-up of tolerances. See (Figure 4) for configuration of clamp withcomplete freedom of adjustment.

The design of riser guides shall also comply with the following requirements:

- the inside of the guide shall be provided with a ribbed polychloroprene liner vulcanised

to the guide body;- the inside diameter of the lined riser guide shall be determined such that the riser canmove in its axial direction without significant restraint;

- risers coated with a polychloroprene coating at the location of riser guides shall beprovided with external Monel sheeting vulcanised to the riser coating over the length ofthe riser guide and 250 mm at both sides in the installed condition. The length of theMonel sheeting shall be sufficient to accommodate the requirement for adjustment ofvertical riser position during installation.

7.3 LOADING CONDITIONS

Supports shall be designed to resist the maximum loads from the risers, the support weightand environmental loads on the support. Riser loads on the supports during hydrotesting ofthe riser shall be taken into account when determining the support design loads.

The supports and supporting structures shall be designed to resist the combined loads fromthe riser, environmental loads acting directly on the clamping structure and its weight for allriser design conditions.

7.4 CORROSION PROTECTION

The corrosion protection of riser supports shall be in accordance with the substructurerequirements, and is a function of the support location, namely, either above the splashzone, or in the splash zone or in the submerged zone.

Riser supports above, or in, the splash zone shall be protected by a coating system inaccordance with substructure specifications.

The design of riser supports in the splash zone shall include a corrosion allowance basedon the design life of the structure.

Riser supports beneath the splash zone shall be protected by the substructure cathodicprotection system and shall be coated in accordance with substructure requirements.

Ribbed linings on the riser clamps shall be used to prevent shielding of the cathodicprotection system.

Electrical continuity straps between the substructure and retrofitted riser supports shall beused.

8/13/2019 31401010 Dec 99

27/47

8. J-TUBE DESIGN

8.1 DESIGN DATA

The following data shall be provided in addition to the requirements of (5):

- pullhead weight, diameter and length;- pull-in cable weight, diameter and maximum tension capacity;- back-tension during pull-in of the riser.

8.2 J-TUBE ROUTING

Routing of J-tubes shall take into account the following requirements and considerations:

- alignment tolerances of J-tube and the pull-in cable or riser;- space and support need to be available above the J-tube for the riser hanger clamp;- space and supports are required for the riser pull-in winch and routing of pull-in cable;- the number of bends should be minimised;- bend angles should be kept as small as is possible;

- for steel risers the bend radius shall be as large as possible. Radii should be typically100 times the diameter and radii of less than 50 times the diameter shall not be used.

NOTE: Reducing the number of bends and bend angle and increasing the bend radii will reduce the frictionforces between riser and J-tube during pull-in and will lead to minimum pull-in and J-tube design loads.

8.3 J-TUBE SIZING AND RADIUS OF CURVATURE

The internal diameter of the J-tube for steel risers should not be less than twice thediameter of the riser.

The riser shall be capable of negotiating J-tube bends without exceeding maximumpermissible strains or without collapsing, buckling or wrinkling.

The combination of internal J-tube diameter, bend radius and bend angle shall be sufficientto accommodate the pull-head.

NOTE: Minimum values for J-tube wall thickness and radius may be governed by the allowable spanrequirement to prevent vortex-induced vibrations.

8.4 PULL-IN LOADS

The pull-in of the riser up the J-tube shall be analysed step-by-step from entry of the pull-head into the bellmouth all the way up the J-tube using a validated riser pull-in program.This analysis shall provide the required pull-in loads, point loads on the J-tube and bendingmoments/strains induced in the riser.

The following forces shall be taken into account when calculating required pull-in loads:

- back-tension during pull-in of the riser;- forces necessary for the elastoplastic bending of a rigid riser;- friction forces between the riser and the J-tube and friction forces between the pull-in

cable and the J-tube;- radial forces in the bend due to loss in tension force around the bend;- bellmouth jamming forces;- J-tube jamming forces, i.e. the load on the J-tube that would result if a pull-head got

stuck, prior to the pull winch stopping.

Back-tension shall include the tension or residual tension in the riser from the layingoperation and friction with the seabed.

Predictions of the contribution of friction forces to the required pull-in loads shall beconservative.

NOTE: Frequently used coefficients of friction are as follows:

8/13/2019 31401010 Dec 99

28/47

Coefficient of friction

riser and seabed 0.4 to 0.6

riser and inside of the J-tube wall 0.3 to 0.65

pull cable and the inside of the J-tube wall 0.2 to 0.4

8.5 STRUCTURAL DESIGN OF J-TUBE AND SUPPORTS

J-tubes and their supports shall be designed and fabricated in accordance with therequirements for the platform structure. Particular attention shall be paid to the modelling ofthe loads at the contact points between the J-tube and the riser/pull-in cable. Supports shallbe designed without large stress concentrations particularly when subjected to fluctuatingloading.

The possibility of fatigue damage of J-tube and riser shall be examined and, if necessary, afatigue analysis carried out to confirm adequate fatigue life and possible requirements forinspection for fatigue damage.

Buckling analysis shall be performed to investigate any possibility of buckling/collapse ofthe J-tube. Bar buckling of the J-tube compression shall also be prevented. Checks shall beperformed on the buckling stability of the J-tube bends, both in-plane and out-of-plane, forthe pull-in load case and for local buckling at the worst loaded area of the J-tube.

J-tubes shall be designed and supported so that vortex-induced vibrations cannot occur.

8.6 APPURTENANCES

8.6.1 Bellmouth design

The purpose of attaching a bellmouth to the J-tube bottom end is to ease the pull-inoperation. The bellmouth acts as a guide for the pull-head into the J-tube, and should have

an entry angle and height above the seabed (if any at all) which accommodate pull-in andlead to acceptable span lengths with respect to vortex shedding and column bucklingcriteria (if applicable). The bellmouth may also serve to reduce the stresses resulting from aminor change in orientation of pull-head and riser as they enter a J-tube. In case a sealbung (8.6.2) is to be used, the bellmouth design needs to be suitable for seal bunginstallation and operation. The bellmouth might also need J-tube flushing facilities for J-tubeinstallation and/or for flushing the J-tube of seawater/inhibitor for corrosion protectionpurposes.

The loading that the bellmouth may experience can be divided into two load cases, namelyinstallation (pull-in) load case and the operational load case.

a) During installation, the bellmouth shall be able to sustain the reaction forces inducedon the bellmouth from the pull force required to free a jammed pull-head.

Bellmouths which are close to the J-tube bottom bend may form a contact point on theriser as it progresses around the bend. In these cases the bellmouth shall be designedto sustain these loads.

b) During operation the loading on the bellmouth is very dependent on what type ofrestraints the seal bung (if any) puts on the pipeline riser. However, the followingloading might have to be considered:

- Pipeline expansion: When the bellmouth structure acts as a clamp fixing the riser tothe end of the J-tube, expansion movement of a pipeline on the seabed imposes abending moment, axial loading and shear force at the bellmouth.

- Gravity: When the bellmouth acts as a fixed support for the pipeline as it spans theseabed or to a support structure, the submerged weight of the line causes bending

and shear at the bellmouth.- Environmental: Wave and current loading acting on the suspended section may

induce shear and bending at the bellmouth.

8/13/2019 31401010 Dec 99

29/47

- Settlement: Differential settlement between platform and seabed may inducebending and shear at the bellmouth.

There are many bellmouth designs in existence and it is difficult to categorise them. Thebellmouth layout is very much dependent on whether a seal bung for prevention of inhibitedwater diffusing into the seawater is going to be used or not, and if so, what type of sealbung.

In some cases a seal bung is not required, and the messenger wire, preinstalled in the J-tube for installation purposes, needs only to be attached to the pull-head padeye outsidethe bellmouth to commence the pull-in operation.

When installing a jacket structure it might be impractical to have heavy and long bellmouthsattached to the J-tubes. In this case, the J-tube may end in a blind flange with themessenger wire attached to its inside. The bellmouth must then be flanged to the J-tubebefore the pull-in operation can start.

(Figures 5A and 5B) illustrate these bellmouth concepts.

8.6.2 Seal design

The primary objective of the seal bung is to isolate the void between the inside of the J-tubeand the outside of the riser/pipeline from seawater. The riser/pipeline within the J-tubewould experience accelerated corrosion if the line was open to the sea. To prevent thisaccelerated corrosion, this void is filled with inhibited seawater, or other suitable non-corrosive medium.

Secondary considerations are the degree of restraint the bung applies to the riser/pipelineand the ability to flush the J-tube of seawater/inhibitor. The flushing consideration may notform part of the seal design.

The seal is designed to prevent diffusion of the contents of the J-tube into the sea. Insatisfying this task the seal must accommodate the following load conditions:

a) Pipeline axial movement

The pipeline usually experiences high axial loads during operation which, if the line isunrestrained, will translate into axial movement. Should pipeline axial movement beexperienced, then the seal can either permit the line to expand or prevent it fromexpanding. Should the seal permit line expansion, then it will prove difficult to provide awatertight seal suitable for a long design life. However, should the seal prevent lineexpansion, the seal will experience high axial loads.

b) Hydrostatic pressure

The seal may experience a pressure differential between the inside of the J-tube andoutside of the tube. This differential pressure can be either positive or negativedepending on the design. If the J-tube is filled with inhibited seawater up to thetopsides, the pressure differential at the seal will be the hydrostatic head due to the

height of water from sea level to the topsides. However, if the J-tube is gas filled, thepressure differential will be dependent on the pressure of the gas in the J-tube.

If the gas pressure is topsides ambient pressure, the maximum differential pressure atthe seal will be the hydrostatic pressure at the seal due to water depth. Thesescenarios are clearly illustrated in (Figure 6).

c) Design life

The seal should maintain its integrity over the design life of the J-tube, which cantypically be 20 years. The seal material should not degrade due to seawater, J-tubefluid content or extended durations of high temperature (from the riser/pipeline).

There are many J-tube seal designs in existence, most of which can be placed into 5different categories. These categories are:

- conical seals;- inflatable seals;

8/13/2019 31401010 Dec 99

30/47

- rubber boot seals;- bellow seals;- integral plug/anchor seal.

Each type of seal is discussed with respect to their advantages and disadvantages duringinstallation and operation.

a) Conical rubber seals

Conical rubber seals are suitable for small-diameter lines where the expansionmovement is low. The seal consists of a rubber sheath of varying cross-section whichis bonded to the riser. This is usually fabricated on a short pup piece which is thenadded in place offshore.

The riser shall be installed carefully so that the conical seal section is accuratelypositioned in the J-tube bellmouth. In some cases, an anchor flange is fitted to restrictthe movement of the riser within the J-tube and to prevent damage to the seal. Thelimits of axial movement and misalignment during installation with which the seal cancope will depend upon the contact length of the seal. Typically, the maximum

permissible misalignment or movement is 50 mm. The principal advantages of thistype of seal are its low cost and ease of installation.

b) Inflatable seals

This type of seal consists of two toroidal inflatable seals. The seals are installed on theinner surface of the J-tube bellmouth. After the riser is pulled in, the seals can beinflated from the surface to close the annulus between the outside diameter of the riserand the inside diameter of the J-tube. The seals can provide a sufficient seal towithstand a differential pressure of typically 5 bar. Axial movement of the riser withinthe J-tube is accommodated by shearing of the toroidal seals. With larger expansionsthe riser may slip through the seals and cause damage to the elastomeric seal

components. Therefore this system is typically limited to axial movement of 30 mm.

c) Rubber boot seals

One of the simplest methods of sealing a J-tube is using a rubber boot, which isinstalled in the J-tube bellmouth. Since the sheath is designed to fit the riser a tight fitshould be achievable. However, the seal is susceptible to mechanical damage duringinstallation and pull-in.

The wear caused by the riser during pull-in can be uneven and render the sealuseless. Once the riser is installed, it is not possible to replace these seals. If this typeof seal can be installed correctly without suffering damage during installation the sealscan withstand differential pressures up to 2 bar and an axial movement of up to30 mm. This method lends itself to the use of a rubber sealing diaphragm, whichpermits the J-tube to be filled with corrosion inhibitor before the riser is pulled through.To pull the riser through, the diaphragm is punctured to allow the riser to pass into theJ-tube. The ruptured diaphragm can form a seal, however an effective seal cannot beguaranteed.

d) Bellow seals

This is another simple method of sealing a J-tube by means of a rubber diaphragm.These seals come in two forms, integral diaphragms and zipped types. The integraldiaphragms must be installed on the riser before the pull-in operation, hence aprotective cover is usually required to ensure no damage occurs to it during pull-in.Should the diaphragm be damaged then it cannot be replaced with a similar seal.However, the zipper diaphragm is installed by use of a waterproof zip. This will allowinstallation subsequent to the riser pull-in and replacement of the whole seal ifnecessary. The seal between the riser/diaphragm and J-tube/diaphragm can be madein a number of ways. The simplest method is to use banding straps, however split

flanges can also be used.

8/13/2019 31401010 Dec 99

31/47

These seals allow greater axial movement compared to simple diaphragm seals and,by increasing the length of the sleeves, can cope with axial movement over half ametre.

These types of seal are suitable for differential pressures of up to 2 bar.

e) Anchor type seal

This type of seal is based on the same principle as the conical rubber seal, with thedifference that the seal is kept in constant compression irrespective of themovements/loads in the riser. This is achieved by an anchor flange which is attachedto the riser behind the conical seal. Two split flanges are attached behind the anchorand tightened to the bellmouth. This prevents the riser from moving at the bellmouth,so ensuring the integrity of the seal. As discussed, this system does not permit anyaxial movement of the riser, but can accommodate relatively high differentialpressures.

8.6.3 Pull-head design

The pull-head is an item which is attached to the end of the riser on one side and to the

pull-wire on the other. The pull-head shall be designed to facilitate the pull-in operation andnot cause damage to the J-tube or the riser. It must withstand the tension caused by thepull-wire and distribute the load to the riser so that these will not get damaged. It must besmall enough to pass through the J-tube bends without any danger of its getting jammed,and incorporate any feature which results in a reduction in riser stresses and pull-in loads.These features are often incorporated by designing a curved pull-head body of hardenedsteel, see (Figure 7).The danger of the pull-head getting stuck in the J-tube bend may beeasily checked by sketching to scale the J-tube bend with the pull-head inside it.

The pull-head needs to be designed for the highest pull-load the system will experienceduring pull-in plus the additional safety factor required. This load may either come from thepull-in analysis or from a pull-head snagging analysis.

Two pull-head designs are illustrated in (Figures 7Aand 7B)and they are used for small

(50 mm to 150 mm) and medium (150 mm to 500 mm) diameter rigid pipelines,respectively.

8.7 CORROSION PROTECTION

The internal surface of the J-tube shall be protected against exposure to untreatedseawater prior to riser pull-in by means of a blind flange that prevents the ingress ofseawater.

At the time of the riser pull-in the blind flange is removed and replaced with a bellmouth. Aseal is fitted to the riser that blocks to the bottom of the J-tube. The J-tube is then filled withinhibited seawater to prevent corrosion of the internal surface of the J-tube or the riser.Provision for sampling the annular water shall be provided.

The external surface of the J-tube shall be protected against corrosion in the same manneras a riser, see (10).

8/13/2019 31401010 Dec 99

32/47

9. FITTINGS

9.1 FLANGES

Flanges shall comply with PTS 31.40.21.34

If bending moments, additional axial forces or shear forces occur at the location of theflange connection, a behaviour (including the gasket with regard to leaking), stress andbolting force analysis according to ASME VIII shall be carried out, taking into account allrelevant loading situations for the flanged connection.

For maintenance purposes, the operating manual for the pipeline system shall detail theflange installation procedures used including the equipment required, the bolt pre-tensionforces to be applied and measurements to be made.

Consideration shall be made for the provision of profiled flange protectors to preventsnagging by cables.

9.2 GASKETS

The gasket shall be a ring type gasket in accordance with ASME B16.20 and shall be madeof a material softer than the flange ring groove. The gasket material shall be chosen forcompatibility with the flange material and for the service conditions. Consideration shouldbe given to the use of ring joint inlays and corrosion-resistant materials for the gaskets.

Consideration should also be given to the use of coatings on the gaskets to improvecorrosion resistance.

9.3 BOLTING

Bolting shall comply with PTS 30.10.02.11

Note: The preferred materials for standard applications are ASTM A 193-B7 and ASTM A 194-2H for non-sourservice conditions, and ASTM A 193-B7M and ASTM A194-2HM for sour service conditions. For specialapplications, e.g. low temperature, other materials may be required.

The bolt tension shall be calculated on the following basis:

the bolt tension shall not cause a stress in the bolt greater than 50% SMYS;

the relaxation of the bolt is a function of the method of tensioning and the coating on thebolt;

the bolt tension shall not lead to excessive yielding of the gasket;

the bolt tension shall be sufficient to ensure the gasket remains seated under the worstcombination of tension, bending and bolt relaxation.

The use of a low-friction coating for ease of tightening shall be considered.

9.4 VALVES

Valves for offshore pipelines shall comply with API 6D.Submarine valves should not be included in offshore pipeline systems because of thedifficulty of inspection and maintenance. To facilitate maintenance, valves shall be eitherflanged both ends or be of the top-entry type and be suitably mounted for ease of access.

Piggability requirements shall be taken into account in the selection of valves.

9.5 BENDS

All long-radius riser bends shall comply with PTS 31.40.20.33

Consideration should be given to the use of long tangents to provide cut material for fit-upoffshore.

8/13/2019 31401010 Dec 99

33/47

10. RISER MATERIALS AND CORROSION PROTECTION

10.1 GENERAL

The service conditions throughout the design life of the pipeline shall be established to

permit the selection of suitable materials based on a technical and economical evaluation.The requirements for pipeline materials shall comply with PTS 31.40.00.10

10.2 LINEPIPE

Amended perCircular 16/02

Carbon steel linepipe shall comply with PTS 31.40.20.37

10.3 EXTERNAL COATING

All risers including Duplex or austenitic steel pipelines shall be coated externally by asuitable anti-corrosion coating, supplemented by cathodic protection for the part of the

system below the water level. The sections located within the spash zone shall beexternally coated with a vulcanised polychloroprene (neoprene). Consideration should alsobe given to PE coating and Monel cladding.

The section above the splash zone and the riser bends shall be coated with a glassflakeepoxy coating system.

Recent QRA studies have demonstrated the benefits of providing passive fire protectionaround the above-sea section of the riser, to prevent escalation due to flame impingement.This aspect should be considered during the design of new risers.

External coating selection shall take account of the following proven temperature limitationsof the available coating systems, unless otherwise agreed with the Principal:

Table 10.1 Coating temperature limits

Coating System Maximumcontinuousoperating

temperature(C)

Maximumexcursion

temperature(C)

Specification

Asphalt enamel 60 70 To be agreed with Principal

Fusion bonded epoxy 70 85 PTS 31.40.30.32

Polychloroprene 100 100 To be agreed with Principal

EPDM 105 105 To be agreed with Principal

Polyethylene andpolypropylene

100 120 PTS 31.40.30.31

Coal tar enamel or coal tar epoxy coating systems shall not be used.

Corrosion coating systems shall be in accordance with the PTSs listed in the above table orproject-specific specifications.

Field joint coating systems shall be compatible with and have good adhesion to the mill-

applied coating, and shall be stored and applied in accordance with the Manufacturersrecommendations.

8/13/2019 31401010 Dec 99

34/47

Thermal insulation materials and their properties shall only be selected in full consultationwith the Principal, taking into account the long-term degradation of mechanical and thermalproperties at operating conditions, such as service temperatures and (external hydrostatic)pressures.

10.4 CATHODIC PROTECTION

Cathodic protection design and sacrificial anodes shall comply with PTS 30.10.73.32 Zincanodes shall be specified and the system shall be designed such that operationaltemperatures of the anodes do not exceed 50 C. Impressed current systems should not beused.

To allow effective monitoring of the cathodic protection of risers and to minimise the risk ofcurrent drain from pipeline cathodic protection systems, submarine pipelines and risersshall be electrically isolated from platforms and onshore installations. For offshore pipelinesisolating flanges are not acceptable and use shall be made of an appropriate type ofprefabricated isolating joint, see PTS 31.40.21.31

Electrical isolation shall be ensured at all points of potential electrical contact, between the

riser and the structure, below the isolating joint.

8/13/2019 31401010 Dec 99

35/47

11. MECHANICAL PROTECTION

11.1 PROTECTION FROM BOAT IMPACT

To prevent boat impact, the locating of the riser on the inside of the jacket structure

adjacent to a leg should be considered. Alternatively, a boat fender should be provided.

11.2 PROTECTION FROM DROPPED OBJECTS

The frequency of damage caused by dropped objects shall be assessed, by means ofspecific drop zones and the probabilities of an object being dropped, of the object hitting thepipeline and of the pipeline sustaining damage. A consequence analysis shall be carriedout and the results of this analysis shall be assessed against accepted risks. For any risksexceeding allowable levels, protection measures shall be designed.

If the expansion loop configuration cannot avoid the platform loading areas or otherpotential dropped-object areas, consideration shall be given to the provision of protectioncovers to the expansion loop. The protection cover shall be designed to withstand theimpact from the heaviest item transferred between the platform and supply vessels.

Protection covers shall allow free movement of the expansion loop for maximum pipelineexpansion.

Consideration should be given to the method of installation of the protection covers toensure that they are not a potential hazard to the expansion loop or to adjacent pipelinesand structures. The covers should be designed to allow easy access and removal ifrequired. Consideration shall be given to ensuring that the cathodic protection systemprovided for the expansion loop remains unaffected by the protection covers.

Alternatively, where this is impractical or excessively costly, the hazard and risk should beevaluated on a quantitative basis as part of the overall risk to the installation. Appropriateaction should be taken where necessary to reduce the risk to an acceptable level.

11.3 PROTECTION FROM SNAGGING LOADS

Consideration should be given to preventing accidental snagging of the pipeline/tie-in spooland avoiding transfer of such loads to the riser system.

The possibility of snagging may be mitigated by avoiding spanning in the pipeline and tie-in/expansion spool and protecting the tie-in spool and pipeline end close to the platform bymeans of burial, rock dumping or covering with mattresses. This is particularly important if,for example, anchor cables are frequently deployed in the vicinity of the platform.

8/13/2019 31401010 Dec 99

36/47

12. INSTALLATION REQUIREMENTS

12.1 RISER INSTALLATION TOLERANCES

The alignment of all the riser clamps shall be verified before riser installation. In cases

where the riser is stalked into position, the position of the clamp shall be adjustable byapproximately 250 mm in all directions.

12.2 INSTALLATION FEASIBILITY

A procedure demonstrating the feasibility of the riser installation shall be prepared. Theprocedure shall demonstrate the following:

the riser installation vessels capacity is adequate (e.g. deck space, lift capacity, etc);

sufficient clearances are provided for the installation vessel;

flexibility is provided in the design to make allowance for possible seabed levelvariations;

clearance is provided to adjacent structures for the tie-in operations;

installation sequence is established including riser handling, up-ending, positioning and

placing of the riser in the clamps; the riser will not be overstressed during any stage of load-out and installation, including

static and dynamic loadings;

minimised interference to platform operations.

12.3 CLEARANCE FOR HYPERBARIC WELDING

If the expansion loop is to be connected to the riser by hyperbaric welding, sufficientclearance shall be maintained from any adjacent pipeline or structure (including theplatform jacket and appurtenances e.g. mudmats and pile guides) to allow positioning of thehyperbaric welding chamber and associated handling frames.

12.4 CONSTRUCTION AIDS

Consideration should be given to the installation of construction aids at the time of thejacket design.

Construction aids for the installation of future risers, subsea tie-in to expansion loops, andhook-up to the topsides section of the riser should all be considered.

12.5 TEMPORARY CONSIDERATIONS

Temporary protection and seafastening requirements should be considered for pre-installedrisers in order to prevent damage during load-out, transportation, installation and setting ofthe platform. Temporary supports/fixings should also be considered for the installationoperation.

In order to minimise installation stresses within the riser, it may be necessary to provideknee bracing on the riser, usually at the bottom bend in order to support the protrudingriser. After installation the knee bracing shall be completely removed in order to minimiseoperational stress levels.

Consideration should be given to the temporary requirements for hydrotesting and pre-commissioning equipment.

8/13/2019 31401010 Dec 99

37/47

13. REQUIREMENTS FOR OPERATIONS AND MAINTENANCE

The riser system should be designed with regard to future inspection, maintenance andrepair.

If intelligent pigs are to be used for internal inspection, bend radii shall meet the followingrequirements:

Nominal pipe diameter, D,(mm)

Minimum bend radius

100 10 D

150 to 250 5 D

300 3 D

Additionally, if intelligent pigs are to be used, the pipeline internal diameter should ideallybe constant throughout, including valves, flanges, tees and other fittings.

Variations in internal diameter (Di) cannot always be avoided in local areas of limited

length, e.g. pipeline equipment such as valves. If changes in Di occur at the location of

equipment, pup pieces shall be used with a Diof the equipment. These pup pieces shall

have tapers to the pipeline Diwith at least a 14 degree transition angle, measured from the

axis of the pipe (i.e. a taper of 1:4).

Consideration should be given to the requirement for possible riser replacement, in theevent this becomes necessary at some time during the life of the structure. If replacementis not possible, as for example with a gravity based structure, consideration should be givento the provision of a spare riser.

As far as practicable,