aTe 6076

A New Approach To Designing Repair Clamps forOffshore Structuresby F.P. Shuttleworth and C.J. Billington, Billington Osborne-Moss Engineering Ltd.

Copyright 1989, Offshore Technology Conference

This paper was presented at the 21st Annual OTC in Houston, Texas, May 1-4, 1989.

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper,as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflectany position of the Offshore Technology Conference or its officers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. Theabstract should contain conspicuous acknowledgment of where and by whom the paper is presented.

I

ABSTRACT

A consideration of the structural assessmentprocedure for offshore tubular steel jackets isgiven, which may be followed in the event of damage,potential static overstress or inadequate fatigue1ife.

In the event that repair/strengthening is necessary,state-of-the-art techniques such'as clamping, weldingand grinding are reviewed in the light of the latestresearch data and operating experience. Performancecharacteristics following repair/strengthening arediscussed with reference to each of the abovetechniques,

Ways in which the design of a repair may besimpl ified and optimised are discussed with referenceto the degree of offshore effort involved in carryingout each of the repair techniques. As part of thisprocess this paper describes a recent review of testdata for grouted and ungrouted stressed clamps. Newformul ae are presented whi ch demonstrate enhancedcapacities.

1. INTRODUCTION

Offshore tubular steel structures may experiencepotential overload (static 01" fatigue) as a result ofa number of possible causes such as increased topsideload due to new equipment, excessive corrosion,insufficient environmental data at the design stageor code updating. (The latter cause is much moreprevalent with the older generation of structures).

This paper is concerned not with the initial causesof such overstress but with the reassessment and '..reappraisal procedures which precede a decision onwhether or not to repair/strengthen and, in the event

References and illustrations at end of paper.

of a decision to undertakE! repair/strengthening,guidance on which technique prOVides the most costeffective solution from a technical standpoint. Dueregard is given to the fitness-for-purpose of thetubular joint in the context of the structure as awhole.

Offshore repairs are costly and should be avoidedwherever possible. The high costs are due to theproblems of supply, diving and associated working inthe vicinity of the sea's surface. Hence a techniquewhich minimises offshore effort is likely to findfavour compared with one which is offshore-labourintensive.

As a step towards minimising costs by optimisingdesign, a new and more rational assessment ofexisting test data on stressed clamps is presented.Thi s reassessment gi ves the justi fi cat ion forreductions in the prestressing requirements for theseclamps which in turn lead to savings in fabricationand installation costs.

The subject of repairs to offshore structures isextensive therefore this paper concentrates on thos.eparticular techniques which are directly applicableto the repair and strengthening of tubular joints.

2. STRENGTHENING AND REPAIR TECHNIQUES

This section contains a brief summary of thetechniques currently available and theirapp1icabil i ty to certai n pr'obl em types. Beforeproceeding, it is helpful to define the terms'repair' and 'strengthening'.

Repair is taken to mean reinstatement to the originalform and condition as intended in the design of thestructure.

Strengthening is taken to mean that, in addition toreinstatement, additional load carrying capacity is

317

A NEW APPROACH TO DESIGNING REPAIR CLAMPS POR OFFSHORE STRUCTURES

2.1 WELD IMPROVEMENT TECHNIQUES

• Toe Grinding

OTC 6076

Fatigue lives of wet welds are lower than thesame details in air.

Large hydrogen entrapment, leading to hydrogeninduced cracking.

Rapid cooling due to quenching by waterproduces hard, crack susceptible HAZs.

Thick walled sections requiring preheat cannotbe welded.

•

•

•

These problems have not prevented the use of wetwelding in non-critical applications in the North Seaand for structural applications where fatigue and

Welds which are comparable with thoseperformed above water, achieved byspecifying comparable properties andtesting requirements.

Welds which are less critical where lowerductil i ty, greater poros i ty and 1argerdiscontinuities are tolerable.

Having lesser properties than types AorB, used where load carryi ng is not aprimary function.

Welds required to meet special needswhich will require unique specifications.

For each of the four categories, inspection levelsare given and permissible defects are quoted. Weldstype Aand Bshall develop similar static tensile andshear properties as the base metal. There are norecommendations given regarding fatigue.

..

Type 0

Type C

Type A

2.2 WELDED REPAIRS

It has long been recognised that underwater wet weldshave inferior mechanical properties to those ofsurface welds made in air.

The problems which have been identified (5) are:-

• Lack of vi sibil i ty due to steam and gasesgenerated.

Type B

peening appears the more promising of the two peeningtechniques due to the greater benefits and lowervariability in the results.

2.2.1 Wet Welding

The specification which governs underwater wetwelding is AWS 03.6-83 (4), wherein four types ofweld are defined:-

318

3 - 53 - 5

14 - 17

Improvement Factor on Life

Hammer Peening

Shot Peening

Technique

Toe GrindingShot Peening (heavy)Hammer Peening

••

At present only toe grinding is covered by theDepartment of Energy Guidance Notes which permit a2.2 factor for increase in life.

Further development work is still needed before thepeening techniques can confidently be appl ied totubular joints underwater, but given the benefits tobe gained it is worthwhile pursuing them. Hammer

The recently published report on UKOSRP II (2)considers the effects of each of these techniquesand reports that the following improvements infatigue life may be expected for properly controlledoperation (3).

provided within the structure. This is done in caseswhere the original strength is inadequate and damagemay recur.

Grinding should be carried out using rotary burrsrather than discs. Discs are quicker than burrs butare much more difficult to control leading topossibly excessive grinding and poor profile.

It is usual with hammer peening to induce surfaceindentations approximately O.5mm deep at the weldtoe. Shot peening is controlled by reference to the'Almen' intensity scale, rather than by specifyinglevels of indentation.

There are three weld improvement techniques availablewhich may increase the fatigue life of a weldedconnection.

2

It must be stated straight away that no improvementin static strength may be expected from the use ofany of the above techniques. Indeed, excessive

. removal of chord wall material at the weld toe overand above 0.5mm below any visible undercut may leadto an unacceptable reduction in static strength ofa tubular joint, particularly if the area affectedrepresents a large proportion of the tubular joint.The UK Guidance Notes (1) limit the grinding to 2mmor 5% of the chord thickness.

OTC 6076. SHUTTLEWORTH AND BILLINGTON 3

notch toughness are not design constratnts.

Typical structural applications where wet welding hasbeen used in the Gulf of Mexico and Africa are bracerep1acement, repai ri ng corrosion ho1es and crackrepair for which welds of Type B were specified.

2.2.2 Dry Welding

Dry welding underwater is achieved by creating awatertight environment around the work space to bewelded. This can be achieved in one of four ways:

in the app1icat i on of weld beads or other shearconnectors for grouted connections but moredevelopment work is needed where good weld quality isessential.

Dry welding could be used, for example, to reinstatethe strength of tubular joints which wereinadequately welded during fabrication, to installnew members into a structure, or to install wrapplates or gussets on understrength joi nts (subject tosatisfactory demonstration of fatigue life in thestiffened condition).

• Cofferdam 2.3 CLAMPING TECHNIQUES

• Dry habitat, internal pressure one atmosphere(habitat welding)

• Dry habitat, internal pressure at ambient(hyperbaric welding)

• Portable dry spot.

In each case sufficient space must be allowed for allthe necessary welding and life support systems, egpre-heating, gas exhaust, post-heating, communicationand monitoring equipment.

There are no special weld quality problems associatedwith cofferdam or one atmosphere habitat weldingsince the welding operations are carried out insimilar conditions to a fabrication yard. There areobvious logistical and planning details which shouldbe fully addressed to ensure smooth running of thework offshore. Design of the cofferdam or habitatneeds special attention to ensure that it canwithstand the head of water and that it can beinstalled and sealed allowing for normal tolerances.

Hyperbaric welding is achieved by installing ahabitat around the workspace and displacing the waterby gas at or above the ambient pressure. The designof a habitat in this case is simplified because onlya small pressure differenti al head need beconsidered. In some case flexible rubber habitatshave been used. Welding procedures have beenmodified to cope with the increased pressure anddifferent atmospheric conditions.

Portable dry spots, mini habitats or hydroboxes inessence only protect the welding head and a smallarea around the weld. The weld quality achievable isintermediate since the weld does not suffer immediatequenching and water is kept away from the arc.However, preheating and interpass grinding is notpossible.

Aparticular use for portable dry spot welding may be

319

Over, the past 15 years thel'e has been considerableresearch effort expended in developing clampingtechniques for offshore tubular steel jacketstructures. All of the techniques have been provenby static and fatigue ~esting and are accepted by theCertifying Authorities. The length of service whichthe older repairs can now demonstrate is furtherevidence of thei r durabil ity, part icul arly in thehostile environment of the North Sea.

Techniques which will be described in more detailare:-

• Grouted Clamps and Connections

• Stressed Grouted Clamps and Connections

• Mechanical Connections

2.3.1 Grouted Clamps and Connections

A grouted clamp is one in which the outer sleevearound the tubular joint is formed in two or moresegments which are placed around the existing tubularjoint. The sections of sleeve are brought togetherby tightening the bolts prior' to injecting grout intothe annular space between the clamp and existingtubular joint. For details see Figure 1. Such aclamp may be applied to repair or strengthen one ormore brace members, or indeed the chord member, ata tubular joint against static or fatigue loading.A grouted connection would be used to connect twotubulars together.

All bolts are tightened before injecting grout and,therefore, grout/steel bond and interlock are theonly methods of load transfer, Shear keys may haveto be applied to the tubulars to ensure adequateload transfer into the clamp or connection.

2.3.2 Stressed Grouted Clamps and Connections

A stressed grouted clamp is a clamp in which the

- - ~ --- -

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A NEW APPROACH TO DESIGNING REPAIR CLAMPS FOR OFFSHORE STRUCTURES

3. REASSESSMENT OF TEST DATA

3.1 STRESSED GROUTED CONNECTIONS/CLAMPS

The load transfer mechanism for grouted connectionsand grouted clamps is essentially through the bondthat develops between -the grout in-fill and thestee1work. It fo11 ows that the connection 1engthwould reduce if the bond resistance is supplementedwith fri ctional resi stance provided by app1 ication ofan external prestress normal to the bond face. Thisled to a concentrated research effort to develop theso-call ed stressed grouted clamps, and thei rextensive application in strengthening/repairsituations is sufficient evidence of theiracceptability by both operators and regulatorybodies.

aTC 6076

3 large scale elastic and fatigue tests onstressed grouted T clamps, under axial or OPBcyclic loads.

• Various grout strengths, both short termand long term

• Load monitoring in studbo1ts, duringstressing, ultimate load testing and withtime

i i )

A number of client specific tests were performed onstressed grouted clamps during their developmentstage. Whilst these data remain confidential, itshould be noted that their findings conform with thepublic-domain data. '

Within reference 6 a detailed assessment of the datawas undertaken, and a relationship for thecharacteristic slip strength per effective slipsurface was derived, as fo110ws:-

• Various inner member radial stiffnessratios

i) Over 75 e1ast ic and ult imate load tests onlarge scale stressed grouted connections,encompassing

• Various clamp length to inner memberdiameter ratios

Descriptive details of stressed grouted clamps havebeen presented inSection 2. The research effortunderlying the design of such clamps is summarisedbelow, and has been extracted from References 6, 7and 8 viz:-

• Vari ous stiffness val ues for the longstudbolts

320

It is not recommended that mechanical clamps are usedto repair tubular joints because very accurate surveyand fabri cat ion work is essent ialto ensure thatdamage does not occur due to misfit when the deviceis prestressed. Mechanical connections may be usedto connect two tubulars end to end or to connect ontoa straight length of tubular member, see Figure 3.

The vari ous components of a mechani cal connectionare similar to a stressed grouted connection. Thestrength of a mechanical connection is obtained fromthe steel/steel friction developed as a result of thestudbolts applying a compressive force normal to theinterfaces.

A mechanical connection is used to connect twotubu1ars together in a similar manner to a stressedgrouted connection. The body of the connection isformed in two segments which are placed around theeXisting tubulars. An external bolting force isapplied to develop frictional resistance on the steelinterface of the sleeve and tubulars. The saddles ofthe outer sleeve are stiffened to allow the studboltforces to be carried without distress -to the saddleitself or to the inner tubular.

grouted sleeve is formed in two or more segmentswhich are placed around a tubular joint or tubularmember. Grout is injected into the annular spacebetween the clamp and the existing tubular joint,which is allowed to reach a predefined strength priorto applying an external bolting force. The saddlesof the outer sleeve are stiffened to allow thestudbo1t forces to be carried without distress to thesaddle itself or to the inner chord member.

Stressing of the clamp is achieved by long studbo1tsplaced between widely spaced flanges. The strengthof a stressed grouted clamp is obtained from. acombination of plain-pipe bond as for grouted clampsand grout/steel friction developed as a result ofthe studbo1ts applying a compressive force normal tothe grout/steel interface. The various components ofa stressed grouted clamp for a tubular joint areshown in Figure 2.

A stressed grouted clamp may be applied to repair orstrengthen one or more brace members or the chord ata tubular joint against static or fatigue loading.It has similar load carrying capacity to a groutedclamp having shear keys but avoids the necessity forunderwater application of weld beads or other typesof shear key. A stressed grouted connection can beused to connect two tubulars together.

2.3.3 Mechanical Connections

4

OTC 6076 SHUTTLEWORTH AND BILLINGTON 5

Pc = 0.22(Fn.C's + 1. 75 x 10-sA.Cs)(1 + 33(0/Tr1)

...... (1)

ii) The constant B represents the bond developedbetween the grout and steel faces. Theconstant A represents the rate of gain incapacity as the stud bolt force increases.

The results of the statistical analysis are shown inTable 1. The analysis gives the following equation

The following became clear in examining the approachadopted in Reference 6:-

• The statistica1 assessments adopt the method ofleast squares of absolute differences. Indeveloping design methodologies for theultimate limit state of structural components,it is more appropriate to adopt statisticaltechniques based on the least squares ofpercentage differences (9). In this manner,appropriate weighting of the test data can bemade without introducing any unintended bias.

• The characteri stic strength has been determinedon the basis of 95% probability of survival for95% confidence level. This is grosslyconservative and does not follow normal,established limit state procedures forstructural components whi ch call s for a 95%probabil ity of survival for 50% confidencelevel (see, for example, 1, 9, 10 and 11).

In light of the above inconsistencies in the approachadopted in Reference 6, a complete reassessment ofthe data has been undertaken herein, and the findingsare reported below.

The relevant data suitable for statistical treatmentare shown in Table 1. The database is identical tothe base used in Reference 6 in the deri vat ion ofEquation 1. A statistical analysis of the Table 1data has been undertaken usi ng the 1east squaresprinciple based on percentage difference, andadopting the following equation:-

..... (3)

i) 84 elastic and ultimate load tests,encompassi ng the parameters noted above forstressed grouted connections

ii) 3 large scale elastic and fatigue tests onstressed mechanical T clamps, under axial orOPB loads.

Pc = 0.26 (Fn.C's + 1.5.A.1O-s.Cs)(1 + 33 (0/Tr1)

....... (5)

Table 2 presents details of the available data onmechani cal connections. These data were subjected toa detailed assessment in Reference 9 leading to thefollowing relationship for the characteristiccoefficient of friction:-

Pc = (0.5193F + 118.24)(1 + 33 (O/T) -1 )...... (4)

The omi ss ion of the grouted annul us from stressedgrouted connections results in the concept ofmechan ica1 connections. Here, the des ign re1iesexc1us ive1y on the fri ct ion developed between thesteel surfaces to transmit the incoming loads .Extensive research into mechanical connections hasbeen conducted (6 and 7), which can be summarised asfoll ows:-

3.2 MECHANICAL CONNECTIONS

Rewriting Equation 4, introducing the surfacecondition factors Cs and C's and normalising the bondstrength term (ie. 118.24) with bond area gives thefollowing equation for the characteristic slipstrength per effective surface for stressed groutedconnections:-

for the mean slip strength:-

Pmean = (0.6039F + 137.5)(1 + 33 (0/Tr1 )

The close fi t of Equat ion 3 to the test data isdemonstrated in Figure 4. The mean of the ratio oftest results to prediction by Equation 3 is 1.0158(close to unity as expected) and the standarddeviation is 0.0938. The sample size is 30.

The statistical constants for various sample sizesare tabulated by Baker (12). For a sample size of30, and using a 95% probabil'ity of survival with 50%confidence, Baker gives a value of 1.6620 for thestati stical constant. Therefore, Equation 3 needs tobe multiplied by 0.8599 = (1.0158 - 1.662 x 0.0938)to give the characteristic strength equation. Thecharacteristic sl ip strength of a stressed groutedconnection can be written as

....... (2)A F + B

The term Pmean is divided by a relationshiprepresent i ng the effect of chord wall stiffnesson slip strength. This relationship is takenfrom Reference 6 and is considered to be validon the evidence from test data.

1)

Pmean1+33(0/T) -1

Note the fo11owing:-

321

A NEW APPROACH TO DESIGNING REPAIR CLAMPS POR OFFSHORE STRUCTURES

3.3 COMPARISON WITH CURRENT DESIGN METHODS

• Increased topside load

OTC 6076

Excessive corrosion

Insufficient environmental data at design

Code updating

Fatigue

Caisson supports fixed rather than guided sothat axial and bending forces develop in thecaisson which induce large out-of-planestresses at the tubular joints in the supportmembers

During this stage it would be valuable to use theactual member yield strengths rather than the nominalvalues since there may be as much as 20% enhancementfor typical offshore tubulars.

A common practice with fatigue induced defects is toundertake fracture mechanics analysis to determinethe growth rates and stability of crack-like defectsat tubular joints. It may well be that incombination with load reduction. as described below.the growth rate is greatly reduced so that no otheraction is necessary. Alternatively the crack mayarrest as it grows around the joi nt into a 1esshighly stressed portion away from the 'hot-spot'. Inthis latter case it will be necessary to survey thetest data to ensure that the tubul ar joi nt hasadequate reserve strength in the cracked state, toensure that static failure does not occur.

•

•

••

•

• Vibrations in slender members due to vortexshedding

• Impact damage caused by dropped objects orsupply vessels

Figure 6 presents in flowchart form the decisionsteps which lead to a final decision to undertake arepair. They are arranged approximately in order ofincreasing total cost which is the sum of onshore andoffshore effort.

It may be possible to directly address the cause ofa potential defect by modifying the structureaccordingly to el iminate the problem. As part ofthis step it is necessary to undertake a redundancystudy to determine the criticality of themodifications which should not lead to unacceptableri sk of fail ure el sewhere in the structure. Thi smethod would particularly apply to members whoseprimary functions were served during fabrication orinstallation and are not essential to the structureonce installed. ego launch bracing. This mayrepresent an acceptable solution even though itinvolves a degree of offshore work.

322

TO

(7)

PRIORASSESSMENT PROCEDURE4.

For mechanical connections. inspection of equations6 and 8 demonstrates an enhancement of 5.5% for allprestress levels. It is recommended. however, thatthe upper 1imi t ing val ue for the coeffi ci ent offriction given in Reference 6 (~c = 0.45C's) is notexceeded.

REPAIR/STRENGTHENING

An initial study should be undertaken to identify thecause of a defect or potential overstress. Suchcauses are many and various and a brief list of thosewhich the authors have experience of is given below:-

~mean = 0.2043 (1 + 20 (D/Tr1 )(1 + 66.1 Kb )

The new equations for stressed grouted clamps givethe same capacity as the current equations for zeroprestress. however. as' the level of prestressincreases the enhancement may be as high as 20% forheavily stressed clamps.

Note that the stiffness term for the chord has beentaken from Reference 6 which treats this aspectadequately. The close fit of Equation 7 to the testdata is demonstrated in Table 2 and Figure 5. Themean of the ratio of test result to prediction is1.0054 and the standard deviation is 0.0504. Thesample size used is 21. For this sample size. thestatistical constant from Reference 12 is 1.6698.giving a factor of 0.9212 to be applied to Equation7. The characteristic coefficient of friction istherefore given by the following equation:-

~c = 0.19 (1 + 20 (D/T) -1)(1 + 66Kb ) ..... (8)

~c = 0.18 (1 + 20 (D/T)-l)( 1 + 66 Kb) •••••• (6)

However. the analyses suffer from the sameshortcomings as identified for stressed groutedconnections. Therefore. the data detailed in Table2 were reassessed using procedures identical to thosefollowed for stressed grouted connections. In thisinstance. the results of the statistical analysisbased on percentage difference gave the followingequation for the mean coefficient of friction formechanical connections:-

6

-~--._- ----~-

OTe 6076 SHUTTLEWORTH AND BILLINGTON 7

Fracture mechanics procedures are also valuable indeveloping an optimised inspection and repairprocedure for a jacket. In the case where more thanone repair is anticipated it may be cost effective todelay less critical repairs until a later date whena number of repairs can be performed together.

Another potentially valuable step is to survey theavailable test data for the particular jointconfiguration under consideration rather than relyingon current codified methods which may under predictthe joint capacity. The UEG Tubular Joint DesignGuide (11) provides a comprehensive database ofstatic and fatigue test results and is, therefore, auseful starting point for such an exercise.

When direct action is not possible and a testprogramme is not worthwhile, the next step in theassessment procedure is to consider undertaking anindirect load reduction programme offshore. Thisentails removing all extraneous items which attractenvironmental loading such as redundant conductors,caissons, sling platforms, padeyes, pile guides, boatlandings, launch bracing. and removing marine growth(annually if necessary). This, as one might expect,is a costly exercise and would be considered only asa last resort, indeed it may be less expensive toundertake a repair.

If, after this procedure, it is not possible tojustify adequate strength or remaining 1ife in thejoint then it is inevitable that some kind of repairor strengthening will have to be considered.

4.1 CONSIDERATIONS FOR OPTIMISATION OF COST AGAINSTBENEFITS

There are no hard and fast rules to say which is thebest repair method for a given defect. Indeed, thedefinition of 'best' may well vary from jacket tojacket. Each repair has its own uniquecharacteristics of cause, environment, water depth,joint type, accessibility, inspection regime, loadingand each jacket will have its own limitations oncranage, labour availability and skills andoperational constraints which all have a part to playin reaching the optimum solution.

This section provides guidance on technical aspectsof the complex equation; it is for the operator tofill in the remainder of the terms in the equationassociated with platform operation. The fitness-for­purpose of the repair must be maintained for theremaining design life of the platform. This musttake into account static strength, fatigue, corrosionand durability of materials.

323

Table 3 summarises some of the primary parameters toconsider in selecting a repair/strengtheningtechni que for a tubul ar joi nt which incorporatesthese factors and gives rankings where appropriate.

4.2 COST CONSIDERATIONS

It is not possible to give absolute cost figures forrepairs as there are so many vari ab1es invo1ved inanyone repair. It is possible, however, toapproximately rank the repair techniques in terms ofoffshore timescale and equipment needs.

This has been done in Table 4 for the repair of atubular joint to overcome:-

• a) Fatigue related lack of capacity

• b) Inadequate static strength

Table 4 includes offshore preparation andinstallation effort and does not allow for the costsof inspect ion over the years fo11 owi ng repair. Ifthe repair is at considerable water depth, it maywell be that inspection is undesirable at thatlocation in which case the emphasis in Table 4 wouldmove towards one of the techni ques havi ng lowerinspection levels as defined in Table 3.

5. METHODS OF OPTIMISING DETAILED DESIGN

Some methods of minimising the extent and cost ofrepair which are available to the designer arediscussed below:-

5.1 LOAD SHARING/COMPOSITE ACTION

Where a tubular joint or member is intact it may beassumed to carry a proportion of the environmentalload after repair. (Note that a repair does notremove the static loads in the members at the time ofinstallation unless this is specifically achieved byjacking or member severance). Thus the joint can beanalysed for a reduced loading and the repair schemedesigned for the proportion of load remaining,thereby making savings in fabrication andinstallation effort.

Similarly a clamp placed around a joint will have theeffect of stiffening the joint by the effects ofcomposite action such that chord wall deformationsare reduced and it, effectively, has a thicker chordwall.

5.2 FABRICATION SPECIFICATION

Stressed grouted clamps are often heavy, complexitems of fabrication. To reduce problems of welder

8 A NEW APPROACH TO DESIGNING REPAIR CLAMPS FOR OFFSHORE STRUCTURES OTC 6076

Corrosion protection of bolts, nuts and washersis by cadmium plating and PTFE coating. Thecost of this type of studbolt is approximatelyone fifth that of Monel K-500 bolts.

Monel K-500 in accordance with BS 3076 (15):1976 Grade NA 18.

This is a cold worked and precipitation heattreated nickel-copper-aluminium alloy.

Minimum Charpy V Notch impact toughness of 20Joules at -100·C should be specified.

No corrosion protection coatings are requiredfor this material.

L7 material is recommended for splash zone andother aggress i ve areas as it has a provenimproved ductility compared with B7 due to itsspecified minimum Charpy V notch impacttoughness of 20 Joules at -100·C. B7 has aspecified impact toughness of 54 Joules at ­20·C. These materi.als are now most frequentlyspeci fi ed because they have shown to be themost re1i ab1e and sign i fi cant1y more economi ca1in comparison with Monel K-500 or other nickelall oys.

This material can suffer permanent set atstress levels above 0.6 x proof stress.

Monel K-500 can also suffer hydrogenembrittlement in ,the event of over protectionby the structural CP system.

Macalloy - A high strength carbon steel withchromium, molybdenum and nickel alloying

2.

1. BS 1506 (14) Grade 621A L7 or B7These materials are hardened and tempered, 1%chromium, molybdenum carbon steel alloys.Maximum tensile strength should be limited to1000 N/mm2 •

bolts, nuts and washers is by cadmium plating andPTFE coating.

A number of materials are commonly available forstudbolts, the costs for which vary greatly. It is,therefore, worthwhile considering the environmentalconditions in which the studbolts will operate beforemaking a final selection. In general studboltsshould have adequate strength, resist corrosion,should not be brittle and should exhibit satisfactorytoughness at the temperature of operation. Threetypes of commonly used studbolt material aresummarised:-

3.

324

- threaded bars which in thefinal installed position haveone or more nuts at each end.

Structural bolts should be ISO-B5 3692 (13) Gr 8.8or 10.9. It is riot recommended that grades higherthan 10.9 are specified. Corrosion protection of

access, inspection and distortion it is advisable touse fillet welding rather than full penetrationwe1ding wherever poss ib1e. Even in fatigueenvironments there are portions of a stressed groutedclamp which do not experience fatigue loading, egosti ffening pl ates supporting the stud bol ts (seeFigure 2), and it is reasonabl e that they may befillet welded. Clamps in non-fatigue environmentsmay be almost exclusively fabricated by filletwelding to minimise costs.

Similarly, if a welded repair involves wrap plates,doubler plates or other forms of stiffener the choiceof welding procedure should reflect the criticalityof the joint, material properties and loading regimeto the extent that, where appropriate, wet weldingmay be the selected method in accordance with therecommendations of AWS 03.6 (4).

5.3 FATIGUE OF BOLTS

Where the clamps are subjected to fatigue loading itis imperative that adequate prestress load is inducedinto the bolts at the time of installation. At notime during the life of the clamp should the initialprestress be overcome as that woul d gi ve ri se toseparation of prestressed contact surfaces whichwould in turn lead to large fatigue stress ranges inthe bolts and potential fatigue failure.

The sleeves of grouted clamps are designed to havemating steel faces and the bolt or studbolt load isdesigned so that a positive contact pressure ismaintained at all times. Hence, the bolt is isolatedfrom the damaging effects of fatigue.

Stressed grouted clamps and connections andmechanical connections function by maintaining apositive contact pressure on the tubulars, thereforethe segments of clamp do not touch. To minimise theeffects of fatigue due to fluctuations in tubularmember diameter under varying load, long studboltswhich minimise stress ranges are recommended whichare tensioned to specified load levels to maintainthe required positive contact pressure.

• Studbolts

Two types of bolt are used in clamps namely:-

• Common structural - those bolts having abolts hexagonal head at one end.

-- -

~~~-_::o~,,="" -_~~=~-'=7:='c,CC=''''~-:::::::=--C~~~C S'~"~-~~"2."-'~"::S ,?",=,,~=,:; c"~~cc --=-~~-_-=~~==~;,~=--:"-,-~::--~~=~~_:_~~~~~-~~~·2=--~_~?-=, --::0 -.~;:~"'=c----~~_=-

OTe 6076 SHUTTLEWORTH AND BILLINGTON 9

elements which is extensively cold worked.Specification is to BS 4486 (16).

Prob1ems have been found in the use of th ismaterial due to its sensitivity to brittlefracture. Its use is not recommended inaggressive environments or where a long life isexpected of the repair. It is, however, thecheapest of the studbolting materials and maybe satisfactory for non critical or temporaryapplications.

Corrosion protection is usually by galvanisingor PTFE coating; cadmium plating is notrecommended where the ultimate tensile strengthexceeds 1000 N/mm2 •

It is necessary to protect studbolts from bendingstresses which would be introduced when oppositeflanges are not parallel. This is achieved by theuse of two part washers with the mating facesmachined to give a spherical surface. These washersshould be placed at each end of the studbolt.

Studbolts and nuts should have cold rolled threadswhich extend over the full length of the item.

5.4 TOLERANCES AND INSTALLATION

At the detailed design stage it is important to makeallowance for normal tolerances on pipe dimensionsand fabrication, ego the normal tolerance on pipediameter is ± 1% (ie. ± 9mm for a 900mm diametermember). Allowances are made in design by selectingthe appropriate nominal annulus thickness in agrouted clamp. If using a mechanical connection,which has no annulus, it will be necessary to performan underwater survey and monitor fabrication closelyto ensure that there will not be a clash duri nginstallation, due to adverse dimensional variationsand combinations.

When installing new members into a structure it isprudent to prOVide a tolerance device within thelength of each member to provide length and angularadjustment to accommodate variations in geometry fromthe design condition. Such a device would be agrouted connection.

Diving work is very inefficient compared to workonshore. The designer should prOVide as manyinstallation aids as possible to speed up theinstallation process. Such items as hinges forclamps, temporary holding clamps, locating devices,annulus adjusting set screws and simple, reliableseals are all worth considering to minimise offshoreeffort.

325

5.5 USE OF NEW EQUATIONS

Benefi t from use of the equat ions presented hereaccrues in that fewer bolts or lower bolt loads arerequired therefore shorter and lighter clamps will bethe outcome.

6. CONCLUSIONS AND RECOMMENDATIONS

This paper summarises the experience of the authorswho have been involved in more than 60 sub-seastrengthening and repair assignments worldwide. Thepaper provides an overview of the different repairtechniques and explains advantages and disadvantagesof each technique. It is concluded that no singletechnique always provides the best solution.

A reanalysis of test data on stressed grouted clampsand mechanical clamps provides enhanced strengthequations which help to reduce clamp size and boltrequirements. The 1evel of improvement can be ashigh as 20% of capacity.

Recommendations are given for many practical detailsencountered during fabrication and installation.Experience has shown that great care is necessary inthe choice of bolting materials and corrosionprotection methods. It is recommended that BS1506Grade 621A L7 or B7 bolts are generally used. Theseoptimise on strength, ductility and cost. Corrosionprotection by cadmium plating and PTFE coating isrecommended as this has been shown to provide longservice life in the marine environment.

NOMENCLATURE

A bond area of slip surface (mm2) or constant

Ab shaft area of bolt

B constant

Cs surface condition factor for bond

C's surface condition factor for friction

D chord diameter (mm)

Eb Young's Modulus of studbolts

Es Young's Modulus of stElel

F total stud load (kN)

A NEW APPROACH TO DESIGNING REPAIR CLAMPS FOR OFFSHORE STRUCTURES

REFERENCES

OTC 6076

11.

15. British Standards Institution. 'Specificationfor Nickel and Nickel Alloy Bars': BS 3076,1916.

,

16. British Standards Institution. 'Specificationfor Hot Rolled and Processed High Tensile AlloySteel Bars for the Prestressing of Concrete' .BS 4486, 1980.

12.

14. British Standards Institution. 'Steel for Usein the Chemical, Petroleum and AlliedIndustries'. BS 1056-621A.

Foster ML. 'Leg Strengthening of a North SeaJacket' . OTC 4880, Offshore TechnologyConference, Houston, May 1985.

Lalani M. 'Rationalisation of Design Practicefor the Ultimate Limit State of TubularJoints' . Integrity of Offshore StructuresConference, Glasgow, September 1987.

UK Department of Energy. 'Report of theWorking Party on the Strength of GroutedPile/Sleeve Connections for OffshoreStructures' . Paper OT-R-8258, CIRIA Report,September 1982.

'Design Guide on Tubular Joints in SteelOffshore Structures'. UEG Pub1ication UR33,April 1985.

Baker MJ. 'Vari abil i ty of the Strength ofStructural Steel - A Study in StructuralSafety' • Part 1 - CIRIA Technical Note 44,April 1973.

13. Bri t ish Standards Inst ituti on. 'Speci fi cat ionfor ISO Metric Precision Hexagon Bolts, Screwsand Nuts, Metric Units'. BS 3692, 1967.

10.

8.

9.

326

UK Department of Energy. 'UK Offshore SteelsResearch Project Phase Two Summary Report'.Report No OTH-87/265, HMSO, 1987.

mean coefficient of friction

UK Department of Energy. 'OffshoreInstallations: Guidance on Design andConstruction'. HMSO 1984 •.

nominal connection length

stressed length of stud bolt

number of bolts

sl ip load (kN)

char,acteristic sl ip strength per effective sl ipsurface (kN)

chord wall thickness (mm)

characteristic coefficient of friction

net normal contact force (kN)

stud bolt stiffness factor = n.A~t

2L.Lb • s

Booth G S. 'Techniques for Improving theCorrosion Fatigue Strength of Plate WeldedJoints' . 3rd International ECSC OffshoreConference on Steel in Marine Structures,Delft, 1987.

4. American Welding Society. 'Specification forUnderwater Welding'. AWS D3.6-83.

1.

2.

5. Gooch T G. 'Properties of Underwater WeldsParts 1 and 2'. Metal Construction, Volume 15Nos 3 and 4, 1983.

6. UK Department of Energy. 'Grouted andMechanical Strengthening and Repair of TubularSteel Offshore Structures'. Report No OTH­88/283, HMSO, 1988.

7. Fern D T. et al. 'Bolted Repair of TubularJoi nts' . Paper 23 of second Integri ty ofOffshore Structures Conference, Glasgow, 1981.

3.

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L

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n

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Notes:: - 1.

2.

TABLE 1

REASSESSMENT OF DATA ON STRESSED GROUTED CONNECTIONS

TotalSpecimen Stud Load (A) (B) (C)

F(kN)

1/1 960 678 717.34 + 0.94521/1 1016 702 751.17 + 0.93461/1 1008 694 746.33 + 0.92992/1 1024 805 756.00 1.06482/1 1000 787 741.50 1.06142/1 984 759 731.84 1.03713/1 140 219 222.06 + 0.98623/1 142 217 223.27 + 0.97193/1 140 217 222.06 + 0.97724/1. 2392 1487 ** 1582.30 0.93984/1 2040 1347 ** 1369.70 0.98354/1 1976 1332 ** 1331.00 1. 00075/1 234 263 ** 278.84 0.94325/1 220 255 ** 270.38 0.94315/1 220 246 ** 270.38 0.90986/1 444 475 405.68 1.17096/1 376 447 364.61 1.22606/1 368 447 359.78 1.24247/1 236 304 280.05 1.08557/1 252 309 289.71 1.06667/1 256 298 292.13 1.02018/1 * 338 358 341.66 1.04788/1 * 341 304 343.47 0.88518/1 * 342 309 344.07 0.89819/1 * 413 449 386.96 1.16039/1 * 444 382 405.68 0.94169/1 * 444 367 405.68 0.904710/1 * 397 407 377 .29 1.078710/1 * 400 412 379.10 1.086810/1 * 400 391 379.10 1.0314

(Aj = P/O +33 (D/Tr1)

(B) = Prediction of (A) usingP/(l + 33 (D/T)-I) = 0.6039F + 137.5

(C) = (A) / (8)

* = long term tests

** = normalised

+ = D/T r 34

Notes: - 1.

TABLE 2

REASSESSMENT OF DATA ON MECHANICAL CONNECTIONS

Specimen K (A) (8) (C)

1/1 0.0061 0.298 0.28519 1.04491/1 0.0061 0.298 0.28519 1.03791/1 0.0061 0.295 0.28519 1.03442/1 0.0061 0.288 0:28519 1.00992/1 0.0061 0.295 0.28519 1.03442/1 0.0061 0.266 0.28519 0.93273/1 0.0061 0.304 0.28519 1.06603/1 0.0061 0.301 0.28519 1.05543/1 0.0061 0.296 0.28519 1.03794/1 0.0061 0.288 0.28519 1.00994/1 0.0061 0.291 0.28519 1.02044/1 0.0061 0.303 0.28519 1. 06255/1 0.0061 0.269 0.28519 0.94325/1 0.0061 0.275 0.28519 0.96435/1 0.0061 0.249 0.28519 0.87316/1 0.0024 0.247 0.23489 1.05156/1 0.0024 0.224 0.23489 0.95366/1 0.0024 0.233 0.23489 0.99197/1 0.0095 0.346 0.33140 1.04407/1 0.0095 0.331 0.33140 0.99887/1 0.0095 0.314 0.33140 0.9475

(A) = J.l / (1 + 20 (D/Tr1)

(8) = Prediction of (A) using

J.l / (1 + 20 (D/T)-I) = 0.2043 + 13.5 Kb

(C) = (A) / (8)

TABLE 3 - SUMMARY OF REPJ'IR TECHNIQUES APPLICABLE TO TUBULAR JOINTS

Applicable Applicable Relative Post Causes Offshore Onshore RequiresTECHNIQUE to Fatigue to Static Insta11 at ion Additional Installation Fabrication Underwater

Induced Strength Inspection Waveload Timescale Needed WeldingDefects DHects levels and Weight

Toe Grinding Yes No Moderate No Moderate No No

Hammer Peening Yes No Moderate No Quick No No

Shot Peening Yes No Moderate No Quick No No

Grouted Clamps Yes Yes High Yes Moderate Yes Only if shearkeys needed

Stressed Grouted Yes Yes High Yes Slow Yes NoClamps

Wet Welding No Yes low No Quick No Yes

Dry Welding Yes Yes low No Very Slow Yes Yes

TABLE 4 - SUMMARY OF WORK EFFORT TO REPAIR A DEFECTIVE TUBULAR JOINT

lack of Fatigue Capacity Inadequate Static StrengthTECHNIQUE

Type of Equipment Offshore Type of Equipment OffshoreRepair Needs Timescales Repair Needs Timescales

Toe Grinding Weld light Moderate Not - -Improvement Applicable

Hammer Peening Weld light Quick Not - -Improvement Applicable

Shot Peening Weld light Quick Not - -Improvement Applicable

Grouted Clamp No Clamp Moderate Moderate Clamp Moderate ModerateShear Keys Needed

Grouted Clamp Clamp Heavy Slow Clamp Heavy SlowShear Keys Applied

Stressed Grouted Clamp Moderate Slow Clamp Moderate SlowClamp

Wet Welding Not - - Applying Moderate QuickApplicable Gussets

Dry Welding Relaying Heavy Very Slow Applying Heavy Very SlowTubular Wrap PlatesJoint Weld

328

GROUT ANNULUS

BRACE

REPAIR SLEEVE BRACE

CHORD

FIGURE 1

TYPICAL GROUTED CLAMP FOR T JOINTS

CAP PLATE -j~~~~~J~

},J."I---SADDLE PLATE

STUD BOLT - ...mfjll~~~~~

SADDLE PLATE

FIGURE 2

TYPICAL DETAILS OF STRESSED GROUTED CLAMP

329

CONNECTING TWO MEMBERS USING A MECHANICAL CONNECTION

CONNECTING A NEW BRACE INTO A STRUCTURE

FIGURE; ~

APPLICATIONS OF MECHANICAL CONNECTIONS

FIGURE 5

REANALYSED DATA ON MECHANICAL CONNECTIONS

FRACTUREMECHANICS

ANALYSIS

NOENHANCEMENT

IDENTIFY CAUSE

,, ~

I...

88

Q NOT '"ISFA'''''~-LTISFACTORYI

IDENTIFY ACTUALMATERIAL

PROPERTIES

POSSIBLEENHANCEMENT

SATISFACTORY

NOTIFICATION OF EXISTING/POTENTIAL DEFECT

INVESTIGATELOCAL MEMBER

REMOVAL

I ST:P I

FIGURE 6

ASSESSMENT PROCEDURE PRIOR TO REPAIR/STRENGTHENING

SATISFACTORY

/

'.10-6039f.1l7-5111033Illr',I

TOTAt sTun LOAD F{kN)

XX

%:'. >l'

)J Mea•• 0- HIJ 110 20 lOll r'll1. 66-' Kb I

200 400 600 600 ,0,00 1200 1400 1600 1fWO 2000 2200 2400

FIGURE 4

REANALYSED DATA ON STRESSED GROUTED CONNECTIONS

HOD

200

1200

400

160a

0.05

0.00 [ ! , , ! , ! t' !! I " " I , I0.000 D.O'Ol 0.002 0,003 0,004 0,005 0.006 0.0.07 0.008 0.009

Kb

~ 1000

I'J:. 800=1-

Ao ~

.:. 6,00

Co>Co>c:>

0.35

0.30

.... 0.2'5

"L.,0.20.

- 0.15c

:C"-

0.10

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