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k- .^' C M P T

Defect Assessment inOffshore St ructures

A Procedureby

F M Burdekin FEng, FRS and M J Cowling

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CMPT Publication: 100/98

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A B O U T C M P TTh e Cen t r e f or Mar i ne and Pe t r o l eum Techn o l ogy ( CM PT ) was s e t up i n Apr i l 1997 a s anot - for -prof i t company wi th underpinning f inancia l suppor t f rom the subscr ip t ions ofindus t ry members . CMPT provides i t s member companies in the ups t ream oi l and gasindus t ry wi th access to advances in sc ience , engineer ing and technology (SET) tha t canbr ing s igni f icant benef i t to the i r bus iness . I t suppor ts the providers of research andinnovat ion in br inging the i r capabi l i t ies and products to the indus t ry , he lp ing them toensure t imely and ef f ic ient de l ivery and re levance to indus t ry ' s needs .C M P T opera tes a v i r tua l " research and inn ova t ion t radin g f loor" deal ing in inform at ion tha tw i l l connec t wha t i ndus t r y needs w i t h wha t t he r e s ea r ch commun i t y and t e chno l ogycompanies can provide , and wi th what the publ ic sec tor can do to ass is t .

A B O U T T H E O F F S H O R ED I V O N O F T H E H A N D

The responsibil i t ies for regulating health and safety offshore were unified in a single body -the H eal th and Safe ty Execut ive - as a resul t of the rec om me nda t ions of the Lord C ul le nenquiry in to th e Piper Alp ha d isas ter . A n ew Offshore Safe ty D ivis ion was se t up as a resul t .Research has p layed an impor tant par t in ensur ing safe ty in the Nor th Sea and wi l l cont inueto do so . A major a im is to under take an in tegra ted programme of projec ts which addressboth the s t ra tegic or gener ic inves t iga t ion of of fshore hazards and the re la ted shor t te rmneeds , for example to support safety case assessment. A risk-based research strategy has beendeveloped by HSE wi th input f rom the Divis ion ' s Research St ra tegy Board to provide amea ns of pr ior i t i s ing research effort an d help in g to ensure tha t va lue for mo ney is obta in ed.

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Contents

A c k n o w l e d g e m e n t sSummar y1.0 In t roduct ion2.0 Stru ctura l analysis

2 .1 Ou t l ine procedure2.2 Dat a required2.3 Modelling

3.0 S-N fatigue design3.1 In t roduct ion3.2 Dat a required3.3 Partial Safety Factors3.4 Asse ssm ent of reliabili ty3 .5 Categor isa t ion3.6 Re-assess inent of a single join t3 .7 Li fe time inspect ion/ repai r p lann ing3.8 System reliabiUty

4.0 Detaile d fracture me cha nics assessinent for fracture4 .1 In t roduct ion4.2 Data required4.3 Fracture assessme nt princ iples4.4 Fracture assessme nt in tubu lar join ts4.5 Safety factors

5.0 Detaile d fracture me cha nics assessmen t for fatigue5.1 In t roduct ion5.2 Dat a required5.3 Fatigue assessme nt princip les5 .4 Procedure5.5 Partial Safety Factors

6.0 System reliabili ty6 .1 In t roduct ion6.2 Join ts wit h equal failure proba bili ty6.3 Join ts with different failure probabili ty

7 .0 Inspect ion and repai r p lann ing8.0 References/bibliographyC o m m e n t a r yCl . O I n t r oduc t i onC3.0 S-N fatigue designC4 .0 Fracture me cha nics assessment for fatigue

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A C K N E N T SThis Procedure resul t s f rom a progra inme of research (The Defect Assessment in Offshore St ructures ManagedPr og r amme) j o i n t ly f unded by gove r nm en t and i ndus t r y unde r t he aus pi ce s o f t he Mar i ne Techno l ogy Di r ec t o r a t e( M TD ) L t d ac t i ng as agen t s f or t he Sc i ence and Eng i nee r i ng Res ea r ch Counc i l ( SE RC ) , now t he Eng i nee r i ng andPhys i ca l Sc i ences Res ea r ch Counc i l ( EPSRC) . MTD has now been abs o r bed i n t o CMPT ( The Cen t r e f o r Mar i neand Pe t r o l eum Techno l ogy L t d ) .Most of the research in the Managed Programme was car r ied out a t e ight univers i t ies wi th the major i ty of thedev elop me nt of th is Procedure being performed a t U M IS T and the Univers i ty of Glasgow. Sponsors of theMa naged P rogram me inclu de AE A Technology, BP, Br i t i sh Gas , Br i t i sh Stee l , Brown &. Roo t , Con oc o (U K) , Ear l& Wright , the Heal th & Safe ty Execut ive , the Minis t ry of Defence , Mobi l Nor th Sea , River Don Cas t ings , Shel lExp l o r a t i on and P r oduc t i on , S i r Wi l l i am Ha l c r ow and Pa r t ne r s , The We l d i ng I n s t i t u t e , and Ve r i t e c .This Procedure arose f rom the del ibera t ions of a sub-group of academics and indus t ry sponsors who formed aMe t hod o l ogy Wor k i ng Gr oup . Spec i a l me n t i on s hou l d be made o f t he i nd i v i dua l con t r i bu t i ons o f Dr M F L i gh t(C ono co) w ho chai red th is group, and the ass is tance g iven by Dr J Haswel l (Br i t i sh Gas ) . Th e help and suppor tp r ov i ded by t he Managed P r og r amme S t ee r i ng Co mm i t t ee Cha i r ma n , Dr B Tomk i ns ( AE A Techno l ogy ) i sgra tefully ackno wledg ed. Th e publ ica t io n was edi ted for CM P T by Judi th MirzoeffAnyone in teres ted in fur ther de ta i l s on th is work should consul t the Programme Manager :Professor M J CowlingGl a s gow Mar i ne Techno l ogy Cen t r eUnivers i ty of GlasgowG l a s go w G 1 2 8 Q QT el: + 4 4 ( 0 ) 1 4 1 3 3 9 0 9 69F ax : + 4 4 ( 0 ) 1 4 1 3 3 0 4 0 15

NoteThe Offshore Safe ty Divis ion of the Heal th and Safe ty Execut ive has suppor ted publ ica t ion of th is repor t toencourage d isseminat ion of the resul t s of research tha t has received publ ic funding. This suppor t does not implyau t oma t i c endo r s emen t by t he HSE o f any o f t he t e chno l og i e s de s c r i bed .Every reasonable ef for t has been made to ensure tha t th is docum ent i s based on the bes t knowled ge avai lable up tothe t ime of finalising the text. However, no responsibil i ty of any kind for any injury, delay, loss or damagewhatsoever , however caused, resul t ing f rom the use of the Procedure presented can be accepted by CMPT, thesponsors or o thers involved in i t s publ ica t ion .

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S U M M A R YThis review provides an assessment methodology procedure for defec ts in of fshore s t ruc tures , to supplement theprocedure given in BSI document BS 7910:1998, Guidance on methods for assessing the acceptabilify of flaws m fusionwelded structures. I t is prese nted in a similar format. T he Proce dure is a set of routin es for establish ing th e structu ralintegrity of welded joints in steel offshore structures, based on current research and best industry practice. I t isoffshore specific in that i t deals with the special characterist ics of such structures in terms of detailed design and thenature of the loading in such appl ica t ions .Routines for assessment of individual joints are combined into system reliabili ty assessments and an overall schemefor inspect ion an d repai r . Th e P rocedure i s suppor ted by a Com me ntar y t ha t g ives the back ground to sec t ions of theme t hodo l ogy .

BS 79 10: 199 8 (d eriv ed from PD 6 49 3:1 991 ) G uidanc e on methods for assessing the acceptabilify of flaws in fusion weldedstructures (ref 8) contains well established routines for the assessment ofthe significance of defects in weldeds t ructures .The present Procedure document has been produced to provide more deta i led and of fshore speci f ic guidance ,inc luding a background Com men tary , th an can be inc luded in BS 7910:1998. This Proced ure i s based on aprogramm e of s ix years ' jo in t go vern men t and indus t ry funded research (T he D efect Assessment in OffshoreSt ructures Managed Programme) under the auspices of MTD. I t has been extended to inc lude subsequent worksponsored by HSE a t UMIST and recent developments associa ted wi th the la tes t revis ion of BS 7910:1998.Th e Pro cedur e presente d in Sect ions 2 to 7 is a specific set of rou tines for establis hing th e structur al integrity ofwelded joints in steel offshore structures. I t is offshore specific in that i t deals with the peculiar characterist ics ofsuch s t ruc tures in te rms of de ta i led des ign and the nature of the loading in such appl ica t ions .The fea tures of the Procedure which cont r ibute to th is approach inc lude:i) Results frotn a very large nu mb er of add ition al stress analyses of various (particularly tub ular) join ts co mm onl y

found on offshore structures,i i) A n in- dep th study of the influen ce of detaile d weld geometry and its effect on local stress distrib ution s and

hence in tegr i ty ,i i i ) Unique t rea tments of res idual s t ress d is t r ibut ions in appropr ia te jo in ts ,iv) New informat ion on the u l t imate load car ry ing capaci ty and hence reserve s t rength of tubular welded jo in tscommonly found on of fshore s t ruc tures ,v) T he use of reliabili ty-based rou tines wh ich ena ble structural in tegrity to be assessed against target levels of

performance, which are geared to the consequences of fa i lure ,v i ) Genera l compat ib i l i ty wi th BS 7910:1998.Th e Proce dure i tself is no t founded simply on the results of the rec ent sponso red researc h but draws to get hetappropr ia te informat ion f rom a wide range of re levant sources . To th is extent the Procedure a ims to be the bes tcurrent ly avai lable in te rms of the overa l l knowledge base . Gen era l s t ruc tura l engin eer ing guid ance i s publ i shed onmethods for assess ing defec ts in welded s t ruc tures , in the form of BSI Document BS 7910:1998. This document wasupdated f rom the previous BS PD6493:1991 and provides a useful compar ison wi th the Procedure descr ibed here .Given tha t the revis ion of BS PD6493:1991 has been known of (and cont r ibuted to) for some t ime, and nowincludes an offshore specific Annex, i t seemed sensible to present this Procedure in a format similar to BS7910:1998. This provides users wi th a reassur ing fami l iar i ty and permi ts de ta i led d i rec t compar ison. This Procedurerefers to BS 7910:1998 at several points, recommending the use of clauses in the BSI docuinent in some cases, butgives mo re specific guida nce wh ere appro priate for oth er cases. Th is is an exam ple of bring ing tog ethe r the results

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' ^ ^f rom the Defect Assessmen t in Offshore St ructures Mana ged Pro gramm e wi th informat ion f rom elsewhere toproduce a uni f ied , but much improved, comprehens ive approach. An impor tant fea ture of the Procedure i s theaccompanying deta i led Commentary which provides background informat ion for the speci f ic guidance g iven in thePr ocedu r e .Sec t ion 2 of th is Procedure represents s tanda rd indus t ry prac t ice for s t ruc tura l analys is , wi th add i t ional warningsregarding obscure factors of safety. This is important to ensure that the resulting data are in a satisfactory anduniform form for the subsequent assessment procedures .Sect ion 3 incorpora tes the t radi t ional fa t igue endurance (S-N) des ign rout ines wi th opt ions to a l low the user toinclude a reliabili ty based analysis. These options vary from a simple application of partial safety factors(mul t ip l iers ) to , for exainple , the assumed s t ress range , to more complex rout ines a l lowing the user to es t imate theeffec ts of uncer ta in t ies in the var ious inputs to the analys is . The overa l l objec t ive i s to provide the user wi th a toolto ca lcula te a fa tigue l i fe t ime for a pred eterm ined level of re l iabil i ty . Gu ida nce is g iven o n the se lec t ion of th ispred eterm ined level of safe ty and the effects of s t ruc tura l redu ndan cy in the re la t io nship be twee n individ ual w eldedjoint and structural system reliabili ty.Sect ion 4 incorpora tes the la tes t f rac ture mechanics based assessment for f rac ture in welded jo in ts in of fshoreappl ica t ions . The sequence of the individual s teps in the analys is fo l lows the pat tern of BS 7910:1998 but inc ludesthe evaluat ion of much grea ter de ta i l in the s t ress d is t r ibut ions in the v ic in i ty of a defec t .Sect ion 5 g ives a s tep-by-s tep descr ip t ion of the f rac ture mechanics based fa t igue assessment for tubular weldedjoin ts con ta in i ng a crack- l ike defec t . Th is par t of t he Procedu re i s suppor ted by a specia l ly develop ed sof twarepackage . Th is software i s recom me nded because of the rep et i t ive natu re of the ca lcu la t ions and t he s ize of the datase ts which are accessed. Sect ion 5 accesses a la rge amount of new data genera ted dur ing the Defect AssessmentM a n a g e d P r o g r a m m e .Sect ion 6 g ives guidance of the assessment of sys tem re l iabi l i ty , appropr ia te ta rget levels and the re la t ionships toindividual component (welded jo in t ) re l iabi l i ty .Sec t ion 7 describes how to com bine th e rout ines wi th in Sect ion s 3 to 6 to produce a schem e of inspe ct ion andrepair.