44.Offshore Construction

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Offshore

Construction44Goodfellow Associates Ltd

Contents

44.1 Introduction 44/3

44.2 Offshore structures 44/3

44.2.1 Jack-up rigs 44/3

44.2.2 Fixed platforms 44/3

44.2.3 Floating platforms 44/3

44.3 Stages of construction 44/6

44.3.1 Fabrication 44/6

44.3.2 Launching 44/8

44.3.3 Transportation at sea: marine operations 44/9

44.3.4 Installation 44/9

44.3.5 Hook-up and commissioning 44/9

44.4 General factors affecting construction

techniques 44/10

44.5 Concrete structures 44/10

44.5.1 Types 44/10

44.5.2 Major requirements 44/11

44.5.3 Concrete construction 44/11

44.6 Construction in the arctic 44/12

44.6.1 Environmental conditions 44/12

44.6.2 Types of arctic structures 44/12

44.6.3 Construction 44/13

44.7 Fabrication/construction facilities 44/13

44.7.1 Fabrication yards 44/13

44.7.2 Dry docks 44/13

44.7.3 Slipways 44/13

44.7.4 Offshore fabrication platforms 44/13

44.7.5 Back-up facilities 44/13

44.8 Analysis 44/14

44.9 Schedule of work: cost factor 44/14

44.10 Codes and regulations 44/14

44.11 Organization and management of offshore

projects 44/15

44.11.1 Project requirements is 44/15

44.11.2 Project organization 44/15

44.12 Inspection, maintenance and repair 44/15

44.12.1 Underwater cutting 44/16

44.12.2 Underwater welding 44/16

44.12.3 Grouted clamps 44/1644.12.4 Concrete repair 44/16

44.13 Cathodic protection 44/16

44.14 Removal of platforms 44/16

Acknowledgements 44/17

Bibliography 44/17


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44.1 IntroductionUn d ers tan d in g offshore construction opera tions requires somefami l iar i ty w i t h th e type an d f or m of the structures involved.For r e a de r s who a r e no t famil iar with such structures, section44.2 briefly describes th e ir genera l fo rm and f u n c t i o n .O f f shor e structures are dominated by oi l and gas productionfacilities as e xp lor a t ion fo r hydr oca r bo ns e x te nde d f ro m land toshal low w aters and mo ved to deeper and more ho sti le enviro n-m en ts such as the North Sea.Other types o f offshore structures include cargo and off load-in g t e r mina l s , offshore wind tu r b ine s , oce a n the r ma l e ne r gyfacilities, mil i tary an d defence-re la ted structures an d some nove lf loat ing structures fo r le isure o r other purposes.T he co ns t r uc t ion a nd ins ta l l a tion te chn ique s va ry de pe ndingon the types o f s t r uc tu re s invo lve d , but in this chapter sometypical examples, mostly re la ted to the oi l industry, a re intro-duced and give a good representa tion of the methods andactivities invo lve d .Construction methods for both s tee l and concrete s tructuresare descr ibed. Reference is ma de to the genera l factors affectingth e techniques wi t h particular re ference to cost , safety andpract ical i ty of operations.

Offsh o re ope r a t ions involve a we l l -p la nne d p r ogr a mme o fw o r k and project organization wi t h effective c o n t r o l an dmanagement. This subject is briefly discussed to de mons t r a te it simpor ta nce in a mult idiscipl inary ope r a t ion o f great com plexity .!Finally, reference is m a d e to codes an d r e gu la t ions andoperations involving inspection, maintenance and repair ofoffshore structures.I t is hoped tha t the reader wil l ga in a ge ne r a l unde r s ta nd ing ofoffshore construction techniques and their impact on var iousfields o f engineer ing b y the examples given.T h e subject h as been addressed purely as an in t r oduct ion tothis topic and readers who are interested in extending theirkn ow led g e f ur t her have access to n um erou s publ ica tions includ-in g those me nt ione d in the b i b l i o g r a p h y at the end of th ischapter .

44.2 Offshore structuresT he o i l indus t r y on ly be ga n to move o f f sh o re in the late 1940s.O f f shor e operations were f irs t carr ied out in the US, where agr a dua l move cou ld be ma de f ro m the swa mps of Lou is ia na .Explora tion resul ts there indicated tha t the oi l a rea extendedoffshore in to the sha l low wa te r s o f the Gu l f o f Me xico . T hemob i le ja ck-up dr i l l ing un i t w as origina l ly developed fo r thisregion.44.2.1 Jack-up rigsT he j a ck-up u ni t is a barge f i t ted with movable legs (Figure44.1). Th e uni t can be towed or se lf -propel led f rom site to sitewi t h the legs in an e levated posit ion. Once a t a dr i l l in g location,the legs can b e lowered to the sea-bed and th e b arge can ' jack'itself up the legs so that it comes out of the water, clear of anyanticipated wave action, ready fo r dri l l ing. When th e wel l isf inished, the opera tion is reversed to make the barge ready form ovin g to i ts next location. Th e length o f the legs determines thewater depth in which th e jack-up c an b e used, b ut they arecomm only designed fo r use in up to 75 m o f water and occa-sional ly as much as 105 m. Reaso nably ca lm w eather is requiredw h e n the units are be ing jacked up and dow n.

In order to enable offshore dri l l ing to be carr ied out in thedeeper w aters (e .g. in the Gulf of M exico) , semisubm ersible anddri l l - sh ip dri l l ing units were developed.

44.2.2 Fixed platformsOnce explora tion dr i l l ing has conf irmed the existence of an oi l -or gasfield, appraisa l dr i l l ing is usual ly required to show if it isla rge enough to be developed commercia l ly . Fie ld developmentca l ls for the dr i l l in g of a series of production wel ls and theins tal lat ion o f e qu ipme nt to cont r o l th e pr oduct ion . T h e usualmethod is to insta l l a f ixed pla tform and to dr i l l devia tedprod uction w ells fro m it. Deviated w ells are drilled inclinedf rom th e vertical and in a d i r ec t ion a wa y f r o m th e pla tform toreach par ts of the reservoir as far away f rom the pla tform aspossible . So metimes sa te ll i te w el ls are dr i l led up to 10 km aw ayand tied back to the p la t f or m b y pipeline. Both steel andconcrete p la tform s have been used in the N orth Sea in a var ie tyof designs. The f irs t f ixed pla tforms instal led in U K w aters wererelatively small uncomplicated stee l s tructures for the southernN o r t h S e a gasfields in water depths up to 4 5 m . These havebecome dwarfed by those subsequently insta l led in the nor thernNorth Sea oi l- and gasf ie lds, in water depths of up to 1 8 O m .T he se ha ve ove r a l l he ights o f a r o u n d 275 m f ro m th e sea-bedand are a b l e to w i t h s t a n d s t o rm w a v e s 30 m h i g h an d w i n d s o f2 4 0 k m / h .A stee l p la tform consists o f a f r a me w or k ca ll ed a ' jacket ' o nw h i c h a deck is moun ted (Figure 44.2) . The jacket is fabr ica tedo n s h o r e an d towe d out to sea on its side, e i ther a f lo a t or on ala rge barge . On reaching i ts location, i t is careful ly up-endedand secured b y piles driven into th e sea-bed. Once this h as beencompleted, the deck is insta l led and modules conta ining thedri l l ing, productio n and accomm odation faci l it ies are added.Concrete p la tforms vary considerably in design and conse-quen t ly in me thod o f construction (Figure 44.3) . Normally , abu oy ant base is buil t in a dry dock and f loa ted in to progressive lydeeper water as the s tructure is buil t up f ro m it. This requiressheltered, deep water close to shore . T h e w e i g h t o f a concretepla t fo rm is severa l hundred tho usand tonnes greater than a s tee lpla t fo rm . A concrete p la tform is f requently designed withchambers for oi l s torage. When completed, with a superstruc-tu r e con ta in ing dr i ll ing , p r oduct ion a nd a ccommoda t ion facili-ties, it is tow ed ou t b y a num b er of large tugs to its location. It isthen b a l lasted do w n unti l i t rests on the sea-bed w here i t remainssecure under i ts own weight. Concrete p la tforms are conse-quen t ly cal led gravity p la tforms. All the f ixed p la t f or ms a r e ,t he re fo re , bottom-supported structures.A n o t h e r approach developed for deeper waters is the guyedtower (Figure 44.4) . The pla tform deck is supported by al ightweight stee l compliant tower , he ld upr ight by guy l inesradia ting outwards. This type of p la tform has been used for afield in th e Gul f o f Mexico.44.2.3 Floating platformsBecause of the very large cost of f ixed pla tforms and thepossib i l i ty of f inding oi l in waters which are so deep that f ixedpla tforms would neither be technica l ly feasible nor economical ,considerable a t tention h as been given to developing oilf ields b yothe r me thods .O ne approach is to use a f loating pr oduct ion p la t f or m.However, it is necessary to restrict la tera l and ver t ica l move-me nts to a m i n i m u m , so as to avoid unacceptable loads on thehigh-pressure ver t ica l pipes known as 'risers' wh ich p rov ide thel ink between the pla tform and the wel ls on the sea-bed.T h e semisubmersible rig is a floating p la t f or m wi th th e decksupported b y vertical columns o n submerged p o n t o o n s w h i c hprovide i ts buoy ancy (Figures 44.5 and 44.6) . By varying thequanti ty of ba l last water in the p o n t o o n s , the rig can be raisedo r lowered in the wa te r . T h e l o w e r th e pontoons li e beneath th ew ater the less they are inf luenced by w ave action. Th is reducesver tica l movem ent and al lows dr i l l ing o r pr oduct ion to cont inue


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F i g u r e 4 4 .1 A t y p i c a l c o n f ig u r a t io n o f a j a c k - u p r ig

F i g u r e 4 4 . 2 A t y p i c a l c o n f i g u r a t io n o f a c o n v e n t io n a l f ix e d s te e lj a c k e t ( p l a t f o r m )


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F i g u r e 4 4 . 3 A c o n f i g u r a t i o n o f a g r a v i t y -b a s e c o n c r e te p la t fo r m

F i g u r e 4 4 . 4 A t y p i c a l c o n f i g u r a t i o n o f a g u y e d t o w e r


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in r ough seas. A semisub mersible r ig is no rmally h e ld in posit ionb y up to twelve very large ancho rs. The design o f the la testsemisubmersible r igs enables them to dr i l l in UK waters a tdepths of 4 5 O m and over , a l l the year round, despite theexceptional ly high waves exper ienced in w inte r . Semisubme r -sible pla tforms can a lso be designed as production facilitiesequipped with process e qu ipme nt .Anchored semisubmersible units used fo r dr i l l ing o r bu i l tw i th production and accom mo dation faci li t ies are in use aroundt h e w o r l d . A n o t h e r f loat ing technique is the use of a tension legpla tform (TLP) w hich is a semisubm ersible type of un it , he ld inplace b y tensioned cables anchored to the sea-bed immediate lybeneath each corner of the pla tform. The pla tform is ba l lastedd o w n w hi le the cables are a t tached and then debal lasted,b r in g in g th e ca b les und e r te ns ion . T h e p l a t f o r m m o v es l ike aninverted p e n d u l u m , w i t h very l i t t le heave. See Figure 44.8.O the r te chn ique s inc lude the use of specia lly bu il t sh ip-shapedvessels, conve r te d ta nke r s an d f loat ing concr e te p la t f or m s .

44.3 Stages of constructionOffsh o re construction can b e categor ized into f ive m ain stages:(1) fab r ica tion; (2) launch ing; (3) tow -out; (4) insta l la t ion;a nd (5 ) hook-up a nd commiss ion ing .

44.3.1 FabricationIn this section, construction of steel structures is discussed inor de r to h igh l igh t the m a in ta sks involve d . Con s t r uc tion ofconcrete structures is covered in section 44.5.Fabrication of steel jackets is generally carried out in land-based fab r ica tion yards w hich have access to w a t e r w a y s , or theopen sea . Such faci l i t ies are in some w ays similar to those in thesh ipbu i ld ing indus t r y w i th d r y docks a nd s l ipw a ys a l lowing thevessels to b e e ve ntual ly la unche d upon comple t ion .

Size and weight of s tructures vary considerably and as aresul t , some can be fabr ica ted in only a l imited numb er of yardsw hic h have suitable faci l i t ies with sufficient dr a ught a long thew a t e r w a y s for the ir t ranspor t .Some typica l s izes and w eights of the jackets are :(1) Steel jacket , Thist le A , Nor th Se a : j a cke t we ight 31 3961,w a t e r de p th 161 m.(2 ) Steel jacket , B rent A, Nor th Sea: jacket w eight 14 22 5 t,w a t e r depth 14Om (Figure 44.7).(3 ) Steel jacket , Indefa tigab le CD, south ern N orth Sea: jacketw eig h t 5361, w ater depth 29 m.The wor ld 's ta l lest exist ing pla tform is the Cognac stee l jacket

F i g u r e 4 4 . 5 A g e n e r a l v i e w o f a s e m i s u b m e r s i b l e drilling p l a t f o r m


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pla t fo rm w i th a he ig h t o f 3 8 5 m . How ever , t he B ul lw in k lep la t fo rm , w h ic h is of a similar design to the Cognac, w i l l b e4 9 2 m ta l l when ins tal led in 1988. This plat form wil l t hen b e49 m ta l ler than th e world 's ta l les t bui ld ing. This record will nod oub t b e b roken ag a in in future years .Limited dimensions and handl ing capaci t ies o f fab r ic a t iony a r d s and dry d oc ks m ay resul t in the need to fab r ic a te th es tructures in m ore than o ne piece. In ad d i t ion , parts of thepla t fo rm m a y b e fabricated separately in o ther yard s . T h e partswil l t hen b e b roug h t tog e ther and mated under separate opera-t ions . Deck s tructures of jackets and mo dules are of ten fabr i -cated and assembled separately. These modules , which couldweigh f rom un d er 501 t o a few thous an d ton n es a re t ran spor tedand are l ifted and installed on the deck of the pla t fo rm s , us in gcrane barges , when th e deck is ins tal led . As an example, th eto tal topside weight of the BP Magnus pla t fo rm , w h ic h c on s is tsof a mul t is to rey deck 75 m square and 32 m h igh, is in the ordero f 3 1 0 0 O t .T o m in im iz e cost, th e m axim um pos s ib l e w ork o n fabrica-t ion, assembly, tes t ing, inspect ion and ins tal lat ion o f var iousc om pon en ts is carried o ut in land. Costs o f of f shor e construction

opera t ion s a re significantly h ig her than the l an d -b as ed w orkand are therefore l imited to essent ial tasks which cannot b ec arr ied out in an y o ther w ay.Fo r fab r ic a t ion o f s teel s t ructures , welding tubulars rangingf r om 3 0 0 m m t o 2m d iam eter o r m ore , an d w i th vary in gthickness of up to 80 mm is involved; an exam ple is the BPMagnus p l a t f o r m in w h i c h tw o o f i t s f o u r legs each has ad iam eter o f 10 . 5m . Weld in g s uc h l a rg e s t ruc tures requi resefficient autom at ic w e ld in g tec hn iques w i th qua l i ty c on t ro l an dstress-relieving in many cases .Fab r ic a t ion o f n od es c on s i s tin g o f s evera l tub u la r m em b ers o fdifferent s izes is one of the most complex parts of the weldingoperat ion. Techniques o f cast ing nodes have been developedw hich enhance their load-carry ing capaci ties b y el imin at inghigh w eld in g s tress es an d s t ream l in in g an d s t ren g the n in g thejoint s t ructure. The design of tubular jo in ts is d iscussed in apub l ic a t ion b y t h e Un d erw ater En g in eer in g Group of the C on -struct ion I n d us t ry R es earc h an d I n fo rm at ion As s oc ia t ion ( s eeB ib l iog raphy) .C overed fab r ic a t ion faci li t ies are ava i l ab l e to a l l ow w ork tob e independent o f w eather c on d i t ion s .

F i g u r e 4 4 . 6 A d y n a m i c a l l y p o s i t i o n e d s e m i s u b m e r s i b l e drilling r igi n o p e r a t i o n

T H R U S T E R S


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F i g u r e 4 4 . 7 S t e e l p r o d u c t io n p l a t fo r m B r e n t A o p e r a t e d b y S h e l la n d E s s o in 1 4 O m o f w a t e r i n t h e N o r t h S e a . ( C o u r t e s y : S h e l l )

Steel structures ar e fabr ica ted in sections which can beaccommodated and handled in the yard. Close tolerances arerequired to enable mating with other sections. Inspection andqua l i ty cont r o l be come in te gr a l par t s of the fabr ica tion opera-t ions, as these structures are required to withs ta nd h igh loa d ingcondit ions with theore tica l fa t igue life e qu iva le n t to 10 timestheir service life.Failures of welds resul t ing f ro m b a d w o r k m a n s h i p , u n p r e -dicted loading condit ions an d p o o r tolerances have providedlessons to the industry, resul t ing in br inging about improve-me nts in welding techniques, more extensive nondestructivetesting and a t te n t ion to deta i l ing o f structures.Large-size structures are genera l ly fabr ica ted and assembledo n pre-installed trestles and rails to enable th e next stage of theope r a t ion , which i s l a unching a nd tow-ou t , to ta ke place.44.3.2 LaunchingWhen fabr ica tion is complete on land, the s tructure is t rans-ferred to w a te r wa y s f or towing a nd t r a nspo r ta t ion to i t s o f f -shor e de s t ina t ion . T h e m e t h o d o f l a unching de pends on the sizeand w eight of the s tructure and the faci l i t ies used for i tsconstruct ion.

44.3.2.1 Load-out from quaysLighter s t r uc tu r e s , o r those which , be ca use o f the d r a ughtl imitat ions of the w a te r wa y s , a r e f a br ica te d on qua y s , a r emove d on to f la t-top barges (moored against th e l oa d-ou tquays ) for transpor t ing to sea . Limita t ions of cranes in fabr ica-t ion y a r d s to ha nd le we ights r a ng ing f rom a few h u n d r e d tosevera l thousand tonnes require th e completed structures to b et r a nspor te d slowly on ra i ls or bogies and loaded on to thebarges. Alternative ly, they c an b e supported o n pads, each o fw h i c h f loa ts on a cushion o f w a t e r o r oi l , using th e principle o fhydraul ic or air f lo tat ion. Re duct ion in f r ict ion, as the resul t o fpads f loat ing on wa te r or oi l cushions, enables th e structure tob e w i n c h e d on to the ba r ge w i th r e la t ive ly sma l l pu l l ing loads .M od ular tra i lers w ith over 700 w heels and capacit ies o f up to1 2 0 0 O t or more have been used for this purpose . Bogies a lsoe na b le the loa d to be d is t r ibu te d to l e ve ls w i th in the loa d-be a r ing capacity of the qua ys ide which is often be low 5.5 t/ m 2 .T h e barge-loading opera tion requires powerful ba l last ingfacilities on barges so tha t they mainta in the ir level against thequays id e unde r cha nging t ide s a nd gr a dua l t r a ns f e r o f the loa do n to the ir f la t decks.Barges with sufficient deck capacities need to be fitted w i t h


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sea-fastenings to secure th e structure during transportation.Extensive deck stiffening is sometimes required to enable thebarge to hold its load safely.44.3.2.2 Load-out from dry docksLoad-out from dry docks requires f looding of the docks a l low-in g the structure to f loat. Limitation in the draught in the drydocks often requires the operation to take place within thelimited period o f high tide. The floating structure is towed out ofthe dock by tugs for transport to sea.44.3.2.3 Launching from slipwaysTh e completed structure rests o n a n u m b e r o f rails w hich extendalong the sl ipway into water , similar to the method used in theshipbuilding industry. Th e structure is freed from it s trestles fo rlaunching, and is gradually winched and al lowed to move intoth e water unt i l it f loats. This technique is particular ly suitab lefo r structures w hich are too heavy t o be transported o n barges,o r have excessive draught and need t o be fabricated an d loaded-out in yards c loser to the open sea. T h e BP Magnus, a self-f loating jacket and piles w eighing 42 0001, was launched in th isw ay f rom the Highland Fabricators' yard in Scotland in 1982.44.3.3 Transportation at sea: marine operationsTransportation of structures, whether floating or transportedon f lat-top barges, is carried out b y a n u m b e r o f tugs. Th e tugsposition themselves in a 'star' fo rmat ion , providing th e p o w e rand control l ing the movement of the structure a long its pre-determined path.Suitab le weather windows are required to ensure th e safety o fthe structure during transportation. The speed of the tow islimited to a f ew kno ts and, depending o n the distance, m ay takea ny th ing f rom several hours to a few days . At the destinationand prior to i ts instal la tion, intensive survey and inspection ofth e sea-bed, subsea template and other structures is made.Back-up facilities are mobil ized, and tr ia l runs are und ertakento ensure that the f inal crucial stages of the operation passw i t h o u t difficulties.A t th is stage, support vessels carrying power, personnel,equipment , divers and inspectors are at th e site to carry ou t thehighly c ontro l led and co-ordinated operation o f setting th estructure in i ts f ina l position. Sonar systems and satellites ar eused to monitor the position of the structure and help to hold i tin a position wi t h i n th e small a l lowed to lerances, which varyf rom a few metres for the first stage o f station-keeping to a fewmil l imetres for the f ina l setting stage in to th e support structuresor templates.44.3.4 Installation44.3.4.1 Installation of the main structureThe method of instal la tion varies and depends on the type ofstructure. For floating, bottom -supported, steel jackets acon-tro l led up-ending operation is carr ied out fo l lowed by fur therballasting; th e structure is then lowered on to the sea-bed.Mainta ining structural safety and stabil i ty during the up-ending operation is crucial . This stage is therefore a well-investigated and tested operation during w hich the movem ent ofthe structure is a lso helped b y l ines from tugs and crane vessels.Structures transported on barges may be lifted either byheavy-lift crane barges and lowered on to the sea-bed, o rsubmersib le barges may be used if they remain af loat. Submer-sible barges can,b y a process o f ballasting, have their draughtincreased until th e structures they carry f loat freely and aretowed c lear .

An a lternative method is to launch the structure directly froma barge equipped with a launching frame at its stern.Crane barges ar e used to drive piles around th e legs of thejackets; piles are guided by pre-installed sleeves arou nd the legs.Pile-driving techniques now al low the use of underwater pilehammers w i th high driving capacities of 200 tonf and bey ond.Piles driven fo r the BP Magnus jacket are a typical example ofth e support system, being 10Om l o n g w i t h 2.1 m diameter an d63 m m plate th ickness. The 36 piles have been driven b y t w o o fMenck's (MHU 1700) underwater hammers, delivering a strik-in g energy of 170 tonf . Each pile has been designed to tak e loadso f up to 60001.The techniques explained in th is section are examples ofinstalling fixed jackets. The installation of other types of struc-tures such as templates, articulated columns, floating structuressuch as semisubmersib les, and TLPs are all different, w i t hdiffering levels of complexity.In th e case o f TLPs (Figure 44.8), fo r example, instal l ing an dtying the tethers to the ir templates on the sea-bed is a complexand len gthy operation. I t can be carr ied out f ro m the p la t fo rmitself o r, a l ternatively, b y pre-insta l l ing th e tethers using craneb arges and f ina l ly mat ing and ty ing them to the main s t ruc tureas a second operation, h as b een shown t o be more ec onomic a l.44.3.4.2 Installation of secondary components (topside)For insta l l ing other components or parts of a productionpla tform such as the deck structure and some subsea compo-nents, heavy-lift cranes are used. Fixed jackets could have deckstructures weighing several thousand tonnes which are trans-ported separately on flat-top barges. The deck is lifted b y o n e o rtwo cranes and is instal led on top of the support structure.Various modules, part of the hydrocarbon production facil i t ieson the deck, and each weighing from a few hundred to a f ewt housand tonnes , are also transported separately and, using acrane barge, are installed on the deck.Early insta l la tion of a l l modules on the deck is not oftenfeasible because of weight l imitations of barges and stabil i typrob lems dur ing t ranspor ta tion at sea.In th e BP Magnus pla t fo rm, the to ta l tops ide pay load o f thestructure w as 31 0001divided into 19 modules eac h w eigh ing upto 22001, some 40 to 50 m lon g.44.3.4.3 Installation of secondary components (subsea)Fo r instal la tion o f subsea modules such as templates o r m a n i -folds, often accurate positioning and mating with existingsubsea structures are required. In such conditions, a guidancesystem is required in addition to cranes to co ntrol their low eringand positioning. Tensioned guide wires combined with guideposts are examples of the methods used for the control ledlowering and po sitioning of the modules subsea. The guide w iresare tensioned by winches f ro m the instal la tion vessel and thelines are tied subsea to the guide posts. The component which isbeing instal led is equipped w ith funnel-shaped guidance sleeveswhich are engaged on to the guide wires and which enable theuni t to be lowered to i ts position guided by the tensioned l ines.A ll operations are c losely monitored by divers or remotelyoperated vehic les (ROVs) carrying underwater te levisioncameras.44.3.5 Hook-up and commissioningT h e term 'hook-up' refers to the operations which l ink th evarious components and parts of the o f f sh o re facilities w h e nthey are all installed.Tying the subsea pipelines to the platform risers, installingand ty ing umbilical l ines, cabling and pipew ork to complete the


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F i g u r e 4 4 . 8 A g e n e r a l c o n f i g u r a t i o n o f a t e n s i o n le g p l a t f o r m

l inking of the topside modules w hich have been transpor ted andinsta l led on the deck separa te ly , a re examples of the hook-upoperation. This work requires a wel l-co-ordinated mult idiscip-l inary taskforce with back-up faci l i t ies such as cranes, servicevessels, divers and ROVs.Several thousa nd ma n-hour s are required to complete thisstage of the w o r k and commission th e faci li t ies . With of fsh oreman hour ra tes ranging f r om 5 to 10 t imes tha t o f land-basedope r a t ions , hook-up and commiss ion ing are costly operationsw h i c h need to be minimized as much as possible.

Th e ma npowe r r e qu i r e d fo r such an ope r a t ion of f shor e couldru n to over 1000 m en fo r ma j or p r o j ec ts. T e mpor a r y a ccommo-d at ion an d t r a nspor ta t ion offshore are required fo r such a la rgen u m b e r o f people w h o m a y stay in accommodation vesse lsmoo r e d c lose t o the p la t f or m . P a r t of the ope r a t ion is sensitiveto sea sta te an d m ay resul t in significant d e l ay ( d o w n t im e ) inc om ple t ion . Selection o f suitable vesse ls which ca n operatesafely close to the pla tform at more severe sea sta tes, a l thoughmor e cos t ly , is mor e e conomica l in the l ong run for severee n v i r o n m e n t s such as those in the N o r t h Sea.

44.4 General factors affectingconstruction techniquesSelection of su i ta b le te chn ique s fo r f a br ica t ion and ins ta l l a t iono f offshore structures are inf luenced b y m a n y f a c t o r s w h i c hinclude:(1 ) M ater ia l (s tee l , concrete o r hybr id s t r uc tu r e , an d o t h e r newmaterials) .(2 ) Economic factors such as the need to br ing the f ie ld topar t i a l pr oduct ion e a r ly an d impr ove ove r a l l ca shf low .(3 ) Cost.(4 ) E n v i r o n m e n t a l c o n d i t i o n s : se a sta tes, wind, current.(5 ) Wa te r de p th .(6 ) Sa f e ty .(7 ) Cons t r a in ts impose d b y regula tory author i t ies , such asvessel ope r a t ion cons t r a in ts , po l lu t ion con t r o l , na v iga t ionres tric t ions , etc.(8 ) Existence o f su i ta b le f a br ica t ion ya r ds / dr y docks w i th suffi-cient space an d load capacity and a va i la b le d r a ught in thew a t e r w a y s fo r t r a nspor t of the structures.(9 ) Socio-pol i t ica l factors which m ay inf luence selection o fyard s an d even th e type and form o f structures.In a d d i t io n , w i t h th e de ve lopme nt o f nove l te chn ique s and newequipm en t and tools , t radit ional methods have been replaced byn ew m e t h o d s an d a re l ikely to c o n t i n u e c h a n g i n g .T h e i n t r o d u c t i o n o f high-pressure f lexib le lines, subseat ren c h in g cr a wle r s fo r t r e nching an d l ay i n g pipel ines, dynami-cally posit ioned vesse ls capable o f mainta ining position a t m oresevere sea sta tes, ROVs, crane barges with heavy l ifting capaci-ties o f 80001 o r m o r e all inf luence n o t on ly f a br ica t ion an dinstal lat ion techniques but have played major roles in changest o the fo rm an d design of the structures.T he h a nd l ing ca pac i ty o f c r a ne ba r ge s e na b le b igge r modu le sto be bu i l t onshor e a nd p r ov ide a r e duct ion in the cost o foffshore hook-up ope r a t ions . The us e o f u n d e r w a t e r h i g h -capacity ham mers has a l low ed the sizes of pi les to be increased,resul t ing in reductions in n u m b e r s and, therefore , savings inmater ia l costs and of f shor e operation costs.Som e o f the de ve lopme nts an d t rends have been descr ibed inp u bl i ca t i o ns listed in the b i b l i o g r a p h y .44.5 Concrete structures44.5.1 TypesConcrete s tructures, by the ir nature are, in general , b ulkier andheavier than those constructed in steel an d invo lve differentconstruction techniques.In order to understand and appreciate th e differences, it ishelpful to re fer to a number of major concrete s tructures andtheir functions, as l is ted be low.(1 ) Concrete gravity p la tforms (rest ing on the sea-bed w i t h n opiling involved).


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(2 ) Flo ating concrete s tructures (semisubm ersible , TLPs orship-shape structures) .(3 ) Arctic caissons.(4 ) Concr e te pon to ons , suppo r t ing va r ious type s o f s t r uctu r e s.(5 ) A r t i c u la t e d b u o y a n t c o l u m n s .In this section, construction of concrete gravity p la tforms isdiscussed to demonstra te the tasks involved.

44.5.2 Major requirementsFor a l l such structures the pr ime considera tions are:(1) Suitable facilities and locations for their con structio n.(2 ) Of f sh o re cons t r uc t ion (i f a pp l ica b le ) , mo or ing and suppor tfacilities.(3 ) M a r ine ope r a tions i nv o l v i ng t r a nspor t , moor ing , ma t ing ofcompone nts a nd ins ta l l a t ion .(4 ) Founda t ions a nd scour p r e ve nt ion .

The weight and size of concrete gravity s tructures increasedsubstantia l ly as the ir applica tion to deeper w aters of 100 to3 0 O m was introduced, and resul ted in the construction ofstructures we i gh i ng in excess of 800 0001 with tops ide loa ds o fove r 3000O t .The Brent p la tform in the North Sea consists of a ce l lu larbase of 90 m square and 54 m high . T he fo u r towers r ise some10 7 m a bove th e base to suppor t a deck with a total area o fa r ound 3 9 0 0 0 m 2 and we ighing 31 0001 (Figure 44.9).

T h e pla tform displaces 436 3001 of wa te r . Cons t r uc t ion todeck level required over 2 5 7 0 0 O t o f concrete and 1 5 0 0 O t o fre inforcing steel.Co nstruction of such large pla tform s in exist ing dry do cks h asbeen impractica l b ecause of the l imi ta t ion in the size an d w eig h tca pa ci ty o f the docks a nd the d r a u ght a va i la b le f or tow -ou t . Fo rthese reasons th e practice h as been to cons t r uc t dry ba s ins wi thaccess to deeper waters and to construct par t of the ba se to ahe ig h t at w h i c h th e a va i la b le w a te r de p th a l low s f lota t ion, tow-o ut an d t r a nspor ta t ion .Cons t r uc t ion of such a ba s in a t Ar doyne r e qu i r e d the r e -mova l o f some 900 000 m 3 o f ma te r ia l .Limita t io ns in w ater depth of 10 to 1 5 m in many coasta lareas and waterways leaves only a few locations suitable in theU K f o r such ope r a t ions . Nor wa y w i th it s she l tered deep f jords ,h o w e v e r , offers good su r r oundings fo r construction o f concretestructures. Stabi l i ty requirements dur ing transpor t dicta te thedepth to which the f loating structure sh ould b e submerged. Suchrequirements acknowledge the need for a deep she l tered si tew h e r e th e part ly completed base c an b e m o v e d , and b e m o o r e d ,a nd w he r e the r e ma inde r o f the cons t r uc tion w or k o f f sh o re ca nb e completed. It should b e remembered that dr a ughts o f 100 to1 5 Om are often required fo r ma j or p la t f or ms .44.5.3 Concrete constructionHigh-grade sulphate-resisting cement concrete (grade 50 o rmor e ) is used fo r o f f sh o re cons t r uc t ion wor k. Dur a b i l i ty inhosti le se a environments requires high grades of cement, aggre-gate an d g o o d w o r k m a n s h i p. T h e large quantities involved posesupply an d storage problem s. Concrete produ ction plan ts w ithhigh output capacity in excess of 100m 3 /h are often required.This can be achieved by using more than one plant to ensurecont inu i ty o f supp ly dur ing b r e a kdowns .Concrete is pumpe d, o r move d b y t r ucks , wi th in th e site. Fo ro f f sh o re construction, severa l pumps are used, each with capaci-ties in excess of 300 m 3/h . The concrete production plants can belocated o n pontoons , moor e d a ga ins t th e p la t f or m. Long pump-ing distance often requires th e addition o f plasticizers andretarders to the concrete.Sl ipforming is the common me thod o f placing concrete, withrates in excess of 50-100 m m / h for the caissons and higher ra tesof 100 200 m m / h for the ma in towe r s . S l ip f or ming o f incl inedsurfaces h as also been developed and has proved to b e practical.Thicknesses o f concrete slabs and walls vary f ro m 500 mm toa few metres. The ducts are introduced wi t h i n the th ick mem-bers to he lp in the dissipation of heat to cope with the high heato f h y d r a t i o n .Bo th re info rcing bars an d prestressing tendo ns similar tothose used in land-based structures ar e used.For the Br e n t o f f sh o re p l a t f o r m , 1000 jacks of 3 t capacityw ere used an d required 1100 m 3 o f concrete to achieve a 1 m l ift .The base slab required 20 000 m 3 of grade 50 cement concrete .Severa l , tow er cranes wi t h the capacity of 10 to 151 we r erequired fo r concreting and ha nd l ing r e in f or ce me nt and f o r m -w o r k .Th e effects of creep and temperature changes require thor-ough investigation fo r both cons t r uc t ion and service life w h e n ,during o il production, par ts of the cellular base space are usedfo r storage o f crude oil at temperatures of 30 to 4O 0C above th esur r ounding sea-water temperature .44.53.1 Deck installationFollowing completion of the concrete p la tform it is ba l lasted-d o w n to enable th e deck structure to b e lifted and posit ioned o nthe towe r s by heavy- l i f t crane barges. Other modules for thedeck ar e brought into the ir posit ions an d insta l led. It is alsoF i g u r e 4 4 . 9 O n e o f th e c o n c r e t e p la t fo r m s u n d e r c o n s t ru c t io n b yM c A l p i n e S e a T a n k a t A r d o y n e P o i n t , f o r u s e in t h e B r e n t field


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sometimes possible to ballast-down the platform unti l only afew metres of the towers are above w ater. The deck may then betransported on pontoons, each with a clearance to enable thedeck to be moved over the towers. Bygradually deballasting thepla t fo rm, the deck can be aligned w ith the towers.Winches installed on the towers are used to perform the finalpulling stages of the deck over the towers, and the final fewmillimetres of the positioning is completed with the help ofjacks.This operation requires delicate control of the platform andthe deck, cont inuous monitoring of the movements and apowerful ballasting system to cope wi th the ballasting ratesrequired.44.5.3.2 Towing to the final positionThe significant draught of the structure is often in excess o f10 0 metres; it is therefo re necessary to select and survey a tow ingroute in order to ensure that sufficient water depth exists alongthe total distance. The effect of current , waves and wind arestudied to ensure tugs have sufficient reserve power to cope withtowing under specified adverse weather conditions. Towingspeed can be as low as 0.5 kn, increasing to 2 to 2.5 kn in saferpassages.Navigat ion, towing an d moni tor ing of the operation m ayrequire a crew of f ro m 30 to 50 men.W h e n th e structure reaches its destination, tugs in starformation hold it in position while, b y gradual ballast ing, th estructure is lowered on to the sea-bed.44.5.3.3 Foundation considerationsIn addition to the common requirements for load-bearing, long-and short-term settlement, stability and keying against shearforces, i t is impo rtant to note that, ow ing to the act ion of w aves,loads on the fou ndat ion are cyclic and affect the drainage of thesoil underneath both in the short and long term. The directeffects of waves on soil, particularly in sha llow waters of up to50m, could also be significant. Variations in pore pressuredepend, among other things, on the densi t ies of the oi l to bestored.Problems o f scour around th e perimeter of the base requirecareful consideration. Va rious metho ds, varying from dumpingstone to the installat ion of manmade mattresses f i l led withgrout , sand or s tone, have b een used w ith varying degrees ofsuccess.44.6 Construction in the arcticO il in arctic zo nes w as f irst discovered in the MacKenzie deltaand Arctic islands in North America. Further s tudies in the U Sin the late 1970s showed that there were substant ial potent ialresources offshore in the A rctic zones, particularly in the Bering,Beaufort and Chukchi Seas.The f irst f ie ld was developed in wa ter dep ths of 1 to 2 0 m .Future discoveries in the lease sale areas involved operating indepths of 20 to 50 m.Structures suitable fo r such relat ively shallow depths b utextremely host i le environments ar e therefore different from th econventional offshore structures. The environmental con-di t ions, part icularly the presence of ice packs, p lay dominantro les and are w or th ment ion ing .

44.6.1 Environmental conditionsT he expected maximum wind and wave condit ions in arcticareas of immediate interest are less severe than those of the

North Sea. The 100-year expected maximum wave height is inth e range of 12 to 15 m for water depths of 15 to 30 m. Stormsurges in excess of 6 m are, however, significant for the design o farctic structures.Ice criteria dominate the design of the structures. The mainfeatures of the arctic ice are:(1) First-year ice. The thickness of ice formed wi th in 1 yearcould be up to 2 m, depending on the area.(2 ) Multi-year ice. This is the ice which h as las ted more than

one melt season and has resulted in the build-up of an icesheet into a thickness of 6 m or over, w ith a diameter of 3 to5 km b eing typical.Collision of two large sheets of ice may result in the format ionof pressure ridges several metres above the water level as icemo untains and their coves could extend several metres into thesoft sea-bed.Multi-year ice-floes could travel at velocities of up to 2 m /sand their impact with an y structure w ould result in an effectivetotal load o f several tho usand tonnes, depending on the form o fice and details of the structure.A m b i e n t temperature reaches a low of - 5O 0C.So far as ground condit ions are concerned, the new featuresparticular to arctic zones are permafrost and gas hydrates . Thepermafrost table could vary f rom a few feet below the mud lineto several metres. G as hydra tes are ice-like po ckets o f natu ra lgas which fi t into th e structural voids in the lattice o f w a t e rmolecules.Freezing an d t h a w i n g o f soi l columns ar e other featureswhich affect ground condit ions to support gravi ty base s t ruc-tures.44.6.2 Types of arctic structuresThe most common types of s t ructures considered as arct icplatforms for drilling o r product ion o f hydrocarbons are :(1) artificial islands; (2) hybrid islands; (3) cone structures;(4 ) tower structures; and (5) floating structures.44.6.2.1 Artificial islandsSince early 1970s, a n u m b e r o f artificial is lands have beenconstructed in water depths of 1 to 20 m. M o s t of these islandsare in the MacK enzie delta, in northe rn C anada. The construc-t ion m ethod h as varied from over-the-ice construction to dredg-in g the loose soil and filling with dredged sand and armourstone. Arm ourin g is part icularly the cause of h igh cost becauseof lack of quarry s tone in the nearby areas.Artificial islands are at t ract ive economically fo r shallowwaters below 10 to 20 m depth. For depth ranges of above 10 m,other types could become more economical.

Ic e pads are anoth er type o f s t ructure wh ich consis t o f layersof ic e formed on top o f one another b y pumping water from th elower depth of water to the surface of the ice-pack. Theth ickness of each layer is in the order of 6 m. The ice-pack co versth e ent ire water depth f or ming a p la t form for the operat ion.44.6.2.2 Hybrid islandsThey include caisson-retained islands in which sand-filledbarges or ship hulls fo rm the central core of the is land and reston beams which ex tend 4 to 5m be low the water leve l . Thebenefit of this type o f island is the reduct ion in volume of fill andshort construct ion t ime.44.6.2.3 Cone structuresThe most common types of is land developed are cone-shaped


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structures. Cone-shaped gravity platforms vary in form andshape and are constructed of steel, concrete or a hybr id of steeland concrete structures. The outer walls are inclined to break iceon impact in the most effective w a y . T he main structure of thecone consists generally of cellular form.44.6.2.4 Tower structuresOther types of platforms are braced-steel structures and con-crete g ravi ty plat forms w ith cylindrical towers. These s t ructuresar e suitable fo r areas with l ight ic e condit ions.44.6.2.5 Floating structuresFloating structures vary in form and include ship-shaped struc-tures, floating concrete caissons and conical floating p la t forms .Most s t ructures in this category are suitable fo r deeper watersand arct ic areas where ice survei l lance and management ispractical and economical. These structures are basically m ooredto the sea-bed with several m oo ring lines.44.6.3 ConstructionWith t emperatu res do wn to 5O 0C, th e presence of ice f loes andlimited open w ater restrict the w orking season to 1-3 m o n t h s .Construct ion operat ions are cost ly and, for both economic andpractical reasons, most structures are designed to be constructedin easier conditions and are towed to location fo r installation.Construc t ion o f artificial islands using arctic dredgers h asproved possible. Use o f suppo rt vessels, ice-break ers fo r t owingand m anagem ent of ice-packs, and tugs enable the p lat form s tob e fabricated in several sections and b e brought together fo rf inal mat ing an d setting o n locat ion.Concrete cone s t ructures w ith total displacement in excess o f50 0 O Q O t are fabricated in segments, using conventional tech-niques of concreting. Similar to concrete plat forms used fo ro ther offshore locations, limitations in d r a u g h t fo r t owing th estructure dictate th e location fo r fabricat ion and the construc-t ion techniques.Ic e loadings on arct ic cone s t ructures are not kno w n preciselyb ut could vary in intensi ty f rom 1000 to 2000 kN /m 2 globalloading and to 12 00 0 k N / m 2 local pressure. These requireconcrete s t ructures to w ithstand high punch ing shears as wel l ashigh bending and shear forces. The s t ructures are thereforeheavily reinforced with high-strength temperature-compatiblesteel as well as prestressing tendons.Concrete h as b e en s h o w n to gain s t rength with t ime in l o w -tempera ture condit ions. This includes compressive s t rength,tensile strength, bo nd s trength, impact resistance and m o d u l u so f elast icity . A pplicat ion o f concrete fo r arctic structures istherefore a viable solut ion.

Low temp erature and presence of ice hav e been used as an aidfo r construction purposes, e.g. ic e roads several ki lom etres longand 10 to 20 m deep. These roads stretch into the sea and formaccess routes to artificial islands. Artificial ice-platforms fo rdrilling in high arct ic areas are ano ther example.Offsh o re construction in hostile arctic areas h as t herefore le dto the development of novel ideas and use o f special equipmentsuitable fo r such conditions. Arctic engineering h as become aspecialized field invo lv ing th e development of material, equip-ment and bet ter understanding of environmental loads such asice loads an d soil condit ions.

44.7 Fabrication/construction facilitiesMa j or facilities sui table fo r fabr ica t ion an d construct ion o f

offshore structures are: (1) land-based fabrication yards;(2) dry docks; (3 ) slipways; and (4) offshore floating facilities.44.7.1 Fabrication yardsLand-based yards are close to waterways w i th loadout quays fort ransport ing the structures to sea. Main features of such yardsare:(1 ) Covered areas for w eather- independent w ork such as s teel-rolling, fabrication, assembly and paint ing.(2 ) Cranes with sufficient reach an d capacity.(3) Quays w ith surface load capacity of 50 to 150 kN /m 2 to copewith heavy loads of several thousand tonnes.(4 ) Access to deep water and open sea for tow ing out s t ructures.Such facilities ar e often required to b e approved b y cert i fyingauthori t ies to ensure that they provide condit ions needed tomeet the necessary s tandards of workmanship and quali tycontrol. Fab ricat ion yards are used primari ly for fabricat ion ofs teel jackets , deck s tructure of the plat forms and a variety ofmodules fo r installation on the decks o f offshore structures.44.7.2 Dry docksD ry docks fo r fabr ica t ion o f offshore structures are, in general,larger structures than those used fo r shipbui lding. These facili-ties are equipped with cranes and other support facilitiesrequired fo r fabr ica t ion o r construct ion o f large an d heavystructures, w h i c h are outside th e capacities of the fabr ica t ionyards, and c an b e f loa ted out fo r t ranspor t to offshore loca-tions. Th e dimensions of Kishorn dry dock, Scot land, are180 x 170 x 11.5 m deep. This facility, with its deep-water m oor-in g site and var ious fab r ica tion and paint shops, is a typicalexample of the dry dock sui table fo r fabricat ion o f large steeljackets.44.7.3 SlipwaysThese facilities are similar to shipbuilding slipways and allowthe fabricat ion in land-based environments . Wh en the con struc-t ion is completed, the structure is loaded-out on rails on to thewater in a similar manner to launching a ship. Purpose-builtslipways, with direct access to open seas, suit large-size struc-tures which are outside the h andling range o f avai lable fabrica-t ion yards and dry docks.44.7.4 Offshore fabrication platformsLarge floating pontoons made o f steel o r concrete have beendeveloped and moored o f f sh o re as fabricat ion yards. The use ofsuch p la t forms is justified w hen other co nventional faci lit ies ar enot available, or there are specific restrictions such as depth ofwater for t ransport to the sea.Those countries involved in the oi l industry , such as the UK ,France, Norway , Hol land , the US, have developed and, att imes, maintained such faci li ties w ith governm ent assistance.44.7.5 Back-up facilitiesA vital key to success is the use of sui table equipment fo refficient an d cost-effective execution of work. Speed of opera-t ion, complet ion o f w o r k o n t ime and achievement o f highs tandards o f wo rkma nship demand tha t t he mos t up-to-dateequipment is avai lable fo r these purposes. Well-equippedcovered areas, automa tic w elding equipment , non destruct ivetesting facilities, all backed-up b y computer services, are ex-amples of what are needed.


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A host of equipment and services is needed of fshore to carryout the various stages of fabr ica tion, mating, transport andinstallation of structures. The f o l lowing is a list of some of themajor facilities required.(1) Heavy-l if t crane barges with capacities ranging f ro m a fewhundr e d to over 120001. Some semisubmersible crane ves-sels available at present have two cranes with total liftcapacities of up to 10 to 12 0001.(2 ) Support vessels fo r specialized work, such as diving support

vessels, inspection vessels, and vessels for carrying powerand control equipment.(3 ) Accommodation vessels o r semisubmersibles as offshorehotels for engineers, inspectors and fitters.(4) Remote ly operated vehicles for subsea operations.(5 ) Tugs fo r towing o r sta t ion-keeping floating structures.(6) Anchor-handling vesse ls .Involvement o f such vessels an d associated equipment is a costlystage of the insta l la t ion opera tion because o f the high d ai ly ra tesinvolved in the ir deployment.

44.8 AnalysisAnalysis i s by no means restricted to the be ha v iour of thecompleted structures in the ir insta l led con dit ion. Th e structureseither as part or complete units are subjected to loads differentf rom the ir normal service condit ion dur ing fabr ica tion, launch-ing, t o w - o u t and insta l la t ion.Sta tic and dynamic loading condit ions are involved whichrequire analysis fo r various purposes, including:(1) Checking stresses (local and global).(2 ) Static s tabi l i ty of the f loa ting structure a t var ious stages ofinsta l la t ion.(3 ) Dyna mic b e ha v iour and stability of the structure subjected

to w ind , w a ve a nd cur r e n t loa ds .(4 ) Load cycles experienced dur in g transpo r t and insta l la t ionand the ir effect o n t h e fa t igue life o f t h e structure .(5 ) Deflections and de f or ma t ions o f structures, par t icular lypipel ines and r isers dur ing ins ta l la t ion.(6) Beh aviour of the guidance systems, such as tensioned guide-wires , i f used for low ering and locating compon ents subsea .(7 ) Beh aviour and response of f loa ting structures and vesse lsw h i c h are used dur ing the insta l la t ion opera tions.For analysis of the condit ions l is ted here , computer programshave been developed and are used for both s ta t ic and dynamicanalys is . T he r e a r e , howe ve r , ma ny ca se s whe r e compute rprograms and techniques are insufficient and model tests areneeded to verify predicted behaviours o f structures.Model- test ing in w a te r ta nks is an e xa mple of the type o f testscarried out for the oi l i n d u s t r y .

44.9 Schedule of work: cost factorFa br ica t ion , a sse mbly a nd ins ta l l a t ion of the va r ious compo-n en ts and modu le s , as well as the ma in p la t f or m s t r uc tu r e , areall complex mult idiscipl inary tasks. T h e w o r k of ten involvesacquis i t ion of some long lead i tems w h i c h need t o b e ordereda nd ma nuf a c tu r e d wel l ahead of t ime.So far as the offshore operations are concerned, many faci l i-ties such as heavy- l i f t crane barges, support vessels and tugs, arerequired. T he se r e qu i re mo bi l iza t ion , m odi f ica t ion an d insta l la-t ion of e qu ipme nt .A w e l l-de ta i le d p r ogr a mm e o f w o r k is required in or de r to

carry out all such tasks. Complex civil engineering projects areno exception, and readers famil iar with the programmes of w o r kinvolved in conventional civil engineering will appreciate theadditional complexity of o f f sh o re construction.In of f shor e work, sensit ivity to we a the r and seasonal seaconditions, involvement of high-cost facilities, such as heavy liftcranes, support vessels and the like, create a de ma nd forthor ough p la nning of the operation.B ar charts and critical path analysis techniques are used tode velop the f o l low ing ke y areas of the opera tion:

(1 ) Duration of each opera tion.(2 ) O rder of wo rk to be carr ied out and identif ica tion ofcritical activities.(3 ) Equipment and facilities required, together with specifica-t ions fo r performance.(4 ) Materials needed.(5 ) Site /plant requirements.(6 ) Requirements and restr ict ions imposed by regula tory auth-orities.(7 ) Ma npowe r r e qu i r e me nts .(8 ) Tendering and se lection of contractors and subcontrac-tors.(9 ) Quality assurance and quali ty-control requirements.(10) Ro ute survey and se lection for transpo r t .(11) Co-ordination o f w o r k .(12) Planning for com pletion and transpor t o f var ious m odules.(13) Approval and cer t if ica tion for a l l s tages of the operation.(14) Management system and cost control .Complex of f shor e structures of ten t a ke mor e tha n a ye a r tocomplete and cost severa l mil l ion pounds in capita l expenditure .The h igh ra tes of cost involved in deploy ment of these faci l i tiesand th e use of ski l led personnel mean that de lays o r m iscalcula-t ions are likely to inc ur high cost penalt ies.T h e tota l capita l cost o f developing of f shor e h y d r o c a r b o nfields varies significantly de pe nding on the de p th o f wa te r ,complexity of the structure and the production system involved ,typical costs be ing, fo r example , 500 mil l ion fo r the F u l m a rf ie ld and 1250 mil l ion fo r the M a g n u s f ie ld . These comparewi t h mu l t i mi l l i o n p o u n d civil engineer ing projects such as theT ha me s Ba r r ie r at 430 mil l ion (1976 price level).T h e cost of the development of the oilfields includes dr i l l ing,pipelines, production and export facilities. The capital cost ofth e p la t f or m a nd the cons t r uc t ion a nd ins ta l l a t ion ope r a t ionsare the r e f or e on ly o n e part o f a la rge capita l investment in thed eve lopm en t of a hydr oca r bon f ie ld .

44.10 Codes and regulationsT he r e a r e ma n y code s w hich a pp ly to the de s ign a nd f a br ica tiono f offshore sys te ms. Specific codes related to the design o fo f f s h o r e structures have been issued b y va r ious a u tho r i tie s in theUK, the US a nd o the r count r ie s , such a s Nor wa y . T he r e a r ea l so r e gu la t ions r e la t ing to ma r ine a nd o the r of f shor e opera-t ions , some of w hich a re specific to par t icular coun tr ies or areas.T h e facilities require t o b e certified as f i t for the purposesspecified fo r of f shor e s t r uc tu r e s , whe the r fo r pr oduct ion o fh y d r o c a r b o n s or other purposes. The cer t if ica tes confi rm th esafety of the operation, safety of the crew, s tructura l an den vi ron m en ta l requirements.T he r e are organizations which assess and issue such certifi-cates. These bodies have set out guidel ines and rules wi t hreference to codes and acts w h i c h a r e to be f o l low e d . Adhe r e nceto such codes an d r e gu la t ions is essential and is one of the


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requirements for a l l stages of the project development, f romconceptual design to commissioning.T h e m a j o r certifying authorities are:(1 ) American Bureau of Shipping (US)(2 ) Bure au Veritas (France)(3 ) D et Norske Ver itas (Norway)(4) Germanischer Lloyd (W. Germa ny)(5 ) Lloyd's Register of Shipping (UK )T h e codes and guidelines cover a broad area ranging f ro menvironmental conditions, loads to be considered, a l lowablestresses, stability, fatigue requirements, methods of analysis,li f t ing operations, corrosion protection, material specification,fabr ica tion an d associated quality control and testing andinstal la tion operations.There are codes and guidelines issued b y a n u m b e r o forganizations in the UK including the Department of Energy,Lloyd's Register of Shipping and the British Standards Institu-tion.In the US, the codes issued by the American PetroleumInstitute, the American Bureau of Shipping, the AmericanConcrete Institute , the American Society of Mechanical Engi-neers and the American National Standard Institute are thema in guidelines.Th e fo l lowing is a shortl ist of some of the codes and regula-tions currently in use.(1 ) American Petroleum Institute API RP2A: Recommendedpractice for planning, design and constructing fixed o f f s h o r eplatforms.(2) British Standard 6235: Code of Practice for fixed o f f s h o r estructures.(3) Det Norske Veritas: Rules for the design, construction an dinspection of o f f s h o r e structures.(4 ) Depar tment of Energy: O f f s h o r e installations - guidance ondesign and construction.(5 ) Lloy d's Register of Shipping: Code for lifting appliances in amarine environment.44.11 Organization and management ofoffshore projectsInterdependency of design, construction and instal la tion tech-niques plays an impor tant part in the development of o f f sh o restructures. Integration of multidisciplinary tasks at a l l stagesdemands well-organized management, co-ordination an d con-tro l o f the w o r k .

44.11.1 Project requirementsLike man y other complex projects, the main groups o r organiza-tions involved are: (1) the client(s); (2) the designers and con-sultants; (3) the contractors and subcontractors; (4 ) supplierso f materials and components; (5) inspectors and approvingauthorities; (6) finance organizations; and (7) insurance com-panies.Management requires a project execution plan and an or-ganized team to carry out the tasks o f planning, organizationand manpower c ontro l , contract administration, quality con-trol, expediting, cost control and liaison and co-ordination.44.11.2 Project organizationManagement can be carr ied out with varying emphases ondecision-making, delegation and construction. Th e matrixwould therefore b e different fo r each approach. T h e most

c ommon approaches for the form of project organization are:(1) Ow ner pro jec t m anagement .(2 ) O wner partia l invo lvement plus project services contractor .(3 ) Management contractor.(4 ) Prime contractor .44.11.2.1 Ow ner project managementIn owner project management, the owner parcels out variousparts of the w o r k to contractors and subcontractors and m a n -ages the entire work directly using his own project team. Thisapproach requires a vast team of engineers and planners f romth e o w n e r s w h o d o n o t of ten have such a pool o f experts.44.11.2.2 Ow ner project services contractorIn the project services contractor approach, the owner sti l l hasan active ro le in the man agemen t and decision-making processesb ut selects a contractor to carry ou t a l l o r most of the projectmanagement services.44.11.2.3 Managem ent contractorIn th e management contractor approach, th e managementcontractor acts o n behalf o f the o w n e r an d carries out a l lmanagement tasks w i th the main w ork b e ing contrac ted out toselected engineering, procurement and construction subcontrac-tors. Th e owner 's ro le in this case is top-level managemen t andsurveillance of the management contractor using his selectedproject team.44.11.2.4 Prime contractorIn the prime contractor approach, w ork is carried out o n a' turnkey' basis b y a contractor o n a design/construct b asis. T h econtractor is, in this case, responsible for the m a n a g e m e n t andexecution of the w o r k , w h i ch he ma y undertake partly himselfwhile sub c ontrac t ing many o ther par ts o f the w o r k to othersub c ontrac to rs .In addi t ion to the above approaches, there are cases wherecom bin ations of these methods are used. Each approach has i tsbenefits and weaknesses. Selection of the r ight approachdepends on the c apab i l i t ie s o f the owner to manage the work ,the type of project, country and location.The number of project managers, planners, engineers, con-struction inspectors and con tract, purchasing, estimating, safetyand adminis t ra t ion staff varies significantly, and r u n s from afe w hundred to a few thousand depending on the project andmethod o f management . The team operates in various locations,i . e . th e central office, land-based sites an d o f f sh o re sites.The project managem ent of the Fulm ar f ie ld in the N orth Seainvolved an in-ho use team of 95 and a site team of 170 people.44.12 Inspection, maintenance andrepairTh e emergence o f certification fo r o f f sh o re structures h as meantthat requirements have been estab lished fo r inspection, main-tenance and repair a t regular intervals during the life of thep la t f or m.T h e inspection o f o f f sh o re structures presents difficultiesbecause they are being placed in ever deeper and more hosti lean d turb u lent wa te rs . A steel platform can weigh 2 5 0 0 O t o rmore an d have a to ta l w eld length o f over 1 km, distr ibuted oversome 900 weld points. Templates, m anifold s, w ellheads, christ-


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m as trees, risers, pipelines, f lowlines and l o a d i n g facilities allrequire regula r inspec tion.A typ ic a l inspect ion rou t ine fo r an o f f sh o re s t ruc ture w ouldinclude the fo l lowing:(1 ) Genera l inspec tion.(2 ) Ma rine grow th inspec t ion.(3 ) Debris survey an d m a p p i n g .(4 ) Sea-bed, scour and structure s tabi l i ty inspection.(5 ) Corrosion damage inspection.(6 ) Cathodic pro tec tion po tent ia l surveys .(7 ) A no de inspec tion.(8 ) Still p h o t o g r a p h y and p h o t o f o r m a t t i n g .(9 ) V i d e o g r a p h y .(10) Nondestruc t ive tes t ing inspec t ion wh ic h m ay inc lude :(a) magnetic particle inspection (crack detection);(b) eddy current inspection; (c ) ultrasonics; (d ) AC-PDmethods; (e) Harwell ul trason ic torch technique; (O radio-graphy; (g) v ib rode tect ion; and (h ) pho togram metry .

Once a defect h as been located, there are numerous repa irpossib il i ties to consider d epending o n the typ e of structure. Steelstructures are general ly repaired by cutting out the defectivearea and re-welding or by strengthening, using grouted c lamps.Reference should also be made to Chapter 42 for fur therin f or ma t ion o n inspection an d repair underwater . Typicalopera t ions involved in the repair o f o f f sh o re structures are asfo l lows.44.12.1 Underwater cuttingThere are four underw ater cutting processes general ly in use: (1)oxy-arc; (2) thermic cutting; (3) gas cutting; and (4) shieldedmetal arc . Of these, oxy-arc is probably the most widely used.Shielded metal arc can cut steels resistant to oxidiz a t ion andcorrosion and nonferrous materia ls, and is useful where nooxygen is availab le . Oxy-hydrogen cutting is per fo rmed wi th ato rc h ra ther than wi th a cutting electrode and an experiencedoperator can achieve a very neat cut in th ick metal . Thermiccutting will b urn th rough a lmost an y materia l , inc luding re -inforced concrete.44.12.2 Underwater weldingThere are three underwater welding techniques:(1 ) Dry hyperbaric welding. Using e i the r th e semiautomatic o rm a n u a l metal arc-welding processes, the weld area can beenclosed in th ree ways: (a ) full-sized hab i ta t ; (b ) m i n ihab itat; and (c) portab le d ry bo x.(2 ) One-atmosphere welding. This technique uses an under -w a t e r c hamb er in w h i c h th e e n v i r o n m e n t is mainta ined at

one a tmosphere . The dry hyperb ar ic we lding and one-atmosphere w elding are the same except that dry hyperb aricwelding is conducted und er pressure.(3 ) Shielded metal arc wet welding. Basically th e same equip-ment is used as for surface welding, b ut w ith insulated cablejo ints and a torch with waterproof e lectrodes.44.12.3 Grouted clampsGrouted clamps are used to strengthen nodes and braces onexisting steel platforms. Th e clamps are bolted together andthen filled with grout. They have been extensively used to repairdefective nodes on older North Sea platforms.44.12.4 Concrete repairA discussion of methods of repair for concrete structures is

given b y the U nderw ate r Engineer ing Group o f CIRIA (seeBibliography). Techniques fo r inspection, maintenance andrepair opera t ions vary depending on the w a te r depth and man yo ther features of the platfo rms. Divers are used to perform someo f these operations in sha l low wate rs wi t h i n th e range o f the i rsafe opera t ion wh ic h , in most cases, is up to 15 m. In deeperwaters, remo tely operated vehic les or rem otely operated equip-ment is used.M a n y tec hniques fo r remote inspection and maintenanc eopera tions h ave been developed recently . These have resulted inthe need to mo di fy details of the structures so that suchoperations can b e carr ied o ut successfully. Design and construc-t ion methods are the re fo re influenced b y inspect ion, ma inten-ance an d repair requirements during th e platform's service life.These operations, apart f rom ha vin g to b e practical , need to b esafe and econ om ical as, in most cases, the costs of divers, servicesupport vessels and o ther equipment fo r o f f sh o re use are veryh i g h , compared w i th inspection, maintenance and repair opera-t ions fo r land-based structures.44.13 Cathodic protectionCathodic protection is the most commonly used corrosionprotection method fo r steel o f f sh o re structures. It is normal lyused in combination with an insulating or protective coating,whe r e the coating forms the f irst l ine of defence. Where thecoating is damaged, how ever, corrosion can occur and cathodicprotection is used to pro vide protection at such lo cations.Cathodic protection uses either sacrif ic ia l anodes orimpressed current to m a k e th e structure cathodic. This causesan electrical current to flow f ro m th e anodes, th rough theelectro lyte , to the cathode (the structure) thereby opposing thena tu r a l electrical current arising f ro m the f low of electricallyc harged ions aw ay f rom th e surface that is corroding.In a sacrificial anode system, the current can be generated bythe use of sacrif ic ia l anodes, such as z inc or a lum inium . Thesewill corrode instead of the structure, b y vir tue o f their strongeranodic reaction w ith respect t o the environm ent . They corrodea t kno w n ra tes , wh ic h means tha t the i r life expectancies can beestimated and maintenance replacement programmes specif iedso that new anodes can be instal led before the o lder ones areentirely used up. A large platform anode can produce around4 A d.c . a t abo ut 25 V. The curren t is transm itted o ver onlyrelatively short distances.The current a l ternatively can be generated by an impressedcurrent system w here an o utside e lectrical pow er supp ly (e.g. at r a ns f or me r rectifier) supplies a current to an au xil iary anode ofsome highly resistant materia l , such as platinum-coated ti ta-nium. An electr ic f ie ld is estab lished w hich inhib its current f low sout of the protected metal . Typical operating pow er for a singleimpressed current ano de m ay b e around 50 A, 20 V d.c . so th athigh power levels can be achieved with only a few anodes andlong distances can be covered.

44.14 Removal of platformsMost North Sea f ixed platforms have planned l ives of 20 to 30years. A few pla t fo rms will therefore b e decommissioned in the1990s with a b unc h ing of decommissioning dates between 2000and 2010. Th e latest estimates put the decommissioning cost o fal l the 250 existing platform s at about $20 bn.There are no set laws regarding th e removal o f o f f shoreplatforms at present. A consultative document recently issuedb y the UK Department of Energy envisages complete removalo f p la t fo rms to a depth of 50 m in the southern North Sea,partial removal providing a m i n i m u m clearance of 55 m below


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the surface in the central North Sea and partial removal to awater depth of 75 m in the northern North Sea.

This is expected to be challenged by the US and the SovietUnion , whose strategic concerns are to minimize the hazards forsubmarine navigation and require total removal.

Only a few small offshore platforms in the shallow southernNorth Sea have been removed to date at relatively low cost. Themajor removal problems will be associated with the 40 or solarge platforms located in the central and northern North Seawhich are located in water of 10Om or more. While most ofthese platforms are steel jackets weighing up to 40 0001, thereare also 18 large concrete gravity-based structures with baseweights of up to 800 0001 and topside weights of up to 500001.

Suitable dumping sites for the platforms have been investi-gated by government and offshore contractors. In the US andJapan, the creation of artificial reefs in shallow waters, whichcould enhance the fish population, have been considered.

However, oil companies are presently trying to increase thelife of existing platforms by bringing new fields on stream usingsubsea completions or unmanned platforms and linking themback to the existing platforms.

Several detailed studies have been carried out to develop cost-ef fec t ive and safe techniques for removal of such large struc-tures. Many of these techniques involve using some of themethods established for handling such structures during theirinstallation. The methods involve removal of the topside equip-ment and the deck structure by floating crane barges and the useof underwater explosives to cut the steel structure from its piledfoundation.

AcknowledgementsGoodfellow Associates wish to acknowledge contributions tothis chapter by M. M. Sarshar, H. D. Parker, L. E. Clarke andR. E. Lawrence.

BibliographyDet Norske Veri tas (1983) Guidelines for the P reparation of Underw aterInspection Procedures. DNV, Oslo.D o n o v a n , J. F., Beam, W . T . a n d N a s h , N . W . (1987) 'The Balmoralsubsea production templa te . ' Paper O T C 5431, Proceedings

o f f s h o r e technology conference, 27-30 A pr i l , H ous ton .

G oodfe l low Associates Lt d (1986) O f f s h o r e Engineering Development ofSmall Oilfields. G r a h a m and T r o t m a n , L o n d o n .Graff , W. J. (1981) Introduction to O f f s h o r e Structures:Design/Fabrication/Installation. G ul f Publ i sh ing C om pany ,H o u s t o n .Ins t i tu t ion o f Civil Engineers (1977) Proceedings, conference on designand construction of o f f s h o r e structures, 27-28 October.ICE,L o n d o n .Jones, M. E. (1981) Deepwater O il Production an d Manned UnderwaterStructures. Graham a n d T r o t m a n , London.Leniham , J. E ., A ust in , R. T. C. and Flanagan, P. J. (1984) T h e rapidinsta l la t ion o f a large N o r t h S e a j a c k e t over a subsea template . 'Paper O TC 4759, Proceedings, o f f s h o r e technology conference, 7- 9May, H ous ton .M a h o n e y , T. R. (1987) ' Ba lmora l , conception to p roduc t ion . ' PaperO TC 5430, Proceedings o f f s h o r e technology conference, 27-30Apr i l , H o u s t o n .Myers , J. J. , H o l m , C. H. and McAllis ter , R. F. (1969) Handbook ofOcean and Underwater Engineering. McG raw H i l l , N ew Y o r k .North Sea Platform Guide (1985) Oilf ie ld Publ ica t ions , Ledbury ,H ere fordshi re , Eng land .North Sea Subsea Construction Guide (1986). Oilfie ld Publ ica t ions ,Ledb ury , H ere fordshi re , Eng land .R a n n e y , M . W . (1979) O f f s h o r e Oil Technology: Recent Developments.N oyes Data Corporation, N ew Jersey.Society of Underwater Technology (1985) The Design an d Installationof Subsea Systems. V o l u m e 2 o f Proceedings, subsea internat ionalconference o n advances in underw a te r t echnology an d of f shoreengineering, London, 15-16 J anuary . G raham an d T r o t m a n ,L o n d o n .Thom as , D. B. J. (1981), 'Offshore steel s tructure repair an dm ain tenance . ' In: D. F a u l k n e r , M. J. C o w l i n g and P. A. Fieze(eds), Integrity of O f f s h o r e Structures. Applied Science Publ ishers ,L o n d o n .U nderw a te r Eng inee r ing G r o u p (1985) Design of Tubular Joints fo rO f f s h o r e Structures, U EG U R 30 (3 vo l s ). C IR IA , London .U nderw a te r Eng inee r ing Group (1986) The Influence of Methods an dMaterials on the Durability of Repairs to Concrete Coastal andO f f s h o r e Structures, U EG U R 36. C IR IA , London .U nderw a te r Technology (1984) Proceedings, international conference ondeepwater technology. Bergen, Norway.Walker , D. B. L. (1980) 'The design an d ins ta l l a t ion of the B uchanField subsea e q u i p m e n t . ' Paper EU R 174, Proceedings, EuropeanO f f s h o r e petroleum conference, 21-24 October, Society ofPe t ro leum Enginee rs , London .

44.Offshore Construction

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