4

In recent years, drilling contractors have faced a seemingly intractable problem:

$200 million. Sedco Forex’s solution is not to build, but to renew. Enhancing their

price of a new rig.

New Lease on Life for the 704

Georges BarreauEric MagnePierre MorvanDang TranMontrouge, France

Paul TranterAberdeen, Scotland

Frank WillifordCollege Station, Texas, USA

Geoffrey BoothPeter MudgeHenryk PisarskiThe Welding InstituteCambridge, England

SemisubmersibleDrillship

DRILLING

Semisubmersibles, or semis as they areaffectionately known, provide an enduringsymbol for the offshore oil and gas industry.Currently numbering about 140, they arethe only effective means of drilling explo-ration wells in deep and harsh waters. Fromthe Canadian Arctic to the North Sea, semisweather the world’s worst storms permittingdrilling operations in water from a few hun-dred to thousands of feet deep (right).

The first semis were built in the 1960s,when oil companies began contemplatingexploration in waters deeper than the reachof a jackup or submersible. Drilling clearlyhad to take place from a floating vessel, butevery kind tried up to that time was toomuch affected by heave, the up-and-downmotion caused by ocean swell. Semissolved this problem by supporting the deckon a few large columns that are joinedbelow water level to pontoons. With theballast tanks filled with sea water, the semifloats deep in the water and is securelyanchored prior to drilling. Pumped dry, thestructure rides high and can be towed to thenext location.1

The design and fabrication of semisevolved rapidly (page 6). An early 1960sdesign with three columns arranged in a tri-angle could work in a 55-foot sea, in 500feet of water and manage a 1500-ton deckload. Sedco built the first of these 135-seriessemis for just $7.1 million.2 A subsequentdesign arranged six columns in an ‘H’ for-mation. Yet another design, by Forex Nep-

tune, arranged five columns in a pentagon.As specifications grew more ambitious, thecost rose in proportion.

In the early 1970s, Sedco and its SanFrancisco-based naval architecture sub-sidiary, Earl and Wright, produced the suc-cessful 700-series design. With eightcolumns arranged in two lines of four andthe rig placed amidships, this mammothwhile anchored could manage 110-footseas, a 2000-foot water depth and support adeck load of 3500 tons. The first was builtfor $36.5 million, but the average cost laterclimbed to $84 million. The 700-series

Oilfield Review

In this article, ANSYS is a mark of Swanson Analysis Systems, Inc.; HYDRA is a mark of Gerdes & CarlsonEngineering; SMACS is a mark of Kvaerner Earl & Wright,Inc.1. For more on marine drilling:

Maclachlan M: An Introduction to Marine Drilling.Ledbury, England: Oilfield Publications Limited, 1987.

2. Sedco merged with Forex Neptune in 1984 to formSedco Forex, the drilling-contractor arm of Schlum-berger Oilfield Services.

3. Roger A: “Perspectives for Drilling Activity in the1990s,” presented at the 8th Latin American DrillingConference, Rio de Janeiro, Brazil, October 14-16,1992.

nExplorationdrilling techniquesfrom deep water toland. Only thesemisubmersiblecan efficiently han-dle both deepwater and harshenvironments.

how to replace an aging semisubmersible fleet when day rates remain depressed and a new semisubmersible costs

704 semisubmersible to North Sea standards and extending its life span another 20 years have cost just 10% of the

Land rigSubmersibleJackup

semis arrived just in time for the North Seaexploration boom for which they were per-fectly suited.

The Sedco/Earl and Wright team followedthis success with an early 1980s designcalled the 600 series. This had four columnsof square cross section and was built formoderate environments at reduced cost. Bynow, the oil bust was not far away. Sedco’s800 series was designed in the mid-1980s tooffer an increased deck load of 4500 tons inthe harsh environments previously servicedby the 700-series semis, but the estimatedcost was around $100 million and nonewas built. For the even more ambitious 900-series semis, designed for a harsh environ-ment but able to drill in 5000-foot water

April/July 1993

while anchored, cost rose to a staggering$160 million, and, again, not one was con-structed. Recently, Sedco Forex reviewedthe 800-series design and estimated currentcost to approach $200 million.

Since today’s business climate precludesconstruction, drilling contractors are beingforced to take a long, hard look at what theyown. The bulk of Sedco Forex’s fleet ofsemis comprises thirteen 700-series rigs andall are nearing the end of their prescribed20-year life span. Two years ago, SedcoForex had to face two key questions. Doesthe 20-year life span really mean the end ofthe road for the 700-series semis, and, if not,

what measures can to be taken to prolongtheir active service?3 To appreciate the solu-tion of these issues, one has to understandthe regulatory nature of offshore drilling andhow it has evolved.

Any mobile, floating structure such as asemi has to have at least two basic certifi-cates—a flag of registration and a classifica-tion of service capability. A flag is requiredto allow the structure to move in interna-tional waters, and addresses many safetyaspects and also details of how the rig ismanned. Classification, originally for insur-ance purposes but now also used to indicategeneral operational capability, is controlledby classification authorities such as theAmerican Bureau of Shipping (ABS), DetNorske Veritas, Bureau Veritas and Lloyds.Experts inspect each semi according to aprecise timetable ensuring the hull is soundand the structure is generally built and main-tained to internationally accepted standards.

5

6 Oilfield Review

nConstruction ofsemis and evolu-tion of their designsince their intro-duction in the mid-1960s. The majoritywere built in the1970s and thosestill operating arenow approachingthe end of their 20-year life span.

Pentagon

135 series

600 series

700 seriesN

umbe

r of

uni

ts b

uilt

1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990

5

10

15

20

25

0

Pentagon

135 series700 series

600 series

nThe 704 inVerolme Botlek’sdry dock in Rotter-dam, November1992.

In addition, the floating structure mustobey local rules if they apply—for example,the 4th Edition Guidance Notes prescribedby the UK Department of Energy4 for anysemi operating in the UK North Sea—and,most recently for UK operations, safetyrequirements resulting from the CullenReport that followed the Piper Alpha disaster.

When the first semis were built, this mazeof regulation was just beginning to evolve.There were few local rules and insuranceclassification had to be pretty much madeup from scratch. ABS initially borrowedmuch of its standard shipping rules to clas-sify semis, and the 20-year life span wasmore an inspired guess than a serious scien-tific estimate. Techniques to assess life spansuch as computer modeling of rig structureand prediction of metal fatigue were in theirinfancy. Today, the situation is dramaticallydifferent. Massive computing capability andtwo decades of inspection experience meanthat fatigue of individual welds can be accu-rately estimated, leading to speedy remedialaction that can double a semi’s life.

Just as significant, these techniques alsoallowed a drastic rethink of the inspectiontimetable, allowing five-year intervalsbetween major inspections rather than theusual four-year cycle that includes two majorinspections. This change allows semis to ful-fill their new role as deep-water tenders,assisting drilling from platforms in harshenvironments such as the North Sea. Once asemi is assigned as a tender to a develop-ment platform, there is no opportunity forrecall to sheltered waters or dry dock forannual inspections. To illustrate these break-throughs, we’ll follow Sedco Forex’s analysisand refurbishment of its 704 semi, as it wasreadied for a five-year contract as a tender toShell Expro’s Gannet platform.5

The 704 was built in Halifax, Nova Scotia,Canada in 1974. In November 1992, whenit was 18 years old, the rig was towed to Rot-terdam and positioned on blocks in the drydock of the Verolme Botlek shipyard, the firsttime it had ever been totally out of the sea(right ). For three months, 400 workersattended to three major projects: first, modi-fications specific to its new function as a ten-der, such as the installation of gangway andcatenary systems for fluids and electricalpower; second, some improvements to itsdrilling capability requested by Shell Exproshould the rig be needed to drill indepen-

April/July 1993

4. The UK Department of Energy is now absorbed intothe Ministry of Trade and Industry.

5. “Tender Moments in the North Sea,” Oilfield Review3, no. 2 (April 1991): 7-10.

dently from the platform; and third, the sub-ject of this article, inspection and remedialaction to prolong the structure’s life and tostreamline inspection.

Refurbishing a semi the size of the 704 isno easy task. The structure is so huge thataction must be focused. Areas of concerndepend on the semi’s history. For example,when a sister rig, the 702, was dry-dockedin Singapore in 1991, remedial work pri-marily involved replacing corroded plateson the columns. The rig had spent most ofits 18 years in the relatively calm but warmseas of the Far East and South America. Cor-rosion was more significant thanstress/fatigue damage. For the 704, whichhad worked mainly in the cold and roughNorth Sea, the situation was the opposite.Corrosion was not expected to be serious,but stress damage was.

Fatigue AnalysisThe main tool for guiding the inspectionand remedial action was an estimate offatigue life for critical welds connectingbraces to columns and braces tobraces—the semi had eight main columnsand 27 braces. Ultimately, an estimate ofeach weld’s fatigue life was needed. Thisrequired a computer simulation of therepeated buffeting the semi had receivedfrom years of violent North Sea storms—acomplex task achieved by Sedco Forex dur-ing a two-year period prior to the dry dock-ing and verified by a classification authority.Modeling the stress history of each individ-ual weld in such a large, complex structureis no easy matter and proceeded in severaldistinct steps.

First, the elastic stiffness of the 704 wasanalyzed using the commercially available

7

1.656

1.724

2.679

3.020

1.724

3.045

2.355

1.637

1.939

2.404

1.674

1.687

Column side

Brace side

1.8852.562 1.902 2.724

nFinite-element modeling of three bracesmeeting the 30-ft column, used to deter-mine stress concentration factors aroundthe connecting welds.

nStress concentration factors around theweld connecting the 6-ft oblique brace tothe 30-ft column for the 6-ft brace underuniform compression. Stress concentra-tions are different on either side of theweld and vary between 1.6 and 3.

nSimple beam-element model of the 704’scolumn and brace structure that preservesthe overall stiffness characteristics of thesemi but without the detail around welds.

finite-element program, ANSYS (above).This permitted a detailed modeling of thecritical connection areas including gussets,stiffeners and reinforcements. The symmetryof the structure simplified modeling a little,but there were still five different column-brace and brace-brace connections to worryabout (next page).

The ANSYS modeling served two pur-poses. First, it provided the detailed picture,

showing how stress concentrates aroundeach weld. This information was captured asa stress concentration factor, a number lessor greater than one depending on whether acertain part of the weld bears less or morethan its proportionate share. As an example,the stress concentration factor around theweld connecting the 6-ft brace with the 30-ftcolumn varies from 1.6 to more than 3 for auniform compression on the brace (top). Itshould be noted that stress concentration isdifferent on either side of the weld, indicatedby the two sets of numbers around thebrace. Stress concentration factors will reen-ter the analysis later.

But the ANSYS modeling also allowedSedco Forex engineers to calibrate a simple

8 Oilfield Review

beam-element model (above) comprisingbeams joined together with joints character-ized by stiffness coefficients that describehow gross load is shared when the rig isstressed—a model that paints the big picturewhile dispensing with the geometrical com-plexity of actual joints and welds.

The first role for the beam-element modelwas to create an even simpler model which,while correctly simulating the shape of thesemi and the distribution of its mass,assumes complete rigidity. Fluctuating stressis now out of the picture. This simplificationis required to calculate the forces exerted onthe rig and its accompanying motion and

2

31

4

5

nStructure of the 704 (above) with five critical areas where columns and braces arewelded together—through the symmetry of the semi’s construction, these five accountfor all the main welds:

brace-column welds at the bottom of the four outside 30-ft columns

brace-column welds at the bottom of the inside 18-ft columns, two separate cases being required because the columns are structurally slightly dissimilar

so-called ‘K’ joints where brace meets brace between the 30-ft columns

double ‘K’ joints where brace meets brace between the 18-ft columns.

inertia as it rides the ocean—calculating theaction of the ocean on a flexing structure istoo difficult. The forces are of three types:the direct action of the waves on the semi’sunderwater structure, the buoyancy of thesemi, and the forces exerted by numerousanchoring cables and chains that maintainthe semi in position.

The basic calculation determines forcesand motion for a single planar sinusoidalwave, unlikely in practice but a necessaryprerequisite for later steps in the calculation.

Both the period of the wave and its direc-tion relative to the semi can be varied. Thecomputations were made using a propri-etary program called SMACS, and indepen-dently verified using an alternative methodby a classification authority (next page, bot-tom). Further verification came from com-paring computed results to model tests con-ducted by Offshore Technology Corporationin Escondido, California, USA in 1972 whenthe 700-series semis were being designed.In these tests, a 1/60-scale model of ananchored 700-series rig was subjected tosinusoidal waves of varying frequency andorientation in a 150-ft [46-m] long wave

tank. The computed results and experimen-tal tests agree remarkably.

A real sea—the technical term is seastate—comprises multiple wave componentsof different periods and a scatter of orienta-tions, so the sea’s net force and rig motionmust be derived by summing the individualeffects of the different components. The trickis knowing how a sea state breaks down intoits components, into its so-called wave spec-trum. Several wave spectra have been sug-gested following observations in differentoceans of the world. In all of them, the spec-

9April/July 1993

1

32

5

4

;

Detail (left) of the three braces meeting a 30-ft brace. Load is transferred betweenbrace and column at the outside shell of the column.

trum varies with significant wave height, ameasure of the sea’s roughness. For the 704,Sedco Forex used the Pierson-Moskovitzspectrum, derived from measurements madein the North Atlantic Ocean (right).

With the ocean’s effect on a rigid rigunderstood, it was time to move back to thebeam-element model to reintroduce stiffnessand varying stress. With HYDRA, anotherproprietary program, the ocean forces com-puted by SMACS were translated into stressvariations everywhere on the semi’scolumns and braces, particularly where theyjoined. Finally, by factoring in the stressconcentrations derived from the very fine-scale ANSYS modeling, variations in wave-created stress around individual weldscould be evaluated.

The final step in the analysis was now athand—estimating the stress history of indi-vidual welds and determining their fatiguelife. The principle for determining fatiguelife is simple and based on extensive

destructive testing in the laboratory. Theessential data for each type of weld arereduced to a simple curve on a double loga-rithmic plot, the so-called S-N curve (nextpage, top). S is the stress range experiencedby the weld, and N is the number of timesthe weld has been cycled through the stressrange. As long as a weld’s S and N valuesplot below the curve, the weld is safe. Butonce the data point reaches the S-N curve,fatigue may be advanced and the weld mustbe considered as having a 1% probability ofprogressing to failure. If the weld has experi-enced multiple stress ranges, as is the casewith a constantly varying sea, then a cumu-lative measure of fatigue based on the sameS-N curve is applied.

The previous calculations provided thestress range for all welds on the rig’s struc-tural frame. The missing link was knowingthe number of cycles a weld experiences.The wave spectrum goes part way toanswering that since it directly gives the

10 Oilfield Review

0

20

40

60

80

0 0.5 1 1.5 2Frequency, rad/sec

Wav

e sp

ectr

al d

ensi

ty

30 20 15 10 7 5 4 3Period, sec

6-m significant wave height

5-m

3-m

1.5-m

nWave spectra obtained by Pierson and Moskovitz for North Atlantic seas.The spectra show wave density versusfrequency or period, for four sea statesdefined by significant wave height.

5 10 15 20 250.0

0.5

1.0

1.5

Wave period, sec

Hea

ve/w

ave

heig

ht r

atio

Model, 0° 45°

90°

Model, 90°

0°

nSimulating theaction of a sinu-soidal wave on arigid model of the704. Inset: Heaveresults (continuouscurves) agree wellwith 1/60-scalewave tank experi-ments (points),made in 1972when the 700-series was beingdesigned, shownversus waveperiod and forthree wave orien-tations. The rig dis-plays resonanceat a wave periodaround 25 sec-onds, which issafely in excess ofrealistic values.

period, or cycle time, of each individualwave component. The problem is that thesea state and therefore the wave spectrumconstantly changes.

The final information needed is a statisti-cal distribution of sea states, a sea state rep-resenting a three- to six-hour lapse of timeduring which the wave spectrum is assumedto remain constant. Observations on the dis-tribution of sea states have been made in allcorners of the world’s oceans—Sedco Forexused data obtained in the North Sea. Thedata are typically presented in a wave scat-ter diagram that shows the probability ofany sea state, defined by significant waveheight and average periodicity (below,right). Combining the wave scatter diagramwith the wave spectra provides a completehistory of the stress cycling. Through the S-N curve, this leads to assigning a calculatedfatigue life in years to every weld in thestructure (below).

11

BCDE

F2GW

Wel

d ty

pe

Str

ess

rang

e S

, N/m

m2

108107106105104

Cycles, N

100

10

F

Unsafe

Safe

89100

108

2228

89

2816

39

67

3328

92

22

84

36

527

34

128

326

200

9545

26320

11695 80

335192 90

78

11

4-ft horizontal brace

30-ft column

K- joint between 18-ft columns

nS-N curves for assessing fatigue life for various weld types, indicated by theletters at right. Stress range, S, is plotted against the number of cycles, N, theweld has experienced. If the point plots on or above the relevant S-N curve,fatigue is likely and the weld is not considered safe. Welds for the 700-seriesrigs are of the F-type.

nWave scatter diagram showing probability distribution of sea states in the North Sea. A sea state is a stationary con-dition lasting between three and six hours and is character-ized by significant wave height and average period.

nExamples of fatigue life derived from the S-N curve—for weldsconnecting a 4-ft horizontal brace to a 30-ft column and for theK-joint between 18-ft columns. Weld areas with fatigue life lessthan 20 years are marked red and received remedial action inthe dry dock.

> 12

10-11

9-10 1 1

8-9 1 1 1

7-8 1 2 2 1 1

6-7 2 4 4 2 1

5-6 1 4 9 7 4 1

4-5 2 11 19 14 6 2 1

3-4 6 27 39 26 10 3 1

2-3 1 17 63 73 40 13 3 1

1-2 3 49 121 99 40 10 2

0-1 19 86 94 41 10 2

<4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 > 11

Zero crossing period, sec

Sig

nific

ant w

ave

heig

ht, m

In Dry DockThese results provided sharp focus to reme-dial work in the dry dock, with welds hav-ing short fatigue lives receiving the mostattention. For every weld with a fatigue lifeshorter than 20 years, a weld improvementprogram was performed that more than dou-bled fatigue life. All other welds wereinspected for cracks and found to be free ofdefects, giving them a clean bill of health.By this means, the structural integrity of the704 was satisfactorily ensured for a 40-yearlife, twice its previous life expectancy.

The improvement program was pioneeredby The Welding Institute (TWI), a UK-basedcenter of welding expertise and consultantto Sedco Forex throughout the dry-dockingof the 704. The program involved grindingthe weld toe with a burr tool or grindingdisk (below). This removes sharp edges thatnormally concentrate stress and increase thepropensity for fatigue crack growth, but justas importantly it also removes slag intru-sions in the toe that are a by-product of the

welding process. Slag intrusions createsmall flaws that can develop into fatiguecracks. In all, parts of 24 welds were sub-jected to toe grinding.

It would be misleading, however, to sug-gest that extending the life of the 704 simplyrequired targeting and treating a handful ofpotentially weak welds. The inspection pro-cess was comprehensive, following strictABS guidelines. In addition, it had toencompass a range of techniques to sub-stantiate the new five-year inspection cycle.

Sedco Forex and TWI first reviewed the704’s vessel construction portfolio, a com-prehensive file of previous inspections list-ing all observed defects and subsequentrepair work. Most defects were found duringthe early years of the rig’s life, as defectscaused by substandard workmanship andthe occasional out-of-specification materialwere found and corrected. There was notone mention of fatigue cracking in the entireportfolio. Although some concern aboutsubsurface flaws in the steel remained,TWI’s experience suggested these wereunlikely at this mature stage in the rig’slife—surface flaws are always more likelythan internal flaws since surface stressinvariably exceeds internal stress.

In addition, TWI reviewed the structure ofthe 704 for any deviation from the designused in Sedco Forex’s fatigue analysis. Onoccasion, disasters have been caused byfatigue cracks in seemingly insignificant andunrecorded structural modifications. Fol-lowing these reviews, TWI suggested theoverall inspection strategy that was subse-quently carried out by an independent third-party. As a result of an incident when asmall defect was revealed that had escapedinitial detection, TWI also recommendedthat future inspections of critical areas beperformed independently by two subcon-tractors. The defect in question was elimi-nated through weld-toe grinding and weld-metal replacement.

The first task once the 704 dry-dockedwas a comprehensive visual examinationallowing rapid assessment of large areas anddetection of gross flaws—none were foundon the 704. Visual examination also drawsattention to poor surface conditions thatmay limit the effectiveness of other detec-tion methods and reveals any deviationsfrom the design drawings. Throughout theinspection, assiduous recording of observa-tions is mandatory.

The visual inspection was followed byapplication of two nondestructive tech-niques. The most common testing method ismagnetic particle inspection, a cost-effec-

tive operation that reveals surface cracksinvisible to the naked eye. In this technique,the weld and the adjoining metal surfacesare first painted white. Once dry, the area ismagnetized by holding a strong magnetnear the weld. Simultaneously, a water solu-tion saturated with tiny, dark magnetic parti-cles is sprayed onto the surface. The parti-cles are attracted to any surface irregularitythat distorts the local magnetic field (above).Outlined by dark particles against a whitebackground, cracks immediately becomevisible. The technique is not foolproof, andcare is required in both conducting tests andinterpreting the results.

The second nondestructive technique,ultrasonic testing, probes beyond the reachof visual or magnetic particle inspection.For the 704, it was used to check the thick-ness of the column steel plating, not a prob-lem with a rig that has spent most if its lifein the North Sea, and also to check forcracks in single-closure welds—welds made

12 Oilfield Review

nPrinciple and practice of magnetic par-ticle testing. The weld is painted whiteand sprayed with a water-based solutionof dark magnetic particles. After a mag-netic field is applied across the weld, theparticles congregate around irregularitieson the weld, such as cracks, making theirregularities visible to the eye.

nPrinciple of weld toe grinding, a reme-dial action that removes the sharp sur-face contour of a weld, decreasing stressand eliminating flaws in the weld itself.The action more than doubles the weld’sfatigue life. Structural modeling identified24 areas on welds requiring this action.

Toe grinding

Plate

Plate

Weld

Flaw

nRight: one of four access holes in braces used to obtain samples of rig steel and weldmaterial for tensile and fracture toughness testing in the laboratory. Left top: schematic of section cut from the brace showing further cuts (red) used toobtain test specimens. Notches indicate location on specimens for fracture toughnessmeasurements—locations progress from weld to brace plate. Two specimens at bottom(blue) are for tensile testing.Left bottom: one of the test specimens before notching shows parent plates, weld metaland heat-affected zone, part of the plate altered by the heat of the welding process.

Plate Weld Plate

WeldPlate Plate

Heat-affected zone

from one side of the joint only—that wereused to close temporary access holes onseveral of the braces. It was important toverify that cracks were not developing frominside the brace, since these could penetratethe brace before becoming visible. All sus-pect welds discovered by ultrasonic testingwere replaced or repaired.

With these nondestructive techniques, the704 received the most thorough four-yearlytype of inspection, called a Type IV, that canbe demanded by ABS—less demandinginspections are sufficient for interveningyears. The challenge remained, however, todevise new testing methodology that wouldallow ABS to certify the 704 for five straightyears without any further inspection.

The Five-Year GuaranteeGuaranteeing five years of trouble-free oper-ation between major inspections requiresstate-of-the-art application of fracturemechanics, as outlined in British StandardsInstitution document PD6493:1991, a dry-sounding piece of literature but one thatrevolutionizes the maintenance and inspec-tion of large welded structures.6

The basic fracture mechanics premise isthat all welded steel structures contain flawsthat during the life of the structure can growthrough fatigue. Failure is prevented byensuring that cracks never reach the sizethat precipitates catastrophic fracture. TheBritish Standards document prescribes howto estimate the rate of growth and also the

April/July 1993

6. PD6493—Guidance on methods for assessing theacceptability of flaws in fusion welded structures. Lon-don, England: British Standards Institution, 1991.

7. Paris PC and Erdogan F: “A Critical Analysis of CrackPropagation Laws,” Journal of Basic Engineering 85(December 1963): 528-534.

nCrack growth during repeated stressingof a weld, calculated according to princi-ples outlined in the British Standards Insti-tution guidelines on fatigue assessment.

Life of crack

Cra

ck s

ize

Tolerable size at inspection

Critical size for failure

5 years

size of crack that eventually fails. With thesetwo pieces of information, the largest tolera-ble crack size at the beginning of the five-year period can be estimated: it is simplythe one that grows to the critical fracturesize after five years. Inspection for a five-year period then relies on detecting andeliminating any crack greater than this toler-able size.

Factors affecting the propagation of crackswere established by Paris and Erdogan in1963.7 Surprisingly, they discovered thatrate of propagation depended on just twofactors, the instantaneous length of thecrack and the stress range through whichthe material was being cycled. Using soft-ware they developed from the British Stan-dards document, TWI was able to deter-mine growth rates for critical welds on the704—Sedco Forex provided the stressranges from its own structural analysis (left).

The next step was determining the criticalcrack size that would lead to failure. Thisrequires a more complex computation, alsoderived according to principles specified in

the British Standards document, involvingthree parameters. First is the peak stress agiven weld is likely to experience during thefive-year period—the figure was providedby Sedco Forex. Second is the tensilestrength of the steels used in the 704. Thirdis the fracture toughness of the steels. Theselast two parameters were measured by TWIusing destructive tests on samples cut fromthe 704 (above).

Tensile strength is a measure of the stressrequired to pull material apart—the resultson the 704 samples exceeded ABS require-ments. Fracture toughness requires the diffi-

13

Fatigue crack

nConfiguration of sample for fracturetoughness, CTOD, testing. A notch is cutinto the sample and a small fatiguecrack initiated below the notch. A bend-ing moment is then applied until thefatigue crack fails.

cult crack-tip-opening-displacement (CTOD)test, which measures how much force ittakes to prize open an existing crack (right).In preparation for the test, a notch is cut intothe sample and then a fatigue crack initiatedbelow the notch. Force is then applied tobend the sample and increased until failure.

Fracture toughness is dependent mostlyon the steel’s crystalline microstructure andtemperature. It was therefore important totest all known steel types used in the manu-facture of the 704—four were isolated—alsothe weld metal, and, lastly, steel situatedclose to the weld that is altered by the heatof the welding process, the so-called heat-affected zone. Fracture toughness decreaseswith temperature, so all tests were con-ducted at the conservative temperature of–10°C [14°F], the minimum air temperatureused for designing North Sea structures andobviously lower than the coldest North Sea.The results showed, as expected, lowerCTOD fracture toughness in the heat-affected zone, and in some cases lower stillin the weld metal.

Two steps now remained. First, the peakstress, tensile strength and fracture tough-ness values were entered into TWI’s pro-gram to compute the largest permissiblecrack size at the end of the five-year period.Then, the history of crack propagation hadto be backtracked five years to derive thelargest tolerable crack size at time of inspec-tion. The results indicated that the combina-tion of toe grinding and magnetic particle

14

detection conducted during the dry-dockinspection would either detect or eliminateall potentially dangerous cracks.

On welds experiencing high stress con-centration, TWI computed the largest per-missible cracks to be 0.3 to 1 mm deep.These would have been eliminated throughthe toe-grinding process, which removed1.5 to 2 mm of the weld material. On weldsexperiencing minor stress concentration,TWI’s computation showed cracks must beless than 1 to 4 mm deep, easily visibleusing magnetic particle detection. The five-year guarantee was established.

This is a long story that could have beenlonger. To concentrate on the key aspects ofthe 704 life enhancement, no mention wasmade of a redundancy analysis to ensurethat if the semi does lose a brace it will sur-vive, nor of a leak-detection system installedin the columns and braces to permit contin-uous monitoring of their water-tightness,ensuring the rig’s buoyancy and providingan indication of premature cracking.

However long a narrative, though, the bot-tom line remains unchanged. Refurbishingthe 704 with an artful mix of theory, dry-dock practice and laboratory experimenta-tion cost a drilling contractor just 10% of thecost of building a new rig. This strategy willhelp guarantee cost-effective exploration inthe North Sea and other harsh areas. —HE

Oilfield Review