OTC-5049-MS-Paper.pdf

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OTe 5049Design Methodology for Offshore Platform Tieback Conductorsby R.B. Manley Jr., Amoco Production Co.Copyright 1985 Offshore Technology ConferenceThis paper was presented at the 17th Annual OTC in Houston, Texas, May6-9,1985. The material is subject to correction by the author. Permission tocopy is restr icted to an abstract of not more than 300 words.

ABSTRACTA methodology is presented for th e d es ig n o ft ieback conductors and internal casing str ings. Theanalysis is done by hand rather than by computer.The methodology considers th e combined loading from

environmental forces, external weights and internals trings in tension or compres sion , thermal expansion, and angular and la tera l offsets. I t also considers th e fac t that i nt er na l s tr in g s in tension dono t cause buckling.INTRODUCTION

This paper presents a comprehensive methodologyfo r the design of t ieback c o n d ~ c t o r s and t h ~ i rinternal casing s trings . The s tabi l i ty and strength

This p ap er is a c om pa ni on to th e paper presented at th e Offshore Europe 1983 Conference ent i t led itA North Sea Template Well Tieback System:Design and Operational Exper ience" (Reference 1).That paper presented th e mechanical design andinstal lat ion experience from t he t ie ba ck of th eseven subsea wells beneath th e Northwest HuttonPlatform. The s tructural d es ig n o f th e tieback conductors and internal casing s trings was performedusing th e methodology presented in this paper.Together, th e two papers give a complete overview ofthe knowledge gained from th e Northwest Hutton Platform tiebacks. The following three paragraphsbriefly describe th e Northwest Hutton Platform andthe t ieback conductor ins ta l la t ion procedure.

The Northwest Hutton Field is l oc at ed i n


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2 DESIGN METHODOLOGY FOR O ~ ~ S H O R E PLATFORM TIEBACK CONDUCTOR OTC 50494. Run 20 in . outer t ieback, engage wellhead

and lock.5. Cut 20 in . and weld on casing head.6. Run and la tch 13-3/8 in . ~ i e b a c k .7. Se t s t r ips and packoff; cut casing andins ta l l casing head.8. Retrieve inser t , flush s ea l a re a.9. Run and lock down 10-3/4 in . t ieback; set

s l ips , cut casing and ins ta l l tubing head.DISCUSSION

Theoretical BasisThe theoretical foundation of th e subject t i e -back conductor design methodology was p re se nt ed i n

Reference 2. That reference, however, was specif i -cally concerned with th e design of standard conductors fo r conventional fixed platforms. As will bediscussed s ho rt ly , t ha t methodology must be modifiedsomewhat for the design of t ieback conductors.Perhaps th e most significant contribution ofReference 2 is t he exp la na ti on o f why t en sions i ninternal casing and tubing s t r ings , that are hung atth e top of th e conductor, do not in duc e b uckl in g ofth e conductor.Reference 2 presents in teraction rat io (unityratio or IR) equations that account fo r th e aboveeffects . The app l icab i l i ty o f th os e eq uations was

che cked by preparing a thorough mathematical modelof th e conductor and inner st r ings, and analyzingth e s t resses in them by means of a nonlinearfinite-element computer program. Reference 2 showsthat one can us e those equa t ions to check the adequacy of th e st r ings, without having to perform acomputer analysis.Design Assumptions and Loads

The f i r s t s_tep in the design procedure is th edevelopment of the design assumptions and loads.

perature and current . The thermal conductivitiesand res is t ivi t ies of th e walls of the strings andth e mate ria ls in th e annuli between them can then bede te rmined , a long w ith th e c on ve ctiv e h eat t ransfercoefficient of the conductor wall to th e moving seawater. A heat transfer computer program can then beused to calculate th e steady s tate t emperature o feach of t he c as in g s t r ings . (Lacking a l l of th eabove, one can make conservative assumptions of th ecasing temperatures.) The in ternal casing stringsare assumed to heat up to the i r steady state temperatures from th e inside out, in turn , in th e derivation of th e maximum thermal thrust in each s t r ing .This is discussed fur ther shortly .

The in ternal casing strings mayor may not bepretensioned. I f one determines that such pretension is desired (to l imit th e compression due tothermal thrust in th e in ternal st r ings, to aid incentral iz ing th e strings in the wellhead at th eplatform cel lar deck, or to aid in establishing th eannular pressure pack-offs) , one should considerseveral factors. Firs t , th e pretension is developedby picking up on th e strings af t e r they are lockedinto th e subsea wellhead. There will be very l i t t l es t re tch fo r th e d es ire d p re te ns io n. Second, much ofth e top tension in the s tr in g w il l be los t due tos li pp ag e o f that string through th e sLips when i t islowered in the sl ips and before th e slips bite.Thi rd , t he re will be an additional loss of tensionin that string as th e combined s tr in gs d ef le ct a xi ally due to that tension. Finally , the inc remen ta lcompressional loads on th e other casing strings dueto th e combined deflection must be accounted for.This too is discussed shortly .

The t ieback internal ( " a ~ i n g " ~ i n g s mayor maynot be ru n with central izers . C e n l r ~ l i z i n g th ei n terna . l cas ing s t r i n g s car, he l p t tie o u te r c on d uc to rr es i s t bending moment, provided that those innerstrings are no t already loaded to t he ir u lt imat ecapacit ies. Such centralizat 'o n wllI also reducethe slight bending moment on th e c0nductors thatresu l ts from th e eccentrici ty of internal casing


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UTe "'.i049 b. MANLEY, JR. 3Analysis Methodology

Tieback Overshot Moment - The l leback overshotbending moments on th e t ieback strings can be calcu-lated by anyone o f s ev er al i nd ete rm in ate s t ructu ralanalysis methods, s uch as th e moment-slope, ormoment distr ibut ion methods, or by means of a computer program. I t is assumed that th e bottom of th econductor i s displaced la teral ly and ro ta t ional ly byth e amount of the offsets (conservatively assumed tobe additive), bu t that the remainder of th e conductor i s la teral ly restrained a t th e conductorguides. For conven ience , only t h r ~ e spans of theconductor (a span is th e distance from one conductorguide to th e next) are considered, as t iebackmoments drop off rapidly with distance above th esubsea wellhead (see Figure 3). The moments arearbi t rar i ly set a t zero at th e top of th e th irdspan, although in real i ty , there will be a smallmoment there. (The designer can analyze as manyspans as desired.) No axial load is considered inth e t ieback moment calculat ion, as th e secondarymoments ( load deflect ion or P-A effect) areaccounted for la ter in th e in teraction rat io equat ions. Also, for convenience in the design process,th e calculation can be made fo r a unit la teralof fse t , a unit angular offset and a unit moment ofiner t ia . Appropriate ratios are then applied asrequired for other offsets and other moments ofiner t ia .Axial Loads-

The worst compressive load in each of th e internalcasing strings occurs as each heats up upon product ion. I f they are pretensioned, the tensi lestresses occurring when each is ini t ia l ly pretensioned (before lowering into th e sl ips) should bechecked as well.

The ex te rna l ax ia l load (See Ref ere nc e 2) onth e outer conductor, at any elevat ion, is th e sum ofth e external weights on to p of th e conductor, andth e buoyant weights of the conductor and a ll of th ein ternal casing and tubing st r ings, from that elevat ion to th e top of the conductor.The maximum in ternal compressive load (See Ref

erence 2) applied at th e ~ of the conductor is th eto tal compressive load on th e top of the conductorimmediately af t e r th e prOduction tubing is ru n andhung off, but whi le th e BOP i s s t i l l in place, lessth e external weights applied a t th e top of the conductor. Both loads were calculated in th e axialload history analysis . The maximum in ternal axialload on th e conductor a t any other elevation is thenderived by subtracting from th e above maximumin ternal load at the top of the conductor, th ebuoyant weights of a ll of the in ternal strings( including t he p roduct ion tubing), from t ha t e le va t ion to the to p of the conductor . Those weightss ub tr ac te d a re considered part of the external load,as noted in t he p reviou s par ag raph .

Following are a few definit ions arid exampleca lcu la t ion s cons is ten t with Figure 4 that i l l us t ra te th e concept of in ternal and ext er na l a xi alloads.

MSL a t 0 ' elevationPlatform cel lar deck at +83' elevationSubsea wellhead @ -458' elevat ion.W=weight of t ieback string or tb g from+83' elevation to -458'

Axial loads are applied to th e conductor inthree ways. The f i r s t arises from external weightsapplied at th e top, such as that of the blowout preventer (BOP). The second type of axial load is aninternal load that ar ises from pretensioning thet ieback in ternal casing strings and hanging product ion tubing. The th ird type of axial load arisesfrom th e thermal expansion t hr us ts t ha t occur whenth e in ternal casing strings heat up upon th e well 'sbeing brought on production. The la t ter l oad p la ce scompressional loads on the in ternal st r ings, and


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without buckling than the outer conductor aLone can. internal bending moment on the outer conductor isThis method of considering the combined strength of then the sum of the internal bending moments fromthe conductor and first inner string is inconvenient the strings in tension, as calculated above, lessfor the design of tieback conductors, however, as the sum of the products of the loads in the internalvariations in the assumed design pretensions, tubing strings in compression times the eccentricity ofweights, etc., can cause any of the inner strings to those strings. The resultant, total, internaL com-be anywhere from strongly in compression to strongly pressive load and internal bending moment are thenin tension. It was found that there was a signifi- appLled to the outer conductor alone, rather thancant jump in the interaction ratio between the con- the composite conductor noted in Reference 2. Andition of the first inner strings being very example calcualtion of the internaL axial loads andslightly in tension to its being very slightly in bending moments applied to the conductor at the topcompression. That jump was due to being able to is shown in the coLumns below. The equations belowinclude that string in a composite conductor when it illustrate how the internal bending moment andwas in compression, but not when it was in tension. internal and external axial Loads on the conductorThis discrepancy should not exist. There is really are calculated at any elevation. These calculationsvery little difference between the two cases. are consistent with Figure 4.

Axial Eccentricity InternalAs a result of the above discontinuity, the Loati:


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OTC 5049 k B. MANLEY, JR. 5

PE(Z) = BOP + (wabove water line Z = plastic section modulus of thed~oll casing (in.3)

+W Y = load factorb13-3/8 = 1.7 when extreme environmentalloads are not considered

+W blo-3/4 = 1.3 when extreme environmentalloads are considered+W ) z ... (3)b5-1/2!! The above equation is a strength rather thanstability equation because the internal casingstrings in compression are laterally supported by

PE(Z)below WL = BOP + (83)(wd ) the outer conductor.20,1When an internal casing string is in tension,

+ (Z - 83)(w ) its maximum axial load at any elevation is simpLyb20 the maximum tensile Load from the time history anal-ysis (or initial pretension before lowering inslips), accounting for the change in axial load down

+ (w +W the string resulting from the strings buoyant unitb13-3/8 blo-3/4 weight. The bending moment considered comes fromthe tieback offsets, and is worst at the bottom of

+W ) z ........... (4) the string. Tieback offsets would generally resultb5-1/2 in sufficiently large deflections across the Longspans considered (if the conductor guides did not

laterally restrain the conductor and inner strings)Internal casing strings that are centralized that it makes Little difference to the tieback over-

will help the conductor resist bending, regardLess shot bending moment in the internaL string whetherof whether those strings are in compression or ten- that string is centralized in the outer conductor orsion (another difference from Reference 2 where onLy not. It is thus onLy sLightLy conservative to cal-the first internal string was considered in the com- cuLate such tieback overshot moments in the innerposite conductor, and then onLy if it was in com- strings as if they were fuLIY centralized in theouter conductor.pression). The extent of that bending resistance Wave bendir,gmoments are anotherheLp is discussed shortLy. story. GeneralLy, the lateraL deflection of theouter conductor from wave loads is small enough thatInternal String Stress Check very little wave induced bending moments would get

into uncentralized inner strings. UnLess an innerOnce the worst axial Load in each of the string is centralized, therefore, no wave moment isinternal casing strings is determined, a stress considered in the inner string.check is made for each for its worst Load condition, Centralizers on inner strings cannot actually


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6 DESIGN METHODOLOGY FolsrlFFSH(N~PT,ATFURMTIEBACK CONDUCTOR OTC 5049

makes the interaction ratio calculated there withwave loading much less than that without waveloading, where a Y = 1.7 is considered. Waveloading is thus usually not a significant factor inthe internal string stress check.

When the internal string is in compression anduncentralized, the maximum compressive load comesfrom the time history anaLysis,and the bending loadis the sum of the bending moments from helical buck-Ling and the tieback overshot bending moment. TheheLical buckling stress is calculated by means ofthe following equation from Reference 3:

_ DrF_b 41

...................................... (6)

where: D = nominal outside diameter of thepipe in question (in.)r = radial clearance between thispipe and the next one out(in.)

F = axial compressive load at thatelevation, from the timehistory analysis, and accountingfor casing weight (kips)

I = moment of inertia of this casingstring (in. 4,

If centralizers are used, r is the cLearancebetween the outside of the centralizers and theinside waLl of the next string out. The use of cen-tralizers will thus significantly decrease the hel-ical buckling stresses. These stresses are,however, usualLy far less than the tieback overshotand wave bending stresses, and thus generally notsignificant regardless of whether or not central-izers are used.

When the inner string is in compression andcentralized, the maximum compressive load againcomes from the time history anaLysis. The bending

Equation L is aLso used to caLculate thereserve bending moment capacity of each inner stringthat is centralized in the outer conductor. Thereserve bending moment capacity of a string is theamount of additional bending moment that the stringcan take in addition to the loads that it is aLreadycarrying. The reserve bending moment capacities ofcentralized strings can be added to the pLasticmoment capacity (M ) of the outer conductor in det-?rmining its capac ty to support bending loads.

The Load factor, Y, is not used in the reservebending moment calculation, as it is applied laterin the outer conductor stress check. It is thusdeLeted from Equation 1 for this calculation. Sincethe inner string reserve bending moment capacitiesare used in the outer conductor stress check, theinner string loads considered in deriving thosereserve capacities are those that occur in the innerstrings at the same time the outer conductor isexperiencing its worst load. These Loads all comefrom the time history, axial load anaLysis.

To determine an inner strings reservestrength, one calculates the aLLowable bendingmoment in the inner string by setting the IR inEquation 1 equal to L.0, deleting Y from theequation, and solving for M. This value must beLess than or equal to M . For P/P less than 0.L5,the calculated value ofpM wouLd beygreater than M .In that case, the alLowable moment is Limited to ~ .One then deducts the actual bending moment in the pstring (sum of helical buckLing, overshot and wavebending moments) from the allowabLe moment to arriveat the remaining bending moment that the string cantake, i.e., the reserve bending moment capacity ofthe string. The reserve bending moment capacitiesof the centralized inner strings are then added tothe M of the outer conductor alone, to arrive atthe tgtal, uLtimate plastic moment capacity of theconductor. This is equivalent to the method pre-sented in Reference 2 for the determination of thebending properties of a composite conductor. The


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OTC 5049 F!. ,yAN- , . IR. 7

The two primary conductor design equations, M1/M2 = ratio of smaller to largerboth from Equation 9 in Reference 2, are as follows: overshot moments at the ends ofthe unbraced portion of theunbraced portion of the span

~R= Y@l+pE) Y(M+MOS) considered. MI/Mz is positiveP + 1.18(1-Y P P )M S1.O .. (7) if the span is bent in reverseY EJep curvature and negative forStrength Check single curvature.M2=MSinyP1 yPE YI0.85M+CMMOS] 9

Equation 8, as defined+ (1-~ PE/Pe)Mp ~lo .. (8)

a oveIR = ~ +pY crm Stability Check P = Euler buckling load (kips)e = A?12E/(kl/r)2

k = effective length factorwhere: PI = internal compressive load in

= 0.8 for bottom span of conductorthe outer conductor aLone at = 1.0 for subsequent spans offbottomthe elevation considered, dueto axial loads in the inner r = radius of gyration of conductor(in.)strings (kips) L = length of conductor span

considered (in.)PE = external axial Load in the E = modulus of elasticity (kips/in2)conductor at the elevationconsidered due to weights Mp = (ZF )on top of the conductor, y conductorplus the combined weightsof the conductor itself and

= ultimate plastic moment capacitythe inner strings from of the conductor alone for thethe elevation considered case with uncentralized innerto the top of the conductor casing strings (in.-kips)(kips) = zFy)conductor +

A = cross-sectional area of the ~Mconductor (in.z) reserveinner strings or

PY = AF the case with internaL casingY strings centralized in= axial yield capacity of the co nduc tor (i n.-k ips )

conductor (kips) Note that the Euler buckling load, Pe, inEq. (7) is the minimum Pe of the spans immediatelyP = the modified criticaL bucklingcrm load of the outer conductor above and below the support point being checked.For the bottom and top of the conductor, there is


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8 DESIGN METHODOLOGY FOR OFFSHORE PLATFORM TIEBACK CONDUCTOR OTC 5049the denominator in the strength equation, ratherthan 0.85 in the numerator, helps to resolve the 2. Stahl, B., Baur, M. P.~ Design Methodology forconfusion. Offshore Platform Conductors, Journal ofPetroleum Technology, November, 1983.

Note that, as discussed previously, the outerconductor is considered aLone in calculating P in 3. Lubinski, A., ALthouse, W. S., Logan, J. L.,Equation 8. No composite conductor is conside$~~ in Helical Buckling of Tubing SeaLed in Packers,that calculation. Journal of Petroleum Technology, June, 1962,Transactions AIME (1962), 225, 655.Conductor Stress Check

Strength checks with Equation 7 are performedat those points where the conductor is laterallysupported, i.e., at the wellheads and conductorguides. The conductor cannot buckle laterally atthose points. Buckling checks with Equation 8 aremade at the mid-spans between the conductor guides.Buckling will generally control over strength at themidspans.

The IR checks must be performed for the entireLength of the conductor, rather than just for thebottom portion, as the loads and span Lengths changealong the conductor, and these changes do affect theinteraction ratio calculations significantly.CONCLUSIONSThe seven Northwest Hutton Platform tieback conduc-tors and their internal casing strings were designedby means of the analysis method presented in thispaper. As noted earlier, those wells were tied backin January and February of 1983, and placed on pro-duction. During the installation, pretensionsgreater than those assumed in the design wererequired in the inner strings to energize theweight-set annular packoffs used in the first well.(The packoffs were change~-to Allen-screw energizedpackoffs in the remaining wells.) The ease ofchanging design conditions in this analysis methodenabled the writer to determine at the site just howmuch help could be obtained by adding centralizersto the inner strings, and how much pretension could


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PlatformWellheadspools

+ P~

Fig.1lieback Schematic(From Reference1)

550ft.

SubseaWellhead

VeRe

uctor Angle

20 TebackConnector

f{

p

$1\

Wellhead. ,!LateralOffset L+ Wellhead Angle

Fig. 2Misalignment Between wellhead & ~eback connector

(From Reference1)


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+83r+24 - >c

o-75 -

- 110~L tco= _1910Ziii

-280

I 71 \\ \ \ \.-375 t t \>-------.--..----I458-400;+ I I I A f I I [-2000 0 4000 2000Bending Moment (In Kips)x Conductor Guide Wave Moment---- Overshot Moment

Fig. 3

20 13-3/8 10-3/4_ 5-1/2. _ _

450.6k I 47.lk t 38.2k+83r

w

B Iw~olf=62.2kXL-

t 312. 8k t

w13_3/ 8wI =31.9k

79k +

Fig.4

1&3/ 4.=28.7k200k

!W5_l/2V,= 10.6k

9.5k ! 189.4k

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