Hirsch Ocean Engineering

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CEAN ENGINEERINGDesign, Driving,And Eva' Of Long Offshore PilingI by T. J. Hirsch, Professor a nd Research En gineer ,

Cizil E ?lgi?leeringDepart)rze?zt, Texas A&M Uiliversity, College Stati on, Tex.

Fig. 1. "Set-up" orrecovery of strength af terdriving in cohesive soil.

2 0 0 4 0 0 6 0 0 8 0 0 1 0 00 I 2 0 0T I M E A F T E R D R IV I N G ( H O U R S )

R A T E O F P E N ET R A T I O N - B L O W S / F O O TFig. 2. Comparison of equivalent and actual are a methods.

B L O W S P E R F O O TFig. 3. Effect of wall thickn ess.

T h e one dimensional wave equation method of pileanalysis h as pro ven t o be a pow erful tool which czfn beused for th e design of th e piles, selection of hamm ersand other accessories for installation, and for evalua-tion of the load capacity of the piling as t hey a re dr ivenin hostile environmen ts.The wave equation can be used along with founda-tion exploration and evaluation techniques to selectthe pile size and thickness which could be expedi-t iously driven to th e d esired pene tration (or capacity)without reco urse to drilling or jetting . Various ham-mers can be evaluated so that the desi red sizes ortypes can be selected. Th e wave equation has also beenused as an aid in th e design an d selection of pile make-up, e xtension len gths , type of pile chasers , pile cush-ions and oth er necessary pile driving accessories. Im-proper choice of such accessories can undermine anotherw ise good pile driving ope ration.

(Continued)

7ooa -

Loa; 0 0 0 -

HARDWOOD CAPBLOCKO TON PILE CHASER5 4 In dlam 2 m wall pp. pi18P I L E L E N G T H 5 8 5 t I1 5 0 f t P E N E T R A T I O N

0 ~ " ' ~ ' ' ~ ' 1 ' ' ' " ~ ~ ~ " ' ~ ~ '0 5 0 1 0 0 W 20 0 2 5 0 3 0 0.B L OW S P E R F O O T

Fig. 4. Com parison of Menck 7000, 2500-4SL, and 2500pile driving ability -platform FA , B P Forties Field.

Technology of the ocean environment for engineeringloperating men


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Finally, when th e pi lings a re be ing dr iven the waveequation can be used along with th e pile driving recordof bloivs pe r foot to evaluate the load capacity of each' pile. This technique was used by PB Petroleum De-velopment, Ltd., during installation of platforms inthe North Sea Fort ies Fie ld. A t Pla tform FA , the seevaluations confirmed the required axial pile loadcapacities in excess of tha t required and permit ted the

use of only nine piles per le g which resu lted in consid-erable construction savings. A s a result of th e drivingcharacteristics of the first 12 piles, relatively simplepile driving criteria were developed which perm ittedquick an d spe edy completion of the remain ing piles.Pile-Soil Inter actio n

I t is important to unders tand tha t the Ivave equat ionmethod ofpi le analysis does not replace a good founda-tion exploration and evaluation prog ram. A goocl soilprofile and strength properties of the various forma-t ions are necessary input for a good wave equat ionanalys is. I t i s also important to un ders tand th a t th esoil resistance at the time of initial pile driving isusually less than t he soil resistance ac ting on the pilesevera l hours o r days af te r dr iving. T his phenomenonof soil "set-up" is illustrated by Fig. 1. When a pile isdriven in clays (cohesive soils) they generally remo ldth e clay and redu ce its friction resistance d urin g con-tinuous driving. An evaluation of soil "set-up" can be) obtained by driving piles to partial penetration, then

MENCK 700 07 5 X Eff~caency13 TON P I L E W A S E R5 4 In dlom 2 8 woll pop pnl.PlLE LENGTH 5 8 5 f1

BLOWS PE R FOOT

Fig. 6. Effect of capblock o n piledriving effectiveness, site FA .

6 0 0 0 .

5 0 0 0 -LP" 4 0 0 0 -zxb a o o o .

DBI -B2-0

R -b

X

MENCK 70 007 5 % E f f l c l r n c yO In. Nar dwood Copblosk

L

.

5 I / 54i n. dlom. 2," . ro l l Pip. Pala IE ~ 1 a n a i o n 2 0 5 fl . l on guoln PIIO 380 ft. lone24 0 ft . Penat rot i on

0 . O . lt n J p * O - I 5 J , = 0 . 2 0Ioi l R . 8 i rt . 20 % Point 80 % Fr~ct i o n

Jp 0 01 215'

0, ' 0 IJp '0 I5

Fig. 5 . Typical pile configuration, site FA .

W z

W

BLOWS PER FOOT

Fig. 7. Effect of pile chaser weighton pile driveability, site FA .

--:?-g

60 OCEAN ENGINEERING NOVEMBER 15,1975

M A I N P l L E ?- M A I N P I L E


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Fig. 8. Hollow 13-ton and solid 32-ton chaser used on 54-in.diam piles at platforms F A and FC.

delaying redr iving for several hours or days . T he in-crease in blows per foot required to redriv e a pile canbe used lv i th a wave equat ion bear ing graph toevalu ate th e increase in soil fr iction d ue to "set-up".Selection of pile size for offshore str uc tu res is usu-ally based 011 inservice design loads whicli produceaxial, shear and bending force s in th e piling. Designs ofthis ty pe usually result in piles w ith various wall thick-ness, with the thickest section usually immediatelyabove and below th e mudl ine. Des igns of this typ e ar eshown in Fig. 2. T hey may not be the m ost economicalor effective when one considers the drivea bili ty of thepile and construction time require d.Fig. 3 shows th at by inc reasing th e pile wall thick-ness below the mudline the pile driveabili ty is in-creased significantly. Th e wave equation an d field ex-per ience has shown that a stiffer pile can overcomegre ate r soil res istance dur ing dr iving. Before the pilewall thickness is f ixed, th e desig ner should carefullyanalyze the pi le for dr iveabi l i ty and cont ingencieswhich may occur durin g installation. To driv e long,high c apacity piles (4000, 6000, or 8000 'tip piles) gen-erally req uire s large ham me rs and stiff piles.Fig. 4 show s a comparison of the pile driveabili ty ofthre e di f ferent s ize pile hamme rs . A t 150 blob\-s erfoot the hlenck 2500 could o vercom e a soil resistance ofabout 4700 kips while th e hIenck 7000 could overcome6400 kips. Since the d esired c apacity of the piles a t theBP For t ies Field was appros imately 5500 kips , theMenck 7000 was select for f inal dr iving a t the des i redfinal penetration. This hammer had the capabili ty ofverifying the desired 5500 kip capacity of the pilesdur ing a redr ive af ter soi l "set -up" had occurred atfinal pene tration.Basically, the Menck 2500 and Menck 2500-4SLwere used for the ini t ial dr iving down to approxi-ma tely 180 ft of penetratio n. Th e final drivin g exten-

81019, FOOT

Fig. 9. Effect of cushion stiffne ss.PILE Cb P WE IGHT

10 K I P S

,

V U LC A N 0 6 0

OoI I I

20 40 60 90 1 0 0 I2 0BLOW 1 I N C H

Fig. 10. Effect of pile ca p weight.

4 5 % E l f

5 HOUR DELAY '

- E N C K 2 5 0 0 - 4 S L4 5 % E f t

6 5 % E l f .2 5 I J

0 2 0 4 0 60 80 IW 12 0 140 160BLOWS PER FOOT

Fig. 11.Typical driving record pile- 2-2, BPplatform FA.OCEAN ENGINEERING NOVEMBER 15,1975


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Lon g Offshore P i l ing

T O T AL S O I L R E S I S T A N C E A T T I M E O F D R I VI N G -K I PS

Fig. 12 . Wave equation prediction of soil resistancevs. penetra tion, pile B2-2- P platform FA.

ion was the n placed on th e piles and th e Menck 7000used to drive t h e last extension to the final c:zsiredpenetration which rang ed from 210 ft to 250 ft a t thevarious p la tforms. When in termediate sand layerswere encounte red a t P la t fo rms F B and F C the Menck7000 basically had the capacity to penetrate them at150 blows per foot . I n a few cases a t P la tform FC theMenck 7000 at 150 blows per foot was not able t opene t ra te the sand laye r a t approx imate ly 200 f tpenetration. This was anticipated, however, and theMenck 7000 verified that these piles were capable ofcarry ing t he desire d axial load of 5500 kips. In antici-pation of this refusal occurring, an added length ofthick wall pile was used in the pile make-up so tha t itwould be located a t the mudline where it was neededfor the larg e service load bending stre sse s at this loca-tion (see Fig. 5) .

By using the wave equation to evaluate pile drivingaccessories such as the capblock and the connectorbetw een th e main pile and driv ing extension, good piledriving effectiveness was maintained. Fig. 6 illus-trates how the asbestos capblock could reduce thedriv ing effectiveness of the M enck 7000 ha mm er by 9%or 10% if used in lieu of the u sual hard wo od capblock.Fig . 7 illustrates effect of the weight of the pile chaseror connector betw een th e main pile and driving exten-sion on th e pile driving effectiveness.If the %in. diam, 32-ton solid ste el cha ser is used,the driv ing effectiveness of th e hlenck 7000 would be

reduced by 7% to 8% when compared to the 13-tonchaser. The 13-ton chaser weight was achieved bymachining out th e cen ter of the solid chaser (s ee Fig.8). If both the 32-ton chas er and asb estos capblock hadbeen used, it is apparen t tha t the driving effectivenessof th e Menck 7000 would b e reduced by appro ximately17% and it would become a marginal hamm er for driv-in g 5500 kip piles.Th e effect of othe r pile accessories on the drivingeffectiveness of a given hamm er can be inves tigated byuse of the w ave equation. Figs. 9 and 10 prese nt ex-amples of the effect of cushion stiffness and pile capweight, respectively.

Field E valuationUse of the wave equation to determine soil resis-tance actin g on a pile during drivin g is relatively new.

Th e method was used quite successfully during con-struct ion of the p la tforms in the North Se a Fort iesField.A typical pile drivin g reco rd of blows pe r foot vs. pilepenetration (see Fig. 11) can be translated into a plot ofsoil resistance vs. pile pe netration (see -Fig. 12) byusing a graph similar to Fig. 4 which rela tes blows perfoot to soil resistance acting on t he pile.Th e information show n in Fig. 12 for several pileswith sev eral different time delays betwe en initial driv-ing and redriving can be used to determine the ulti-mate static load capacity of the piling. One must re-OCEAN ENGINEERING * NOVEMBER 15,1975


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i"ong Offshore Piling SO l L RESISTANCE - KIPS 8000 9000IM A X COMPRESSIVE CAPACITYPREDICTED BY C P T METHOO o SOIL RESISTANCE DUR ING CONTINUOUS ,OPTIMISTIC SOIL PROFILE-SOIL PLUG DR IV I NGt- 50 - PILE PE NE T R AT E D UNDER OWN WEIGHT -WW

0 SOIL RESISTANCE W R I N G RE-DRIVINGLL M I N COMPRESSIVE CAPACITY AFTER SOIL S E T - U P - TIMEA METHOD - ESSIMISTIC SOILPROFILE - N O SOlL PLUG DELAY INDICATED BESIDE DATA POINT -

-e-012 hr. -- hr -- --- - --A*_ -.4 Q ~ 4 ! --50Fig. 13.Sum mary of wave equation pr etlic tiol~ s f soilresistance vs p enetration of first 12 piles- P platform F A .

member th a t the r a v e equat ion ana lys is can be used topredict the soil resistance acting on the pile duringdrivin g a t a given blow count pe r foot. One can care-fully evaluate the soil "set-up" proper t ies s h o ~ m ydata s imilar to Fig. 12 and d etermine the soil "set -upfactor" which can the n be applied t o the soil resistan ceacting on the pile during continuous driving a t finalpenetration. A more direct method of obtaining theultimate st atic load capacity is to red rive a pile a t finalp en e t r a t i o n a f t e r s ev e r a l d ay s d e l ay i n o r d e r t oevalu ate the soil "set-up" a nd final ultim ate capacity.Both methods were used on the Nor th Sea For t i esField. Both methods require that pi les be redr ivenaft er a sa tisfactory tim e delay t o obtain soil "set-up."Fjg. 13 shows a summ ary of the wave equat ion pre-dictions of soil resistan ce vs. pene tration for the firs ttwelve piles on Platform FA. Superimposed on thisf igure a re th e maximum and minimum pile compres-sive capacities predicted by t he soil cons ultants usingclassical foundation exploration an d e valuation tech-niques . Fr om Fig. 13 i t can be seen tha t the pi lespene t r a ted kom 50 to 70 f t under th eir own weight .The se piles remained in the soil a day or m ore beforepile driving commenced. T he solid data p oints indicatethe magn itude of soil resistance a cting on the piles du r-ing cont inuous dr iving. T he open da ta points indicatethe soi l res istance dur ing a re-dr ive af te r several hoursof t ime delay. I t can be seen that in the clay thesoil resistance durin g continuous driving is abou t one

half the static soil resistance which would developwhen th e clay reconsol idates and se ts up.When t he pi les pene trate t he s i l ty sand layer the soilres is tance increases rapidly. W hen th e pi les ar e dr ivencont inuous ly into the sand layer and s topped, i t isappa rent f rom Fig. 13 th at th e soi l res is tance encoun-tered dur ing cont inuous dr iving can be increased bythe anticipated am oun t of soil "set-up" in the clay toarrive at th e final pile load bearing capacity. aAcknowledgement. This ar t icle was adapted from pape r OTC 2247presented by the auth or at the Seventh Annual Offshore TechnologyConference, Houston, Tex., May 5-8, 1975. Co-authors of the OTCpaper were Albert M. Koehler of Brown & Root Inc. and V.J.R.Sutto n, BP Petroleum Development Ltd.

About the AuthorT . J . Hirsch isa professor ofc i v i l e ng ine e r ing a l zd re-search engineer with TexasA & M U n i v e r s i ty . H e h a sbeen act zvely ellgaged i n pilefounda tion research an d de-velopm ent for over 15 years.He holds B S , M& and PhDdegrees from Texa s A & M a ~ t dis considered one of the pio-neers in the developnze?zt ofthe wave equation method ofanalys is for the design andevaluatio n of pil ing.

66 OCEAN ENGINEERING NOVEMBER 15,1975

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Hirsch Ocean Engineering