LABORATORY TESTING FOR OFFSHORE STRUCTURES ESSAIS EN LABORATOIRE POUR DIMENSIONNEMENT OFFSHORE. The Norwegian Geotechnical Institute Oslo, Norway

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1 XIII ICSMFE, 1S94. New Delhi, India / XIII CIM STF, 1994, New Delhi, Inde LABORATORY TESTING FOR OFFSHORE STRUCTURES ESSAIS EN LABORATOIRE POUR DIMENSIONNEMENT OFFSHORE s L j i i u Tm Lunne Trnd By Suzanne Lacasse The Nrwegian Getechnical Institute Osl, Nrway SYNOPSIS: Labratry testing is a very imprtant part f ffshre sil investigatins. Fr large imprtant prjects with tight time schedule, special tests in the ffshre labratry shuld be cnsidered. Offshre labratry testing is als imprtant fr a cntinuus evaluatin f sample disturbance and fr assessing stress histry and in situ stresses f the sil in situ. This is essential input fr the mre advanced nshre labratry testing, including cyclic lading. The well-knwn fact that UU tests may be misleading fr determinatin f in situ undrained shear strength is again dcumented. It is shwn that cnslidating back t in situ stresses is the preferred apprach. Labratry tests shuld as much as pssible simulate the stress changes fr the in situ situatin, even fr pile design. Hwever, UU tests are still used fr pile design, due t the large empirical database n which the API guidelines are based. Gas in the prewater significantly influence the results f labratry tests even if it is nt in free frm in situ. Recmmendatins are given fr hw sil cntaining gas shuld be handled and tested t achieve as representative results as pssible. INTRODUCTION Labratry testing is a very imprtant part f ffshre site investigatins. Based n previus knwledge abut the site and the structure cnsidered, the number and types f tests in the ffshre and nshre labratries shuld be cnsidered during the early stage f the planning fr the investigatin, since this may deteniiine the number f brehles required. During the field wrk, sample quality shuld cntinuusly be evaluated and a reasnable amunt f sil material shuld be used in the ffshre labratry. Samples fr nshre testing shuld be sealed and handled carefully during shipment t the land based labratry. Fr advanced nshre labratry testing, with static and cyclic tests, it is particularly imprtant that the in situ stresses are reliably assessed fr designing the cnslidatin phase. Samples that are suspected t cntain gas shuld be treated with special care. The test prgramme shuld establish the required tests with representative cnslidatin and shearing stress paths. Different test types and stress paths need t be cnsidered fr different structures and lad situatins. The paper discusses examples frm the Nrth Sea. OFFSHORE LABORATORY TESTING The extent f labratry testing ffshre shuld be decided at an early stage. It is nrmal practice t carry ut a limited range f rutine tests in the nbard labratry (e.g. Andresen et al., 1979). These include sample descriptin, water cntent, ttal unit weight, and index shear strength by pcket penetrmeter, fall cne, trvane and in sme cases uncnfined cmpressin tests. A range f mre advanced labratry tests are als run ffshre in sme cases (e.g. Amundsen et al., 1985): X-ray f samples Cnslidated Anistrpically Undrained, CAU, triaxial tests Direct Simple Shear, DSS, tests Cnstant Rate f Strain, CRSC, edmeter tests Gm measurements with bender element The main arguments fr ding these special tests ffshre are: a) cntinuus evaluatin f sample disturbance. It may be pssible t imprve sample quality by mdifying rate f drilling, mud pressure during drilling, and/r sampling equipment, b) t assess in situ stresses and stress histry befre starting the mre advanced nshre labratry testing prgram, c) t speed up labratry testing and reprting including having a reprt available at the end f the field wrk, d) t test samples a shrt time after retrieval as the time elapsed between sampling and labratry testing may influence the test results. EFFECTS O F SAM PLE DISTURBANCE The stress changes that ccur when a tube sample is retrieved have three cmpnents: (i) (ii) (iii) Shear strains are impsed n a sample as the tube is pushed r driven int the grund. The ttal bundary stresses are reduced due t sampling and gradual equalizatin f pre water pressure within the sample. Other stress changes ccur as a result f mechanical disturbance, bth when taking the sample, during transprt and when extruding it in the labratry. The strains caused by tube sampling were studied by Hight (1992). He cncluded that clay plasticity, stress histry and the gemetry f the sampler have the largest influence. NGI has at times used pressurized canisters t minimize the time the sample is under zer ttal stresses. This is especially imprtant in gassy sils where expansin f the gas may cause serius sample disturbance. Stress changes which ccur as a result f mechanical disturbance can in mst cases be minimized by using the right equipment and careful handling. Fr instance, the intrductin f the "hard tie* (Amundsen et al., 1985) system t better cntrl the drill string penetratin and cmpensate fr tidal 1677

2 variatin imprved the sample quality significantly. Figure 1 cmpares,, edmeter test results n samples taken with and withut the "hard tie" system. P' strains than the pistn samples (Fig. 2). This effect wuld be even m ri imprtant if blck samples and push r hammer samples were cmpared. Fig. 1 Effective axial stress, kn/m2 Example n imprved sample quality by use f "hard-tie" system during drilling. Varius methds have been prpsed t assess the likely level f disturbance t samples. Visual inspectin f extruded samples and radigraphy (Lacasse et al., 1984) f sil in sampling tubes n bard the drilling ship are rapid methds t check the quality and hmgeneity f samples. Hwever, disturbance nt bservable upn visual inspectin may still be significant. Measurement f strains during recnslidatin t in situ stresses is a mre quantifiable methd. Table 1 belw shws a classificatin system used at NGI: Table 1 Range f OCR Classificatin f sample disturbance accrding t vlumetric strain, at p0 in CAU triaxial tests. (OCR = vercnslidatin rati). Depth interval m Very gd test if v < % Acceptable test if < v < % Likely t be very disturbed if e. > % Fig. 2 Effect f sample disturbance n peak shear strength and axial strain at failure in anistrpically cnslidated triaxial tests. Cmparisn f field and labratry measurements f dynamic shear mdulus may als give a gd indicatin f pssible sample disturbance f clay prvided that the samples are cnslidated t the in situ stresses (Hight, 1992). Measurement f the dynamic shear mdulus can be dne ffshre as well as nshre by bender elements in edmeter, direct simple shear r triaxial tests (Dyvik and Olsen, 1985). The mst imprtant effects f sample disturbance are reduced sil mduli, reduced shear strength and higher failure strains. Recnslidatin f the samples back t in situ stresses will tend t "repair" the sil. Figure 3 cmpares results frm uncnslidated undrained (UU) tests and anistrpically cnslidated undrained triaxial tests (CAU). The rati between the shear strengths btained with the tw methds is presented as functin f the vlumetric strain after cnslidatin (CAUC-tests). Each pair f tests cme frm the same pistn sample and are assumed t be equally disturbed. The figure clearly shw that with increasing sample disturbance (increasing vlumetric strain) the shear strength frm uncnslidated tests will be cnsiderably lwer than fr cnslidated tests. X X X a < Nrth Sea sites Nrmally cnslidâted cli*y Slightly vercnslidated clay x Overcnslidated clay Vlumetric strain after cnclidatin t p'0, % fr CAU triaxial tests * X Table 1 is useful when cmparing the relative quality f samples frm the same sil type. The abslute value f the strains will depend, hwever, n the stress path fllwed, the specimen size and the lcal sil structure, as discussed by Hight (1992). Fig. 3 Effect f sample disturbance n the relatinship between undrained shear strength frm uncnslidated and cnslidated triaxial tests. Lacasse et al. (1985) cmpared the quality f blck samples 300 mm in diameter with the quality f 95 mm pistn samples fr three Nrwegian marine clays with plasticities between 5 and 40%. Triaxial test results, shwed that the blck samples gave higher shear stresses at lwer failure 1678

3 EFFECT OF STURAGE, TRANSPORT AND RECONSOLIDATION During the 1984 deep water (320 m) sil investigatin at the Trll East field (Amundsen et al., 1985), tests dne very shrtly after sample retrieval n deck were cmpared t tests in the nshre labratry sme time later. Based n inspectin f n bard X-rays, parallel samples frm the same sample tube were selected, carefully waxed and sealed fr transprt t N GI's labratry in Osl. Tests with as indentical as pssible test cnditins and prcedures were then perfrmed. Figure 4 shws the precnslidatin stress determined ftm the CRSC tests using the Casagrande methd. Figure 5 shws stress-strain curve >and maximum shear stress ( s j frm nshre and ffshre DSS tests as a functin f depth. 30 Offshre Onshre - / px P'c \, Precnslidatin stress, 15 P'c (k P a ) Offshre pistn sample Onshre pistn sample " Push sample P' Trll field Uncnslidated undrained (UU) tests n unladed samples shwed that cmpared t the in situ sil lwer shear strengths and larger strains t failure were measured. This behaviur became mre prnunced with increasing sample age and was accmpanied by dissipatin f residual negative pre pressures in the samples. A summary f the results as shwed in Fig. 6 led Kirkpatrick and Khan (1984) t cnclude that 'th e uncnslidated undrained test is misleading fr predicting in situ behaviur where stress relief is appreciable.' Hwever, tests by Kirkpatrick and Khan cnfirmed that the in situ behaviur culd be reprduced with reasnable accuracy when the samples were cnslidated back t the in situ stresses. This cnclusin was valid fr strage up t 28 days (which was the maximum used fr the CAU tests by Kirkpatrick and Khan), prvided n change in water cntent tk place during strage. Istrpic cnslidatin n the ther hand did nt give satisfactry results. Kirkpatrick et al. (1986) extended the abve study t vercnslidated kalin and illite labratry manufactured clays and fund mre r less similar trends as fr nrmally cnslidated samples Precnslidatin stress. p'c (kpa) 20 Fig / Precnslidatin stress, pc, as a functin f depth frm ffshre and nshre CRSC tests Offshre Onshre 23m depth Shear strain (%) Shear strain (%) 10 g-20 a 30 e 1. Offsh e Onsh re I Undrained shear strength, su (kpa) Fig. 6 Stress-strain and prewater pressure fr kalin and illite samples (Kirkpatrick and Khan, 1984). Graham et al. (1990) als did tests n recnstituted illite clays with strage times up t 1 week. Their cnclusin was als that UU tests were misleading but that recnslidating the samples back t in situ stresses gives reliable values f s and A while sme differences may be expected in the stiffness rati E^/s^. There are numerus ther studies that shw the inadequacy f UU fr the measurement f in situ strength. EFFECT OF GAS IN SEDIMENTS Axial strain, e (%) Fig. 5 Results frm ffshre and nshre DSS tests. N significant differences between the results f ffshre and nshre tests were fund. The effects f stress relief during sampling and subsequent strage n the results f triaxial tests have als been investigated by Kirkpartick and Khan (1984) fr nrmally cnslidated labratry manufactured kalin and illite samples.. In situ behaviur was simulated by cnslidating the specimens in the triaxial cell and then testing these undrained withut unlading. Samples were prepared frm blcks f material and stred fr varius times befre being tested in undrained cmpressin. In recent years there has been a grwing cncern with gas in sediments. Gas may be bigenic (usually less than 100 m r s belw sea bttm) r petrgenic. The petrgenic gas riginating in the reservirs at large depth (say mre than m) may migrate up thrugh the sediments in free frm thrugh faults r discntinuities r in disslved frm by diffusin. Research prgrams at Oxfrd University and NGI have shwn that gas bth in free frm and in slutin in the pre water may affect the sil behaviur and the results f labratry tests. Rad et al., 1992 (Fig. 7) shw the effect f methane gas n the cnslidated undrained triaxial behaviur f dense sand. Test N. 19 was perfrmed n a sample with n free gas but the pre water was cmpletely saturated with methane gas (tj = 100%). Test N. 16 was perfrmed n a sample with free methane gas (S = 90%). The tw CIU tests n samples with gas have significantly lwer shear strength than the CIU test (N. 6) withut gas. 1679

4 affect the test results (except when cavitatin ccurs), the initial pre pressure has a strng influence n the respnse f gassy sils t undrained lading. Hence, the initial pre pressure shuld be made equal t the in situ value. The degree f sil saturatin (S) and the water-gas saturatin (ij) f the test specimen shuld be similar t the actual in situ values. In the case f fully saturated sil, the in situ )j-value may be estimated utilizing the in situ BAT equipment (Rad and Lunne, 1992). When the in situ sil is nt saturated, the degree f saturatin can be rughly estimated based n the degree f saturatin f the specimen in the labratry and knwing the ttal pressure reductin. In this case the rj-value shuld be assumed t be 100%. The ttal stress path used during the test shuld be identical t that expected in the field. This is imprtant since different stress paths used during lading may result in different extent f gas exslutin/slutin and expansin/cntractin, and thus, misleading results and design values. It is als recmmended t measure the vlume change f the specimen even during undrained tests. TEST PROGRAM FO R DIFFERENT STRUCTURES Sil investigatins shuld be planned in such a way that adequate infrmatin is btained abut vertical and lateral distributin f the sil layers, cupled with a gelgical understanding f the site t help interpret the stress histry and in situ stresses. Fig. 7 Axial Strain, ea, % Effect f disslved r free methane gas (C H J n cnslidated undrained triaxial behaviur f dense sand (Rad et al., 1992). The specimen with free gas exhibits a lwer peak strength than the ne with gas in slutin. The residual strengths are, hwever, abut the same. This appears t be due t the fact that the pre pressure reductin is initially strnger fr specimen 19 than fr specimen 16. As the test prgresses, gas bubbles are als generated in specimen 19 and the pre pressures and strength agree better. Rad et al. (1992) als perfrmed cyclic triaxial tests which indicated that gas in ccluded r intercnnected frm strngly affects (negatively) the respnse f dilative dense sands t cyclic lading. Lse sands are nt as much affected by gas in slutin in the pre water as that f the dense sand. Rad et al. (1992) give the fllwing practical guidelines fr handling and testing f samples cntaining gas: Sampling and Specimen Preparatin - Sample disturbance caused by gas exslutin and expansin during retrieval, transprt and strage shuld be prevented as much as pssible. Since gas exslutin is time-dependent ne pssibility is t test samples immediately nbard. Anther less ideal but mre practical apprach is t place sample tubes in steel canisters and t pressurize them t the in situ stresses immediately after retrieval t avid further disturbance during transprt and strage. Ideally, sil samples shuld be placed in an hyperbaric chamber pressurized t the pre pressure f the sample. This is pssible nly in well funded research prjects. Alternatively, the pressurized sample may be frzen befre releasing the pressure and placing the sample in the test chamber and repressurizing. Mre practically, the pressure shuld be released immediately befre placing the specimen in the test chamber, and its pre pressure shuld be brught t the in situ value as fast as pssible. Labratry Testing - In cntrast t labratry testing f saturated specimens where the magnitude f the initial pre pressure (i.e., ttal stresses) des nt Labratry tests in additin need t be carried ut fr the purpse f deriving sil parameters t be used in the getechnical analysis fr varius fundatin prblems. The apprach preferred is t bring back the labratry samples t the density and stress state befre sampling and then subject the sample t the same additinal stress expected in situ. It is useful t establish a set f preliminary design parameters upn the cmpletin f the field wrk based n the results available at the time, and t run thrugh preliminary fundatin analyses with variatins in key parameters. The results f these analyses will increase awareness f which layers and which sil parameters are mst critical. In the subsequent labratry prgram, emphasis can be given t carrying ut a sufficient number f tests and the mst adequate test type(s) in the varius layers. Piled jackets. The API rules are generally used fr establishing axial and lateral bearing capacity f piles. In clayey sil the undrained shear strength is ften based n the UU test. In previus sectins the inadequacy f the UU test t represent the in situ sil strength has been clearly illustrated. Hwever, due t the large data base with UU tests and measured skin frictin f installed piles, UU tests cntinue t be used. In mst cases it will be a very cnservative apprach. Experience frm a large number f Nrth Sea sites shw that frequently the rati between shear strength frm UU-tests and CAUC-tests is 0.75 t 0.8. One can establish a design prfile based n CAUC-tests and scale this dwn t establish a UU-prfile if such a prfile is required. A shear strength prfile based slely r uncnfined tests will reflect relative sample disturbance rather than real variatin in shear strength with depth. Skin frictin shuld als be calculated based n labratry tests that mre fundamentally mdel what ges n in the grund adjacent t the piles. The NGI-methd (Karlsrud and Nadim, 1990) is such an apprach accunting fr three main factrs: Remulding and distrtin f the clay caused by pile installatin. State f effective stresses after dissipatin f all excess pre pressures. Varying shear strength with distance frm the pile. This apprach invlves perfrming direct simple shear tests n remulded and undisturbed sils t be able t simulate the variatin f s away frm the pile as illustrated in Fig. 8. An advantage f the NGI-methd is that it can als include the effect f cyclic lading. A testing prgram t derive pile skin frictin (including effect f cyclic lading) accrding t the NGI-methd fr a typical Nrth Sea site with stiff 1 680

5 I T Zne A z ne B ^/(RR-clay)^ ^ (Disturbed clay) ic Shear strength a'ac (crrespnds 'hc alng pile) ^ I Pile wall / Zne C (Undisturbed clay) Impsed r _ / shear stress t = t 0 ~ Fig. 8 r '1 r2 Principle f direct simple shear tests t mdel clay adjacent t piles (frm Karlsrud and Nadim, 1990). Fig. 9 Example f labratry prgram fr DSS tests fr NGI pile design methd. vercnslidated clay is illustrated in Fig. 9. (This prgram shuld be run in additin t the standard prgram giving input fr design parameters accrding t the API guidelines). G ravity base structures. Design parameters need t be established fr a range f fundatin analyses, including but nt limited t: installatin phase, mainly skirt penetratin settlements stability against verturning and sliding behaviur during dynamic and cyclic lading In the fllwing the effect f cyclic lading will be given mst emphasis. Figure 10 gives a simplified picture f the shear stresses in typical elements alng a ptential failure surface beneath a gravity structure (als anchr, jack-up spud can). The sil elements fllw varius stress paths, and they are subjected t varius cmbinatins f average shear stresses, t and cyclic shear stresses, Tcy. Herein, r dentes the shear stress n the hrizntal plane in the direct simple shear (DSS) test and n the 45 plane in the triaxial test. The average shear stress, r is cmpsed f the initial shear stress in the sil prir t the installatin f the structure, r = 0.5 (1-KJ p, and additinal shear stress, At., induced by the static lad (e.g. the weight f the structure) where p is the vertical effective verburden pressure, and K is the cefficient f earth pressure at rest. The sil is cnslidated under the initial shear stress, r. The shear stress due t the static lad, A t., will first act under undrained cnditins, but as the sil cnslidates, this shear stress will als act under drained cnditins. Fr sand the cnslidatin ccurs rapidly, but fr sft clays it may take years befre cnslidatin is cmpleted. Fig. 10 S. lvlcy W cy / DSS (Cmpressin). -E3-' /D S S t f 0 A rw r ^m e \ Vxa\y \j Time Ta TRIAXIAL (Extensin) a Ta Time Simplified stress cnditin fr sme elements alng a ptential failure surface belw a gravity base structure (frm Andersen and Heeg, 1991). The cyclic lading will cause redistributin f the static stresses in the sil, and r, may als vary with time during the cyclic lad histry. T determine the sil prperties needed fr fundatin analyses, it is necessary t duplicate the in situ stress cnditins fr the varius elements as clsely as pssible (triaxial and DSS tests under varius cmbinatins f r. and t ). The use f labratry test results t arrive at sil design parameters fr fundatin analysis fr cyclic effects are described in detail by Andersen and Haeg (1991). Table 2 gives an example f a cmplete labratry prgram required fr establishing the full range f sil design parameters fr a gravity base structure. The extent and distributin f the varius labratry test types will vary with sil cnditins, platfrm gemetry and impsed frces. -cy The cyclic shear stress, r^, is caused by the cyclic lads. In a strm, the wave height and perid vary cntinuusly frm ne wave t anther, and the cyclic shear stress will als vary frm cycle t cycle. 1681

6 Table 2 Example labratry testing prgram fr a gravity base structure. Depth Interval m Sil Unit Sil Descriptin Oedmeter Triaxial tests Direct Simple Shear Dynamic Tests Required length f CRSC 1NCR Static Cyclic Static Cyclic Res. Clumn Bender Elements sample m Cmpr. Extens. Length f sample required per m bring m/m A SAND, fine t medium IB CLAY with sand layers A CLAY, silty, sandy, very stiff, vercnslidated B CLAY, silty, sandy, stiff, less vercnslidated C CLAY, suty, sandy, very stiff, vercnslidated SAND, silty with seams f clay CLAY, very sandy, vercnslidated CLAY, very hard, highly vercnslidated * N undisturbed samples Ttal N. f tests R EFER EN C ES Amundsen, T., T. Lunne, H.P. Christphersen, J.M. Bayne and C.L. Barnwell (1985). A deep water sil investigatin at the Trll East Field. Internatinal Cnference n Offshre Site Investigatin 85, Lndn. Prc. pp Andersen, K.H. and K. Haeg (1991). Defrmatin f sils and displace ments f structures subjected t cmbined static and cyclic lads. Prc. f tenth Eurpean Cnference n Sil Mech. and Fundatin Engineering, Flrence May, Vl. 2, pp Andresen, A., T. Berre, A. Kleven, T. Lunne (1979). Prcedures used t btain sil parameters fr fundatin engineering in the Nrth Sea. Marine Getechnlgy, 3(3), Dyvik, R and T.S. Olsen (1989). G measured in edmeter and DSS tests using bender elements. Prc. Int. Cnf. n Sil Mech. and Fund. Eng., 12. Ri de Janeir 1989, Vl. 1, pp Graham, J., M.R. Jamiesn, D.Y.F. H and F. Azizi (1990). Influence f strage and recnslidatin prcedures n clay prperties. Getechnical Testing Jurnal, Vl. 13, N. 4, December 1990, pp Hight, D.W. (1992). A review f sampling effects in clays and sands. Sciety f Underwater Technlgy, Cnference n Offshre Sil Investigatins and Fundatin Behaviur, Lndn Prc. Karlsrud, K. and F. Nadim (1990). Axial capacity f ffshre piles in clay. Offshre Technlgy Cnference Hustn. OTC Paper N Kirkpatrick, W.M. and A.J. Kahn (1984). The reactin f clays t sampling stress relief. Getechnique, Vl. 34, N.l, Kirpatrick, W.M., A.J. Khan and A. A. Mirza (1986). The effects f stress relief n sme vercnslidated clays. Getechnique, Vl. 36, N. 4, pp Lacasse, S. T. Berre and G. Lefebvre (1985). Blck sampling f sensitive clays. Eleventh Internatinal Cnference n Sil Mechanics and Fundatin Engineering, San Francisc, August 1985, Vl. 2, p Lacasse, S., K. Iversen, G. Sandbaekken and P. Mrstad (1984). Radigraphy ffshre t assess sample quality. Nrdiska GeteknikermStet. LinkQping, Statens Getekniska Institut. Prceedings, Vl. 2, Rad, N.S. and T. Lunne (1992). Gas in sils: Detectin and i)-prfiling. Accepted fr publicatin in ASCE, Jurnal f Getechnical Engineering. Rad, N.S., A.J.D. Vianna and T. Berre (1992). Gas in sils: Effect f gas n undrained static and cyclic strength f sand. Accepted fr publicatin in ASCE, Jurnal f Getechnical Engineering. Sandbskken, G., T. Berre and S. Lacasse (1985). Oedmeter testing at the Nrwegian Getechnical Institute. Presented at ASTM Sympsium n Cnslidatin f Sils Labratry Testing. Frt Launderdale, Flrida, USA, January, Sills, G.C., S.J. Wheeler, S.D. Thmas and T.N. Gardner (1991). The behaviur f ffshre sils cntaining gas bubbles. Getechnique, Vl. 41, N. 2, pp