Mdellng the seepage flw durng cassn nstallatn n a natural seabed Faramarz, Asaad; Faz, Khyar; Drar, Samr; Mehravar, Mura; Harreche, Ouahd Dcument Versn Publsher's PDF, als knwn as Versn f recrd Ctatn fr publshed versn (Harvard): Faramarz, A, Faz, K, Drar, S, Mehravar, M & Harreche, O 216, Mdellng the seepage flw durng cassn nstallatn n a natural seabed. n Prceedngs f the 24th UK Cnference f the Asscatn fr Cmputatnal Mechancs n Engneerng 31 March 1 Aprl 216, Cardff Unversty, Cardff. pp. 15-153. Lnk t publcatn n Research at Brmngham prtal General rghts Unless a lcence s specfed abve, all rghts (ncludng cpyrght and mral rghts) n ths dcument are retaned by the authrs and/r the cpyrght hlders. The express permssn f the cpyrght hlder must be btaned fr any use f ths materal ther than fr purpses permtted by law. Users may freely dstrbute the URL that s used t dentfy ths publcatn. Users may dwnlad and/r prnt ne cpy f the publcatn frm the Unversty f Brmngham research prtal fr the purpse f prvate study r nn-cmmercal research. User may use extracts frm the dcument n lne wth the cncept f far dealng under the Cpyrght, Desgns and Patents Act 1988 (?) Users may nt further dstrbute the materal nr use t fr the purpses f cmmercal gan. Where a lcence s dsplayed abve, please nte the terms and cndtns f the lcence gvern yur use f ths dcument. When ctng, please reference the publshed versn. Take dwn plcy Whle the Unversty f Brmngham exercses care and attentn n makng tems avalable there are rare ccasns when an tem has been upladed n errr r has been deemed t be cmmercally r therwse senstve. If yu beleve that ths s the case fr ths dcument, please cntact UBIRA@lsts.bham.ac.uk prvdng detals and we wll remve access t the wrk mmedately and nvestgate. Dwnlad date: 4. Sep. 218
Prceedngs f the 24 th UK Cnference f the Asscatn fr Cmputatnal Mechancs n Engneerng 31 March 1 Aprl 216, Cardff Unversty, Cardff MODELLING THE SEEPAGE FLOW DURING CAISSON INSTALLATION IN A NATURAL SEABED Asaad Faramarz¹, Khyar Faz¹, Samr Drar¹, Mura Mehravar² and Ouahd Harreche³ 1 Department f Cvl Engneerng, Schl f Engneerng, Unversty f Brmngham, Brmngham, B15 2TT 2 Wlfsn Schl f Mechancal and Manufacturng Engneerng, Lughbrugh Unversty, LE11 3TU 3 Department f Cvl Engneerng, Faculty f Engneerng, Islamc Unversty n Madnah, KSA A.Faramarz@bham.ac.uk ABSTRACT Seepage plays a crucal rle n the nstallatn f suctn cassn fundatns and ths has been cnsdered by several researchers va expermental, numercal and analytcal studes. In mst f the avalable numercal and analytcal studes, the actual seabed prfle has been mdelled usng a number f assumptns and cnsderng a lmted number f dealsed cndtns t smplfy the prblem. Hwever, t s well establshed that the behavur f seabed, and sl n general, s a cmplex and nnhmgeneus ne and that these assumptns can affect the relablty f the results. Ths paper examnes the nstallatn f suctn cassn fundatn nt a realstc and natural seabed prfle. It dentfes the changes n seepage flw at dfferent stages f nstallatn and the rle f crucal parameters such as permeablty n the nstallatn prcess s dscussed. The results shw that a sgnfcant ncrease n resstance t penetratn ccurs at the nterface layers. Keywrds: suctn cassn; nstallatn; natural seabed sl; permeablty 1. Intrductn Suctn cassns are relatvely new types f fundatns whch have applcatns n ffshre l and gas ndustry as well as ffshre wnd farms. They are upturned buckets f cylndrcal shape made frm steel. Installatn f these fundatns n seabed sl s acheved by pumpng ut water trapped n the cassn hllw space whch enables ther penetratn nt the seabed after ntal penetratn under self-weght. Seepage ccurs n permeable sls durng suctn cassn nstallatn, as dscussed by Hgervrst [3], leadng t a reductn n the sl effectve stress and hence ts resstance t penetratn [8]. Hwever, n sls wth lw permeablty, nstallatn s manly acheved by the dfference between pressure nsde and utsde the cassn, caused by pumpng ut the trapped water n the cassn cavty, and the effect f seepage s lmted [8, 1]. Installatn n mult-layered sl prfles, e.g. very lw permeable sl verlad by sand, s expected t be mre challengng snce the reductn n resstance t the cassn tp penetratn s restraned by restrctns f flw n the lwer less permeable layer. It s unclear t what extent the lwer less permeable layer mpacts the seepage flw and thus the resstance t the suctn cassn tp penetratn. Installatn aspects f suctn cassns n hmgeneus clay [2] r granular materals [4] are extensvely studed by many researchers. Hwever, lmted dcumentatn, references r gudelnes are avalable fr nstallatn n stratfed sls [5, 6]. In the present paper, a fnte element (FE) mdel s develped t study the effect f a mult-layered seabed n the nstallatn prcess f suctn cassn fundatns. The results are cmpared wth thse btaned frm nstallatn n a sngle layer sl and the dfferences are hghlghted. 2. Frmulatn f the Nrmalsed Seepage Prblem In rder t draw cnclusns regardless f specfc dmensns, an FE mdel f a suctn cassn nrmalsed wth respect t the cassn radus (R) s develped. The cassn heght and penetratn depth nt seabed are dented as L, and h respectvely. The frst case represents cassn nstallatn n hmgeneus sl wth permeablty k and saturated unt weght γ sat. Fr the secnd case f a twlayer sl prfle, the depths f the upper and lwer layers are taken as 2m and 4m respectvely. The permeablty f the upper layer s taken as 1 tmes hgher than that f the secnd layer. Fgure 1, shws a vertcal sectn thrugh the merdan plane f the cassn-sl system where a cylndrcal system f crdnates z z / R, and r r / R s used. Fr the cassn penetratn depth, the dmensnless parameter h h / R, s adpted [7].
Fgure1: Nrmalsed Gemetry and Fnte Element Mdel; Hmgeneus (left) and Layered (rght) T study the effect f prewater seepage n sl resstance t cassn penetratn, lateral pressures n nsde and utsde cassn wall caused by seepage s nvestgated. Under seepage cndtns prduced by an appled suctn, the lateral effectve pressure ( h ) actng n the cassn wall at depth z, nsde and utsde the cassn s respectvely gven by: ( R, z) K( z h ( R, z) K( z h z z g ( R, ) d ~ ( R, z)) g ( R, ) d ~ ( R, z)) where K s the ceffcent f lateral earth pressure, s the effectve unt weght f the sl, ~ s the enhanced effectve stress caused by the effect f shear resstance that develps n the sl-cassn nterface and g ( R, ),and g ( R, ) dente the vertcal cmpnent f the pressure gradent n the nner and the uter sdes f the cassn wall respectvely. Seepage causes the frctnal resstng frce actng n the cassn wall t decrease by a magntude gven as a functn f the nrmalsed penetratn depth h by the expressn: F s (1) (2) h Fs L ( ) ( ) 2 Z L Z dz 2R Ks tan (3) Where: Fs F s (4) 2 (2R Ks tan ) z L ( Z) g (1, ) d (5) z L ( Z) g (1, ) d (6) and δ dentes the angle f frctn at the nterface sl-cassn. Suctn magntude at mudlne level nsde the cassn cavty are dented by s. In the next sectn, the nrmalsed decrease f frctnal resstance ( F s ) s calculated usng the results f the FE mdels and the abve relatnshps. 3. Results and Dscussn Fgures 2a and 2b, shw the dstrbutn f nrmalsed excess pre water pressure P arund the cassn fr the nrmalsed penetratn depth h =1 and 3 n a hmgeneus seabed.
Water seepage cause by suctn prduces a hydraulc gradent whch n bth faces f the cassn wall vares wth depth. Fgures 2c, and 2d shw the dstrbutn f P fr the seabed wth layered sl prfle at the same depths. Frm these fgures t can be seen that the cnturs f excess pre water pressure have been affected due t the presence f a less permeable layer. Ths wll accrdngly have an mpact n the penetratn resstance as t can be seen n the next sectn. Fgure 3a, and b depcts the cnturs f vertcal cmpnent f the scaled pressure gradent g p / z fr nrmalsed penetratn depth h = 1, and 3, crrespnds t a hmgeneus seabed. It can be bserved that the hghest gradent magntudes are cncentrated arund the cassn tp. At shallw penetratn depths, hgh gradents arund the cassn wall affect the whle penetratn depth. As the penetratn depth ncreases, these gradents tend t lcalse arund the cassn tp. Fr cmparsn, Fgure 3c, and 3d shws smlar cnturs fr a seabed layered prfle. Frm these fgures, t can be seen that dscntnuty n the permeablty prfle at the nterface between the tw layers s reflected n the prfle f the pre pressure gradent. Ths n turn affects the magntude f frctnal sl resstance as a functn f nrmalsed depth as wll be shwn later. Fgure 2: Excess pre water pressure fr h=1, 3; (a, b): hmgeneus; (c, d): layered sl. Fgure 3: Nrmalsed pressure gradent fr h=1, 3; (a, b): hmgeneus; (c, d): layered sl.
Fgure 4 shws clearly that the reductn n the magntude f lateral frctn resstance ( F s ) ncreases as the cassn passes thrugh the layer wth hgher permeablty. An ncrease (.e. 5%) n resstance t penetratn ccurs at the nterface between the sl layers. Ths result shws clearly that the suctn pressure requred culd be affected by sudden changes n permeablty n layered sls. 4. Cnclusns Fgure 4: Effect f Suctn-Induced Seepage n Sl Resstance at Cassn Wall Ths study cnsdered the effect f sl prfle permeablty n the predctn f sl resstance t cassn penetratn. Tw sl prfles were cnsdered, namely hmgeneus sl wth cnstant permeablty and a layered sl prfle where lw permeable sl s verlad by a 2-meter thck layer f hgh permeable sl. The effect f suctn-nduced seepage n sl resstance t cassn penetratn was nvestgated usng the nrmalsed slutn f seepage arund the cassn wall. It has been bserved that takng nt accunt the actual varatn f permeablty n layered sls may lead t a mre accurate estmatn f the crtcal suctn. References [1] B. Zhu, D. Q. Kng, R. P. Chen, L. G. Kngand, Y. M. Chen. Installatn and lateral ladng tests f suctn cassns n slt. Canadan Getechncal Jurnal, Vl.48, pp.17-184, 211. [2] G. T. Hulsby and B. W. Byrne. Desgn prcedures fr nstallatn f suctn cassns n clay and ther materals. Getechncal Engneerng, 25, 158, 2: 75 82, 25. [3] J. R. Hgervrst. Feld trals wth large dameter suctn ples, Prceedngs f the Offshre Technlgy Cnference, Hustn, Paper OTC 3817, 198. [4] K. H. Andersen, H. P. Jstad, R. Dyvk. Penetratn resstance f ffshre skrted fundatns and anchrs n dense sand. Amercan Scety f Cvl Engneers, Vlume 134, pp. 16.116. 28. [5] M. Senders, M. F. Randlph. Thery fr the nstallatn f suctn cassns n sand verlad by clay. Offshre Ste Investgatn and Getechncs, Prc. 6th Int. Cnf. Lndn, 27. [6] O. Ctter. Installatn f Suctn Cassns fr Offshre Renewable Energy Structures, A thess submtted fr the degree f Dctr f Phlsphy. Magdalen Cllege, Oxfrd, 29. [7] O. Harreche, M. Mehravar, M.A. Alan. Suctn cassn nstallatn n sand wth strpc permeablty varyng wth depth. Appled Ocean Research, Vl 43, pp 256-263, 213. [8] R. Rmp. Installatn-effects f suctn cassns n nn-standard sl cndtns, Master Thess, Delft Unversty f Technlgy, 213.