This paper presents the results from a study into the scour development around a monopile and three different large (20m diameter) marine foundations in currents and waves.  The relevant hydraulic processes are discussed and the results from a mobile bed physical model investigating the scour process presented

Whitehouse, Richard J. S.

English

Whitehouse, R.J.S.

HR Wallingford Ltd.: ePrints at HR Wallingford

Marine scour at large foundations

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Conference Paper, Published VersionWhitehouse, Richard J. S.Marine Scour At Large FoundationsVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/99931Vorgeschlagene Zitierweise/Suggested citation:Whitehouse, Richard J. S. (2004): Marine Scour At Large Foundations. In: Chiew, Yee-Meng; Lim, Siow-Yong; Cheng, Nian-Sheng (Hg.): Proceedings 2nd International Conferenceon Scour and Erosion (ICSE-2). November 14.–17., 2004, Singapore. Singapore: NanyangTechnological University.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Paper for ICSE2, id 53-PRE-A2671MARINE SCOUR AT LARGE FOUNDATIONSRICHARD J. S. WHITEHOUSEHR Wallingford, Howbery Park, Wallingford, Oxon OX10 8BA, UKThis paper presents the results from a study into the scour development around a monopile and three differentlarge (20m diameter) marine foundations in currents and waves.  The relevant hydraulic processes arediscussed and the results from a mobile bed physical model investigating the scour process presented.1 IntroductionIn pursuit of renewable energy targets there is presently interest in alternativefoundation options for windfarm turbines placed in shallow coastal waters where tidalcurrents and wave activity produce a mobile seabed.  When a foundation is installed onthe seabed the local marine velocity field is disrupted, additional turbulence is generatedand the potential for sediment transport near the foundation is increased leading to localscouring [1,2,3,4].The “live-bed” situation where sediment is just mobilised everywhere is consideredto be the regime for which the scour depth is deepest around a slender cylinder diameterDm.  The largest scour depth S occurs in wave-current conditions with a currentdominated “live-bed” condition [4], up to S/Dm = 2.From a review of available predictors it became clear that there were presently noaccepted equations for the scour depth around composite monopile-caisson structures inwaves and/or currents.  It was proposed that the most productive way to proceed was toundertake laboratory testing to determine how the scour behaviour differed betweendifferent foundation types in current, waves, and current and waves conditions. Three factors need to be considered for a scour assessment: What are the key external features of the structure and what are its dimensionsabove the seabed level and below the soil surface, e.g. presence of skirts? What are the environmental conditions – metocean and soils? What are the implications of scouring taking place?In the present study the structure is a monopile tower with outside diameter Dm 4.6mjoined to a circular suction foundation caisson with an outside diameter Dc of 19m.  Theheight of the top of the caisson above seabed level is 2m and the skirt tip depth is 9.5m.Penetration of the skirt to this depth around a “plug” of soil and maintenance of theseabed level is one of the requirements for foundation stability. Three different monopile-caisson connection geometries were proposed including a girder top with radial fins, aconcrete slab top, and a conical top (Figure 1).  For comparison the situation with a 4.6mmonopile directly placed in sand bed was also considered.The water depth was selected as 10m as this represents typical water depths at anumber of potential UK windfarm sites.  Borehole records showed that the surficial layer2is typically fine to medium sand, up to 20m thick with an average median grain sized50=0.338mm.  A flow velocity of approximately 0.4ms-1 is required to mobilise thissediment and expected peak tidal currents up to 1.5ms-1 are well in excess of the thresholdof motion for the sediment. A typical nearshore wave condition typical of 1-year returnperiod was selected with Hs = 4.5m and Tm =7s which produces bottom orbital velocitieswell in excess of threshold.  Thus a vigorous scour environment is expected.Figure 1 Foundation models used in testing – L-R: concrete-conical-girderThe implications of scour taking place around the foundations are a loss of skirtdepth.  Thus it was important to understand how scour might develop around thefoundation.  The paper discusses the flow-structure interaction for the foundations, thetest methodology and an inter-comparison of the results from the tests, before drawingconclusions.2 Flow structure interactionThe variation of velocity with height in seabed boundary layer approaching amonopile structure drives a flow down the face of the monopile.  A recirculating vortexforms as the flow impinges on the seabed which wraps around the monopile and trails offin a downstream direction creating a “horseshoe” vortex.  These mechanisms and the linkto the mechanics of the scour are dealt with in detail by Sumer and Fredsøe [4, 5].The flow-structure interaction of the composite monopile-caisson foundation isdepicted in Figure 2.  The main effect is that the caisson foundation actually protects theseabed from the intense flow field around the base of the monopile.  However, flowacceleration and vortex action and hence scour occurs directly around the wall of thefoundation.  The flow over the top of the caisson will generate a recirculating zonedownstream akin to flow separation over a rearward facing step and the associatedreattachment point downstream will be a point of high turbulent shear stress.  In additionthe turbulent wake from the monopile will extend downstream and also interact with thebed increasing the potential for scour in that region.3Figure 2 Schematic flow-structure interaction for foundationThe different connection details between the base foundation and the monopile willproduce different hydraulic interactions.  Geotechnical aspects of foundations have beencovered by Byrne and Houlsby [6].3 MethodologyTests were undertaken with constant wave and/or current forcing to providecomparative data on scour at the different foundation types.  The modelling was done in alarge test basin at HR Wallingford with waves generated at 90º to the current.The main features of the test basin were a re-circulating uni-directional currentsystem with control structures to widen and steady the flow and produce a smoothtransition onto the sand bed, plus sump and sediment trap at downstream end. The wavepaddle generated irregular period long-crested waves to the required wave spectrum(JONSWAP) and conditions were calibrated from wave-probe measurements taken at thetest section.The foundation models were built at a geometric length scale of 1:40 and thecorresponding model velocity and time-scales were 1:6.3. These were selected asappropriate for simulation of bed scour.  The monopile and foundation models were fixedin position to prevent settlement or displacement once scour had occurred.  The sedimentused in the model was a fine quartz density sand of a median diameter of d50=0.111mmand the model sand bed was 0.24m deep, 5.0m long and 3.0m wide.  The wave-alone andcurrent-alone conditions were capable of mobilising the bed sediment, i.e. “live bed”conditions, which was confirmed by observations of the bed during testing and thesediment was also transported in suspension in the water column.  These two factors wereimportant to reproduce process similitude.The depth of the scour hole at eight radial locations (A-H – see Figure 4) adjacent tothe foundation wall was measured during every test to determine the time development.44 ResultsThe test conditions are presented in Table 1 and the results (e.g. Figure 3) aresummarised in Table 2.  The results are scaled to prototype/field-scale using the scalefactors stated earlier.Table 1 Measured conditions during tests  (scaled up). *assumed – same flow boundary conditions as Test 5Test nr - Structure H1/3 (m) Tm (s) Current speed (ms-1)1  - 4.6m OD Monopile 0 0 1.72  - Girder top 0 0 1.73  - Girder top 3.9 7.1 1.54  - Girder top 3.9 6.9 05  - Concrete top 3.8 6.9 1.76  - Conical top 4.1 6.9 1.7*Table 2 Summary of measured scour depths.Test Structure Condition Maximum scour depth @ end oftest duration @ radial location(s)Maximum scour depthafter 10 hours @ radiallocation(s)1 Monopile Current 4.3m @ 36.1 hrs @ D 3.4m @ G2 Girder top Current 3.5m @ 28.6 hrs @ C 1.5m @ C3 Girder top W+C 9.3m @ 44.4 hrs @ G 3.0m @ G4 Girder top Waves 0.8m @ 18 hrs @ C 0.6m @ B/C5 Concrete top W+C 7.5m @ 38.1 hrs @ F&G 2.9m @ F6 Conical top W+C 8.5m @ 27.5 hrs @ G 4.9m @ FComparison of scouring at monopile and foundation in current (Tests 1 and 2)In Test 1 a local scour hole formed around the monopile and in the early stage ofdevelopment the majority of the eroded sediment was transported into suspension andcarried downstream by the current, with some of it depositing to form a lenticular shapedaccumulation of sediment behind the structure.  The time development curve was thecharacteristic exponential decay experienced for a cylindrical pile [3]; i.e. fast ratesinitially slowing down with time to a reasonably stable maximum equilibrium depth inthis test of around 4.3m or 0.94Dm (Figure 3).In Test 2 the scour development was nearly linear with time. The maximum scourdepth was 3.5m which is 0.18Dc, 0.37 times the skirt depth or 0.30 times the totalfoundation height.  As expected from the assessment of the flow-structure interactionscouring was intense on the seabed adjacent to the foundation and the turbulent wakeproduced scour pits that trailed away from foundation.  The rate of scour depth5development up to 10 hours was slower for the foundation than the monopile due topersistent vortex and eddy shedding around the monopile (Figure 5).Figure 3 Result of scour test with current (Test 2)Foundation scour with waves and current (Test 3)In Test 3 sediment was suspended to the height of the upstand on the footing and thentransported by the current between positions A and B.  The deepest scour followed alinear development in time and took place faster than under the current only condition.The deepest scour did not reach an equilibrium situation as observed with the monopileduring the test.  The reason for the change in shape is due to the fact that as the scourdepth grows, so too does the degree of flow disturbance.  Initially, there was only 2m ofthe foundation above the bed level and hence the flow disturbance was low.  However, asthe scour hole grew, so too did the length of skirt that is exposed to the flow.  Themaximum scour depth was 9.3m which is 0.49Dc or 0.8 times the foundation height.There was a radial variation in scour development with the sheltered down-current side atlocation B where sediment erosion was counter balanced to some extent by accretion,yielding a smaller scour depth of just over 3m.Foundation test with waves (Test 4)In Test 4 the time development was linear and then constant, but the overall scour depthswere much smaller with waves only.  The maximum scour depth was 0.8m which is0.04Dc or 0.07 times the foundation height.  The ratio S/Dc is similar to the situationexperienced with full water depth cylinders of the same diameter [4,7].Different foundation connections with waves and currents (Tests 5 and 6)In Test 5 the results showed that the time development in this test was relatively linear,with some small step-like variations.  The steps in the time development are due toperiods of time when the sediment transport out of the scour hole is counterbalanced by6occasional inputs of sediment avalanching into the scour hole.  It is quite similar to thescour produced with the girder top (Test 3) after an equivalent length of time.  Thisindicates that the caisson top detail does not appear to have a significant influence on thescour development.  The maximum scour depth was 7.5m which is 0.39Dc or 0.65 timesthe foundation height.  In the first 5 hours of the test it was noted that the scour depth atthe down-current locations A and H are deepest and that whilst the bed at location Hcontinued to deepen the bed level at location A increased by a smaller amount due to theinfluence of sediment deposition.Figure 4 Scour depth time development at monopile in live-bed current – Test 1Figure 5 Scour depth time development at structure in live-bed current –Test 2-2-10123450.0 5.0 10.0 15.0 20.0 25.0 30.0Time (hrs)Scour (m)ABCDEFGH-10123450.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0Time (hrs)Scour (m) ABCDEFGHcurrentBACEFGDH7In Test 6 scour was observed to develop quickly around the conical caisson.  Thetime development data showed that overall the scour develops quicker with this kind offoundation compared with the girder top or concrete top; in the phase of scourdevelopment to 5m the scour around the conical foundation was roughly a factor of 1.8faster than with the girder or concrete top.  The scour around the foundation after 20hours was generally deeper than with either the girder top or concrete top.  The maximumscour depth was 8.5m which is 0.45Dc or 0.73 times the total foundation height.5 DiscussionThe results for the current-alone scour around the foundation (Test 2) can becompared with the situation with a full-water depth cylinder of the same diameter as thebase.  The maximum scour depth in the “live-bed” regime could be of the same order asthe cylinder diameter.  Clearly the limited height of the foundation leads to an expectedreduction in the scour depth which is confirmed by the present tests.The results from the 1:50 scale laboratory tests of Wilson and Abel [8] are relevant tothe present study.  They examined scour around three-circular pontoon footings of a jack-up rig in a steady current under live-bed conditions representing 25m of water.  Thefoundations were 24.5m diameter and 8m high and the scour depth with all units scaled tofull size was 6.5m.  Settlement of the model to 2 to 4m took place during the testing.  Themodelled scour depth to foundation diameter ratio equates to 0.27Dc or 0.8 times thefoundation height.  The scour depth is controlled by the height and diameter of thefoundation and by the settlement which promotes scour as the blockage caused by thefoundation moves down into the bed.  The initial above-bed blockage is higher in thiscase and combined with the settlement promotes deeper scour than for the current-onlyscour test for the present foundation.6 Concluding RemarksThe results obtained in the physical model tests provide a clear insight into thepotential scour development around suction caisson foundations in (long-crested, random)waves and/or a steady current.  The scour development around the 19m diameter skirtedcaisson foundations supporting the 4.6m diameter monopile showed an approximatelylinear increase in scour depth with time.  This is because the flow contraction against the2m upstand on the skirt initiates the scour process which is then reinforced as more of theskirt becomes exposed.  The deepest potential scour depth with the caisson was 9.3m inTest 3 with waves and currents and corresponds closely to the 9.5m skirt initiallyembedded in the bed.  It is interesting to note that Test 2 with only the current produced amuch lower scour depth than at comparable times in Test 3.  In both cases the current wasrunning at about threshold of motion for the sand in the model for which it is commonlyaccepted that the deepest scour occurs [3,4].  The scour development around the girdertop and concrete top models was similar.  The scour development with the conical topwas approximately 1.8 times faster (Test 6) than the girder top (Test 3).8The results provide good indications of the potential scour that can develop in “live -bed” current and wave conditions, or current dominated regimes.  The foundations areless susceptible to scouring in wave dominated regimes.  With currents and waves scourprotection will be required to prevent scour from causing a drop in the seabed leveladjacent to the footing and can be expected to be successful if appropriately designed.  Ina site assessment the local scour and regional scale erosion and accretion patterns ofsediment need to be considered.  General changes in seabed level can take place due tostorm events, the changes in the profile and position of marine dunes, subtidal channelsand shoal areas or sandbanks will need to be considered.  All these factors need to beconsidered in site selection and in designing scour protection measures.AcknowledgmentsThe work described in this paper was undertaken as part of a larger programme ofresearch within the joint UK Department of Trade and Industry and industry fundedproject on suction caisson foundations. The views are those of the author and notnecessarily those of the funding bodies.  The author acknowledges the contributions fromcolleagues at HR Wallingford, especially Scott Dunn, Suzie Clarke, John Alderson,James Sutherland, Belen Blanco and Richard Soulsby, and the members of the projectconsortium:  SLP Engineering Ltd, Shell Renewables Ltd, General Electric Wind Ltd,Fugro Ltd, Aerolaminates Ltd, Garrad Hassan and Department of Engineering Science,University of Oxford.References 1.   Health and Safety Executive. (1997). “Technical Review of Subsea Scour.” OffshoreTechnology Report OTO 956955.  Prepared by SLP Engineering Ltd and HR Wallingford Ltdon behalf of the UK Health and Safety Executive. 2.   Hoffmans, G.J.C.M. and Verheij, H.J. (1997).  “Scour Manual.”  Balkema, Netherlands. 3.   Whitehouse, R.J.S. (1998). “Scour at Marine Structures.” Thomas Telford. 216 pp. ISBN07277 26552. 4.   Sumer, B.M. and Fredsøe, J. (2002). “The Mechanics of Scour in the Marine Environment.”World Scientific, Singapore. 5.   Sumer, B.M. and Fredsøe, J. (1997). “Hydrodynamics around Cylindrical Structures.”  WorldScientific, Singapore. 6.   Byrne, B.W. and Houlsby, G.T. (2003). “Foundations for Offshore Wind Turbines.”  Phil.Trans. Roy. Soc., vol 361, December, 2909-2930. 7.   Rance, P.J. (1980).  “The Potential for scour around large objects.”, in Scour PreventionTechniques around Offshore Structures, Proceedings of a one day seminar, 16 December1980, Society for Underwater Technology, London, 41-53. 8.   Wilson, N.D. and Abel, W. (1973).  “Seafloor scour protection for a semi-submersible drillingrig on the Nova Scotian shelf”, Paper OTC 1891, Offshore Technology Conference, Dallas.

Marine Scour At Large Foundations

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Marine Scour At Large Foundations

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