The article cites a study that offers information on the variability of sand wave characteristics in the North Sea. The sand waves variability includes a statement that pipelines may start vibrating due to turbulence generated under the free span and navigational channels often need to be dredged for ships to pass safely. The study used multi-beam measurement of three fields in the North Sea in which sand waves occur. Moreover, the study concludes that understanding the variability of sand waves can help determine the optimal depth of a pipeline trench

Blom, A.

Hulscher, S.J.M.H.

Koning, M.F. de

Mark, C.F. van der

Stolk, A.

English

van der Mark, C.F.

de Koning, M.F.

University of Twente Research Information

Pipeline & Gas Journal sInternational/Offshore ReportSea Bed Sand Waves Studied--To Help Pipe ¡ne PlannersBy C.F. van der Mark, M.F. de Koning, A. Bfom, S.J.M.H. Hulscher, and A. Stolk, The NetherianF(ir pipeline laying and operations onthe bed of llie North Sea. the industryneeds information OÊI the average geo-metric properties of sand waves andtheir variability. Németh et al. (2003) outlinedhow Important il is lo have more insight intosand wave behavior. Illustrated here is theimportanee of sand wave variability using twocif their examples.1. Pipelines sometimes have to cross a sandwave field. Migrating sand u-aves canform a threat if they expose a pipeline.Free spans may develop, causing stressesin the pipeline due !o gravity and waterflow. The pipeline may start vibrating dueto turbuienee generated under the freespan. This may cause the pipeline to bendor break. Onee exposed a pipeline canbe damaged by a ship anehor or fishinggear. Knowledge on the deepest troughsthai may occur in a sand wave field helpsus in determining how deep a pipelineshould bo buried.2. Navigational chuméis often need to bedredged so as to be wide and deep enoughfor ships to pass safely Naiitieal charts pro-vide, among other things, infomiaiion onthe areas with the shallowest water depths.Infonnation on tbe highest sand waves orhighest crest elevations which can oectirin a sand wave field may result in a moreefñcient monitoring and dredging strategy.The objective of the study was to get moreinsight into Ihe variability of sand wave charae-tenstics in the North Sea. We performed a dataanalysis to investigate probability density func-tions, eoefficients of variation and extreme val-ues of the following geometric properties; sandwave length, sand wave height, erest elevation,trough elevatioti. and sand wave asymmetry.Data ProcessingWe used multi-beam measurements of threetlelds in the North Sea in which sand wavesoccur (Figure 1 ). We considered six areas in theNoordhinder sand wave field (Figure 2), two areasin the Twin field, and the area Hcomorfi (Figure3). Figure 2 shows a three-dimensional sand wa\'epattern, whereas Figure 3 shows a two-dimen-sional sand wave pattern in which the crest unesare more or less parallel. We classified the sandwaves of the Noordhinder area and alsii the Twinarea as short-crested sand waves mid ihe ones inthe EcomorO area as long-crested sand waves.In order to draw longitudinal bed elevationprofiles from these measurements, we firstFigure 1: Ecomorf3Noordhinderdetermined the orientation of thesand wave field using a part ofthe digipol method (RliCZ. 1997,chapter 4). For each orientation(from 0-Í80 degrees) we deter-mined the gradient in bed eleva-tion. The angle at which the high-est gradient is found is assumed tocorrespond best with the directionperpendicular to the crest lines andis assumed to be the orientation ofthe sand wave field.Given the orientation of the sandwave field, longitudinal bed eleva-tion profiles are processed usingthe bedfonn tracking tool (BTT)of Van der Mark & Biom (2007).The BTT detennines geo-metric properties of sandwaves from the measuredbed elevation profiles as ^objectively as possible.We analyzed sand wavelength X. sand waveheight A, erest elevationtlj.. trough elevation Ti(.and sand wave asymmetryA in the detrended bedeievation profiles (Figure42 44489Figure 3:25 27490 491X co-ordinate (km)492S831 5832 5833 5834Y co-ordinate (km)5835 58367 8 I'i/H'lhie & Gasjiiunitii I April 2008 / www.pgjonline.com1650 1700 1750X co-ordinate (m)1800NormalGammaRayWghExponentialLognormalGumbelWeibullFigure 4: Figure 5:0 1 2 3 4Dimensionless trough elevation (-)4). Sand wave asymmetry is computed as A =IXI -XI)IX.Probability Density FunctionsWe determined the probability density func-tion (PDF) of measured trough elevations in onearea. We then fit Normal. Ganinui, Rayleigh.Weibuil, Hxponcntial. Log-normal, and Gtimbeldistributions to the data using a maximum like-lihood estimation. Figure 5 shows an exampleof fitted distributions for trough elevationsoccurring in one of the Noordhinder areas. Theappropriateness of the fitted distribulions wasdetermined for the probability density functionof trough elevations. We plotted probabilitydensity ñjnctions for all nine sand wave fields,and for each geometric property. For sand wavelength, sand wave height, erest elevation, andtrough elevation we found that the Weibull andCiamma distributions provide the besl fit. Forsand wave asymmetry the Normal distributionprovided the best fit.Coefficients Of VariationFor each geometric sand wave property thecoefficient of variation was determined. ThecoefTicient of variation C is defined as C =a/(i, in which a denotes the standard devia-tion, and \x the mean value of the geometricsand wave property. Figure 6 shows the rela-FIND ITv̂With the most complete product line availableWHEN IT COMES TO GAS DETECTION J AND N HAS EVERYTHING YOU NEED!SENSIT GOLD CGI TRAK-IT III CGISMART-CALCALIBRATION STATION SENSIT LMDSupplemental Leak Detection • Leakage Survey/Pinpointing • Confined SpaceEmergency Responder • Pipeline Purge • Combustion SafetyAnd N Enterprises, Inc1 T D851 ransport rive • Valparaiso, IN 46383 • 888 4SENSIT (473-6748)Ph: (219) 465-2700 • www.gasleaksensors.com/•f/tcHiu' & (ins Journal I April 2008 / www.pgjonlme-com 7 9Pipeline & Gas Journal sInternational/Offshore Report•a ISOS' EcomorONoordhMwTwn0 BO 100 150 !O0 250 300 350 4 «Mean sand wave length (m)Figure 6:tionship between standard deviation of sandwave length and the mean sand wave length.It appears that the long-crested sand wavesof Ecomorß have more regular sand wavelengths than the short-crested sand waves ofthe other areas. This is also the case for sandwave height, crest elevation, trough elevation,and sand wave asymmetry.Furthermore, if we only consider the short-crested areas, we find that the coefficients of vari-ation of sand wave length, sand wave height, crestelevation, and trough elevation are more or lessconstant values, while the coefficient of variationof sand wave asymmetry is not. The coefTicienl ofv-ariation of sand wave length C^ equals more orless 0.55 (Figtire 6), The coefficients of variationof sand wave height crest elevation, and troughelevation are C¿ = 0.49, Cr|c = 0.59. and Cr|c =0.64. respectively. The slandard deviation of sandwave asymmetry appears to he more or less con-stant (CA = 0.35). independent of the mean .sandwave asymmetry.A constant coefficient of variation of sand wavelength means tiiat tlie standard deviation in sandwave length aui be estimated if the mean sand wmelength and the coefficient of variation for sand wavelength C;̂  are known. The same holds for sand waveheigiit, cn;st elevation. ;md trougli ele\'ation.Extreme ValuesWe analyzed the 5"/í) highest and 5% longestsand waves, the 5"/i. deepest troughs and 5%highest crests. For shon-crested sand waves wefound that, for sand wave length, sand waveheight, crest elevation, and trough elevation.the 95% values are linearly related to the meanvalue. For instance, for sand wa\'e length wefound that the 95"/íi coefficient for sand wavelength C)^.95 = (?w)5 - pX) / |A = 1.1. Likewise,we found that the 95% coefficients for sand waveheight, cresi elevation, and trough elevation areC^c.95 = 0-9- CT^^.95 = 1.0. and C^c,95 = '-2-respectively. This means that we can estimate theextreme geometric sand wave properties, giventhe 95% coefficient and the mean value.Note that the given coefficients of varia-tions and 95% coefficients are only valid forshort-crested sand waves.Planning PipelinesKnowledge on the variability of sand wavesmay help us. for instance, in determining theoptima! depth of a pipeline trench. The optimaldepth depends on factors such as dredgingcosts, pipeline construction costs, monitoringcosts, and risk {Németh et al.. 2003). Consider,for instance, a sand wave field through whicha pipeline has to be constructed. Assume thatit is known from previous monitoring surveysthat the sand waves are short-crested, and thatthe sand waves migrate, but do not grow ordecay, as there are no dredging activities andnet currents are more or less constant.We also know from a previous survey thatthe mean sand wave heiglit is more or less fourmeters. Our data analysis shows that mean crestelevation is smaller than the corresi.x)nding meantrough elevation such thai a sand wave height offour meters corresponds to a mean trough eleva-tion of 2.5 meters and a mean crest elevation of1.5 meter. From our analysis we may concludethat the standard deviation in trough elevations is:G( = C«j |i™ = 1.6 m. The 95% trough eleva-tion is: ^^ 95 = (Cr t̂_95 + I ) n,̂ t = 5.5 m.This means that in an area oí" more or less !(K)CATALOG. NO.P95-525PHD053510,000 lbs,40,000 lbs.20,000 lbs.Separa tes... Pries...& More!GEARENCH is pleased to announcethe addition of the POP-IT' Tool toour line of quality products. ThePOP-IT" is the most versatile flangespreading and prying tool on themarket. This unique product israpidly gaining popularity ina wide range of industriesincluding petrochemical,refining, pipeline, powergenerating, as well asemergency rescue.Call us todayfor details. „GEARENCHwww.gearench.comClifton, TX(254)675-8651 • fax (254)675-6100 • sales@gearench.comERRAN.OUR SURPLUS IS YOUR GAIN.Exterran is the world's largestcontract compression companyand a premier provider of sales,operations, maintenance, fabrica-tion, service and equipment for oiland gas production, processing andtransportation applications.Our avaiiabie surplus equipmentFOR SALE includes gas compressionunits, cylinders, frames, engines,coolers, vessels, etc., includingsome pipeline type equipment fromthe Houston Pipeline System.Our surplus is your gain.Call Mickey McOonald at 7ia-335-7357 or &mail atmichey,mcdonald@eKtefran.com to review our current inventory for sale.www.exterran.com/e-surplus8 0 Pipeline & Cias Joninat I April 2008 / www.pgjonline.comsand waves, five of llicse siind waves will have atrough elevation that is equal to or laiïier than 5.5meters. Taking into accotmt the risk one is willingto accept (which may depend on the type of liquidthat is transported), one may decide to place thepipeline at a depth of more or less 5.5 metersbelow the mean bed elevation. If the pipelineis exposed by one of these five deep migratingtraiighs, this trough will probably noi be muchdeeper than 5.5 meters, and the free span lengthwiîl be small. As this example illustrates, oursimple relationships may help in yetting a firstestimate for the optimal deptli of a pipeline.ACKNOWLEDGEMENTThis research projec! is part of VICl project RoughWaler (STW TCB.6231) and is supported by iheTechnology Fouiidalioii STW. the applied science division(if NOW and the tedinology program of ihe Minislry nfEconomie Affairs. Tlie work by ihc second author servedthe patiiyl fulfillnient of the requirements for tlie M.S.degree in civil etigineoring ai University of Twentc. theNetlicrlands. We acknowledge tlie North Sea llii-eetorateof the Ministry of Tran.'iporl. Public Works, and WaterMiuiiigemtnt for providing us with the bathyniclric data..iuthors: C, F. van der .Mark i.s im-olwii in WaterEngineering df Mami^'ment at the Vnive¡s¡t\' ofTwcntt: PO. Bax 217. 75(K) AE. hischeik. theNctheiiiintb. Entail: C.Evumh'rMaiiúi'iituvnte.nl.MJ'\ de Koningis imnhed inWater Euffiieeiing& Management at the Unhvrsity ofTwente. PO.Ba\2¡7. 75mAE. Emchede. Ihe Netherlands.A. Blom is associated with the EnvimnmentalFluid Mechanics Section. Delft University ofTechnology. Delft, the Netherlands.S.J.M.H. Huhcher is involved in WaterEngineering & Management at the UniversityofTwente. PO. Box 217. 7500 AE. Enschede.the Netherlands..4. Stolk is in the North Sea Directorate.Ministry of Transport. Public ii'orks. and WaterManagement. The Hague, the Netherlands.REFERENCESHuischer. S. J. M. H. {1996). Tidal induced targe-scale reg-ular bed fonn patterns in a three-dimensional shallow watermodel. J. üeophys. Res., 101 (C9), pp. 20,727-20,744.Némeih, A. A.. Hulsclier. S. J. M. H,. and De Vriend.H. J. (2002). Modelling sand wave migration in shallowshelf seas. Com. Shelf Res., 22 (18-19), pp. 2795-2806.Németh, A. A., Hulsghcr, S. J. M. H., and De Vrirad, H. J.(2003 ). Offshore sand wave dynamics, engineering problemsand fliture solutions. Pipeline Ga.s J.. 230 (4), pp. 67-69.RIKZ (1997). User ttianual, Digipol Vl.O. Tech. rep.,National Institute for Coastal and Marine Management(RIKZ), The Hague, the Netherlands.Van der Mark. C. F and Blom, A. (2007). A new andwidely applicable tool for determining the geometricproperties of bcdforms. CE&M research report 2OO7R-003/WFM-0()2 ISSN l5hS-4i>52. t'niversity ofTweme,Enschede, the Nethcrljiuts.There is only one crediblesoftware package source..When you need to perform fast,yet complex and accurate, inter-ference analyses. •--Right-Of'Way Afullyautomated analysispackage Ihal can handlemultiphase networks,pipe-enclosed cables,pipelines, rails, commu-nication lines or insu-lated & bare conductors.It includes SESTLC, anintegrated package forquick interferenceevaluations and SES-Enviro that examineselectromagnetic envi-ronmental impacts.# CAD-based import. GUImated design package that determines cur-rents 8 voltages caused by induction andconduction anywhere along the right-of-way.% Automated faults yield maximum values suchas pipeline or rail GPRs. rail-to-rail voltages,induced longitudinal & ieal^age currents,interference on telecom lines, tower GPRs.broken rail GPDs, etc.# Dedicated, flexible tools simplify and auto-mate every aspect of your work. Exceptionaldocumentation, context sensitive help andtechnicai support bring you peace of mind,when you need help.# Flexible data reporting and plot utilities vividlyillustrate all computation results.www.sestech.cominfo@sestech.com1-800-668-3737; 1-514-336-2511World Leader in Grounding & EMIAdvanced Heating Solutions•RuffneckHeateis tor the Harstiesl EnvirDnmenls Eleclric Exploslon-Proof Heaters Explosion-Proof Gas Catalytic HeaterGCaloritechEngineered Eledric HealCCI Thermal manufactures gas afid electric heaters for the hydrocarbon industry in spot,space and process applications. We are committed to bringing our customers the bestheating products availiable. Call us today or visit our website for more information.1.800.661.8529 • www.ccithermal.com • infoigccithermal.comEdmonton Oakvilte Orillia GreensburgPipetine & Gas Journal I April 2008 / www.pgjonline.com 8 1

Sea Bed Sand Waves Studied To Help Pipeline Planners

Modelling sand wave migration in shallow shelf seas.

Offshore sand wave dynamics, engineering problems and fliture solutions.

Tidal induced targe-scale regular bed fonn patterns in a three-dimensional shallow water model.

User ttianual, Digipol Vl.O.

https://ris.utwente.nl/ws/files/6437653/sea_bed_sand.pdf

Sea Bed Sand Waves Studied To Help Pipeline Planners

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