Walney Offshore Windfarm Substation P1 Case Study - FoundOcean

Walney Offshore WindfarmFoundOcean helps DONG Energy reach a milestonewith the installation of the windfarm substationSubsea and offshore groutingfor the global energy construction industries

Milestone cemented for Walney with theinstallation of the Offshore Substation P1ABOUT THE WALNEY OFFSHORE WINDFARMThe Walney Offshore Windfarm is 15km from thecoastline of Walney Island in a north west to southeasterlydirection covering approximately 73km 2 .The Walney 1 Windfarm consists of 51 turbines eachwith a capacity of 3.6MW, powering 160,000 UKhouseholds. When Walney 2 comes online the totalwill increase to 320,000 UK households.The windfarm turbine array consists of a number ofrows of wind turbines connected by one cable fromWalney 1 and one from Walney 2 which will carrythe power to shore. The substation has a typical fourleg jacket style foundation. The foundations for theturbines are monopiles.Key Project FactsOperatorDONG EnergyContractorVSFRegionIrish SeaWater depth -30mTotal cement 51.48 tonnesCrew size 4Equipment12V Grout Mixer with builtin grout pump1 x 40 tonne cement silo1 x 35 tonne cement siloLab and RefrigeratorUmbilical Hose WinchHosesGrout specification 10.04 MPa @ 24 hours78.56 MPa @ 28 daysCuring temperature 8 o cMobilisation port Rotterdam, NetherlandsInstallation vessel RambizFOUNDATION DESCRIPTIONThe jacket is of steel construction with 4 legs. It issecured to the sea bed by 4 main piles driven throughthe sleeves. The piles are OD 1,829mm driven throughsleeves of ID 1,990mm. The connection between thedriven piles and the jacket was made by injectinga cement grout into the annulus around each pile.Thus, the structural integrity of the platform wasdependent on the correct placement and retentionof good quality grout in the annulus of each pile.The lower end of each annulus was equipped withtwin grout seals to retain the grout. Each sleeve hada primary and secondary grout line through whichgrout could be pumped into the annulus.A conventional grouting system has been designedfor the installation of the jacket. The pile sleeves areequipped with primary and secondary grout linesterminating at elevation +16.3m. The lower end ofeach pile sleeve is equipped with an Oil States typegrout seal, the purpose of which is the retention ofthe grout in the annulus whilst it is curing.QUALITY CONTROLDuring mixing, the grout density was monitored atintervals using a pressurised slurry density balance.Three samples were taken during the grouting ofeach annulus: one after 10 bbl had been pumped,one halfway through grouting, and one towardsthe end of grouting. For each sample the densitywas recorded and three 75mm grout cubes weremanufactured. Each cube was marked to identifythe platform, leg number, time and date of castingand the specific gravity. The samples were curedunderwater at 8 o c ± 0.2 o c.THE MIXING SYSTEMCement was stored in bulk pressurised silos anddelivered to a surge tank above the mixer. Waterwas delivered to the mixer from the barge fire

Mixing the cement and seawaterWalney Substation P1 topside and foundationsRambiz places the jacket’main pipe work and measured through a waterflow meter. Cement was metered into the mixerunit from the surge tank by a variable speed rotaryvalve. The vortex action generated in the mixertank immediately assimilates fresh materials as theyare drawn through the mixer housing and mixedby a high shear mixing action. The grout was thentransferred to a 1,500 litre capacity holding tankfrom where it was drawn off by a progressive cavitygrout pump and delivered via a flexible hose.THE ENGINEER’S PERSPECTIVEWe arrived at Rotterdam and the first two deliverytrucks had only delivered 65 tonnes of cementinstead of 75. We requested another 10 tonnes tobe delivered and this arrived a few hours later. Wethen loaded all the cement and equipment ontoRambiz ready for mobilisation before heading toBarrow where we would meet up with the vessel.Onboard we set up all the hoses, silos and mixingequipment. We checked these for any damagethat may have incurred during transportation.All equipment worked well but the polarity ofelectricity had to be reversed before groutingoperations could commence. Space on the deckwas very limited but working closely with the projectengineers we came up with a workable solution.All sleeves were grouted in accordance with ourGrouting Procedures. We had anticipated thegrouting to take a total of 12 hours but with theintroduction of our newest 12V batch mixer, this tooka mere 8 hours. We started grouting at 06:13 andhad completed all grouting operations by 14:40.We cleaned the grouting equipment and were onstandby while awaiting the 12 hour slump test toconfirm no additional top up of grout was needed.Overall the project ran ahead of schedule with theFoundOcean crew spending just 48 hours on boardfor the grouting operations. We worked outsidenormal operating hours to ensure the project wascompleted to a high standard. The client praisedour efficiency, grouting and lab equipment, andprevious working knowledge of the onboardpersonnel.> Visit www.foundocean.com for more case studies

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