Norway is on the way towards green energy transition. Harnessing offshore wind energy is one of its key resources due to stronger and consistent wind at the Norwegian seacoast. Which requires more research to deploy offshore wind turbines safely at coastline. Monopile foundation for offshore fixed turbines already proved its worthiness by covering more than 80% part of entire installed fixed offshore wind turbines till the depth of 30m. Now it is the time to test it on 60m depth of sea which is quite closer the depth of some parts of Norwegian Continental Shelf. In this master thesis, a case study of installing an XXL monopile foundation using Alfa Lift floating vessel at Sørlige Nordsjø II site has been studied. A XXL monopile of 105m in length and 8 to 11.5m in diameter has selected to be installed at 60m depth. The Alfa Lift floating vessel was selected due to its unique ability to accommodate XXL monopile. Its length of 245m provides enough strength to face Norwegian sea conditions and its crane capacity of lifting 3000 ton of monopile is suitable for handling large size monopiles. This XXL Monopile Lowering operation has been modelled in OrcaFlex. Which offers a wide range of tools and objects to model multiple maritime operations. In this typical XXL monopile lowering operation, floating vessel has been modelled by vessel object, XXL monopile modelled by 6D buoy object, gripper device modelled by shape object and its hydraulic cylinder is modelled by link as spring object and Crane wire has been modelled by line object. A time domain simulation has been run with lowering rate of 0.1 m/s. Simulation is run for total 800 seconds, from which first and last 100 seconds are steady states whereas 100 – 700 is a lowering phase and 600 – 700 is a landing phase. In this time, Monopile tip approach from zero sea level to -60m depth and lands on seabed. This study applies Morison’s equation for analysing hydrodynamic effects. At first stage, time history based dynamic response has been identified for monopile x and y displacement motion, crane wire force, gripper device contact force and landing contact force. At second stage, sensitivity studies for extreme responses have been performed by varying peak period from 5 to 13 seconds. Finally, an allowable sea states have been determined by considering monopile x displacement motion as operational criteria which should not be more than 1m. At last, a scatter diagram based on 10 years meta-ocean data has been present with highlighted allowable sea states in case of gripper stiffness of 8000 and 4000 KN/m.

M.Sc. Maritime Operations, Western Norway University of Applied Sciences

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