Stress analysis of a monopile foundation under the hammering loads

Abstract

The installation of monopile is a very important process in offshore wind turbine construction. Duringthemonopilebeingdrivendownbyhydraulichammer,hugeenergytransfersfromham mer to pile, which may cause monopile inclination. In order to keep the monopile steady in horizontal direction, a gripper device is applied between the monopile and installation vessel. Thegrippercontactswithmonopilebyfourhydrauliccylinders. Inthemosttime,thehydraulic cylinder contacts with the smooth surface on the monopile, and it only bears axial load. While, the monopile is welded piece by piece, its surface is not perfectly smooth due to welds. The welds collide with the hydraulic cylinder and induce a vertical force on the hydraulic cylinder, whichmaydamagethehydrauliccylinder. This thesis analyzes the installation of monopile by HLV and hydraulic hammer. It presents modelling and calculation for the collision problem between welds and hydraulic cylinder. A finiteelementmodelformonopile,weldsandgripperisbuiltinAbaqusCAE.Theexternalloads including hammer load, soil resistance and cylinder force is obtained from hand calculation or previousthesisandthenappliedintothefiniteelementmodel. Theeffectsofchangingdifferent parameters (such as welds thickness, hammer energy, initial penetration depth etc.) is carried outforanalyzingthemethodforreducingthegripperstressduringcolliding.