Forced Torsional Vibration of a Monopile for Its Extraction

Abstract

The offshore monopile decommissioning demand will become definite in the coming years. Our responsibility is to ensure the rights and duties of other legitimate uses by completely removing the ageing monopile from the seabed to continuously redeveloping offshore wind farms within the same location. The growing number of past, present, and future monopile installations opens up the challenges and opportunities to be responsible and lead the decommissioning market. With the goal of complete removal, a novel GDP technique can be the win-win solution for offshore wind operators and contractors to extract the monopiles completely from the seabed using torsional and axial vibration
This thesis seeks to understand the torque and normal force to safely clamp a monopile during a torsional vibration so that the monopile continuously slips over the soil. Gradual soil failure along the pile-soil interface's full depth due to the monopile's torsional motion is a possible theory to explain the failure mechanism. When an upper part of the pile successfully moves relative to the soil, kinetic friction occurs until the soil resistance is larger than the shearing at one point. If more shearing is added by adding more torque, more layers below will be broken while the upper part keeps sliding due to lower friction than static friction. While the linear elastic theory of solid and thin shell bodies is used within a 3D FE modelling in Ansys to couple the soil and pile, the clamping force due to the GDP shaker is decoupled from the analysis. Failure criterion is defined outside the simulation so that the gradual soil failure is done through several simulations assuming discrete soil layers.
The FE model is constructed and verified by analytical calculation through the semi-infinite cavity-pile-soil, wave reflection, and finite cavity-pile-soil-spring-dashpot problems. Several cases of gradual soil failure are simulated and show that the torque amplitudes form a distribution. Firstly, a probabilistic sense is proposed to interpret the torque amplitude and search for the optimum depth of the soil failure. Secondly, a convergence check is made with the help of an analytical shell-spring by considering more soil elements by virtue of good correlation of the shear stress between the analytical and FE model. It eventually suggests that a convergence of the torque amplitude can be achieved, which reinforces the theory of gradual soil failure. The interpretation suggests that the current GDP shaker is one step closer for a monopile extraction test with typical monopile dimensions that correspond to a typical 1 m diameter. A first approximation of the required torque and clamping force is then proposed to benefit the analytical model for larger diameters up to 6 m.