Offshore Wind Turbine Monopile Foundation Installation with a Dynamic Positioned Vessel

Abstract

After years of using fossil fuel, the transition to renewable energy sources need to be made to limit the increase in temperature and to support the future energy demand. To support and speed up the transition phase from fossil fuels to renewables it is necessary to decrease the costs. Offshore Wind Turbines are widely used for the production of renewable energy and several Offshore Wind Turbine projects are planned for the future. Most of the Offshore Wind Turbines are founded by monopile, large steel tube, to support the wind turbine. Nowadays, these monopile be installed by either jack-up vessel or moored floating vessel. However, these installation method come with a major drawback: the installation procedure is time consuming. A new installation method is propose to reduce the installation time. This thesis focus of the feasibility to install the monopile with a dynamically positioned (DP) vessel. The
required station keeping situation is faster achieved with a DP vessel. Due to the footprint of the DP vessel relative to an earth fixed position, a vessel motion compensated pile gripper is used to maintain the upright position of the monopile and to decrease the interaction forces between vessel and monopile. Adding the monopile to the vessel is an off-design condition for the DP controller. During the early hammering phase of the monopile, the monopile have limited interaction with the soil and is unstable. The upright position is maintain by the gripper frame. The forces from the gripper frame on the monopile are reaction forces on the vessel. Beside these forces, environmental forces are acting on the monopile and via the gripper frame acting on the vessel. The forces on the vessel could lead to unstable behavior and/or increased vessel footprint. A simulation model is build to investigate the behavior of the DP vessel during the operation. A industry used DP simulator and a simulation model of the Bokalift1 is used. A model of a typical shallow water and deep water monopile is build. A hydraulic based gripper frame is simulated with an inclination controller and a induced vessel motion controller which need
to maintain the upright position of the monopile. The inclination controller is tuned with a higher bandwidth compare to the bandwidth of the DP controller to prevent motions of the monopile is the same frequency range as the linear motions of the vessel. The forces from the gripper frame are fed into the Kalman filter of the DP controller. This is done to prevent a drift of the vessel when the gripper frame starts acting on the vessel. In all simulation cases with governing environmental conditions, the vessel could maintain stable behavior. The rotations of the monopile are in the same frequency as the first order wave forces on the vessel. This relative high frequency motions to not significantly amplify the position of the DP vessel. However, despite the fact of feeding the Kalman filter, a larger drift is observed in case of the large, deep water monopile in the operation stage when the gripper frame force is introduced to the vessel. This increase the requirement on the envelope of the gripper frame. The requirement on the gripper frame is given in terms of power, force and envelope based on governing environmental conditions. The requirements on the gripper frame are assumed to be within an acceptable magnitude. The operation seams to be promising in the future.