Simplified model of contact force between a service vessel and a fixed monopile in irregular Waves

Alistair Borthwick, Denis Kelliher, Ariel Edesses

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Simulation of vessel-wave-structure interaction is important in the context of the safe transfer of personnel and equipment from supply vessels to offshore structures, such as the monopile support structure of an offshore wind turbine. Presented here is a simple model for determining the loading on an idealized supply vessel abutting against a fixed vertical surface-piercing cylinder representative of a monopile structure supporting an offshore wind turbine (OWT). The paper is an extension of research by [1] in which the motion of the supply vessel was simulated for a static scenario and without the inclusion of water wave kinematics. It is assumed that the vessel and monopile are small diameter structures with respect to the wavelengths of ocean waves under operating conditions, and that the vessel remains stationary (due to controlled thrust from its engine), and that the monopile is fixed to the ocean bed. Irregular waves are simulated from a one-sided wave spectrum for a given wind speed and mean water depth. By using linear wave theory and stochastic processes to simulate a randomly varying sea surface elevation, time series of free surface displacement and horizontal and vertical water particle kinematics are estimated at the spatial locations of the vessel and monopile. Linear wave theory is a reasonable approximation for the non-breaking relatively low amplitude waves experienced by repair vessels as they approach turbines under operating conditions. Morison et al.’s equation [2] plus the additional constant motor thrust are then integrated numerically over the submerged lengths of the monopile and idealised vessel to estimate the loading time histories for systematic variations of fetch wind speed, water depth, and incident wave angle between the vessel and turbine. Other previous research has focused on collisions between service vessels and turbines but not on the vessel thrusts needed to maintain contact between vessel and monopile during routine maintenance. An introduction to a simplified numerical solution to this problem is presented here.
Original languageEnglish
Title of host publicationVI International Conference on Computational Methods in Marine Engineering, Marine 2015, Rome, Italy, 15-17 June 2015.
Publication statusPublished - 2015

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