The Boarding Control System: Modelling and Control of a Surface Effect Ship for improved accessibility to Offshore Wind Turbines

Abstract

The main contribution of this thesis is to introduce a control system that enables access to offshore wind turbines for operation and maintenance (O&M) in higher sea states than what is possible today, while maintaining an acceptable level of risk. The system is implemented on-board on a Surface Effect Ship (SES) which introduces a new craft-concept to the turbine transfer vessels. Catamarans and Small-water-area twin hulls (SWATHs) are currently dominating the market. The control system is denoted the Boarding Control System (BCS) which is a control algorithm that utilizes certain sensors and hardware to control air- ow actuators on a SES. The controlled actuators regulates the air cushion pressure to counteract, and compensate against, the vessel motions set up by sea wave propagations. Hence, we perform motion damping in the vertical plane which ultimately improves the accessibility to wind turbines. The work presented in this thesis influenced the decision to build two vessels of the Wave Craft class, build no. 21 and 22 by Umoe Mandal. The prototype, Umoe Ventus, is currently operating at Borkum Riffgrund 1, a wind-farm in the German sector of the North Sea. The SES-dynamics related to the BCS is mathematically modelled and a controller is designed. Stability investigations are performed and system performance are given through simulation, model- and full-scale experimental testing. The performance of the BCS proves that accessibility to offshore-structures is possible in higher seas compared to the case where the system is inactive. Today it is possible to access turbines in up to 1:5 - 1:75m signifcant wave height (Hs). The SES with the BCS is tested in up to 2m Hs with no sign of reduced safety for offshore personnel while boarding a turbine. The model-test results indicates that access is possible in up to 2:5m Hs and at least 3:2m in long-crested seas (Section 2.5).