Assessment of Marine Operations for Offshore Wind Turbine Installation with Emphasis on Response-Based Operational Limits

Abstract

This thesis addresses methodologies for assessment of response-based operational limits in terms of allowable vessel responses or sea state parameters, and numerical analyses of current and novel o_shore wind turbine (OWT) installation activities. A generic and systematic approach that allows for identi_cation of critical events, limiting (response) parameters and assessment of response-based operational limits of marine operations was developed. The approach is based on numerical simulations of the actual operations. Frequency and time domain techniques or operation-speci_c numerical analysis methods may be applied. An e_cient method for assessment of the operational limits of mating operations has been proposed. The method relies on a number of crossings that a mating pin is allowed to perform out of a circular boundary (docking cone circular perimeter) in a given period of time. By carrying out a quantitative assessment of the system dynamic responses, the critical events and corresponding limiting parameters are identi_ed. A characteristic value of a limiting parameter needs to be determined based on extreme value distributions for a target exceedance probability. This depends on the type of operation and consequences of failure events. The characteristic value is compared with the allowable limit of the limiting parameter, so the environmental conditions can be identi_ed. The limits of these environmental parameters represent the operational limits of an installation activity. In this thesis only the e_ect of wave actions are considered, while current and wind actions are not included. The OWT installation activities studied in this thesis are executed with a oating heavy lift vessel (HLV). Three main activities were considered: the monopile (MP) initial hammering process, the transition piece (TP) mating operation, and the entire installation of the tower and rotor nacelle assembly (RNA). For the installation of the MP and TP, standard operational procedures were employed, while for the tower and RNA, a novel and e_cient single lift installation concept was developed. This novel concept is based on the principle of the inverted pendulum and requires a cargo barge, a medium-size HLV and a specially designed upending frame. The need for huge (in terms of lifting height and capacity) o_shore crane vessels is eliminated. Moreover, it has been shown that the novel concept is a feasible and valid alternative for current installation procedures. The current procedures employ jack-up vessels for sequential installation of wind turbine components; however, these activities are not studied in this thesis. The generic approach was applied to the MP, TP and tower and RNA installation activities. For the MP initial hammering process, it was found that the critical events are failure of the hydraulic system and out-oftolerance inclination of the MP. For the TP mating operation, the critical events are the failure of the bracket supports and the failed mating attempt between the TP bottom tip and the MP. During the tower and RNA installation, structural failure of the hoist wire, structural damage of the hinged supports, and a failed mating attempt of the upending frame are identi_ed to be critical events. For various limiting parameters, the operational limits were established in terms of allowable limits of sea states, which are a basis for assessment of the operatiblity. A methodology for weather window analysis and assessment of operability of marine operations was also developed. This methodology includes the response-based operational limits and accounts for sequence, continuity and duration of the activities, which are shown to be important in the analyses. The operational limits of the MP and TP installation were used for weather window analysis and assessment of the operability during the planning phase. Moreover, it is shown that weather forecasts can be used to identify workable weather windows and support on-board decision making during the execution phase. The methodologies provided in this thesis are systematic and e_cient for modeling of current and novel OWT installation activities with the aim of establishing response-based operational limits. These are necessary for planning and safe execution of OWT installation activities.