Floating Offshore Wind Subsea Balance of Plant Cost Savings Using a Risk Based Assessment Approach

Abstract

Abstract A risk based sampling approach for subsea inspections on the Balance of Plant (BoP) equipment in a commercial scale floating wind farm is presented. A lifecycle cost analysis is performed to compare a prescriptive inspection approach to a risk based sampling approach to demonstrate the cost savings benefit of the sampling approach. The conceptual base case wind farm for the study has 60 × 15MW turbines on three-column semi-submersible floating foundations designed with a 30-year design life. The BoP equipment includes the floating foundations for the turbines and Offshore Sub Station (OSS), mooring lines, and inter array and export cables. The prescriptive inspection approach for the subsea BoP equipment is based on typical regulatory requirements such as those proposed by ABS, current fixed wind practices, and those used for Oil & Gas (O&G) assets. The risk-based sampling approach is based on selected key risks/failure scenarios that drive inspection activities and frequency. A BoP subsea inspection lifecycle cost savings of over 30% can be achieved by using a risk based sampling approach instead of a prescriptive inspection approach. The risk assessment for the sampling approach is primarily driven by consequence cost considerations. Cost comparisons are made between un-detected threats that result in failure versus threats detected in time to mitigate or plan repairs in advance before failure occurs. The risk cost is the cost of the failure event times the probability of occurrence. Comparison is made to the cost delta for different inspection frequencies. Floating wind is a new industry with little reliability data available. However, O&G experience provides useful failure rate data that is used in this study. The risk based sampling approach considers degradation mechanism trends on inspected units that are extrapolated to other non-inspected units based on correlation between units. Monitoring can also affect the consequence of failure by providing early detection. Risk assessment should be conducted in the design phase as less conservative designs may save cost initially, but the higher risk may drive more frequent inspections. The cost savings data presented in this study is of great benefit considering that O&M costs can amount to a substantial 30% of the total lifecycle cost. It is a novel study since it is specific to floating wind, which is a new advancement, and therefore does not have many papers available compared to fixed wind. It will also demonstrate key ways in which O&G knowledge can be transferred to floating wind.