CASE STUDY
Cost, Risk and Failure Implications of Different Mooring Spreads and Number of Mooring Lines for FOWT
Client(s): Offshore Renewables Energy Catapult
Dates: 2022 - 2023
AIMS
The aim of this project was to investigate the implications of different mooring system topologies on the cost, risk and failure performance of FWT arrays comprising 15MW turbines. Two hypothetical scenarios involving 0.9GW arrays located in 100m (NE of Aberdeen) and 500m (West of Shetland) water depths were considered.
Approach
The following stages were carried out:
1. Representative designs were identified using TTI-MR’s proprietary multi-objective optimisation (MOO) mooring design tool. Consideration was given to chain catenary, semi-taut and taut mooring systems comprising 3, 6 and 9 mooring lines.
2. The short-term consequence of failure of these designs (in terms of platform motions, line tensions etc) was determined through ALS simulations. The long-term consequences of through the lifetime of a project (25 years) were then assessed via Monte Carlo Next Event analysis, with consideration given to multiple sensitivities including weather windows, vessel cost & availability and component reliability etc.
3. Stakeholder engagement was also carried out to garner sector views on the subject, including risk appetite, design philosophy and perceived challenges in the sector and also to confirm some of the key assumptions used in the study.
Key Outcomes and Impact
The scenarios studied demonstrated that decisions made at the design stage (i.e., redundancy provision) can minimise overall lifecycle cost when the consequence(s) of failure(s) are considered. Furthermore, in order to draw conclusions which are broadly applicable to the FWT sector lifecycle assessments need to be carried out on multiple designs. Perhaps the most pertinent overall conclusion from the mooring identification work is that, the design of these mooring systems is a compromise and it must be thought of as such.
A copy of the public summary report can be found here.
Industry Relevance
The MA02 project is highly relevant because it took a detailed look into systems could be suitable for commercial-scale turbines at similar sites. With this, (and other projects listed here), the team have amassed significant experience in the design landscape.
The project also demonstrated the importance of mooring line redundancy provision on maintaining array power production capability.