The UK has an ambition to develop 50GW of offshore wind in its waters by 2030. The Norwegian government will also allocate development areas for 30GW of offshore wind by 2040. This demonstrates that the world is moving towards a greener future, and a future that will need reliable, functioning, and tested solutions for dynamic cables.
One of the key features of a reliable design is to have control of the failure modes. So why do offshore power cables fail? Failures can happen on a regular basis, and originate from all the phases of a project, from specifications, design and engineering, manufacturing, to installation and operation. This happens with both experienced and in-experienced parties, and one of the challenges is that the industry keeps the root cause of cable failures confidential. There is huge potential in mitigating risk through collaboration and knowledge sharing.
The results of a DNV root cause analysis shows that installation, production, and design account for 63% of the failures of general cable systems, whereas this accounts for 72% of subsea cable failures. It’s worth noting, due to limitations of data, the sample of root causes for subsea cables is significantly smaller than for all cables in general. The failure rate for cables today is high, even for static cables. Dynamic cables systems are more complex, and the market is moving fast, stretching today’s technologies.
What are some of the origins of failure and how can we mitigate the risks?
There are often gaps and unclarities in the standards during the design and engineering of cables. The quality level is dependent on the supplier’s interpretation of these standards, their tools, and their design methodology which is often confidential. Lack of knowledge and experience with dynamic cables leads to un-proven designs. Complex cable systems require cross-discipline expertise in design selection and decision, for example regarding electrical, thermal, mechanical, hydrodynamic, and geotechnical competencies. To mitigate that risk, we need to have clear technical specifications from buyer to supplier, e.g., clear definitions of functional requirements. There needs to be a high level of quality control during design and engineering, i.e., qualified third-party verification and certification to review design, procedures, and documentation. This would help to check that functional requirements are met and solve design issues in an early stage of the process.
Some of the most common causes of failure during manufacturing stems from the imperfect control of the process which results in failure to observe important issues. Non-conformities are not reported, and use of non-qualified procedures related to repairing and factory. A lack of quality control of incoming materials and components and checks of homogeneity causes issues down the value chain. During installation knowingly or unknowingly exceeding the design limits related to bird caging, over bending, axial compression and exceeding of the sidewall pressure is a problem. Insufficient control of the cable laying operation, as well as jointing failures stemming back to workmanship, material handling, and workability of design, are all root causes of failure. Measures such as prequalification of suppliers, agreed manufacturing, installation, inspection, and test plans needs to be in place. There needs to be clear definitions in place to verify that functional requirements are met.
Independent review of tests, inspection plans, procedures and design must be done. Third-party follow-up and observation of manufacturing and testing can help to root out issues before installation takes place.
The need for electric power is growing at an exceedingly high pace. Governments are setting ambitions which the industry must meet and to do so, we need to have control of all the aspects of the process and potential failure modes. The industry needs to come together to share and collaborate, and to develop proper guidelines to account for risk, assure quality and interface management.
Lars Helge Verde is an analysis engineer working with risers, cables and wellhead fatigue in the Riser Technology department of DNV. His background is a master’s degree in mechanical engineering from The Norwegian University of Science and Technology and he has over two years of experience in the field.