Guest editorial

EIC guest editorial

Subsea power cables Critical infrastructure and mitigating risks during installation

Lying across the global seabed is a network of subsea power cables running thousands of kilometres, transporting power to industry and millions of households. Most people are probably not aware of the critical role subsea cables play in our lives, being ‘out of sight, out of mind’.

They are normally buried under the seabed and are not as visible in the same way as a power station or the huge offshore wind turbines you can see dancing offshore, such as those in the Moray Firth. According to a 2019 report published by Wind Europe, offshore wind renewable energy accounted for 15% of power generation across Europe with an increased capacity of 15.2GW. This is a significant increase of clean energy generation over the last 10 years which would not be possible without subsea power cables.

The construction of an offshore wind farm is a complex and expensive process. It requires high value assets such as the wind turbines, foundations, sub-stations and subsea cables to be transported offshore using large vessels and barges. These assets can be owned and insured by multiple parties, whose understandable concern is to manage and mitigate the risks involved in the construction. Incidents can be very costly, and a faulty power cable presents significant problems to the owners, contractors and insurers, due to the costs and time to repair or replace the cable and the consequential delays in the project.

The mechanical, structural and high voltage integrity of a cable design is confirmed by a type-test process. This is a range of physical tests performed prior to the full manufacturing of the cable. Type-testing is used to confirm the limits for the cable, with respect to its handling, installation and the environment that it can withstand.

Cables can experience a wide spectrum of motions and accelerations on a vessel during installation and are handled several times prior to that. It is essential that the handling, transport and installation of the cable does not cause any exceedance of the permissible limits identified in the typetest or any other limits specified by the manufacturer.

For owners and insurers, this is where the concern lies; ensuring that all handling and installation operations are done in a manner that respects these limits.

Subsea power cables are installed using a specialised Cable Lay Vessel (CLV). It is critical that the compatibility of the CLV and the cable is confirmed before the cable is loaded onto the vessel and installed. Parameters such as carousel bend radii, instances of multi-plane bending and other vessel specific characteristics must be checked with respect to the permissible limits for the cable, as defined by the type-tests.

Subsea cable being pulled-in and laid onshore

During the cable installation process itself, the CLV is operating offshore in a hostile and dynamic environment. There are many variables, but among the most significant of these is the weather. The motion of the vessel at sea, during the voyage to site and critically, at the installation stage directly affect the forces that act on the cable. With a cable, suspended from the vessel during the laying process, installation is a critical operation and one that requires detailed analysis and risk mitigation. Owners and insurers therefore seek to protect their interests (that is manage their risk) and to ensure that the cable is installed using industry best practices.

One of the main methods to help reduce incidents and damage to cables, is to identify best practice for the specific project, this is where the engagement of a Marine Warranty Surveyor (MWS) for the planning and execution of installation operations will yield the greatest benefit.

The question is often asked ‘who or what is an MWS’? The MWS is an independent body appointed to satisfy the requirements of the Marine Warranty Clause in the Construction All Risks (CAR) insurance policy. Their scope typically includes independent review and approval of all procedures and related engineering, confirmation of the suitability of vessels and equipment, witnessing and approval of site preparations and critical operations, onshore and offshore.

An MWS should be involved from the moment the insured asset crosses the quay until it is safely secured on the seabed or returned to shore. While the MWS, as an independent party, cannot design, they can make recommendations as to best practice. By monitoring the application of best practice and establishment of the correct principles, the MWS can help mitigate the risks in the construction phase of the project. Since the cost of any incident can be very high, many companies consider the MWS as a vital part of their risk management strategy.

The MWS should be involved as early as possible, during the engineering and planning phases, so that they can provide input as soon as possible before things progress too far and it becomes expensive to make changes. The MWS reviews the engineering documents and procedures for an operation, and once satisfied, approves them. Prior to the start of the operation, the MWS will confirm that preparations have been conducted according to the approved procedures and then issue a Certificate of Approval (COA). This confirms to the insurers that the insured has complied with the terms of the warranty clause.

However, best practice will also evolve as the industry expands and learns from each project. Based on our experience, the testing of a cable should be developed to better understand its ability to withstand the necessary handling, transport and installation cycles. For example, it would be valuable to have clear data on the maximum sea-states that a particular cable can safely withstand and under what conditions the cable might be stressed beyond its acceptable limits. In other words, is there a risk that the design life of the cable could be compromised during handling and installation? Such tests would involve subjecting a cable to multiple spooling and un-spooling operations, simulation of dynamic environments, and passage through multiple planes during the laying process itself.

Subsea cable being unspooled from a chute

Currently, it appears that subsea cables are satisfactorily designed for their operating environment, i.e. buried in the seabed where they shouldn’t move. However, the extent to which they have been designed for the multiple, temporary handling phases prior to that, is less apparent. With insurers concerned about through-life cable failure claims, which have high costs and cause significant impact to the power transmission grids, the industry has an excellent opportunity to enhance the understanding of this. Analyses of the conditions that cables experience during handling and installation will help to better inform the cable manufacturing industry, installation contractors and their insurers and will improve the risk management for cable installations, reducing claims and improving premiums.

Waves Group is actively involved in subsea cable laying for large offshore wind farms and power supply interconnectors. It has worked extensively on major offshore wind farm projects in Europe and further afield such as Beatrice, Hornsea One, Formosa, Triton Knoll, Seamade among many others. As well as regularly providing MWS services, Waves Group also has an engineering team dealing with the design for transport and installation of large, high value assets, including subsea cables.

Waves Group also sees the results of cable damages, whether incurred during the construction phase or due to other events during its operational life; investigating the cause of the damage and managing the response to incidents, including the repair, on behalf of its clients. This includes inspection and assessment of subsea cables through the use of high-resolution 3-D surveys, to independently and rapidly identify damage and gather related evidence.

To learn more about Waves Group and its services please contact mail@waves-group.co.uk