Reducing the risks in offshore wind farms - Best practices for laying subsea cables

Authored by Jarek Klimczak, Jochen Nikolai Raab and Jan Scharrer, Senior Marine Risk Consultants at AXA XL

Offshore wind is a critical technology for powering the decarbonization of the global economy.

Given rapidly falling costs and strong public policy support for offshore wind power, its installed capacity is set to increase substantially in the coming years. Analysts predict, for instance, that its total capacity will reach 330 GW by 2030, up from 34 GW in 2020, and twenty-four countries will host offshore wind installations, compared with only nine today. Total investments in offshore wind are projected to exceed USD 1 trillion over the next decade, and by 2030, as much money will be invested in offshore wind as in onshore wind power.

Undersea cables are vulnerable

These optimistic projections notwithstanding, constructing and operating massive wind turbines in harsh offshore environments is inherently risky. Due to the technology's relative newness and complexity, the learning curve for owners/operators, contractors, suppliers and insurers in this emerging power sector has been steep.

However, the industry’s challenging growing pains have also yielded important lessons that will help reduce future risks, limit costly breakdowns/failures and improve operating efficiencies.

One of the most critical lessons learned concerns the vulnerability of undersea cables linking the turbines to onshore transformers.

In fact, while subsea power cables represent about 10 percent of a wind farm’s overall costs, cable failures account for about 80 percent of the payouts for offshore-wind insurance claims.

When a subsea cable is damaged or severed, the entire network of turbines it serves goes out of commission until the line is fixed or replaced. And because the lead time for procuring new cables can take up to two years, these outages can last for an extended period, especially if spare cables weren’t stockpiled in advance. Also, there are only a limited number of specialized cable-laying vessels, and securing one’s services can add to the time it takes to repair/replace damaged cables.

Best practices for minimizing the risks

Cable laying is a complex operation, encompassing preparation, geological surveys, and mobilization of such specialized assets as cable-laying vessels, barges, excavators, and highly trained crew and contractors. Once installed, subsea cables can be damaged or destroyed by numerous threats: changes in the seabed or surrounding marine environment; fishing, shipping or related activities; severe storms; and improper operation of the turbines (e.g., when the combined speed of the carousel and tensioner creates bending curvature below acceptable limits).

To help insurance buyers and brokers manage and mitigate the myriad risks associated with subsea cables, AXA XL’s Marine Risk Consultants have developed a Risk Consulting Bulletin delineating best practices for planning, installing and maintaining subsea cables. This material is based on their first-hand experience assessing cable failures, along with extensive research on the factors contributing to these breakdowns. The most significant best practices are highlighted below.

Many of the recommended practices and offshore standards are based on the Cable Burial Risk Assessment (CBRA) guidance developed by the Carbon Trust. (The UK government founded the Carbon Trust in 2001 to accelerate the move to a decarbonized future. Today, the Carbon Trust partners with leading businesses, governments and financial institutions to accelerate their route to Net Zero.) The CBRA “offers a standardized, repeatable and qualitative method to improve risk management of subsea cables for offshore wind farms, improve conservative estimates of residual risk, and ultimately reduce the installation and insurance costs for subsea cables.”

Developing the cable engineering ground model

Many different variables and scenarios must be considered when installing subsea cables for offshore wind farms. This entails gathering and analyzing extensive information on the relevant seabed conditions—from beneath the turbine foundation to where the line makes landfall. The surrounding maritime environment and the area’s general activity must also be assessed in detail.

The geological, geophysical, and geotechnical information gathered should then be used to develop a cable engineering ground model to refine the route and understand burial-depth limitations, navigational restrictions, and operational exposure risks. The critical variables include:

• Burial-depth parameters/limitations. If the cables are buried too shallow, the risk of damage from fishing and shipping increases. On the other hand, if they’re too deep, the electrical current traveling through cables can drop due to the insulating effect created by the seabed.
• Sediment mobility. Understanding the potential for sediment mobility is essential since a cable exposed to scouring is much more likely than a buried cable to be damaged by fishing or shipping activities, e.g., via anchor snagging. Cables can also experience fatigue damage due to sediment mobility and scouring created by seabed currents in sandy areas. And mobile sediments can lose their strength, or “liquefy,” due to wave loading, which poses a risk to cable flotation and loss of burial depth.
• Other existing cables. In many cases, subsea cables have already been laid between the offshore wind farm and the shore landing, and if two assets touch, both lines can rapidly fail. Thus, any cable crossing another must be laid to ensure adequate separation between existing assets and the new cable.

Other considerations

A Qualified Warranty Surveyor must be present during cable laying operations, and adequate funds must be allocated for their services.

Loss prevention and repair plans should be prepared in advance. The former should include a rigorous schedule of surveys, supported by site data and ocean information, to help identify and remediate potential issues. The latter should encompass such things as storing extra cable lengths in strategic port locations and determining in advance which vessels and crews could repair the subsea cables.

Resolving claims concerning subsea cables requires deep knowledge and understanding of cable design, manufacturing, installation, and in-situ behaviour. As more giant offshore wind farms are deployed, the number and complexity of these claims will increase. Consequently, insurance buyers, brokers, and insurers have a common interest in appointing experts with the requisite expertise on undersea cables.

In addition to AXA XL’s Risk Consulting Bulletin on subsea cables, other resources on this topic include the Carbon Trust’s Cable Burial Risk Assessment and publications produced by the International Cable Protection Committee (ICPC). The ICPC is the primary organization focused on protecting the world’s submarine cables, and it has developed practical recommendations for such issues as how to install new cables near other cables.