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Shining a light on how to improve subsea cables

WORLDWIDE: Cabling failures have cost millions of euros in delays and resulted in numerous contractual disputes, leading to damage to power cables being identified as a major insurance risk in the offshore wind sector.

Shore to sea… Cable installation guidelines will help cut costs  by preventing expensive errors
Shore to sea… Cable installation guidelines will help cut costs by preventing expensive errors

Cable-related problems most often arise from inadequate risk identification, sub-standard design, lack of planning and deficiencies in how procedures are applied.

Information on cabling issues is usually closely guarded, and little is available in the public domain. But a look at high-profile cases reveals some common problems: poor interface design at wind turbines and substations; the use of unsuitable cable installation vessels, equipment and methods; cable laying and burial delays, frequently weatheror soil-related; damage to cables during installation; cable or joint failure soon after commissioning; sediment movement leading to continued cable exposure and free spans or excessive soil cover; as well as third-party anchor damage.

Both array cables in wind farms and export cables to shore have been affected by these issues. When plotting the occurrences on a timeline from the early 2000s to the present day, a correlation can be seen between the rising number of incidents and the amount of wind capacity installed, although the situation has slightly improved in the last few years.

Sharing information

Yet, when consulting with a wide range of stakeholders in offshore wind, DNV GL discovered that, while the immediate causes appeared to be related to underestimating the risks as described above, the roots of the problem were actually lack of knowledge and skills, poor engineering and project management, and non-existent work standards (see below).

Two years ago, the CableRisk joint industry project (JIP) was set up to introduce a publicly available guideline for cable installation and maintenance, with contributions and involvement from 15 partner organisations, coordinated by DNV GL. The project hosted review sessions, inviting guest speakers from relevant stakeholder groups such as insurers, developers, foundation designers and service providers to contribute. Interim results were disseminated at various wind-energy conferences.

The draft guideline was sent out for review to 500 individuals from 200 organisations, based in Europe and beyond. A total of 900 comments were received and, where necessary, addressed in the guideline.

Risk reduction

In February 2014, the 145-page guideline, Subsea power cables in shallow water renewable energy applications, was published. The recommended practice provides guidance for all lifecycle phases of subsea power-cable projects, with a focus on static service in offshore wind. It defines minimum requirements, but overall promotes a risk-based approach whereby risks are reduced to acceptable levels as defined for each individual project.

The guideline applies to the entire length of the cable and its surroundings, including assessment of project conditions, planning and execution of works, as well as asset management later on, and the document contains numerous examples of possible solutions. It defines key terms used in design documents or agreements, such as "burial depth", and can serve as a reference document between stakeholders, such as contracting parties.

The comprehensive guideline is expected to benefit the entire offshore wind industry. The document references existing guidance in the form of IEC and ISO standards and guidelines published by bodies such as the International Council on Large Electric Systems, the International Marine Contractors Association (Cigre) and the International Cable Protection Committee. It thereby leverages information from a number of sectors, including offshore oil and gas and telecommunications, which had not been consistently achieved before.

Different conclusions

While insurers have reasoned that more powers provided to marine warranty surveyors can largely resolve the issues, the JIP came to a different conclusion. The problems will only disappear if they are addressed early, during conceptual and design phases. Further, a stronger focus on planning is required, as well as working on quality assurance processes.

Frequently, a project's objectives and boundary conditions have not been documented. Site conditions need to be established through a combination of desk-based studies and surveys. Most, but not all, parties involved understand by now that submarine cables require dedicated offshore survey campaigns, separate from those conducted for wind-turbine foundations. Some steps to consider include:

- The optimum burial depth should be determined by applying a risk-based approach, yielding an adequate and economic burial with a consistent level of protection

- Minimum burial depths stipulated by authorities can be counter-productive, for example in hard soils

- The cable interface at the wind turbine or offshore substation should generally receive more attention, particularly the cable installation process

- Mock-ups and onshore trials for cable pull-in can help spot potential problems

- The cable laying and burial processes should be monitored and controlled to ensure no cable parameters are compromised; a range of tests can ascertain a fault-free installation

The focus on cabling has frequently been on capital rather than lifecycle cost. Even if safely buried,cables still require some maintenance activities, including monitoring of cable parameters, inspections and potentially remedial work, such as reburial.

This type of industry collaboration shows that offshore wind is maturing. The incremental improvements in industrial practice that stem from this collaborative approach are a critical element of the push towards lowering the cost of energy for offshore wind.

STILL TO DO — AS OFFSHORE WIND FARMS MATURE, FURTHER CABLE ASPECTS NEED TO BE WORKED OUT

The following important topics still require further investigation.

For instance, the appropriate rating of power for cables transmitting variable renewable energy is not effectively covered by IEC standards.

Risk-based burial protection design requires validation, as well as the backfill typically achieved in a trench during burial and over time.

Some mechanical properties of the cable, such as squeeze loads during installation, should be specified in an agreed way. Long-term storage and associated re-testing of spare cable is another topic that requires clarification, and once offshore wind farms reach the decommissioning stage, aspects like cable removal will need to be expanded on.

There is ongoing work with a number of interested groups. DNV GL intends to align legacy DNV and GL codes and maintain the cable guideline; future updates will cover new technological developments and reference newly published guidance.

JIP partners

Bohlen & Doyen, Boskalis Offshore, Dong Energy, Electrabel GDF Suez, Iberdrola, Inch Cape (EDPR, Repsol), JDR Cable Systems, NSW, Offshore Marine Management, Siem Offshore Contractors, Tekmar Energy, Tideway Offshore Solutions, Van Oord Offshore Wind Projects and VSMC

Thomas Boehme is a principal engineer at DNV GL and was project manager for the joint industry project.

To download the guidelines, go to https://exchange.dnv.com/servicedocuments/dnv and search for DNV-RP-J301

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