“Offshore cables are an area where the industry has experienced frequent cost overruns and information from insurance brokers and underwriters indicates that 80% of European offshore wind farm insurance claims have been cable-related.” BIS ‘Review of cabling techniques and environmental effects applicable to the offshore wind farm industry’, 2008.
The US OW industry is a young business. It has not got the depth of experience that the European industry has. Many areas need to develop, including cabling. This is a critically-important function that the sector will have to master.
Cable Installation and Maintenance - The Unique Challenges
Preparing, constructing, laying and maintaining cable systems needs long time horizons, extensive skill, technical ability and experience. The process presents unique challenges:
Operations in deep water
Working safely and effectively
Complex operations with multiple contractors
Stakeholder and environmental needs
Engineering a robust subsea installation solution is critical. This is achieved primarily by investing heavily at the planning stage of a project in order to protect against possible future costs – to both owners and insurers. The best practice is to create a project implementation plan including all critical path items, based on the provision of cable route services, including desk top studies, permitting, route surveys, environmental assessment reports and route engineering.
Nevertheless, faults occur and cable downtime has a significant financial impact. It is imperative that repairs are undertaken swiftly to avoid long-term disruption to the wind energy power output. Physical threats to cables – such as anchor strikes, dragging fishing nets and erosion – are considerably more common than internal faults. One study by the international association of electrical power experts, Cigré, examined offshore interconnectors linking electricity transmission systems for wind farms. It documented 49 reported incidents between 1990 and 2005 worldwide; 41 of these were caused by external parties – such as anchor strikes.
According to a European Offshore Grid Infrastructure Study, subsea power cables suffer an average of 0.5 – 2.0 faults per year per 1,000 km of installed subsea cable.
After suitably careful surveys and route planning the cables need to be installed. This is a difficult process and can be impeded by various factors. Specialist cable laying vessels and equipment, including trenchers, ROVs and of course specialist divers are often necessary to complete the process successfully.
Weather restricted operations
Accident or contractor error
Congestion in the cable route
The last factor needs explanation. Now that there are many arrays in European waters there have been examples where cables have been too close together or have to cross other undersea cables. This means that careful route-planning will be necessary.
The Bureau of Ocean Energy Management (BOEM) has published guidelines for best practice: ‘Offshore electrical cable burial for wind farms: state of the art, standards and guidance and acceptable burial depths, separation distances and sand wave effect’, 2011.
Offshore Wind Submarine Cable Spacing Guidance:
State permitting requirements for cable installations vary from state to state. There are no prescriptive state requirements for the spacing of cables offshore. Depths for cable burial may be mandated by the state, for example New Jersey has a minimum burial depth requirement in state waters of 5 ft (1.5m). Cable burial depths at landfalls are engineered per site with stakeholder input, for example the utility, town or municipality. States have significant influence and discretion in cable permitting, installation and maintenance.
There are a number of stakeholders that can influence the routing, spacing and installation of cable assets. It is important to note that federal agencies, state agencies, commercial fishermen, recreational fishermen, tribes, and NGO environmental groups often have very different interests and concerns with regards to subsea cables. For project purposes good practice requires early engagement with each stakeholder organization to introduce the project and discuss their interests and concerns.
The cables may well have influence on conservation of species and habitats, avoidance of disturbance of spawning, mating, migration, with special concern for endangered, threatened or charismatic species. The last thing a wind farm project needs is negative publicity because it has caused disruption to local wildlife.
This phase involves the laying of the array cables, cables to the Offshore Substation Platforms (OSPs), and export cables and the landfall cable being connected to the electricity grid. This part of the operation is where adverse weather can cause significant difficulties.
Skilled personnel in many different specialities are needed and this will be a sector that the US will need to train people to perform these tasks. In previous blogs we have examined the progression towards providing training for new industries.
Operations and Maintenance
As part of overall Operations and Maintenance plans, developers must consider the repair and maintenance of adjacent cables and in particular the risks associated with the fault location, recovery, repair and deployment of the repair location on the seabed.
With a bundled HVDC cable pair there will be a requirement to repair two cables and possibly a fiber optic cable, with the assumption that all three cables will be laid out on the same side of the cable route. Often replacement is the only viable solution to cable damage.
Overall, cabling is another area where European expertise will be necessary to ensure that arrays are deployed properly and power takeoff is achieved. However the US trajectory would be to have domestic content and US personnel to work on these systems in the future.
The US offshore wind market is probably the most rapidly-developing renewable energy sector in the world. Follow #USOW20 for the latest news and expert opinions.
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By Julian Jackson – writer on technology, arts, blockchain and cryptocurrencies