But with planning, experienced personnel and the right technology in place, many of the common problems with cable installation can be managed or even eliminated — provided best practice can be identified and applied to this neglected and often out-of-sight area of offshore operations.
To avoid the common pitfalls in subsea cable installation, careful planning is key. And the focus must be on risk and risk mitigation throughout the project.
At the concept stage, planning should cover everything from project management to developing a key supplier index, covering both the global and local supply chain (see list, below). Looking at the big picture means an exhaustive cable installation overview covering types of assets required, installation methods, port options and existing cable route mapping. Other issues to consider include fishery and shipping interests, permit scheduling and developing a maintenance methodology.
Next comes the feasibility stage, which should identify risks, project impacts and plan steps to mitigate these. Procurement details need to be organised, including vetting of suppliers and supply terms.
Also needed are a "constructability" site review and analysis, an economic comparison of construction methodologies, specifications for asset options, a health and safety plan, crewing and support services scheduling, a mobilisation plan, dive-support services specifications, a port liaison and options study and an emergency recovery plan.
The export cable needs to connect to the grid on land. The development may have several grid connection points or just one at this stage.
Data on existing cabling at the site, such as telecommunications cables, must be acquired and contact made with cable owners. Proposals for agreements with existing seabed infrastructure owners should also be drawn up. Delaying this can have a serious impact on final costs and timelines as prior users of the seabed have a great deal of influence and control over how proposed new cables will interact with existing installations.
Technical aspects to consider include a detailed route plan; burial depth; cable protection; a risk assessment and mitigation strategy; and cable-length estimates. Liaison work must include community presentations and discussions with fishing, shipping and other maritime users.
Before installation commences, vessels need to be selected, major suppliers prequalificationc checked, orders placed and terms and conditions for suppliers drawn up. Installation-related services include method selection, a detailed cost-control plan, and an asset selection and modification schedule.
The full installation programme should be laid out at this point, along with the development of a health-and-safety programme and documentation. Recovery procedures, the nomination of key crewing and support personnel, and the creation of an offshore mobilisation schedule must be accounted for as well.
The negotiation of crossing agreements, establishing the fixed and final cable routes; protection and burial detail; risk-control procedures; development of the final cable specifications and estimates; and the final subsea model are also part of this phase.
The right people and equipment
Cable installation is a highly specialised activity, so staffing a project with experienced people , who can manage the project professionally and react effectively when things go wrong, is vital. As with the offshore oil and gas industry in its infancy, in the early years of offshore wind operation there were few trained people to carry out the tasks required, but some specialists have taken their knowledge of the subsea telecommunications cable industry and adapted those skills to offshore wind cabling.
People's core competencies will include safety at sea, vessel safety and tower access. Additional training may be required for working in confined spaces and rescue techniques. Safety is crucial. A typical project team will consist of project managers and engineers, offshore installation managers, technicians, health-and-safety advisers, surveyors and draughtsmen.
Power cables are heavy but easily damaged and need careful handling. Once the cable is loaded on to a vessel, proper handling requires the right equipment, from carousels, back-deck configurations and protection assembly to trenching, ploughing and subsea operations observation.
Cable for offshore wind comes in two types, which are stored on board the vessel in different ways. A coilable cable can be laid into a static tank or basket and be pulled out of the tank with the help of cable engines positioned within the cable pathway.
A non-coilable cable needs to be stored within a rotating basket or horizontal reel. This method is more expensive because equipment requirements are more complex and the power carousels or drum systems introduce additional technical risk from equipment failure and the need for competent staff to operate it.
Deploying the cable from the vessel to the turbines requires lift equipment to ensure the bend radius is not compromised as the cable leaves the vessel. Before starting the pull-in operation, additional lifting equipment and winches can be deployed to the tower to aid the pull-in procedure. The cable, although large and bulky, is not as robust as some might think, so the cable ends need to be protected and a sheaf added to spread the pull-in load and act as a barrier to water ingress.
The whole installation process needs to be observed by a crew. For this part of the operation a durable inspection-class remote-operated vehicle is used. Equipped with additional survey sensors, these machines are designed to work in the harshest offshore maritime environment.
There are two ways to lay a cable: simultaneous lay and burial or surface-laid and buried post-installation.
For simultaneous lay and burial, a cable plough is used. This is a very large machine that opens up the seabed for the cable to be positioned at the bottom of its furrow. Cable burial depths can be up to three metres below the seabed depending on conditions. Ploughing ensures that the cable is adequately protected from the outset and is typically used on longer export cables. Ploughs can also be jet-assisted where water is used to pre-cut or under-cut the furrow and lessen the pull required and so increase the speed of the operation. With simultaneous lay and burial only one vessel is required, although there is usually an element of post- lay burial with additional protection at the beginning and end of the cable.
Although the second method — surface and post-lay burial — is slower than simultaneous burial, it tends to be the preferred option. This technique usually uses separate vessels for lay and burial equipment. Either vessel may also be used to deploy additional forms of protection such as mattresses for cable or pipeline crossings.
For post-lay burial a jet trencher or mechanical cutting machine is used. Jet trenchers use the power of water to cut a trench into which the cable falls. A jet trencher can be used to bury the cable in differing seabeds of sands and clays to a depth of up to three metres below the seabed. Recent advances in the arrangement of nozzles and pressures and flows from the water pumps have seen jet trenchers being used in stiff clays to dig a trench one metre deep. The jet trencher is now often the preferred method of burial.
For areas where the seabed is firmer and rock formations occur, a mechanical trencher is used. This is usually a lot heavier than a jet trencher and uses either a chain or wheel embedded with picks to chip away at the seabed, depositing the cuttings away from the newly formed trench. A mechanical trencher has the cable loaded into the machine, while a jet trencher is non-intrusive with jet legs sitting astride the cable.
There may be areas a jet or mechanical trencher cannot access or where additional protection is needed. In these cases mattressing or rock dumping may also be required to add protection from external damage.
The heart of the operation
Most important of all is the selection of the cable-laying vessel, the platform from which cabling activities will be executed. For inshore cable installations a barge is normally used and equipped to operate using anchors as its means of propulsion and keeping station. Because of their shallow draft, they are able to get to the shallowest areas of an offshore wind-farm site. In the early North Sea Round 1 and 2 sites, barges were the vessels of choice. They are not self-powered and need tugs for transit and positioning. However, once operational their manoeuvrability is adequate to install export or inter-array cables.
It is only since developers have moved into deeper waters and further from shore that dynamic positioning vessels with their own propulsion have been used. For export cables from site to shore, a barge is equipped with a carousel, plough, pulling winch for the plough and an inspection-class remote-operating vehicle. In deeper water a more traditional dynamic-positioning construction vessel can be used, equipped to complete all aspects of the wind-farm cable-installation process.
So successful cable-laying can be achieved through proper planning and the use of the right people and the right technology, ensuring that the cable element of an offshore wind project does not become an expensive Achilles heel for the offshore wind industry. Cables may be hidden from view beneath the waves, but they should be at the front of the offshore developer's mind.