Cables: Designing efficient cable links – cautionary notes

Offshore renewable energy is evolving rapidly, with offshore wind being one of the pioneer sectors. Energy generated from offshore wind farms needs an efficient transmission system of subsea power cables to connect to the onshore network.

Energy transmission networks can form the most expensive components of offshore wind farms. Setting aside the actual manufacturing and installation costs, the potential costs related to a failure and consequent production outage can be significant. Although cable systems represent only around 10% of the CAPEX of a wind farm, they account for nearly 80% of the insurance claims.

As offshore wind farms move into deeper water and further offshore, the design and installation of electrical systems take an ever more important role. Engineers are faced with the challenge of developing cables capable of transmitting power over long distances to the landfall onshore, and also experiencing the harsher marine environment. It follows that the design and analysis during the development studies are key to a practical and efficient system solution.

Preliminary studies

During the pre-development phase, whilst obtaining the various required planning permissions, the developer will have carried out preliminary engineering studies. It is in this ‘predevelopment phase’ that power cable risks should be looked into. Hazard and operability (HAZOP) studies must be conducted to evaluate potential subsea power cable hazards and to develop adequate mitigation plans.

The majority of cable damage relates to external hazards such as fishing, anchoring, and other impacts. Risk matrices and probabilistic studies categorise the various risks into ranks of significance. These provide the basis for an educated approach on installation aspects, cable routing, precautions and operational limitations, all of which will impose certain design requirements.

As an example, if a planned cable routing goes through a high marine traffic area, the appropriate armour package must be specified to provide additional protection to the power cores. Cable routing studies also evaluate the condition of the seabed enabling the optimum route that will minimise these potential cable hazards and define an appropriate installation method.

A transmission network can be designed with redundancy to minimise the hazardous consequences of risk occurrence. Understanding the advantages of redundancy can result in an alternative design to increase the reliability of the system. Reliability assessments can be based on existing industry guidelines for typical failure rates of the various system components.

The final choice of cable redundancy will be dictated by full power system studies. The downtime costs of the installation during an outage event, and the possible interconnection to neighbouring farms or networks are among the key parameters to be considered.

Given the cost of downtime and the ultimate knock-on effect on the cost of electricity (in p/kWh), preventative engineering studies will always be cheaper than costly repairs.

AC – HVDC

Traditionally, offshore wind power has been transmitted to onshore networks via AC subsea power cables due to the relative short distance to shore of early wind farms. As wind farms are now planned further offshore, the additional losses due to the increased length occur primarily from the heavy currents required to charge and discharge the cable capacitance each cycle developing in AC power cables.

Shunt reactors and appropriate power electronics systems absorb these charging currents that reduce the capacity of the cable. Power studies are used to optimise the system and specify the voltage level for power transmission through an optimised cable cross-section.

On long cable routes, HVDC power cables are the alternative to AC designs. This is due to there being a crossover point at a cable length of over 80kms where it becomes more efficient to use HVDC rather than HVAC. HVDC systems transmit power with fewer losses but are constrained by heavy and complex power electronics systems at each end, required to convert between AC and DC. These systems can greatly increase the cost of the development.

Equipment characteristics

The cable needs to operate in a specific environment; whilst maintaining the operating temperature of the conductors within acceptable operational limits. Iterative electromechanical design processes are necessary to optimise a cable crosssection.

The design must also be capable of sustaining the loads during installation, handling, operation, and possible repairs. Understanding the torsional stability and looping propensity of subsea power cables is critical during cable installation.

The accurate prediction of the stiffness of the cable allows the estimation of installation loads and the prediction of the dynamic performance of the cable especially if the cable has a free span and/or dynamic sections exposed to current and wave loading.

All cable handling must respect the predicted mechanical properties, such as the minimum bend radius (MBR). Finally, the mechanical characteristics of the cable determine the design of accessories at the cable link, such as end terminations, hang-offs, I- or J-tubes, bend stiffeners, etc.

Tests must be carried out to verify the predicted electrical and mechanical characteristics of the design. These tests can take place at different stages of the cable development and may include verification, FAT(Factory Acceptance Test), type, routine and post-installation tests. Specific tests are often agreed between the manufacturer and the owner, who may be present during testing to verify the test conditions and validity of the results.

These tests act as a reference during cable operation, hence adequate QA/QC planning must be in place for record keeping. Cable manufacturers must be also consulted on accessories and splices to be selected for the specific cable design.

The contractor’s engineers may also perform system studies to qualify optimum solutions. Accessories, such as Gas Insulated Switchgear (GIS) terminations, must also be tested to verify their performance. A number of marine operations are required for cable installation. Vessels and their equipment are used to carry and lay cables.

As wind farms move further offshore and the generated power increases, longer and bigger power cables will have to be deployed. There is only a limited number of vessels that can carry cable lengths of 6,000t total weight (representing a typical weight of a 132kV export AC power cable of more than 50km length).

Vessel rates can significantly affect the overall project cost and with competing demand for such vessels from the oil and gas industry, installation and economic planning must take place early in the project.

Concluding from the above, a feasibility assessment of the development and installation of long subsea power cables needs to consider a number of parameters, such as manufacturing capabilities and costs, installation methods and equipment characteristics.

A team of knowledgeable engineers need to coordinate the system design for a site-specific solution that is both operable and cost efficient. Studies addressing all of the above will become increasingly important for future offshore wind farm developments, where the cables are expected to span over longer lengths and deeper waters for an extended service life of as long as 50 years.

Thanks to Maria Kallivretaki and Adrien Montfor at BPP-Cables