FEATURE SPONSOR
OFFSHORE INTEGRATED GRIDS
A ROUTE TO LOWERING SUBSEA
ARRAY CABLE LIFETIME COST Offshore windfarms are getting bigger. Over 100 wind turbines in an array are now common, with applications in place for even larger installations.
The power output from each turbine is also increasing, meaning that the 1GW offshore windfarm is not far away. These are large steps forward
towards meeting the UK’s renewable energy generation targets. However, larger windfarms bring greater cost, both in terms of initial outlay (CAPEX) and through operation and maintenance (OPEX).
CABLE ROUTING
Cable routing to connect each of the turbines to the offshore substation (OSS) is becoming more complex. There are many variables to consider, each with potentially huge impacts on the overall lifetime cost of the cables for the windfarm. Minimising CAPEX is an obvious target, but recent work carried out by Jee Ltd has highlighted some of the savings that can be made throughout the life of the installation by investigating options with a slightly higher CAPEX, giving a lower OPEX and greater ROI overall.
OPTIMISATION TOOL
Jee has developed a subsea cable routing optimisation tool which considers
inputs such as cable rating, route configuration (loop, string, branch or combination), mean time between failure (MTBF) and mean time to repair (MTTR), soil conditions such as sandwaves and conductivity, and general unit costs and durations for installation.
The company is able to input the
proposed route path and the tool can determine the initial CAPEX required, the cost of losses due to cable failure and repair (including loss of revenue), and finally the cost due to operational losses, i.e. cable inefficiencies. Selecting the appropriate cable size for the configuration is key in order to balance the cost to procure against the sum of the losses. The tool can compare route configurations and help determine a lowest lifetime cost, including the effect of net present value (NPV).
CASE STUDY
Jee was approached by Scottish Power Renewables to determine the optimised routing solution for their 102 turbine offshore windfarm East Anglia ONE and worked closely with Scottish Power Renewables, using industry best practice and workshops to engage all stakeholders.
The company was provided with the turbine layout, seabed characteristics, turbine utilisation and the cable specifications. From this, looped, string and branched cable configurations were investigated, varying key inputs such as cable size, burial depth and connectivity. The seabed surveys indicated the presence of wrecks (and therefore potential war graves), areas of metallic debris, thought to be potentially unexploded ordinance (UXO) and reefs of Sabellaria (a marine worm). These obstacles were avoided while keeping overall cable lengths short. The tool was used to present a number
of routing options to Scottish Power Renewables, showing that an overall optimised lifetime cost could be achieved by considering options with higher CAPEX but far lower OPEX. It showed Scottish Power Renewables how looping (meaning the cable route starts and ends at the OSS) compares with branching (two cables terminating at one turbine) and simple string options. The company was also able to present
the predicted maintenance costs by forecasting areas of exposure from mobile sandwaves, and the effect of varying parameters such as MTBF, considering that this value is largely subjective but can have a great impact on the results. The cost-benefits of adding cable redundancy to the system to minimise downtime was also investigated.
OPTION PRESENTATION
Ultimately, Jee found an optimised routing solution and presented options for alternatives which Scottish Power Renewables could explore in-house with their own commercially sensitive data. The company’s renewables expertise includes route planning, cable protection design (CPS), HDD/landfall engineering, on-bottom stability, cable installation analysis, integrity management and decommissioning plans.
Jee Limited SCAN/CLICK
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MORE INFO
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WHITE PAPER
www.windenergynetwork.co.uk
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