Explained — Technology · 02.1
Subsea Cables
SubseaLocated or occurring below the surface of the sea · The highest-risk, hardest-to-fix component in a wind farm.
Subsea cables carry every watt the farm generates. They are also the single biggest cause of unplanned downtime and insurance claims across the sector, with cable failures driving a long-documented majority share of loss events. A single cable fault can take a string of turbines offline for months while a cable repair vessel is mobilised, a window is found, and the damaged section is recovered, jointed, and reburied. Insurance markets know this, which is why cable cover has hardened sharply over the last five years.
There are two distinct cable systems in any offshore wind farm, the array (or inter-array) cables that connect turbines to each other and to the offshore substation, and the export cables that carry the aggregated farm output to shore. They are different voltages, different designs, installed by different vessels, and insured separately.
Array Cables
Array cables run between turbines in daisy-chain strings, typically five to eight turbines per string, terminating at the offshore substation. Each cable is a three-core AC design with fibre optic elements integrated into the filler, used for control and SCADA data to every turbine. The conductor is usually aluminium for larger sizes and copper for smaller ones, insulated with cross-linked polyethylene (XLPE) and armoured with steel wire against abrasion, fishing gear, and anchor drag.
The industry standard array cable voltage has been 33 kV for roughly two decades, but 66 kV has now become the default for new projects. Raising the voltage halves the current for a given power flow, which cuts resistive losses and allows longer strings with more turbines per cable. 132 kV array cables are already in planning for 20 MW-plus turbine farms and larger arrays.
Export Cables
Export cables transport the combined farm output from the offshore substation to shore. Depending on farm capacity and distance to the grid connection point, export cables are either HVAC or HVDC. The choice is one of the most consequential design decisions on the whole project and is covered in detail on the Export System page, but the cable-level differences are:
HVAC Export Cable
Three-core AC, typically 220 or 275 kV in the UK, smaller-bore variants at 132 or 155 kV in mainland Europe. Generates significant reactive power (acts as a long distributed capacitor), which limits transmission distance to around 80 to 100 km before reactive compensation becomes uneconomic.
HVDC Export Cable
Two single-core DC cables (one positive, one negative), typically ±320 to ±525 kV. No reactive power problem, so viable over hundreds of kilometres. Paired with expensive converter stations at both ends, which is why HVDC only pays back at large capacities or long distances.
Dynamic Cables
Floating wind turbines move. The cable connecting a floating turbine to the seabed has to accommodate that motion continuously, for 25 years, without fatigue failure. Dynamic cables are a specialist category, with enhanced armour, specific bend-stiffness design, and buoyancy modules fitted in "lazy wave" or "steep wave" configurations that hold the cable in a shape that absorbs platform motion without overstressing the conductor or insulation.
The technology is mature enough for the floating demonstrator projects built to date, but the operating data set is still small, and dynamic cable reliability is a closely watched question as floating wind moves toward commercial scale. Most of the known floating wind project delays of 2024-2025 have been commercial rather than technical, but the supply chain for dynamic cables is narrower than for static ones.
Cable Protection Systems
Where cables exit a buried section and enter a foundation (the J-tube or I-tube entry), they cross the interface between the protected, buried cable and the exposed secondary steelwork. This is a known fatigue hotspot. Cable Protection Systems (CPS), typically bend-stiffeners or distributed buoyancy sleeves, are designed to control bend radius at this interface. CPS design was underestimated for the first generation of UK offshore wind farms, and CPS failures caused widespread inter-array cable damage on several operating projects. Current CPS designs are significantly more robust, but it remains a non-trivial engineering problem on every project.
Installation and Burial
Cables are loaded onto a Cable Lay Vessel (CLV) in one continuous length from the factory, transported to site, and laid along the planned corridor. Burial to a depth of 1 to 3 metres below seabed is achieved using one of three methods:
- Jetting: a subsea tool fluidises the seabed sediment with high-pressure water, allowing the cable to sink under its own weight. Works well in sand, poorly in stiff clay or rock.
- Ploughing: a towed plough cuts a furrow and lays the cable in it, with sediment backfilling naturally. Faster than jetting in appropriate soils, but limited by seabed slope and hardness.
- Trenching: mechanical cutters or chain trenchers cut through harder material. Slower, but needed for any cable route crossing stiff clays, chalk, or boulder fields.
Where burial cannot be achieved (cable crossings, bedrock sections, seabed exits), rock placement or concrete mattressing is used to protect the cable externally. Every unburied metre is a risk, and rock berm inspection becomes an ongoing O&M task.
Why Cables Fail
Cable failures cluster into a handful of causes. External interference from dropped anchors and fishing gear accounts for a substantial share. CPS-related fatigue at foundation entries was historically the next biggest category. Manufacturing defects, installation damage (sharp bends during lay, crushing in the carousel), and thermal damage from poorly placed rock armour make up the remainder. Lightning strike damage is a factor on some sites.
Insurance claim data consistently shows cable failures as the largest loss category by both frequency and cost. Repair cost is dominated not by the cable itself, but by the vessel time needed to locate the fault, recover the cable, cut out the damaged section, joint in a new length, and rebury. A typical major cable repair is 4 to 12 weeks, with insured losses well into the tens of millions.
Supply Chain
Global subsea power cable supply is concentrated in a small handful of suppliers: Nexans, Prysmian, NKT, Hellenic Cables, LS Cable, and Sumitomo for the large-capacity end of the market, with Chinese manufacturers supplying domestic and some export demand. CLV capacity is similarly concentrated, with a short list of dedicated cable lay vessels across operators like Jan De Nul, Prysmian, Van Oord, and Boskalis. Both bottlenecks, factory capacity and vessel availability, are currently tighter than project demand, and lead times for HV export cables have extended to three to four years for major orders.