Explained — Technology · 01.2
Foundations & Substructures
FoundationThe structure transferring loads from a building to the ground below · What keeps a 15 MW turbine upright in 40 metres of seawater for 30 years.
The foundation is where offshore wind's engineering costs, installation complexity, and site constraints collide. Water depth, seabed geology, and wave climate decide which foundation type is viable, and the next-best option is usually much more expensive.
There is no universal foundation. Around six concept families are in commercial use, each suited to a particular depth and soil combination. The choice cascades through vessel requirements, port logistics, and tower design.
The Water Depth Challenge
Water depth is the primary driver of foundation type. In shallow water, a monopile is almost always cheapest. As depth increases, steel and driving energy scale non-linearly, and at some point a jacket or gravity base becomes cheaper. Beyond roughly 70 to 80 metres, fixed foundations become prohibitively expensive and floating becomes the only option. These boundaries shift as fabrication methods and installation vessels improve.
Foundation Types
"In Europe, monopiles represent 77% of wind offshore foundations" (Sánchez et al., 2019, cited in ScienceDirect 2026). Current XXL monopiles can exceed 11 metres in diameter.
Monopile
A large steel tube driven into the seabed, the default choice wherever water depth and soil conditions allow. In Europe, monopiles represent 77% of offshore wind foundations. Current XXL monopiles can exceed 11 metres diameter, 100 metres in length, and 2,500 tonnes in weight (CS Wind Offshore has manufactured TP-less monopiles up to 123.6m long, 10m diameter, 2,515 tonnes). Driven by hydraulic impact hammer or vibratory hammer to reduce underwater noise.
Two variants:
Monopile + Transition Piece (MP/TP) — Traditional design. A separate transition piece sits atop the driven monopile, connected via bolted flange or grouted connection. Corrects pile verticality, provides boat landing and J-tube cable entry, carries secondary steelwork. Critical advantage: all instrumentation (metocean sensors, accelerometers, SCADA, navigation aids) can be pre-installed in the yard because the TP is craned into position, not driven. Early grouted connections led to widespread failures in the 2010s. Modern TPs use bolted flanges.
TP-less Monopile — Emerging design. "The adoption of TP-less monopiles is steadily increasing, accounting for 35% of installations in 2024, up from just 12% in 2012" (Sea Impact, November 2024). Tower connects directly to monopile. Can reduce foundation CAPEX and installation costs by approximately 10% (Skybox Offshore, 2024). Eliminates bolted flange inspection (150+ bolts on large piles). Constraint: no instrumentation can be pre-installed because it falls off during violent piling. All sensors must be retrofitted offshore, adding time and weather risk. Requires improved driving accuracy and larger installation vessels.
Strength: lowest cost per megawatt at shallow depths, simple fabrication, proven track record, dominant supply chain.
Weakness: steel mass scales poorly with depth, piling noise (marine mammal impact), not viable beyond roughly 50m depth.
Jacket
A lattice steel structure with three or four legs, each pinned to the seabed by driven pile or suction bucket. Jackets dominate at 30-70m depths where monopiles become impractically heavy, and they're standard for oil/gas-derived offshore substation substructures. More complex fabrication (more nodes, welds, yard time), but steel-per-megawatt is lower than an equivalent-depth monopile. Jackets are "the fastest-growing segment as demand for offshore wind farms expands further offshore into more challenging environments" (Market Research Future, 2026).
Strength: efficient at 30-70m depths, lower steel mass than monopile at depth, mature oil/gas technology.
Weakness: complex fabrication, concentrated supply chain, longer lead times.
Gravity Base
A concrete structure that sits on the seabed under its own weight, ballasted with sand or rock. Used historically in Danish and Belgian waters, occasionally revived for sites where piling is problematic (hard rock, UXO risk, noise-restricted zones). Requires seabed preparation, heavy to transport, but avoids piling. A niche and declining solution as monopiles and jackets have become more cost-effective.
Strength: no piling noise, suitable for hard rock seabed, proven from oil/gas.
Weakness: requires seabed preparation, very heavy, expensive for deeper water.
Suction Buckets
A large steel cylinder, closed at the top, lowered onto the seabed. Water is pumped out, and atmospheric pressure drives the bucket into the sediment. Used on jacket legs in several North Sea projects and in some single-bucket monopile-style designs. No piling noise, faster installation, reversible for decommissioning. Limitation: soil conditions must be suitable, installation window narrower than piling.
Strength: no piling noise, fast installation (minutes vs hours), fully reversible.
Weakness: soil-dependent (requires clay or sand), narrow weather window, less proven at scale.
Floating Foundations
Where water depth exceeds roughly 70 metres, fixed foundations stop making economic sense, and floating becomes the only route. Floating opens access from a third of the global continental shelf to effectively all of it. As of 2024, approximately 278 MW of floating offshore wind capacity is operational globally, less than 0.4% of total offshore wind (Norton Rose Fulbright, 2024). The technology works, the supply chain is immature, the economics are not yet close to fixed-bottom.
Semi-Submersible
A multi-column steel or concrete hull, partially submerged, ballasted for stability. Most mature concept, port-assembled, towed to site. Used by WindFloat Atlantic, Kincardine, Hywind Tampen.
Strength: no deepwater assembly needed.
Weakness: high steel mass, large footprint.
Spar Buoy
A single long cylinder, heavily ballasted at the bottom, stable like a fishing float. Used by Hywind Scotland and Hywind Tampen.
Strength: excellent stability, simple geometry.
Weakness: needs deepwater site for upending and turbine installation, limits port options.
Tension Leg Platform (TLP)
A buoyant hull held down by taut vertical tendons to seabed anchors. Minimal motion, small footprint.
Strength: very stable, small wake on the turbine.
Weakness: complex installation, tendon fatigue is critical, no commercial offshore wind deployment yet.
Barge
A large flat-bottomed pontoon, stable by waterplane area. Used in Ideol's floating concept.
Strength: simple, shallow draft for port access.
Weakness: larger hull motions, more challenging for dynamic cables.
Floating wind has had a slower commercialisation path than fixed-bottom. 2024-2025 saw several high-profile delays and cancellations across UK, Norway, and Iberia. The technology works, the supply chain is immature, economics are not yet close to fixed-bottom. Commercial-scale floating farms remain a 2030s story.
Geotechnical Engineering
Foundation design starts with the seabed, not the turbine. Site investigation campaigns, geophysical surveys to map the seabed surface and shallow sub-bottom, followed by geotechnical boreholes and cone penetration tests (CPTs), build up a soil model for every foundation location. The cost and schedule of the investigation itself runs into tens of millions of pounds for a full-scale UK project, and getting it wrong leads to either over-engineered foundations (capex waste) or under-designed ones (catastrophic risk).
Soil types drive pile design. Dense sands allow shorter piles, soft clays need longer ones, chalk (common in the UK southern North Sea) behaves unpredictably under cyclic loading and has caused repeated design headaches at projects like Thanet and London Array. Boulders, channel infill, and legacy unexploded ordnance are additional site-specific risks.
Installation Vessels
Foundation installation is vessel-limited. Heavy lift jack-up vessels or floating crane vessels capable of handling 2,500 tonne monopiles are a small global fleet, effectively a supply-constrained market dominated by Cadeler, Van Oord, DEME, Jan De Nul, and a handful of others. The same vessels are then needed for turbine installation, and fleet utilisation directly affects project schedules across the entire European pipeline. Vessel availability is frequently the binding constraint on how fast offshore wind can build out.