Installing an offshore wind turbine is a multi-stage process that begins years before any steel enters the water. It starts with surveying the seabed, continues through foundation placement and cable laying, and ends with assembling turbine components piece by piece using specialized crane vessels. A single turbine unit, including its foundation, takes roughly 6 to 17 days to install depending on the foundation type, and an entire wind farm can take over a year to complete.
Surveying the Seabed
Before anything is built, engineers need to know exactly what’s beneath the water. The seabed’s composition determines which foundation type will work, how deep piles need to be driven, and where cables can be buried. The primary tool for this is the cone penetration test, where a probe is pushed into the seafloor to measure soil resistance and stiffness at precise depths. These tests are typically performed from a vessel equipped with a seafloor-mounted system, and they’re repeated at dozens or even hundreds of locations across the wind farm site.
Engineers are looking for things like sand density, clay stiffness, and whether the seabed contains boulders or shallow bedrock that could block pile driving. In the North Sea, one of the world’s most active offshore wind regions, datasets of over 4,000 individual penetration measurements across 200+ test locations have been used to characterize sites. Borehole drilling and laboratory testing of soil samples supplement the probe data, though sample disturbance during retrieval can make lab results harder to interpret.
Choosing and Installing the Foundation
The foundation anchors the turbine to the seafloor, and the right choice depends on water depth and soil conditions. Three types dominate the industry:
- Monopiles work in water up to about 50 meters (160 feet) deep. They’re the simplest design: a single large steel cylinder hammered into the seabed. Sand and clay soils are ideal. Shallow bedrock, boulders, or coarse gravel layers rule them out.
- Jacket foundations handle depths up to 60 meters (200 feet). These are lattice-style steel frames pinned to the seabed with multiple smaller piles or suction caissons. They tolerate a wider range of soil conditions than monopiles, though boulder-heavy sites still cause problems.
- Suction buckets are limited to shallower water, around 30 meters (100 feet), and work best in medium-stiff clay or fine sand. Instead of being hammered in, they’re lowered to the seabed and sealed by pumping water out, creating suction that pulls them into the soil.
Monopile installation involves a hydraulic hammer mounted on a crane vessel, driving the cylinder into the seabed with repeated high-energy strikes. This is one of the loudest phases of construction. To protect marine life, particularly mammals like porpoises and whales that are sensitive to underwater noise, crews deploy bubble curtains around the pile. These systems pump compressed air through a perforated hose ring on the seabed, creating a wall of rising bubbles that disrupts sound waves. Bubble curtains can reduce noise levels outside the curtain by more than 25 decibels at certain frequencies, with the greatest effect above 200 Hz. A monopile for a 15 MW turbine within a large wind farm takes an average of about 5.9 days to install.
Jacket foundations take longer. Based on data from 17 jacket-foundation wind farms, the median installation time for one complete unit, including both foundation and turbine, is 17 days.
Laying the Subsea Cables
Two types of cables connect the wind farm to the grid. Array cables run between turbines, linking them to a central offshore substation. Export cables carry the collected power from that substation to shore. Both need to be buried in the seabed to protect them from anchors, fishing gear, and currents, typically at a depth of 5 to 6.6 feet.
Three approaches are used for burial. Simultaneous lay and burial is the most protective, since the cable is covered as soon as it touches the seabed, but it’s also the slowest. Post-lay burial places the cable on the seafloor first and buries it in a separate pass. Pre-lay trenching digs the trench before the cable arrives.
The tools match the soil. Jet trenching uses high-pressure water to fluidize soft soils like silt or loose sand, letting the cable sink into place. Jet plowing cuts a trench with a blade-like share while simultaneously feeding cable into it. Mechanical trenching uses a chain fitted with cutting teeth and handles harder ground. The choice comes down to what the seabed survey revealed about soil type at each cable route.
The Vessels That Make It Possible
Offshore wind turbines can’t be assembled with ordinary ships. The industry relies on wind turbine installation vessels (WTIVs), which are essentially floating cranes that can lock themselves to the seabed. These jack-up vessels extend steel legs down to the seafloor, then lift their own hull out of the water to create a stable platform. This eliminates wave motion, which is critical when hoisting components that weigh hundreds of tons to heights above 100 meters.
Early projects repurposed jack-up barges from the oil and gas industry, with crane capacities between 200 and 1,300 tonnes. But as turbines grew larger, those vessels became inadequate. Purpose-built WTIVs now exceed 100 meters in hull length, carry cranes rated for 800 to 1,500 tonnes, and include dynamic positioning systems that hold the vessel precisely in place during transit and initial setup. Their large working decks can carry multiple sets of turbine components, reducing trips back to port.
Assembling the Turbine at Sea
Once the foundation is in place and the WTIV is jacked up beside it, the turbine goes together in a specific sequence. The steel tower sections go up first, lifted by the vessel’s crane and bolted to the foundation’s transition piece. Modern large-scale turbines use two or three tower sections, each weighing well over 100 tons.
Next comes the nacelle, the housing at the top that contains the generator, gearbox (if present), and control systems. On a current-generation 14 to 15 MW turbine like the Siemens Gamesa SG 14-222, the nacelle alone weighs around 500 metric tons. It’s lifted as a single unit and secured to the top of the tower.
The blades are installed last, one at a time. On that same 14-15 MW turbine, each blade is 108 meters long, roughly the length of a football field. They’re bolted individually to the rotor hub, which is already attached to the nacelle. This is the most weather-sensitive part of the entire process, because a 108-meter blade acts like a massive sail. Crews must work within tight wind and wave limits to do it safely.
Weather Windows and Operational Limits
Every phase of offshore installation has weather thresholds, but blade lifting is the bottleneck. Without specialized damping equipment, blade installation is limited to significant wave heights below 1.0 meter. With a tuned mass damper, a device mounted on the crane that counteracts swaying, that limit rises to about 1.6 meters, and installations have been carried out at wave heights up to 2.0 meters. Wind speed is capped at around 12 meters per second (27 mph) regardless.
These limits matter enormously for project timelines and costs. In rough seas like the North Sea, weather windows that meet all criteria can be scarce, particularly in winter. Simulation studies that model different wave height limits show that even small improvements in allowable conditions, from 1.0 to 1.3 meters, significantly reduce the total number of days a vessel sits idle waiting for calmer weather. That idle time is expensive: WTIVs can cost hundreds of thousands of dollars per day to charter.
From First Steel to First Power
A full-scale offshore wind farm with dozens of turbines follows an overlapping construction schedule. Foundation installation typically starts first and proceeds across the site while cable-laying vessels work in parallel. Turbine assembly follows once enough foundations are ready, with the WTIV cycling through positions across the array. Commissioning, the process of testing each turbine and energizing the electrical connections, happens as units are completed.
For a large project of around 1 gigawatt using 15 MW turbines (roughly 65 to 70 units), the turbine and foundation installation phase alone takes approximately one year, based on an average pace of about 5.9 days per monopile-supported unit. Add in the cable work, substation installation, and commissioning, and the full offshore construction window typically spans 2 to 3 years, following several more years of surveying, permitting, and onshore preparation.