Explained, Ports and Vessels
How an Offshore Wind Farm Gets Built and Maintained
Once the components exist, they have to be staged, shipped, lifted into place and then maintained for a quarter of a century. That work depends on two scarce resources, a small number of suitable ports and a specialist fleet of vessels, each purpose-built for a particular job. Neither is easy to expand quickly, and both shape what a project can actually do and when.
This page sets out the ports the industry uses and what makes one fit for purpose, the main vessel classes and what each is for, the role of the Marine Warranty Surveyor and its link to insurance, and the contract strategies owners use to secure it all. It is the companion to the supply chain page: that one covers what the parts are and where they come from, this one covers how they reach the seabed.
Ports, and what makes one fit for purpose
Very few ports can support offshore wind construction. The components are enormous and heavy, the vessels that carry them are large and deep-drafted, and the assembled turbines are shipped standing up. A port either meets a demanding set of physical criteria or it does not, and most do not. A useful way to understand ports is by the role they play, because that role changes across a project’s life.
Manufacturing and marshalling ports
Manufacturing ports host the factories that make blades, towers, nacelles or foundations, and need very large flat areas (up to around 500 hectares) with direct deep-water, high-load quayside. Marshalling ports are the staging points where globally sourced components are received, stored and pre-assembled before being loaded onto installation vessels. The established European hubs include Esbjerg (Denmark), Cuxhaven and Eemshaven (Germany and the Netherlands), Vlissingen, Hull, and Cromarty Firth in Scotland. Esbjerg alone has handled more than 60 wind farms and over 24 GW, with a 1.2 km dedicated quay and over 750,000 m² of port area.
Construction and laydown, the fit-for-purpose criteria
This is where most ports fall out of contention. A construction port has to take heavy quayside loads, deep-drafted vessels, and offer enough open laydown for components that are stored lying down and take up huge space. The typical requirements for a large project:
The scarcity is real. A Crown Estate Scotland assessment found that several ScotWind zones had three or fewer suitable marshalling ports within an economic sailing distance, a capacity gap that exists regardless of where components are made. Proximity matters because every extra hour of sailing eats into weather windows and vessel day-rates.
O&M ports
Once a wind farm is running, it needs a much smaller operations base, close to the field, hosting the crew transfer fleet and the operations team for 25 years or more. These ports prioritise proximity and reliable access over heavy-lift capability. Esbjerg, for instance, also serves as an O&M base with capacity for more than 30 crew transfer vessels.
Vessels, by the job they do
Offshore wind uses a fleet of specialist vessels, each designed for a phase of the work. The diagram below shows the four you will hear about most, drawn to rough relative scale, before we take each class in turn.
WTG installation vessels (WTIVs)
These are the jack-up vessels that install the turbines. They sail to site, lower legs to the seabed and lift their hull clear of the waves to form a stable platform, then crane the tower, nacelle and blades into place. The defining tension in this market is that turbines have grown faster than the fleet: average turbine size rose from around 3 MW in 2010 to 8 MW by 2021, and 15 MW machines are now installing with 20 MW+ on order, exceeding the lift height and capacity of most older vessels. Only a portion of the fleet can handle the largest turbines, and around 70% of new-build orders are going to Chinese yards. The newest vessels, such as Cadeler’s A-class and M-class, carry cranes lifting 3,000 tonnes or more and can transport seven 15 MW turbine sets or five 20 MW+ sets per trip. Cadeler installed the first commercial Siemens Gamesa 14.7 MW turbines at Moray West and the first Vestas 15 MW machines at Baltic Power.
Heavy-lift vessels, and how the topside drives the design
Foundations (monopiles, jackets) and offshore substations (OSS) are installed by heavy-lift vessels, the largest of which are semi-submersible crane vessels like Heerema’s Sleipnir and Thialf and Saipem’s S7000, with dual-crane capacity into the tens of thousands of tonnes. The number of these giants is tiny, and they are expensive and heavily booked. That scarcity reaches all the way back into design.
An offshore substation topside is not a free design choice. Real platforms range from around 1,100 tonnes (Neart na Gaoithe) through 2,700 tonnes (Inch Cape’s double transformer module) up to 9,500 tonnes for a large HVDC topside (Dogger Bank C). The heavier the topside, the fewer vessels can lift it, and vessels with the necessary crane capacity often lack the deck space to carry the platform as well.
A design principle, how many offshore substations?
The question “how many OSSs, and how big?” is settled in the design phase, and vessel reality is one of several forces pulling on the answer. One large topside may force the project onto one of the handful of heavy-lift giants, with the cost and scheduling risk that brings; two smaller platforms may fit a more available, cheaper vessel class. But splitting is never free: more platforms mean more steel, more interfaces, and a different inter-array cable architecture, with shorter array runs but more collection points. Project phasing pulls too, a phased build may favour several smaller OSSs that can be installed and energised phase by phase, rather than one platform that waits for the whole field. Onshore integration of a heavier, more complete topside cuts offshore commissioning, but only if a vessel can lift the integrated weight. The electrical architecture (HVAC versus the much heavier HVDC) sets the baseline. It is a genuine multi-variable trade-off, not a rule.
Siemens Energy’s Offshore Transformer Module (OTM) is the worked example. It is a lightweight, modular substation: the lighter single-circuit versions (around 1,200 tonnes at Beatrice and Triton Knoll) are designed to sit directly on a monopile foundation, sidestepping the jacket and the heavy-lift-giant bottleneck entirely. The heavier double-circuit version (Inch Cape, 2,700 tonnes) still needs a jacket and a Sleipnir-class vessel. The same product family spans the decision.
Cable-lay and protection vessels
Cables are installed by cable-lay vessels (CLVs) carrying thousands of tonnes of cable on giant turntables, followed by separate trenching and burial vessels that bury the cable for protection, and rock-installation vessels that lay a protective rock berm. The contractor field splits between cable-makers who install their own (Prysmian, Nexans) and marine contractors (Jan De Nul, Boskalis, Van Oord). Vessels are getting dramatically bigger to handle long HVDC export runs: Jan De Nul’s Fleeming Jenkin and William Thomson, at 215 m and 28,000 tonnes of cable capacity, are billed as the world’s largest, delivering in 2026–27, and Boskalis ordered a 24,000-tonne-capacity CLV in 2026. The global CLV fleet is forecast to grow from 37 vessels in 2025 to 48 by 2030. Cables matter out of proportion to their cost: roughly 80% of offshore wind insurance claims relate to cable issues.
Survey and support vessels
Before and during construction, a range of smaller vessels do the groundwork: geophysical and geotechnical survey vessels map the seabed and sample its composition to inform foundation design, ROV support vessels carry remotely operated vehicles for subsea inspection and intervention, and guard and safety vessels protect work sites and assets. These are less specialised to offshore wind than the installation fleet, much of the tonnage is shared with the wider offshore energy market, but they are essential to de-risking the build.
The O&M fleet, CTV, SOV and CSOV
Once built, a wind farm is served by three vessel types that trade off cost against capability and distance from shore:
| Vessel | Role | Typical profile | Charter |
|---|---|---|---|
| CTV, crew transfer vessel | Daily transfer of technicians turbine to turbine; advantage when the farm is close to shore | Fast catamaran, 12–24 (now 30+) technicians, lower day rate | Short-term / spot |
| SOV, service operation vessel | Floating base for maintenance, the “mothership”, deploying daughter craft and using walk-to-work gangways | ~80 m, 60–90 personnel, 14-day offshore cycles, DP and heave-compensated access | Long-term to operator/OEM |
| CSOV, commissioning SOV | Larger SOV for the construction and commissioning phase, bigger crane and more accommodation | Up to ~120 personnel, larger deck and crane | Short-to-mid-term |
The SOV-as-base, CTV-as-daughter model lets operators cover large, spread-out farms efficiently. The fleet is young, the first modern SOV entered service only in 2017, and growing fast: around 70 new SOV units are on order, more than half with alternative-fuel capability and 25 designed for methanol. Edda Wind leads with 10 CSOVs and three SOVs. The fleet also splits into tiers: Tier 1 purpose-built vessels with integral cranes and gangways, and Tier 2/3 converted oil-and-gas tonnage serving both markets.
The Marine Warranty Surveyor, and the insurance link
Behind every high-risk marine operation stands a figure that outsiders rarely hear about but who holds real authority: the Marine Warranty Surveyor (MWS). The MWS is an independent third party that reviews and approves the riskiest marine operations, load-outs, tow-outs, heavy lifts and installations, before they go ahead.
The authority comes from insurance. A project’s construction all-risks and marine cargo cover typically carries a marine warranty clause: the policy requires that the high-risk operations be independently reviewed and approved by a competent surveyor. The MWS acts on behalf of the underwriters, not the owner or the contractor, and its deliverable is a Certificate of Approval (CoA) issued for each operation once the engineering, procedures, vessel suitability and weather windows have all been checked and met. In practical terms, no MWS sign-off means the warranty condition is not satisfied and the cover may not respond. That is why an independent surveyor can hold up a multi-million-pound operation until it is satisfied.
The work follows established standards (notably DNV-ST-N001 for marine operations and DNV-SE-0080 for the survey process), and the named firms include DNV (Noble Denton), ABL Group (LOC), Global Maritime, RINA and ABS. The stakes are not academic: insured losses in offshore wind construction have, in some markets, exceeded premiums, which is exactly why insurers insist on independent oversight of the operations most likely to go wrong.
Contract and charter strategies
How an owner buys all of this, the turbines, foundations, cables, substations and the vessels to install them, is itself a strategic choice, and the industry has moved over time. The core decision is how much risk to keep and how much to pay someone else to carry.
Multi-contract versus EPCI
At one end is the multi-contract (or multi-package) approach: the owner splits the project into many discrete packages, turbine supply, turbine installation, foundation design, fabrication and installation, cables, substations, grid connection, and tenders and manages each itself. It offers the lowest outturn cost and the most control, because the owner is not paying a contractor’s risk premium, but it demands a large, capable in-house team to manage the interfaces between packages, which is where the risk concentrates.
At the other end is EPCI (engineer, procure, construct, install): a few large wrapped contracts, each an end-to-end scope, placed with big, experienced contractors who take on the interface risk in exchange for a premium. Only large contractors can lead these, usually the installation contractor.
The pattern of who chooses which is well established. The most experienced developers, Equinor, Ørsted, RWE, ScottishPower Renewables and Vattenfall, tend to multi-contract, particularly on balance-sheet-funded projects, because they invest in the in-house skills and prefer to own the risk rather than pay for someone else to. Independent developers and less experienced utilities lean toward EPCI to minimise their own risk and make a project financeable. In the UK, EPCI has become less common than multi-contracting for fixed-bottom wind as the industry has matured, but it is expected to return for the first floating projects, where the risks are less understood. Contracts themselves are typically heavily modified FIDIC Yellow Book or adapted LOGIC forms borrowed from oil and gas.
Charters and the rush to reserve vessels
Underneath the construction contracts sit the vessel charters, and here scarcity has rewritten the rules. With suitable installation vessels in short supply and long lead times, securing one has moved to the top of a project’s critical path, often before the project has even reached financial close. The instrument for this is the Vessel Reservation Agreement (VRA), which locks in capacity ahead of a firm contract. Cadeler, for example, signed a VRA with Ocean Winds to install around 30 turbines at the BC-Wind project in the Polish Baltic, a deal valued at €48–56 million, years ahead of the work. SOVs are typically taken on long-term charter by the operator or turbine OEM, while CSOVs and CTVs are chartered for shorter construction or commissioning windows.
Ports and vessels look ahead
The defining theme is a race between demand and capacity. Turbines, foundations and cables are all getting bigger, projects are moving further offshore, and the fleets and ports have to grow and upsize to keep pace. Three shifts to watch:
Turbine size has outgrown much of the fleet
First, vessel newbuilds and upsizing. The orderbook is heavily skewed to vessels capable of the largest turbines and cables, but build capacity is concentrated (around 70% of WTIV orders go to Chinese yards), and Europe still faces an estimated 40% shortfall of suitable high-capacity vessels against demand. Cable-lay capacity is growing 37 to 48 vessels by 2030, with individual vessels far larger to handle HVDC.
The service fleet is scaling fast
Second, decarbonisation of the fleet itself. More than half of new SOV orders carry alternative-fuel capability, with methanol the front-runner, and fully electric CTVs with offshore charging are emerging. Second, floating wind changes the vessel mix: jack-ups cannot install turbines in deep water, so floating relies on quayside or dry-dock assembly and tow-out, semi-submersible crane vessels, and heavy tugs and anchor-handlers, with new demands on port wet-storage and deeper quays. Third, the US market remains constrained by the Jones Act, which requires US-built, -flagged and -crewed vessels for point-to-point carriage, the first compliant WTIV, Charybdis, only reached sea trials in 2025.