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How many vessel-days does it take to build an offshore wind farm, per turbine or per megawatt? I notice that when an offshore wind farm is being built, there are quite a few vessels milling around: vessels for shuttling crew, surveying, heavy-lifting cranes, cable-layers, accommodation, pile-drivers.

Looking at all those vessels, and the total capacity, is there a typical number of vessel-days (i.e the number of days spent, per vessel, summed across all vessels) per unit of installation - per turbine, or per megawatt?

I've been watching the vessels move on marinetraffic.com, and keeping an eye on the 4C Offshore register of vessels used on offshore wind farms: even on completed wind farms such as the London Array, there's a lot of ongoing boat activity:

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This is a part answer to your query as there seems to be a lot of challenges and variables that make it difficult to determine the total vessel hours. Below is some detail that approximates the number of vessels per turbine.

The Crown Estate document A Guide to an Offshore Wind Farm provides in detail the ships required from the planning to maintenance phases, and their running costs in both a handy table and diagram. Specifically, and relevant to the issue of vessel-days

There is no single way to build and operate an offshore wind farm,

The main challenges being

scale, water depth and distance from shore

These factors as well as conditions, seasonality etc would have a major effect on the amount of vessel-days, as you have described. The document provides an approximation of the cost per 500 MW segment per ship for each stage, listed below:

Survey stage: Geophysical survey vessel; Ornithological and mammalian survey vessel

Foundation stage: Foundation installation vessel, Array cable-laying vessel

Turbine and substation stage: Substation installation vessel, Turbine installation vessel

Operation and Maintenance: Maintenance vessels

The European Offshore Wind report suggests that the costs per vessel should decrease as new specialised wind-turbine-specific vessels are developed and used.

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