P.3 Resource and metocean assessment

Function

Resource and metocean assessments provide atmospheric and oceanographic datasets to inform the engineering design of a wind farm, the potential future energy production, and to fully describe the likely operating conditions at the proposed wind farm location.

What it costs*

Costs for resource and metocean assessment are about £7 million for a 1 GW offshore wind farm

Example costs for elements of this include:

• Structure: £3.5 million
• Sensors: £2.9 million
• Maintenance: £660,000

Who supplies them

Floating lidars: AXYS Technologies, EOLOS, EOLFI, Fraunhofer IWES, Fugro, IDS Monitoring, RPS and ZX Lidars.

Lidar units: Leosphere and ZX Lidars.

Metocean campaigns and buoys: AXYS Technologies, Fugro, Partrac and Sonardyne.

Reference data provision: The Met Office, StormGeo and Vortex.

Resource campaign management and design: Arup, AXYS Technologies, BMT, Briggs Marine, DNV, DORIS, Fugro, Global Maritime, HR Wallingford, K2 Management, Mott MacDonald, Natural Power, Oldbaum, RPS, Venterra, Wood, Wood Thilsted, Xodus and ZX Measurement.

Key facts

Measurement systems are installed at the project location to collect wind and other relevant meteorological data. Meteorological sensors measure wind speed (with instruments at a number of heights or measuring over a range of heights with one sensor), wind direction, temperature, pressure, humidity, solar radiation and visibility. Measuring wind speeds at different heights provides critical information about the wind speed profile at the site, aiding decisions about the turbine and floating substructure design. Wind speed data is required to at least the proposed hub-height of the wind turbines, which is 130 m or more above sea level for a 15 MW turbine. Metocean buoys are installed in and around the floating offshore wind farm site to collect metocean data. Metocean sensors include wave, sea level and current sensors (for example acoustic Doppler current profiler) which are sometimes sea bed positioned. These record the full wave data spectrum including velocity, direction, and period. Multiple sensors are used to provide spatial coverage and redundancy. This information is crucial in establishing whole system dynamics including substructure design types, turbine ratings, vessel types and operations and maintenance (O&M) strategies.

Long-term reference datasets are required to describe the climatology of the proposed site over a longer period typically more than 15 years. Data is usually collected for a period of at least one year to reflect seasonal variation in wind resource and metocean conditions.

These combined data sets are used in the offshore wind farm system design process, the turbine selection process and to predict the annual energy production (AEP) of the offshore wind farm. Metocean data is also used to inform the vessel selection and operational strategies for the site and is made available to vessel operators and marine planners during the construction and operational phases.

A key interface exists in determining the long-term site conditions between wind resource and metocean disciplines. The output from this interface is the extreme wind and wave climate for the proposed site.

Traditionally assessment was done using hub-height wind masts which are fixed to the sea bed and require a subsea structure, but the trend is towards the use of floating lidars as they are cheaper and easier to install. 

Lidars are a type of remote sensing anemometry device which uses lasers to measure wind speed and direction at up to 300 m above sea level. Floating lidars are moored buoys on which lidars are mounted. This allows the lidars to be deployed and change location as per demand.

When using lidar as the primary measurement instrument, supplementary modelling may be used to inform site conditions such as turbulence and horizontal wind gradients.

Wind and metocean measurement systems require power supply to run sensors, data storage and telemetry. For low power systems this is often achieved with solar PV panels, small wind turbines and battery storage. Larger systems use diesel generators or hydrogen fuel cells.

Current state of the art campaigns integrate measurement and modelling techniques across both oceanographic and wind resource disciplines. The study can be further broadened to look at further issues such as turbulence, atmospheric stability conditions and the influence of neighbouring offshore wind farms on the proposed site wind conditions.

Resource and metocean systems require maintenance, including inspection, cleaning, and refuelling (where diesel generators or hydrogen fuel cells or similar are used). Maintenance visits are typically carried out two to four times per year. Systems are designed to operate autonomously, with onboard power, data and communications systems.

Sensors provide data on meteorological and oceanographic conditions at the site of interest. Data loggers provide data storage, processing, and remote communications capability.

What’s in it

• Anemometers
• Buoys
• Data loggers
• Lidar systems
• Maintenance 
• Meteorological sensors
• Metocean sensors
• Wave measurement sensors