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Conducting Site Surveys For Offshore Wind Farms
A Multidisciplinary Approach of Geophysical, Geotechnical and Metocean Surveys Guides the Assessment of Offshore Wind Sites

AUTHOR:
Ing. Mattijs de Lange Bsc.
Consultant
SEAknowledge
Domburg, The Netherlands

Industry giants have long drilled for oil and gas, the gold of the sea. Since the beginning of the new millennium, a new energy source has gained traction: offshore wind. The seas have enough space, the offshore wind yield is higher than onshore, and the visual impacts, often a difficult hurdle to pass on land, are less of an issue.

Though it seems a bit trivial to say, what is needed most for wind energy is wind. The second consideration is space. Both are available at sea. The general attitude of the public is they donít want to see them from their backyards or their seaside apartments. This means new offshore wind farms are all planned at distances exceeding 12 nautical miles, where they can only be seen as small dots on the horizon on a very clear day.

The civil engineering-related questions this new offshore industry raises remain significant. Megawatt-scale wind turbines are bolted on top of piles rising at least 50 meters above the surface, with typically 50 to 100 of these turbines in an average wind farm. Imagine the forces involved with an offshore wind turbine that has a 70-meter-high mast and three 50-meter blades, which must operate in wind speeds of up to 25 meters per second on a pile driven 50 meters into the seabed. Then imagine the corresponding waves and currents in these conditions. Add in the fact that many projects have a life span of at least 25 years, during which issues could arise with stability and maintenance of the seabed. These combined factors mean the survey industry is needed more than ever to make ambitious offshore wind projects a reality.

Expected Survey Demands and Requirements
The governments around the North Sea are all active in planning offshore wind energy. By 2020, a combined expected capacity of 55 gigawatts is needed in the North Sea alone. Up to 10,000 wind turbines need to be installed in order to deliver this amount of capacity. Governments have set goals for their respective countries: 25 gigawatts in the U.K., six gigawatts in the Netherlands, three gigawatts in Belgium and 20 gigawatts in Germany.

To provide engineers with sufficient input to create robust designs, crews need to conduct geophysical surveys over the sites, perform cone penetration tests (CPTs) at all turbine locations and drill boreholes at 10 percent of all turbine locations.

A straightforward calculation shows a potential of approximately 10,000 CPTs and approximately 1,000 boreholes for the geotechnical survey market. Based on an average of five to nine megawatts per square kilometer, a total of about 6,000 square kilometers of geophysical surveys are expected in the North Sea alone.

Multidisciplinary Planning Approach
In the long run, cutting corners in properly surveying a site doesnít save money. Companies have come to realize that full-scale geophysical, geotechnical and environmental site surveys are necessary for the successful completion of a project.

It could be said that the layout of a wind farm is, on average, designed three times over: The first design is for the wind, the second for the environment and lastly for the foundation.

Foundation Selection. In an initial layout of an offshore wind farm, the choice of a turbine gives a rotor dimension and a choice of three foundation types: monopile, gravity structure and jacket structure. A monopile, which will be the focus of this article, generally consists of three components: a nacelle with a tower, a foundation pile and a transition piece to connect the tower with the foundation pile.

Survey Planning. A good amount of desk research is needed in the early phases of planning. Project workers need to research various sources, such as the public domain, consultancies and universities. This will lead to a decision to perform a preliminary site assessment.

Key elements in this process are to assemble information on the geological model of the site, define the preliminary key geological processes, assemble the preliminary metocean conditions, identify geotechnical risks and areas of insufficient information, and outline the scope of the preliminary survey.

A graphic artistís drawing of an offshore wind farm.

Because of differences in the seabed and environmental conditions, every project is different. For instance, on Belgiumís exclusive economic zone, offshore wind farms are planned on existing sandbanks. The key elements include the bathymetry and dynamics of megaripples on the banks. In another instance, the various types of chalk on the east coast of the U.K. will prompt different foundation design challenges than an offshore wind farm in the Baltic, where there is a variety of complex geology with boulders and till.

Typical products of the preliminary site assessment phase are bathymetry maps (data sets), geological maps showing the various layers and reflectors of the area, wind speed maps based on numerical models and investigative borehole information.

Though the source and nature of these data sets vary much in their parameters, preferably the end product should be accessible with a GIS. These data sets are not only used for civil engineering purposes as described. For instance, the financing process of an offshore wind farm also requires site assessments to be submitted to financial institutions, as foundation stability over the lifetime of a project is an important parameter in financial risk assessment. To continue this article please click here.



Mattijs de Lange was educated as a hydrographer and civil engineer at the University of Amsterdam. Since 1983, he has been involved in multidisciplinary sea projects for the dredging, oil and gas industry, and, since 2006, for the offshore wind industry. He shares his passion for the sea as chairman of the nonprofit education group SEAfoundation and can be found working on various assignments for the industry via the consultancy and project management support business SEAknowledge.



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