Geotechnical site investigations are conducted following the geophysical survey to use the information obtained to target soil and rock strata boundaries, engineering properties and specific sea floor features.
What it costs
About £2.1 million for a 450 MW floating offshore wind farm.
Who supplies them
Fugro, G-tec, Gardline and Horizon Geosciences.
Geotechnical studies are predominantly intrusive and include methods like drilling boreholes to collect soil and rock samples, and cone penetration testing (CPT).
Geotechnical investigation is generally the most expensive part of floating offshore wind farm survey work, making it a substantial at-risk investment for developers. Typically, the geotechnical surveys are performed in phases to add value to the project risk mitigation process.
Geotechnical surveys require specialised equipment and skilled personnel. The scope of the investigation depends on the type of foundation being considered and the variability in the sea bed characteristics.
Boreholes and CPTs are carried out to investigate the physical characteristics of the sea bed. Surface push CPTs are also used as a rapid method to gather sea bed soil stratigraphy. Cable routes are typically investigated using vibro-cores and CPTs to a depth of 5 m.
Offshore laboratories are used to obtain basic soil parameters and samples are taken there for detailed testing once they have been collected. Often soil dynamics tests are performed to monitor the soil behaviour under the constant dynamic loading on the foundation by the wind, waves and current.
Resultant data from the geotechnical surveys are combined with results from the geophysical survey, to improve the geological model prior to the design and installation of anchors. Geotechnical data is also used later in combination with heavy lift jack-up vessel information to determine the risks and feasibility of conducting heavy lift construction activities.
Fixed offshore wind farms require geotechnical data to depths of 50 to 70 m to inform the design of monopile or jacket foundations. Floating offshore wind farms use anchors which are typically not installed as deep as monopiles, although this does depend on the type and design of anchor selected (see B.3.1 for further information). This means that geotechnical surveys are generally needed to shallower depths for floating offshore wind farms compared to fixed.
In the early floating offshore wind projects, developers are expected to be cautious and conduct geotechnical surveys for every anchor placement. The sample and survey frequency may decrease in time as developers gain confidence and experience.