PO178

GROUNDMODEL AND GEOHAZARD ASSESSMENT FOR OFFSHORE WIND FARM AND THE IMPORTANCE OF MULTI-DISCIPLINARY STUDIES: THE CASE OF DIEPPE LE TRÉPORT WIND FARM, OFFSHORE FRANCE

Ulysse Lebrec1, Alexandre Nicolas Bandini-Maeder1, Maarten Vanneste2, Rasmus Tofte Klinkvort 2, Antonio Borges Rodriguez1, Guillaume Sauvin1, Terry Griffiths3, Ragnhild Christin Hansen1, Carl Fredrik Forsberg1, Philippe De Schoesitter4, Chris Blommaert4
1Norwegian Geotechnical Institute Pty. Ltd., Perth, Western Australia, Australia, 2Norwegian Geotechnical Institute, Oslo, Norway, 3Aurora Offshore Engineering Pty. Ltd., Perth, Western Australia, Australia, 4Eoliennes en Mer Dieppe le Tréport, Courbevoie, Ile-de-France, France

Abstract

Knowledge of seabed sediments and rocks is essential to properly and safely design offshore facilities. A poorly constrained soil model may lead to erroneous estimation of the soil properties and subsequently to unfit foundation designs causing expensive counter measures. Therefore, it is necessary to have a multidisciplinary (geotechnics, geomechanics, geology, geophysics, geomorphodynamics, etc.) approach to assess the stratigraphy of shallow sediments and rocks properties.

The Dieppe - Le Tréport wind farm, located in the French part of the English Channel, is a good example on how multi-disciplinary studies assist in developing an integrated ground model to help addressing geohazards, risks and provides insights into scoping for detailed follow-up investigations and laboratory testing. The wind farm lies about 15 km offshore in water depths between 5 and 25 m LAT, covering an area of 91.5 km2. The wind farm will consist of 62 wind turbines for a total installed capacity of 496 MW.

The regional geology is characterized by thick deposits units of Cretaceous chalk. The chalk is covered by Paleogene-Neogene and Pleistocene fine sands and laminated clays. The seafloor is composed of Quaternary coarser sand and gravel forming distinct and mobile sediment waves.

Together with the seabed morpho-dynamics study the model was used to establish a geohazard and risk register. In alignment with the investigation program requested by the French state in the tender documentation, the results of the studies define the scope for complementary site investigations data that ultimately will serve as input for the foundation detailed design.

Method

Interpretations of the wind farm soil conditions are performed in three stages.

First, all available data are reviewed and quality assessed. The assessment of the morpho-dynamic conditions and the creation of the ground model are then performed jointly in order to insure the coherency and the consistency of the final model.

Bathymetry, sub-bottoms profiling data, side-scan sonar, magnetometer data and geotechnical data were used across the study area. All data were integrated in a GIS allowing an efficient data management.

Finally, the results are used to establish a geo-risk register that shows data gaps and mitigation strategies to assist in the optimisation of the next field campaign.

Results

The differential analysis of recent and historical bathymetry surveys allows the quantification of the shallow sediments morphodynamic behaviour and the identification of inter-annual variability caused by storms, along with the prediction of future bedforms. The geophysical data allows the discrimination of shallow mobile sediments from deeper consolidated deposits.

The following topics were assessed for creating the ground model:

Results are integrated in a GIS which allows the delineation of soil provinces based on combined criteria (e.g. slope, relief, sediment properties, etc.). These soil provinces are subsequently used to define a fit-for-purpose follow-up detailed site investigation.

Conclusions

The present study of the soil conditions at Dieppe Le Tréport wind farm allowed the assessment of the soil properties and the identification of potential geohazards, including potential data gaps. It was then possible to define a site-specific geotechnical investigation.

The work performed across this study area shows the importance of multi-disciplinary integration and how the utilization of GIS software can help defining fit-for-purpose site investigation and hence reduce costs and uncertainties associated to offshore foundation design.

Multi-disciplinary studies are only possible if an efficient data management system is set up during the early stages of the project and allow all part involved in the project to have access to the latest development of the work performed.

Objectives

The paper will show delegates how to manage multifaceted datasets and in a multi-disciplinary project in order to have a consistent seafloor and sub-surface ground model. The presented approach helps assessing geohazards and risks across the first stages of a project and assists in the selection of foundation concept for the design of offshore windfarms. In addition, it enhances the optimisation of the planning of offshore site investigations.