Narrative :

The aim of this consultancy was to assist the Client in assessing the applicability of gravel pads consisting of pre-cut rock as one of the possible mitigation measures to allow safe jack-up activities in view of the installation of the Wind Turbine Generators (WTG) at the OWF of Saint Nazaire (France). To support this mitigation measure, HAEDES BV was contracted to assess the hydraulic stability of the gravel pads subject to met-ocean conditions occurring between the time of the pre-cut operations and the time of the installation of the  of the WTG-turbines themselves. This analysis includes the evaluation of potential damage to the gravel. Included in the scope was the evaluation of the applied methods and hindcast analysis based on in-situ measurements of met-ocean conditions and gravel pad deformations (obtained via in- and out-surveys).

The scope of the present assignment was to perform an analytical study in which the hydraulic stability of the gravel pads was assessed under different conditions based on different design standards and state-of-the-art methods. Both static and dynamic stability were applied for the semi-probabilistic and full probabilistic analysis. Hereto project characteristics, including met-ocean site conditions and joint probabilities were defined. The influence of each of the design parameters was assessed and an estimate of the survivability of the gravel pads was provided. A comparison between different analytical methods was performed. The outcome was discussed in view of applicability of the installation and the environmental impact assessment. Our approach was characterised by the combination of close collaboration with the client and our in-depth knowledge of the available methods and their limitations and applicability ranges.