Tidal turbine power generation is still a technology in way of development, for many issues have not been tackled yet. One issue the project STHYF is interested in is the stability of electrical power export cable, taking the power from tidal turbines to shore. Because of the strong dynamics of tidal sites, even if sediment transport is significant, the sediment deposit is hardly possible, so providing a chaotic rocky seabed. Consequently, the export cable cannot be buried when it crosses the high current zones. The cable is then subject to a flow component normal to its axis, thus introducing radial drag as well as lift force due to the asymmetric flow around the cable. When the waves are long enough in shallow water, an oscillating component is added to the ambient flow down to the seabed boundary layer, leading to oscillating efforts. For some lift-to-drag combinations, the cable may be able to lift and/or slip, and to move step by step with the current. Therefore a potential risk is cable abrasion, along with the risk of adding extra mechanical tension to the cable and at its extremities. Another potential risk is for the cable route to move to another one, which can lead to unwelcome situations such as free-span or blockage against or between rocks.

Different data base and models have been set-up for pipeline stability study in wave dominant coastal area. The Danish Hydraulic Institute (DHI) has conducted an extensive experimental campaign, aiming at measuring the efforts on a pipe laying on a seabed under constant plus oscillatory flow solicitations, so representing a data base reference. It is now well-known that the standard Morison formulation of hydrodynamic loads fails to reproduce the non-linearities and the phase between excitation and loads, especially on the lift force. A model answering to a relatively high level of requirements is a model called Wake2, which latest documented development for regular waves has been published by Sabag in 2000. An extension to irregular waves has been published by Aristodemo in 2011.

Within STHYF, an extended model has been developed on the basis of Sabag Wake2 model for regular waves, with an extended range of validity. Thanks to a CFD model that has been developed, and to an experimental campaign that is planned at IFREMER Boulogne facilities, the extended Wake2 model is valid for higher KC values and higher seabed roughness. Moreover, the experimental and CFD campaigns should give a sufficient amount of data for deriving a simple coefficients-based correction law, allowing the new Wake2 model to estimate efforts on a cable separated from the seabed by a certain distance. In particular, the seabed roughness influence on the flow confined between the cable and the floor is exhibited.

Extended Wake2 model presents a compromise between timesaving and precision in force calculation, making it perfectly suited for STHYF methodology. This methodology assesses the cable stability for several configurations, all along the cable route. It has been developed with the objective of being applicable in an industrial environment, at the stage of testing a wide range of cable routes to elect the safest one. The constraints for that methodology were then to find a force calculation model sufficiently refined for taking into account some local specificities (seabed roughness, current and turbulence properties), but still preserving a small computation time, crucial at that design stage.