Marine Scour and Offshore Wind: Lessons Learnt and Future Challenges

The drive for developing marine offshore renewables has led to specific requirements for scour hazard assessment relating to the associated foundation structures and the cabling necessary for in-field transmission and power export. To date within the United Kingdom (UK) a number of demonstrator projects have been constructed covering wind, wave and tidal generation. However, only offshore wind has been developed at large-scale at present as part of two rounds of commercial development of offshore wind farms (OWFs). In June 2008, The Crown Estate, responsible for licensing seabed use, announced proposals for a third round of offshore wind farms to develop an additional 25 GW of energy to the 8 GW already planned for under Rounds 1 and 2. The size of these Round 3 developments will vary, but the largest of these zones will involve the construction of around 2500 seabed foundation structures. Under Round 1 and 2 developments monopile and jacket type foundations have been used, although several other European (non UK) wind farms have been built using gravity base foundations. For a wind turbine the foundations may account for up to 35% of the installed cost. Therefore, one of the future challenges for large volume installation of offshore wind is the control and minimization of these costs. For tidal energy devices one of the principal requirements for many of the devices proposed is their placement in areas of strong tidal energy, and this has implications not only for the stability of the foundation option, but also for the construction methodology. Similarly wave energy devices are designed to be located in shallow, coastal environments as either floating or bottom mounted systems. These devices, by design, are intended to be located in environments with strong wave action. This may be substantial during storm events, which has implications for the integrity of the anchoring system keeping the wave device on station or the design of the device if it is seabed mounted. This paper will explore the lessons learnt from existing offshore wind farm developments as this represents the principal body of collected monitoring data. Using these data the paper will outline some of the challenges facing the offshore renewable industry in respect of the foundation designs and specifically the requirements for scour hazard assessment using the combined experience from those developments currently operational or under construction.