PART 5 - MOORINGS & FOUNDATIONS
The penultimate section in our review of the European Commission's workshop on future emerging technologies in the ocean energy sector. All ocean energy systems need something to react the loads experienced by the device against. Some devices are rigidly mounted to the seabed, for example a horizontal axis turbine on a tripod, a seabed mounted oscillating surge converter or an oscillating water column. Other devices are floating and rely on mooring lines for station keeping. Either way these foundations face their own challenges in the loads they experience and how they affect the performance of the energy system. Unfortunately, the lessons learned from other offshore platforms (oil and gas for example) are not always transferable to marine energy devices as those platforms are designed for more benign, deeper seas and are not reacting loads and extracting power.
Many floating tidal, wind and wave devices use a system of mooring lines (akin to the accompanying figure, or the Orbital Marine tidal turbine) for station keeping. However, these mooring lines also react the loads from the device and thereby have an effect upon performance, which makes effective design more challenging.
The variety in sites; water depth, wave loading, device type, means of reaction, seabed conditions, space envelope, all contribute to an almost unique mooring design for each new site and device. Some will favour catenary moorings or tension moorings, but almost universally they will be required to prevent snatch loading and allow the device to change heading according to the conditions.
Rigid seabed foundations need to directly react the loads from the device and the extreme loads on these structures can be many times greater than the mean loads. This results in design for an unlikely extreme load event that may never be seen, so there is material in that structure that is unused.
Fixed foundations also face the challenge of being subsea for 20+ years during which time significant material can be lost to biofouling and corrosion. Appropriate design for the marine environment using coatings, materials and anodes is integral to the success of
these foundations.
A point affecting both fixed foundations and mooring systems is the need for an involved marine operation to install them. Heavy fixed structures may require expensive vessels to lift, lower and install. Moorings and often foundations require a subsea drilling operation to locate the anchor points as drag embedded anchors are not always suitable in the near shore zone.
Present projects focus largely on mooring systems and, in particular, reducing snatch loading and footprint of the mooring spread for more compact arrays.
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