The mooring design must first resist the magnitude of the forces imposed by the tidal device. As well as the loads imposed by the tidal stream on the device, it also needs to take account of possible wave-induced or turbulence forces. Another consideration is the compliance of the mooring system, especially with regards to limitations imposed by the flexibility of electrical cables and to the proximity of adjacent devices.
Conventional mooring types are as follows:
Catenary MooringsA wealth of experience exists with using this type of system including the mooring of semi-submersible oil platforms. For this type of mooring to work effectively a significant weight per unit length is required in the mooring line to create near horizontal tension at the seabed. Drag anchors are then employed to resist the horizontal loads. These moorings require a large plan area of seabed and are not ideal for the proposed tidal device application. They also produce "spring" in the system, which is not desirable in this context.
Taut Elastic MooringsThese types of moorings have a background in berthing applications. Synthetic fibre ropes, which are practically neutrally buoyant, remain essentially straight and rely on the elasticity of the material to provide the necessary compliance. This feature will be utilised as part of this flexible mooring design. The diagram below shows the differences between catenary and taut line moorings, which are also summarised as follows:
"The major difference between a catenary mooring and a taut leg mooring is that where the catenary mooring arrives at the seabed horizontally, the taut leg mooring arrives at the seabed at an angle. This means that in a taut leg mooring the anchor point has to be capable of resisting both horizontal and vertical forces, while in a catenary mooring the anchor point is only subjected to horizontal forces. In a catenary mooring, most of the restoring forces are generated by the weight of the mooring line. In a taut leg mooring, the restoring forces are generated by the elasticity of the mooring line." (Vryhof Anchors, 2010)
Tethered Surface Buoy MooringsA surface floating structure is used to maintain the mooring lines under tension, thus reducing the heave motion. However, this type of mooring operates in the volatile region on the sea surface, being subject to turbulence and combined wave and wind action. The feature of the buoyancy of a structure to maintain tension in the mooring line is adapted in this design through the use of a submersed buoy thus avoiding the worst of the surface interaction.
Dynamic PositioningThis system uses a set of computer-controlled propellers to actively maintain position of the device over a group of sonic transducers on the seabed. Another method is to attach a number of mooring lines from the sea floor to the device and computer-actuated winches on board the device control positioning. While power is available at the device to accommodate these types of moorings the expense of control equipment would be prohibitive and would contravene a design aspiration for simplicity in this case.
Design ApproachThis design combines aspects of both the Taut Elastic and Tethered Surface Buoy moorings. It delivers tension to the mooring line using a submerged buoy that is tethered to the seafloor. This arrangement provides all the support to the device. The buoyancy provided by the buoy is sufficient to maintain the neutrally buoyant device in its desired working position.
A distinct advantage of this system is that it minimises excursion i.e. it keeps the footprint of the entire system to a minimum.
Reliability will also be a key feature of any design. This will be achieved by maximising the life of the mooring equipment while minimising expensive maintenance operations.