There has been a lot of discussion about the ability of a WEC design needing capacity to withstand extreme storm wave conditions. I agree this subject, and its overall successful inherent design, is paramount to the ultimate survival, and invest-ability into the WEC company who makes it. I will even go so far as to say, at the glee of the industries naysayers, that it may be the Achilles heel, as failure of the mooring system, results in catastrophic loss. The naysayers will also interject, and expound at great length the lack of actual at sea test data for specific and actual WEC applications, albeit existance of high experience and use of the systems for other non WEC applications. It is a touchy subject, one that even I am reluctant to discuss, as it makes the WEC investment seem more of a go- no go endeavor, if you assess failure risk as high, rather than go eventually, at what price, as the mooring system is heavily depended upon for success. One good thing is, Even going back to the days of Keefs ancestors, the marauding Pirates, (in search of Elephant testicles<inside joke>?) there has been a real need for the secure and reliable ability to moor your Ocean going device (Pirate testicle collection ship), and man has been learning ever since. There is a lot of data, and different designs, materials, that have been used in various marine environments throughout mans foray into the Oceans, and a lot of mooring knowledge can be attributed to the Oil and Gas industry, gleaned thru necessity during off shore oil drilling activities, for mooring of giant oil platforms, and on station ships, in varying oceanic sub floor mediums. Some of these mooring designs have stringent conditions, due to the application to a producing oil platform, prone to leaks and spills as a result of excessive movement. Some of these highly successful and proven technologies can be implemented in WEC mooring technologies, with great success, and others require modifications. Obviously, the Oceans motions of surge, sway and yaw necessitate an external restoring force (mooring) in order to return or maintain original equilibrium position and Station Keeping. Mooring systems for WEC use have advanced as the WEC deployment location has progressed to higher wave flux. Shoreline, Nearshore, and Offshore are three common classifications.
The two major requirements for a WEC mooring are to withstand the environmental and other loadings
involved in keeping the device on station, and to be sufficiently cost effective so that the overall
Economics of the device remain viable. The mooring should not adversely affect the efficiency of the device, should allow for removal of single devices with out affecting other devices, materials should be corrosion, sunlight and marine growth resistant, and of adequate strength, fatigue life and durability for the operational lifetime, and not allow tension loads on electrical transmission cables, and be of redundant design allowing failure of one line or cable without significant impact as a whole to the system.
As we know, The OPT PowerBuoy technology comprises a semi submerged
floating buoy consisting of an upright spar unit, surrounded by a vertically mobile toroidal
(doughnut-shaped) float, with an onboard power conversion system and a submerged heave plate.
The device is moored by a three point mooring system consisting of three gravity based anchors. (GBAs)
These GBAs will be positioned in triangular orientation around the proposed test berth. An Auxiliary Subsurface Buoy (NYSE:ASB) will be attached to each GBA via a tensioned vertical mooring line made from synthetic rope. Mooring lines made from the same material will be linked in pre-tension from each ASB to the spar unit. Each mooring line will be approximately 135m each in length. Bridle lines will be fitted between each mooring line and the base plate to further limit horizontal motion of the spar unit. The GBA design has been developed to allow cost effective deployment and retrieval using a relatively small capacity barge.
Navigational Risk Assessment (NRA) has been undertaken for the Billia Croo site by Abbot
Risk Consultants in line with Maritime and Coastguard Agency’s guidance
The largest waves at Orkney originate from the 300°N offshore sector and a number of other analyses have confirmed that the 100 year return period storm wave is approximately 28m. General climate
characteristics have been extracted from Kirkwall 1971-2000 meteorological averages with average
annual rainfall of approximately 1030mm/year, and average annual wind speeds are recorded at 13.6
knots (at 10m).
Robert E. Harris, Lars Johanning, and Julian Wolfram have contributed extensively to WEC mooring design: High suitability WEC mooring designs are outlined on pg 6, Table III, with mooring components and suitability in Table IV, pg 7.
A Point Absorber WEC is relatively small compared to the wave length and is able to capture energy from a wave front greater than the physical dimension of the absorber The point absorber has an advantage, in that it does not have a principal wave direction and is able to capture energy from waves arriving from any direction. Thus, Point absorber moorings can be simpler, than terminator or attenuator WEC designs, because the keeper station for the terminator or attenuator designs additionally has to allow the unit to ‘weathervane’ into predominant wave directions, but this is not necessary for point absorbers. Point Absorbers also are predominately PE absorbers, designed to capture the up and down motion of the waves, in contrast to KE absorbers, and as such also have a lower mooring footprint per KW absorption, than other designs such as terminator, or attenuator designs.
Development of Efficient design techniques for Mooring system optimization for WEC and Arrays is ongoing. This is a study from ‘SeaChange, a marine Knowledge, R&D facility from Ireland with MCS Kenny.
The South Western Mooring Tests Facility (SWMTF) is a unique installation funded by the South West RDA and ERDF, part of PRIMaRE
The facility aims to significantly aid the research and development of suitable mooring configurations for wave energy devices in support of wave energy device developers and the South West RDA’s Wave Hub project. The test facility will be deployed in Falmouth Bay.
Kevlar and steel rope mooring comparisons by DuPont
Disclosure: Long OPTT