OWC Colloquium

Second TCD Colloquium in Rock Engineering for Ocean Wave Energy

By Niall McMahon
October 18th, 1997

This is a redacted document drawn from my notes taken at the meeting. Private information and sources have been obscured.

Niall M. McMahon.


In attendance, representatives from: Trinity College Dublin; Chalmers University of Technology, Gothenburg; the Hydraulics and Maritime Research Centre, University College, Cork; the Trenchless Technology Center, Louisiana Technical University; ESB International.

The G-Drill and OWC Chambers

This involves the drilling of a pilot hole through the centre part of the planned chamber from the top of the cliff to the cliff face. If the positioning of this hole and the geology encountered is favourable, then this hole will become the centre of the unit and completed to a diameter of 12" (30 cm). A series of "helper" holes and contour holes will then be drilled. To create the required chamber, isolated rings of explosives will be placed at regular intervals in these drillings, designed to blast out segments sequentially. Each segment has 2 rings of explosives, one ring in the "helper" holes and the other in the contour holes. The segments are blasted out in 2 operations; first the inner ring is detonated, pushing the rubble inwards and downwards through the 12'' central bore. The 2nd outer ring is then detonated pushing the material between the two rings in and down as well. This blasting process can produce a cavern of any length right to the ground surface if necessary. The resulting chamber, however, will not be geometrically perfect with rough walls and an imperfect ceiling in terms of fluid flow dynamics.

G-Drill techniques are considered to be the most suitable choice in the construction of such a cavern. G-Drills (named with the first letter of the Swedish inventor's surname) use water instead of compressed air to drive the bit. Developed in the 1980s, the G-Drill brings the cuttings back to the surface in the form of slurry. This removes the problem of dust (which is unwanted if this project is to be as environmentally sound as possible). The slurry produced drained to produce sediment and trucked out of the site once dried.

The advantages of the G-Drill are:

This drilling - blasting technique is very flexible; if a pilot hole is deemed not suitably located for the central bore, it can become one of the outlying contour holes and a new pilot hole drilled.

Other problems that might be encountered during the drilling operation include the distortion of the structure due to gravitational and geological effects. He pointed out however that the important task in his view was to keep all the boreholes parallel to each other even if the structure as a whole may be slightly distorted.

Wave Tank Tests

The Hydraulics and Maritime Research Centre developed a method of calculating wave energies likely to be acting on the system. This necessitated a study of wave action around the coast, conducted with help from Met Eireann. Firstly ray trace diagrams were used to show power levels, indicated by the distances between the lines on the trace. With data supplied by Met Eireann's offshore wave monitoring system, a method of relating inshore wave heights to offshore readings was developed, allowing predictions of average wave powers at cliff faces around the coast to be made. The study concluded that 90% of the wave power incident on the Irish coast comes in from the west, north-west, and south-west directions, and that the power levels were sufficient to seriously consider construction of an OWC generating station.

An OWC will need to handle high and low energies equally well and should be situated in deep waters.

As at October 18th 1997, 500 tests had been carried out at UCC. So far tests on arrays have not been conducted. As the exact geometries of the array are not yet finalised, any array testing without knowing the layout would not yield any useful information.

When constructing the model, the need to compare readings with those from OWC projects (such as the Azores project) was kept in mind. Other influencing factors such as friction were neglected. A brief run-down of model specifications and the data acquisition system was then given.

The analysis conducted before construction of the Azores project, governing compressibility effects was outlined, and graphs showing power predictions for sub-resonant, resonant and super-resonant conditions were displayed. These clearly showed the asymmetric dispersion of the point of peak power between the suction and the compression "strokes" of the OWC. This dispersion varied from moderately asymmetric at sub-resonant conditions to highly asymmetric at super-resonant conditions.

Mathematical Analysis

Meaningful results can be obtained from relatively straightforward fluid dynamics. The use of the Continuity, Bernoulli and Momentum equations yields predictions that are similar to results obtained from experimental data.

Shaping the OWC Cavern

Shaping the results of drilling and blasting are required to minimise energy losses during operation. This can be achieved with directional drilling. The production of a "clean wine bottle" shape is feasible, but this may be expensive.

Tuning Clusters of OWC Chambers

Tuning means both optimising the phase matching between separate OWC chambers and the matching of OWC chambers to the input waves. OWCs must be tuned to an optimal wave pattern; it is not practical to have an OWC tuned to storm conditions or to calm seas. A "cut-off" amplitude will have to be specified for optimum consistent generation. In a regular sea, the OWC could be tuned to the main frequency, whereas in an irregular sea, one particular frequency (irregular seas are basically many monochromatic conditions superimposed on one another) must be singled out for the tuning of the OWC.

In stormy seas the OWCs could be kept out of phase, while being phase locked in normal operating conditions.

With respect to the tuning of each OWC to the others, there is a minimum or optimum separation between OWC chambers to prevent interference from reflected waves. If the separation is not sufficient, out-flowing waves from one OWC will adversely affect the inflow to its neighbours, so a minimum distance between units must be established.

There is a question about whether to use a flow system to guide airflows from the OWCs to a single turbine or whether to treat each OWC as a stand alone generator and combine the electrical outputs. A stand-alone design is perhaps the most feasible; it would not be practical to attempt to bring the airflows into phase with one another.

Physical Conditions at Preferred Sites

Regions with favourable geological wave action, suitably layered rock or homogeneous rock (preferred rock types including quartzite) were sought out. Several sites were short-listed, including Donegal, Loop Head and The Burren. Other sites such as Dingle were ruled out due to violent seas.

Cost of Electricity

The power generated by Whittaker/ Queen's University OWC design was priced at 8p per unit kilowatt hour (power from conventional generating stations being in the region of 2p per unit kilowatt hour). Various locations around the west coast were considered (The east and south/north-east coasts deemed as lacking sufficient wave energy to warrant their study). Most of Galway was discarded due to shallow waters and the island sites were ruled out due to their remoteness. Other sites which yielded approximate costs of more than 10p /unit were also discarded.

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