Description Of The UK Christchurch Bay Wave Force Project And Some Initial Results-Part 1
Authors: Bishop John R. 
Published: December 2007

The Christchurch Bay Project is intended to provide wave force data that fill the gap between provide wave force data that fill the gap between small scale laboratory experiments and the fullscale conditions. The site for the Tower provides a reasonable model of North Sea conditions except for the largest waves which are depth-limited.

Experiments with the gravity foundations were carried out simultaneously with the wave force experiments. Mention is made of some important aspects of engineering design and construction. The paper then presents some early results. These include comments on analysis techniques, a brief description of some time histories and some force coefficients from a 'mean square' analysis method.

Introduction

The Christchurch Bay Tower, Fig 1, is a small offshore structure designed specifically for research into wave forces and gravity foundations. The wave force experiments are intended to fill the gap between small scale laboratory work in rather simplified waxes and the full-scale structure in the complex waves of the real sea. The objectives have been discussed in reference (1) and the relation of large scale experiments to laboratory tests has been discussed in (2). The early history of the project is dealt with by Wheatley (3). The emphasis in this paper will be on the engineering aspects and comments will be made on the functioning of the Tower in Christchurch Bay and the problems that have arisen. Some results will be presented at the end of the paper. These comment on some characteristics of paper. These comment on some characteristics of the data and then show some values of force coefficients which are emerging from the early analysis work.

Christchurch Bay was chosen as the site for the Tower (Fig 2) because it appeared, from analyses of wave buoy data, to offer wave conditions which are reasonably representative of the North Sea but at a reduced scale. The predominant winds are south-westerly and these predominant winds are south-westerly and these have a fairly long fetch along the English Channel. This meant that we would experience wave heights in the 2 to 3 m region with a reasonable frequency of occurrence and these heights would, in the 9 m water depth of the test site, be a reasonable representation of the highest North Sea waves of 30 m height in water depths of 100 to 150 m. The worst storm waves at the Tower site are limited to a height of about 7 m by the water depth. These highest waves are not representative of North Sea conditions but the limited height caused by the water depth was considered advantageous as it would restrict the range needed for the various instruments and hence would permit better accuracy in waves of 2 to 3 m. The site was also attractive as it was near to the National Maritime Institute's trials base at Hythe near Southampton.

WAVE FORCE EXPERIMENTS

The elements of the Tower concerned with the wave force experiments are shown in Fig 3. Forces are measured on the large column, which has an outer diameter of 2.8 m, by sleeves at five depth stations. This column is in the inertia regime in all but the largest waves. The small column has a diameter of 0.48 m and has force-measuring sleeves at 4 depth stations. This column is in the drag regime in wave heights greater than about 2.5 m. Instruments for measuring the wave particle velocity are mounted on the small column. The measurement of particle velocity presented a most difficult problem as there were no suitable existing instruments, and this was a key parameter in the experiment design. We eventually fitted two types of instrument, firstly an electro-magnetic type which had been originally designed to measure quasi-steady velocity and secondly, a perforated ball device. This latter instrument had its origin as a device for measuring turbulence in wind and we developed a marine version for this project. The velocity is obtained from the project. The velocity is obtained from the measured drag force and we have found the instrument to be very reliable, although the data processing is a little complex. processing is a little complex. P. 177