Floating Breakwaters-A Wave Tank Study

Abstract

There is a need in the offshore industry for a means of protecting certain operations and for reducing time lost because of weather. A floating breakwater may be the answer in many cases. A laboratory study is presented that establishes the most important parameters affecting a breakwater's performance. Introduction There is a need in the offshore petroleum industry for a means of protecting certain operations and for reducing the time lost because of weather. Among these operations are platform setting, pipeline laying, tanker loading and unloading, and transferring personnel and equipment between platform and boat. One way this need could be met is with a floating breakwater. A breakwater interferes with incoming waves and thus provides a sheltered area in its lee. In the process of interfering, it reflects, dissipates, and transmits certain portions of the wave energy. The amount of energy reflected, dissipated, or transmitted depends on the particular breakwater involved, the mooring system, wave height and period, and the water depth. Rigidly fixed breakwaters, such as the rubblemound or masonry type, provide the highest level of protection. They are very expensive to build and install and are not mobile. Floating breakwaters, on the other hand, provide lesser protection; but they are less expensive and can be moved from one location to another. It is possible that floating breakwaters could be used in locations where a rigid, bottom-founded structure would be out of the question. Potential benefits from the use of floating breakwaters includecost savings because of a reduction in down-time for tanker terminals, and platform setting and pipeline laying operations, platform setting and pipeline laying operations,protection for loading and unloading men and supplies, andreduced likelihood of pollution. There has been a considerable amount of experimental work done on floating breakwaters. Included is that of Kato et al., in which the effects of four different breakmeter shapes (cross-sections) were evaluated. Their work indicated that an inverted trapezoid gave the lowest wave transmission and that the natural frequency of rotation about an axis parallel to the wave front was an important variable. Two excellent sources became available after the beginning of this study. "Transportable Breakwaters - A Survey of Concepts" gives a review, in outline form, of published and unpublished reports and data on 106 breakwater concepts. Also discussed are the effectiveness and potential of specific breakwater classes. "Recent Designs of Transportable Wave Barriers and Breakwaters" gives the status of portable breakwaters, with emphasis on recent designs. It reviews the performance studies for several breakwater types and describes the important parameters on the basis of past theoretical work. Both papers give extensive lists of additional references. The performance of a floating breakwater is influenced by a large number of variables; i.e., shape, mooring type, weight distribution, submergence, skin permeability, and size. This work was set up to define permeability, and size. This work was set up to define the effects of these variables on wave transmission, reflection and dissipation, and mooring-line force. Tests were made over a range of wave heights and periods. periods. JPT P. 269