Crack Propagation Analysis in Underwater Laser Drilling

Abstract

In recent years, a state-of-the-art method of rock drilling has been developed by Japan Drilling Co., LTD. (JDC) and Japan Oil, Gas and Metals National Corporation (JOGMEC). In that method, a pulsed laser of the wavelength that is absorbed by water is used. Bubbles are generated in the water by the absorbed energy. The rock surface is irradiated by the laser beam through the bubbles. It was reported by the experiments under atmospheric pressure that there are two processing types in this method: melting and spallation. In the former, rock is partially melted and removed by a water stream caused by the bubble formation or the bubble extinction. In the latter, rock is spalled by thermal stress and removed in the same way as in the former case. In this study, the processing mechanism in the spallation type was clarified first by the thermal stress analysis based on fracture mechanics. Then, the possibility of laser drilling at the sea bottom which is several thousand meters from the sea level was also investigated, because the experiments are very difficult. As a result, the following conclusions were obtained: In the tensile stress field of the rock inside caused by laser irradiation, the rock is spalled by crack propagation in mode I. In the compression field generated in the neighborhood of the rock surface, a rock is spalled by crack propagation in mode II. These spallations occur also under the condition of high pressure that the bottom of the sea was assumed. Contribution of crack propagation in mode II to the spallation increases as the depth becomes large.KeywordsUnderwater laser drillingCrack propagationSpallationGraniteThermal stressFracture mechanics