A Study on Suction Breaker and Scouring of a Submersible Offshore Structure

Abstract This report shall state about the following two studies.A study on suction and suction breaker of mobil offshore structure when it is about to get free from soft cohensive sea bed.A study on sand scouring of the submersible offshore structure caused by tidal current or wave while the structure is operating on sandy sea bottom. The former is made for the purpose of performing the floating operation safely and efficiently. The latter is made for the purpose of preventing accidents caused by scouring in sit-on-bottom position. The results are as follows:On Suction and Suction Breaker:Suction mechanism became clear and calculation of subtion force in case of no breaker became possible.We could obtain quantitative and qualitative conclusion concerning arrangement, flow rate and pressure of suction breaker.In case of using suction breaker the spud bottom is desirable to be LMJ shaped or W shaped.On ScouringIt was observed that astringency is involved in the scouring.The process and condition of scouring became clear When the unit is objected to wave, current and both of them at the same time.Flat bottom is disadvantageous against scouring, and the skirt is effective against scouring below the bottom.The hemisphere spud with skirt is most advantageous against scouring. Introduction When the submersible offshore structure sits on the sea bed soft cohesive soil, it penetrates deep into the soil; therefore when freeing bottom it causes a large suction force which not only makes floating operation difficult, but also causes many accidents such as capsize arid wreackage. And in case of on sand, foundation the. foundation may sometimes be scoured by tidal current or wave which may cause the structure to turn over, or may cause a great local stress upon the structural materials. What this study aims at is to study character of suction and scouring by means of offshore structure experimental tank, to develop efficient suction breakers, and to examine the preventive method of scouring. And as to section breaker, field tests have been given on a large model in the sea for practical applications. Suction Breaker Test Laboratory Model Test Models and Testing Soil In order to study the character of suction, and to compare the effectiveness of suction breaker, we have designed and manufactured 5 kinds of small models shown in Fig. 1, which has a bottom area of 2.69 ft2, made of steel, and coated with paint. As the testing soil, the sea silt in Hiroshima Bay was. reconsolidated and applied.