大水深ライザーの応答挙動と設計に関する考察

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Research and development of deepwater risers for scientific drilling and offshore oil development are now under way in Japan. Depending on the purpose of riser, target water depth varies from 2000 m of offshore oil development to 4000 m of scientific drilling. Water depth of 4000 m is far deeper than the world record of deepwater drilling with riser, and the technology to be developed in the research and development will be one of the most advanced one. This technology will be also applicable for other fields such as sequestration system of carbon dioxide into deepwater and so on. This paper discusses overview of dynamic elastic response of a deepwater riser and design of the response. It becomes clear that a lighter and more rigid riser is generally desirable for deepwater from a viewpoint of elastic response. It also becomes clear that the distribution of buoyancy material must be designed so that the initial tension distribution is optimized and compressive axial force becomes hard to excite under dynamic tension fluctuation in hung-off condition. Two types of coupled response of riser, riser-mud coupled response and longitudinal-lateral coupled response of riser, which might have significant effects on the design and operation of a deepwater riser are chosen and examined. From a series of calculation, it is understood that dynamic response of mud has significant effects on the dynamic tension induced in the riser. Natural frequency of mud column is around 12 seconds for 4000 m riser case and a sharp peak is observed in the frequency response of riser tension. The peak height is still significant even when highly viscous mud is used. In a hung-off condition LMRP will need to be opened under severe weather condition. It is shown that the lateral response, so called parametric oscillation induced by tension fluctuation under longitudinal vibration, will not be a serious problems for real 4000 m riser case. Deepwater risers are designed so that the compressive axial force will not be induced even in hung-off condition. Under this condition, some amount of lateral drag force which is usually expected for normal configuration of riser will suppress the lateral response. But it is also shown that if a large longitudinal response is applied or lateral drag is very small, lateral large response can be induced.

大水深ライザーの応答挙動と設計に関する考察

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