Dynamics of large diameter riser

Abstract

Structural analysis and material selection for an intake riser pipe with a large diameter in an artificial upwelling system are performed. The most suitable material of a riser pipe for intake of a large amount of deep seawater is selected through the consideration and estimation for a commercial pipe with a larger diameter than 1m in minimum. From an evaluation of pipe materials, a GFRP pipe is appraised to be the best for a riser pipe with a large diameter. An eigenvalue and structural analyses for a GFRP pipe with the numerical method are carried out to understand the characteristic of the dynamic behaviour. From the modal analysis the basic behaviour property of two types of the riser structures is investigated. For a low density and large diameter riser, such as HDPE pipe, with a buffer, it is found that the total added mass of a riser is so large that a buffer attached at the low end of a riser does not affect the behaviour of a riser. The basic dynamic behaviour properties of a GFRP pipe are found to be excellent in the eigenvalue analysis as 1st and 2 n natural periods for a GFRP pipe deviate from the high energy domain of wave. The results show in the static analysis that it should be increased a weight or modified the design of GFRP pipe to maintain a limit water depth of its end of a GFRP pipe under the sea condition of faster than 1.5m/s of the current speed. The combined stress distributed along the riser pipe under both the wave force and top excitation by upper structure is the largest in the condition of 15 second of wave period. But the maximum stress along the riser pipe is found to be smaller than the allowable stress of a GFRP pipe. Therefore, GFRP pipe can be recommended to the candidate material to develop a large diameter riser pipe. ？ 2010 by The International Society of Offshore and Polar Engineers (ISOPE).