Concept to Design: A Novel Seawater Intake Riser System

Abstract

FLNG cooling process uses large volume of cold seawater drawn from 500–1000 feet below the mean sea level utilizing several vertical risers (20in–30in ID) hanging from the vessel. These risers have requirements very different from conventional risers supported at hang-off and seabed. The design challenges are hang-off loads (in both strength and fatigue), minimum bend radius (MBR) and interference. Hang-off loads can be lowered using low stiffness and light weight risers, but the associated deflections and riser motions will be excessive leading to low MBR and potential interference issues. On the other hand, interference and MBR issues can be addressed by using suffer and heavier weight risers, but this will increase hang-off loads and stresses at the platform base. Weight mass not only affects the dynamics but also provides valuable restoring moment thus helps to reduce static displacement due to current forces. Stiffness affects the bending deflections and helps achieve an acceptable MBR. Based on this, a novel alternative design for seawater intake is to use a single large diameter (60in-70in) steel caisson riser. This paper will briefly outline the preliminary concept verification and concentrate on the engineering of the concept in terms of design optimization, cost/schedule, construction, and installation. Other materials such as High Density Polyethylene Pipe (HDPE), Glass Fiber Reinforced Plastic (GFRP), will be examined for feasibility. FLNG process technology is continuing to develop to be safe, compact, and cost-efficient which also increases the possibilities of developing a number of stranded deep water gas reservoirs around the world. FLNG processes will become more efficient and cost effective with the proposed novel seawater intake system. Cost effectiveness will be evaluated in a qualitative manner.