In order to achieve overall cost effectiveness, all FLNG (Floating Liquid Natural Gas) projects must use large volume of deep sea cold water for their cooling process. This has prompted FLNG projects hanging several risers (about 20 to 30 inch diameter) vertically straight down from the platform to a depth between 500 to 1000 feet.
Static and dynamic motion analyses of these risers reveal conflicting design requirements. Low stiffness and light weight help to decrease stress and moment reaction on the FLNG vessel and riser, while associated large motions cause problem with interference between risers. High stiffness and heavier weight help to increase minimum bend radius (MBR) and decrease the riser motions and interference. Weight mass affects the dynamics but provides valuable restoring moment thus reduces static displacement due to current forces. Present work describes details of a concept development study, based on the pertinent design parameters, static and dynamic tuning of riser connection, and the development of an innovative solution.
This study has demonstrated that simplest alternative solution is a single caisson type riser employing available large diameter (60 to 70 inch) pipes with different options of connections at the platform. Other potential materials of choice can be Glass Fiber Reinforced Plastic (GFRP), High Density Polyethylene Pipe (HDPE), and high strength steel, or a hybrid combination of steel with titanium or aluminum. Various options of constructions and installations have also been examined.