Concept design of a foldable Cold Water pipe for a 10 MW OTEC installation

Abstract

Ocean Thermal Energy Conversion (OTEC) has been heavily researched and developed over the last decade. However, no commercial scale for OTEC has been reached yet. OTEC is a clean and renewable source of energy which utilizes the seawater temperature difference between the upper and deep ocean layers to generate electricity. The heat exchangers and the cold water pipe design are considered to be the most challenging part. The cold water pipe is often stated to be the largest risk. The installation of the cold water pipe, the high investment costs of the cold water pipe and the large forces acting upon the cold water pipe are crucial factors for the future success of OTEC. In this thesis, a new kind of cold water pipe concept is introduced. The FLEX-hose concept consists of a thin sheet of nylon, supported by concrete rings and polyester cables. The total length of the foldable concept design is 1000 m. The nylon sheet is attached to the concrete rings and the cables are attached to the concrete rings. The cables run through the concrete rings and can be used during installation to lower the pipe. Deployment of the foldable pipe is expected to be relatively easy thereby lowering the installation costs. The Multi Criteria Analysis performed in this thesis show the FLEX-hose foldable concept to be attractive compared to a foldable spiral concept, a flexible composite concept, a rigid composite pipe and a HDPE pipe. However, the inclination angle occurring while pumping may not become significant, meaning above 7.5 degrees, since this will lead to lower energy efficiency. In addition, no current design is available on the market, meaning heavy research and development is needed for the foldable concept to succeed. Possible challenges for the FLEX-hose concept that have been established through this research include instability occurring due to buckling and a pressure drop becoming larger than 0.6 bar, thereby significantly reducing the net power of the OTEC plant. The maximum allowed angle of inclination of the sheet to stay below 0.6 bar is calculated to be 7.5 degrees at a flow speed of 2 m/s. During pumping a discharge of 20.000 kg/s is pumped through a thin walled pipe with a diameter of 3.5 m, which leads to a radial displacement of the nylon sheet. Collapse and too significant deformations of the thin walled sheet are prevented by applying 10 MN of pre-tension in the thin walled sheet. However, in order for the nylon sheet to stay in the linear elastic regime, the calculated wall thickness is 0.04 m, thereby making foldability questionable. A total of 200 concrete rings is present in the design, one every 5 m over the total length of 1000 m of the pipe. In addition, a large clump weight of 12 MN is needed at the bottom of the pipe. The total system weight of the foldable pipe with concrete rings is calculated to be 1840 mT, which could be installed by an offshore installation vessel. Finally, creep has also been established to be a challenge due to the life-time of 30 years for the cold water pipe. A synthetic plastic material such as nylon experiences relaxation of the material when stressed for a longer period of time, leading to large deformations.