Cold-Water Pipeline Deployment for Offshore OTEC Power Plants

Abstract

Ocean Thermal Energy Conversion is a relatively new renewable energy technology, aiming at producing electricity by combining deep cool sea water with tropical warm surface water. One of the biggest technical challenge for offshore OTEC plants is represented by the installation and deployment of the cold-water pipe, which is required to bring cold sea water to the power plant at the sea surface. The pipe is approximately 1000-meter-long, with an external diameter between 4 and 10 meter and a free hanging configuration from a floating structure, which results in a complex dynamic behaviour. Large bending stress, high-tension loads, and the potential buckling risk must be reduced to ensure a safe and secure pipeline deployment. Pipelines of this size have never been installed before. This project, carried out at the company Bluerise, has the objectives to investigate how a very large diameter cold water pipe can be installed, and to assess the technical feasibility of the deployment method. This research project provides an initial investigation on how a very large diameter cold-water pipe can be designed and installed for large scale offshore OTEC plants. After identifying the best material and best installation method from a technical and economical point of view, it presents several approaches, both analytical and numerical, that can be used to model the pipeline during deployment, each with its own strengths and weaknesses. Based on the models comparison, this project also provides an optimization design of the chosen float and sink installation method, consisting of the use of counter weight and multi-point liftings. In general, the (non-linear) beam theory represents a relative quick way to estimate the behaviour of the pipeline during deployment, giving the most reliable results among the models considered. In terms of installation capabilities, bending curvature and combined stresses from the base case result in a safe configuration, being lower than the maximum values allowed for HDPE material. From this analysis, a large diameter pipe can be safely installed through the float and sink method. Further research will be required to deeply investigate the installation of the cold-water pipe. In this study the models are judged and compared based on the results from just one static configuration. This analysis should be conducted for every phase of the pipeline deployment. This can be done by considering a range of the pull force values exerted by the tug boat on the pipeline, instead of considering only one value. It is recommended to extend the pipe-lay analysis also to the dynamic part. Waves and current influences on the pipe are not considered in this work. It would be very interesting to execute a model test of the deployment and validate the model with actual experiment. Solving the residual challenges for the cold-water pipe might represent the answer to the question on how to make the development of large scale OTEC plants a reality.