Installation Method Analysis for a Large Diameter Cold Water Pipe for Land - Based OTEC Plants

Ocean thermal energy conversion (OTEC) is a technology which uses the temperature difference between cold deep water and warmer surface water in the ocean to generate electricity. This technology is most applicable in remote tropical locations, as it can produce a continuous year-round base load that supplies the remote region with reliable and sustainable energy.

One of the main challenges of large scale OTEC plants is the installation of the cold-water pipe (CWP), which is used to pump the cold water from the depths of the ocean to the plant. Bluerise is currently developing 10 MW land-based OTEC plants. For these plants, the CWP has an internal diameter of 2.5 meters and is installed over the seabed up to a water depth of 1000 meters . A CWP of these dimensions has never been installed yet and thus the optimal installation method is unknown. The main aim of this graduation was to find the optimal installation method and the preferred material for these large cold-water pipes.

First, research was conducted to evaluate several materials, assessing similarities and experience in other industries and the specific requirements for OTEC. In parallel, several existing and new installation methods were investigated and discussed. In order to compare these materials and methods, a base case project was used that Bluerise is currently developing in Curacao for which parameters such as seabed slope and water density are known. The different options for materials and installation methods were evaluated using a multi-criteria analysis taking into account non-technical aspects such as risks and costs.

Based on this analysis, HDPE was identified as the preferred material. Regarding the installation, it was found that the “modified Float and Sink” and the “Hold and Sink”-methods were optimal, both being similarly ranked. As the details of these installation methods are not yet known in practice, it is not yet clear if the CWP can be successfully installed using either of these methods - thus an installation model was developed for the CWP based on Euler–Bernoulli beam theory. This model simulates the pipe installation and describes the pipe in the vertical direction, as depicted in the figure below. In the model, forcing or loading (such as buoys and ballast) can be variably applied to the CWP thereby allowing for the simulation of any possible installation method.

In addition to this model, scale model tests were done at MARIN in Wageningen to investigate the practical aspects of the CWP installation, as well as to collect data for the validation of the Matlab model. As the computer model was validated, it was found that 3-dimensional effects, observed during the scale model tests, had a large influence on the drag and displacement of the pipe. The numerical model consistently overestimates the velocities seen during the tests at MARIN, however trend-wise the numerical model gives similar results when simulating different installation methods for the pipe.

As the main issue for the pipe is the bending, the key challenge is to minimize bending during the installation. Taking this into account, it was found that the best installation method was a combination of the “Float and Sink” and the “Hold and Sink”-method, as the shape and position of the pipe can meticulously be controlled by a combination of buoys (attached to the pipe) and vessels (manipulating the pipe end). It was found that with this combined installation method, the total Von Mises stress is kept well under the limit for HDPE. Nevertheless, it is recommended that these results are further verified using a more detailed model that takes 3-dimensional effects into account so that the response of the cold-water pipe can be calculated more accurately.