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.

For remote islands with high energy prices in the tropics, Ocean Thermal Energy Conversion (OTEC) can be a reliable energy source to supply a renewable baseload for the energy grid. Also, due to the increasing cooling demand, there is a need for sustainable cooling around the equator. This can be supplied by seawater air-conditioning (SWAC). For the cold water supply of such projects, there is a cold water pipe required that could reach to a depth of up to 1000m. The cold water pipe material that is investi¬gated in this thesis is high-density polyethylene (HDPE). The installation of these large diameter cold water pipes is challenging and requires careful attention, as it is one of the most expensive components of an ocean thermal energy project. A dynamic 3D geometrically non-linear Euler-Bernoulli model is developed that allows for large deformations. In this way, the lowering procedure, including lateral current actions on the pipe can be modelled in order to check the structural integrity of the pipe during installation. The model is validated by comparing it to scale model tests in MARIN, which results in a good comparison between the numerical model and scale model tests. Additionally, the model is compared to a geometrically non-linear Timoshenko beam model to estimate whether shear deformation plays a role for large diameter HDPE pipes. Both models showed a good comparison and the for the bending radii that are of interest, shear deformation is negligible. Making use of the numerical Euler-Bernoulli model, a case study is performed for a seawater air-conditioning project in Curaçao. Different installation methods are compared, where the subsurface cur¬rent velocity is an important parameter. This velocity determines the required amount of ballast of the largest section of the pipeline. For high current velocities, the ballast is high, such that there are multiple holding points required along the pipe in order to make it sink in one piece without exceeding the design stress of HDPE. Ballast can be applied by means of concrete blocks along the pipeline. The pipe can be controlled during installation by either buoyancy modules or vessels with tug lines. Another option is to reduce the specific gravity of the pipe and to apply post ballasting by means of rock dumping. The best solution de¬pends on site specific conditions, where detailed current velocity profiles are desired to choose the most cost effective solution. During installation, the currents will have an impact on the lateral deflection of the pipe, which can lead up to a deflection of several hundreds of meters if no measures are taken. Several vessels are required along the pipeline during installation to make sure the pipe is installed on the planned trajectory.