Numerical Modelling of a Large Diameter Cold Water Pipe Installation for Land Based OTEC and SWAC

Abstract

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.