Installation of a Large Diameter Cold Water Pipeline for a 3MW Onshore Based OTEC Plant

Abstract

Ocean thermal energy conversion (OTEC), is a renewable energy resource that uses the thermal gradient of the ocean to generate electricity. Warm surface water and cold deep sea water, which can be found at depths of approximately 1000 [m], are used to generate electricity in a thermodynamic Rankine cycle. Due to its dimensions, the installation of the cold water pipeline is one of the most challenging aspects of an OTEC plant. Allseas Engineering B.V. is planning to install a 3 [MW] onshore based OTEC plant on Bonaire. For this OTEC plant, a cold water pipeline with an outer diameter of 2.25 [m] is required that pumps up water from 950 [m] water depth. High density polyethylene (HDPE) is used as the material for the pipeline. HDPE is buoyant and therefore requires additional downwards force to be installed below the sea surface. Two installation methods are considered for this cold water pipeline: the ‘hold and sink installation method’ and the ‘pull down installation method’. A numerical non-linear Euler Bernoulli beam model is used to optimize both installation methods. The Von Mises stress criterion is used to assess whether the structural integrity is maintained during the installation.
The hold and sink pipeline is ballasted using concrete ballast weights to provide the necessary downwards force. The seabed stability criterion is used to determine the required amount of ballast. Hold points are attached along the length of the pipeline that provide a vertical upwards force to control the sinking velocity. A pull force is applied at the offshore end to reduce pipeline bending stress and to reduce the lateral deflection that results from the sea current. Using the hold and sink installation method, the pipeline can be successfully installed without exceeding the design stress.

The pull down installation pipeline is divided into two sections: a ballasted section of 450 [m] and the remainder of the pipeline that remains afloat. A concrete ballast weight is installed at the seabed, at the final position of the offshore end of the pipeline. The ballasted section is installed using an installation method that is frequently used for HDPE pipelines called the float and sink method. A chain is connected to the offshore end of the pipeline. The chain is connected to a pull cable that runs through the anchor box at the seabed to a crane vessel at the sea surface. The unballasted section is then pulled down to the anchorbox, where the chain is secured in the anchorbox and the pipeline remains in a reversed catenary shape during its operational life. In the transition zone where the transition between the ballasted section and the floating section of the pipeline occurs, the Von Mises stress exceeds the design stress. The Von Mises stress results primarily from pipeline bending, therefore additional bending stiffness is applied in the transition zone. The maximum pull force is limited to the weight of the anchorbox. The required pull force to install the pipeline exceeds the allowed pull force. Additional ballast weights are attached to the free span of the pipeline to reduce the required pull force. The pipeline can be installed without exceeding the design stress when the bending stiffness in the transition zone is increased and the required pull force is reduced.
A preliminary multi-criteria analysis is conducted as an initial comparison between the two installation methods. From this analysis no obvious preferred installation method can be selected. A recommendation is made to expand this preliminary multi-criteria analysis and include a detailed cost estimation of the installation methods. Furthermore, it is recommended to include a detailed operational lifetime analysis on the structural integrity of the pipeline for both installation methods.