The Ocean Thermal Energy Conversion (OTEC) power plant in the Lakshadweep islands being established by National Institute of Ocean Technology would be using a large diameter high-density polyethylene (HDPE) pipeline of 4km-long approx. for drawing cold water from the deep sea. Cold water pipelines are crucial components of OTEC plants. These pipelines need to withstand the harsh marine environment and must be corrosion-resistant and thermally insulated to minimize loss of temperature of the deep sea cold water being transported. In shallow water regions, waves and currents cause movement in these pipelines. Therefore, the on-bottom segment is crucial for maintaining the stability of the pipeline. Estimation of wave loads is crucial for to determine the necessary weight on the pipe and reduce movement for stabilizing the pipe. This paper focuses on estimating the weights necessary for resisting lateral forces induced by waves and currents, as per guidelines outlined in DNV RP F109 and analytical methods. The recommendations provided in DNV RP F109 for vertical stability in water and soils, as well as absolute lateral static stability, were used in assessment of pipeline stability against waves and currents. Design parameters such as external diameter, wall thickness, and weight of the pipe, along with environmental conditions, were considered for weight calculations. Analytical methodologies for determining hydrodynamic forces and total weight for the on-bottom section under different environmental conditions, using airy wave theory and shallow water wave theories, are presented. The drag, lift and buoyant forces have been calculated to estimate the hydrodynamic forces acting on the pipeline. This comprehensive approach contributes to understand weight estimation techniques for pipeline design and on-bottom stability assessment.
Ocean Thermal Energy Conversion (OTEC) uses the temperature gradient in the ocean to produce electricity, utilizing the heat from sun-warmed surface waters and the cold from deep seawater, typically found around 1000 meters deep. The temperature differential necessary for efficient OTEC operations ranges between 20°C to 22°C. Leveraging this thermal gradient, cold seawater extracted from depths of 1000 meters is used in shore-based Low Temperature Thermal Desalination (LTTD) plants with inverted catenary pipelines off Lakshadweep Islands. These pipelines are designed to withstand environmental forces like currents, waves, and seabed conditions, ensuring their structural integrity, stability and operational safety over their service life. Research by Rognoni et al. (2008), Jyothi Prakash et al. (2014), and Ajeesh et al. (2015) provides analyses of the inverted catenary pipeline systems used in LTTD plants. Additionally, Ameh SE and Ameh NI (2019) discusses the fundamental aspects of on-bottom stability design for submarine pipelines. Daniel (1989), McHale (1979), and Lewis et al. (1988) discuss the cold water pipeline for the shore-based OTEC plant in Hawaii and various segments in the pipeline. Gref Rocheleau and Dale Jensen (2015) investigated into finite element modeling, design considerations, and repair strategies for large-diameter deep-sea cold water pipelines operating at depths of 670 meters.
