Hydrodynamic loading of slope-mounted OTEC pipelines: A review of offshore design experience and model test data requirements

Abstract

Proposed slope-mounted pipelines for OTEC proof-of-concept experiments have much larger diameters than the pipelines currently being used in the offshore oil and gas industry. Test data taken from the ocean engineering literature on subsea hydrocarbon pipeline design are thus not directly applicable to these large-diameter OTEC cold water and discharge pipes. Furthermore, the proposed location of these pipes (on or above a steeply sloping seafloor having highly irregular bottom topography) represents a very different hydrodynamic environment than is usually found on the sediment-covered shelves where hydrocarbon production and loading facilities are located. Some experience with this aspect of the problem has been gained by land based OTEC demonstration plants (past French and present Japanese), coastal mariculture research facilities, and the Transmediterranean Pipeline project. Again, however, the pipelines associated with these projects are also relatively small in diameter. Test data taken in support of the design of ocean outfalls and power plant cooling systems may be more directly applicable, due to the large pipeline diameters involved. These pipes are generally installed in shallow shelf waters, where wave and current loads can be avoided by trenching the line or ballasting it with armor stone. Hydrodynamic loading estimates are therefore made to determine the degree to which these lines must be buried, and only a limited amount of data exists for exposed and off-bottom configurations. These data, however, do not appear to be directly applicable to steeply sloping, irregular seafloors. A review of the various types of pipeline projects mentioned above indicates that although applicable analytical methods do exist for estimating pipeline hydrodynamic loads, at-sea test data are required to validate these methods for large-diameter cold water and discharge pipes in typical OTEC offshore environments. A test program is thus outlined, specifying the types of data required for preliminary design of these pipeline systems.