Model Testing of OTEC Plant Platform and Cold Water

Abstract

INTRODUCTION In 1975 the Department of Energy (then ERDA) initiated a significant effort to develop ocean thermal energy conversion (OTEC) as a commercial energy source. Significant technical effort has since been devoted to investigation and solution of technical problems associated with OTEC. Certain areas, such as heat transfer, biofouling of heat exchangers and structural loading of the cold water pipe (CWP), have emerged as key OTEC technical problem areas. The design of OTEC plant CWPs which can operate without failure for 30 to 40 years is a difficult problem, particularly when the severe environmental conditions which can occur at most proposed OTEC sites (Puerto Rico, the U.S. Gulf of Mexico, Hawaii, etc.) are considered. Over the past four years a number of analytical methods have been developed for analyzing CWP dynamic and static loads. These methods have been used as a primary basis for most of the CWP designs and studies carried out to date. The more widely used methods are based on different methods of approach and assumptions, and hence are of different potential accuracy for different CWP designs. It is essential that analytical methods be carefully validated if they are to be used with confidence to design CWP's. Scale model test data provide the best basis for validation, and validation efforts to date have relied primarily on data from laboratory and at-sea model tests. This paper discusses model testing of OTEC plant platform/CWP configurations in waves, with emphasis on simularity or scaling requirements and two sets of laboratory tests carried out at HYDRONAUTICS Incorporated. The use of the test data for validation is also considered. SCALING REQUIREMENTS FOR OTEC MODELS Three types of scaling are applicable to seakeeping or other model tests of OTEC plant platforms and CWP's. These are: dynamic scaling; hydrodynamic scaling, particularly of the CWP; and structural or elastic scaling of the CWP. The first two types are considered in most ocean engineering testing, while the latter type is typically not considered. This section considers scaling or similarity cosiderations for OTEC models with emphasis on CWP elastic or hydroe1astic scaling. A more detailed discussion of scaling is given by Barr and Johnsonl DYNAMIC SCALING For any test in ocean waves, which is the environment of greatest interest for OTEC plants, it is essential to observe Froude scaling, in which the Froude number, F, is identical in model and prototype: (available in full paper) where V is a characteristics velocity, l is a character istic length and g is gravitation acceleration. This scaling makes acceleration invariant between model and prototype and requires that (Available in full paper) where the subscripts m and p denote model and proto type, respectively, and ? is the linear scale ratio of prototype to model size. An important implication of Equation [l] is that: (Available in full paper)