OTEC Project Area Survey

ABSTRACT The Preliminary Design Phase Field Survey of the Oahu OTEC project consisted of precision bathymetric surveying, sub-bottom profiling bottom imaging with the Sea MARC II system And bottom sampling. The survey was design and carried out in close conjunction with the design of the major sea water OTEC pipes and resulted in advances in the design of the large diameter cold water pipe. INTRODUCTION The Ocean Thermal Corporation, a subsidiary of BASIX Corporation, plans to build and operate a commercial scale, shelf-mounted OTEC facility off Kahe Point, Oahu, Hawaii. The preliminary design of the major seawater pipes for this facility is described by Philpot et al [1]. This design work was preceded by a conceptual design phase and a Peliminary Design Field Survey. Information needs related to engineering problems and. data gaps detected during conceptual design were used to plan the field survey. The survey was conducted and the data were analyzed in close conjunction with the engineering requirements of the preliminary design work. OBJECTIVES The Preliminary Design Field Survey had the following objectives:Substantially improve the resolution of project area data concerning bathymetry, bottom features, stratigraphy, mechanical properties of the marine soils, and geohazards;Evaluate the relative merits of three potential Cold Water Pipe (CWP) routes with special emphasis on .the shape of the bottom along a steep submarine escarpment zone;Define the geohazards which are of concern to the CWF; andEstablish a basis for a detailed Final Design Phase Field Survey. BACKGROUND The outline of the project area was defined by practical limitations during the conceptual design phase. Various field data and technical issues existed prior to the Preliminary Field Survey. General Project Area The overall project area extends approximately 1.2 miles to either side (north and south) of the existing Kahe Point conventional power station and 3.5 miles offshore. A narrow shelf extends from the shoreline to the 380 foot depth contour which is approximately 1.1 mile offshore in the center of the project area and 1.6 mile offshore at the northern and southern boundaries of the area. The escarpment zone is an area of very steep slopes between the 400 and 700 foot contours. The area seaward of the escarpment, called the deep offshore zone, is part of a subsea plateau which extends well seaward of the project area. Previous Data Bathymetry, sub-bottom data and other information previously collected in the Kahe Point area were gathered during Phase I (Conceptual Design). Bathymetric data included several US Coast and Geodetic Survey and NOAA/NOS bathymetric: charts a compilation of. bathymetric charts' by Pararas-Carayannis [2] and a bathymetric map reproduced by Normark et al [3] based on a high resolution geophysics survey conducted between 26 and 30 January 1981. These data were supplemented by surveys conduct during the conceptual design' phase.