Early in 1979, as part of a study for the National Science Foundation, tests were conducted on the deep ocean working vessel GLOMAR EXPLORER, to determine vessel motion response and stationkeeping characteristics. Wave and motion response spectral densities from these tests are presented along with a comparison of actual significant wave heights and responses compared to those obtained from the spectral densities. A comparison between actual vessel motion response and theoretical motion response as determined by the NSRDC six degree of freedom motion response computer program (HANSEL) and an irregular seas response analysis is also presented.
With regards to stationkeeping, measured wind force resolution ("x" and "y" forces) curves are presented, which indicate the magnitude of data spread. Also a table of typical measured data is presented. In addition, a set of smoothed wind and current force coefficients are presented for the vessel modified to a proposed drillship configuration.
The National Science Foundation has proposed that the dynamically positioned GLOMAR EXPLORER be converted to a deep ocean drilling and coring vessel to be used in the proposed worldwide Ocean Margins Drilling (OMD) Program. Studies have been made by various organizations over the past three years regarding the feasibility of a scientific Ocean Margins Drilling Program in general and the utilization of the GLOMAR EXPLORER as a drilling and coring vessel for the OMD Program in particular.
The results of these studies indicated that it would be feasible to conduct an Ocean Margins Drilling Program utilizing a dynamically positioned drillship. It was also indicated that the GLOMAR EXPLORER would be a suitable vessel for the proposed program, but that the thrust available for dynamic positioning is inadequate for stationkeeping in some of the proposed drilljng sites. See Tables 1 &2 for vessel particulars. 1,2
It was also determined that the vessel motion response characteristics as well as stationkeeping limits would be required to determine the overall OMD Program scheduling.
Full scale motion response and stationkeeping tests were conducted in order to determine the GLOMAR EXPLORER's motion response and stationkeeping characteristics and limits. In January 1979, a program was developed for obtaining and installing additional instrumentation aboard the GLOMAR EXPLORER for carrying out these tests during the forthcoming mining mission to be carried out by Lockheed Ocean Minerals. Instrumentation was installed aboard the vessel to provide time records of wave height, wave period, wind direction relative to the vessel heading, wind speed, pitch, roll, heave, yaw, vessel position relative to the sea floor reference (x, y), compass heading and the RPM and current of all thruster motors. The instrumentation installation was completed, tested and calibrated in February 1979.
Tests and measurements were conducted at a site location in the Pacific (13.44°N Latitude, 125.14°W Longitude) during the time period of February 18, 1979 through March 11, 1979. These tests were carried out on a non-interference basis while the GLOMAR EXPLORER was conducting deep ocean mining tests at this location.
The tests were conducted with the vessel at various headings to the wind and seas. On occasions, the vessel was allowed to turn broadside to the wind and seas in conditions that allowed the vessel to be driven off station, thus establishing absolute stationkeeping limitations.