This paper describes the design and development of a motion-compensated geotechnical drill rig that was tailored specifically for site investigation work in the Canadian Beaufort Sea. Fieldwork was undertaken during the summers of 1981 and 1982 at potential sites for caisson-retained exploration islands (drilling platforms). The approach to site investigation is described, together with the results of laboratory and in situ testing. The drill rig proved to be reliable and the unique motion compensation system permitted down-hole work to be done in a manner that is comparable to onshore methods. The value of using a variety of in situ tools is apparent when on-site decision making is necessary and the test results prove the inadequacy, in certain circumstances, of reliance on laboratory test data. The flat-dilatometer is a relatively new in situ instrument and the results of its first-time application in the Beaufort are encouraging.
The necessity of offshore geotechnical engineering in the Canadian Beaufort Sea has increased in recent years and is likely to continue in the future as commercial production becomes possible. To date, site investigations have been undertaken for temporary exploration structures.
Project work described in this paper was undertaken for Esso Resources Canada Limited in connection with foundation studies for caisson retained islands. A short open-water season, relatively poor weather and costly support facilities place limitations on the site investigation methods and the speed with which site evaluation can be undertaken is of the essence. It is necessary to utilize highly reliable drilling equipment and to resort to in situ test procedures which yield geotechnical information in a minimum amount of time. In this regard, a motion compensated drill rig has been developed and is described, together with an approach to site investigation, the use of in situ test equipment and a brief comparison of test results from various down-hole tools.
Mobile Augers and Research Ltd. (MARL) completed construction of their DGD-2000 rig in a six week period during 1981. It is a rotary top drive model that incorporates the draw works only of a Damco 2000. The power train, control system, mast and mud pumping system were designed and fabricated by MARL. Heave compensation was incorporated from designs by Sea Core Ltd. of the U.K. and they assisted in the fabrication. Heave compensation was necessary to meet the requirements for soil sampling and testing. The system is a tension-leg concept and is described in principle in Figure 2. The drill rig is illustrated in Figure 3 and its technical specifications are described in Table I.
The rig has a depth capacity of at least 600 m and was fitted with Christensen drill pipe and wire-line systems. In principle, the heave compensation consisted of a 10 tonne, cable-supported clump weight at the sea floor connected by cable to a winch at the base of a "ladder" type frame within the drill mast.
