The design and operation of a geotechnical stability monitoring system for a steel drilling caisson is presented. A methodology is developed whereby this system is integrated into the safe operation of the structure, and the long term development of design criteria.
Details are provided on the design of instrument grids which measure contact stresses, berm deformations, and structural deformation in response to environmental loads.
The performance of these grids is evaluated through a review of behavior under wave, ice and explosive loading.
Since 1980, Canadian Marine Drilling Limited (CANMAR), the contracting arm of Dome Petroleum Limited (Dome), has designed, constructed, and operated three hybrid drilling islands in the severe Arctic environment of the Canadian Beaufort Sea. These units were all installed on berms in water depths greater than 20 meters, and included the multiple concrete caisson and berm system of Tarsiut, and the single steel drilling caisson and berm system at Uviluk and Kogyuk (Figure 1). These structures were constructed in water depths of 21, 31 and 28.5 meters respectively.
The concept of an artificial waterline penetration system resting on a steeply-sided submarine berm is integral to both of the CANMAR hybrid systems. The Tarsiut topside unit incorporated a multiple, concrete caisson system (MCS) (Weaver-and Berzins, 1983) while That deployed at Uviluk is a monolithic, ice reinforced, steel structure created by modifying the forward section of a very large crude carrier. The steel structure has been acronymic ally termed the SSDC, for Single Steel Drilling Caisson. Both caisson approaches result in reduced fill volumes and a minimize the effects of wave and current erosion. The SSDC is an improvement over the MCS with regard to mobility, freeboard, and permanent installation of the rig package.
The prime purpose of each of these structures is to permit hydrocarbon exploration during the previously unexploitable Arctic winters and thereby extend the drilling season beyond the three to four month period of a conventional drillship-based operation. The management of Dome cited the research and experience value of such projects as a close secondary importance to the above because of:
the limited data base for these environs, and
the ultimate desire to construct suitable offshore production facilities in this region.
In order to ensure safe drilling operation and obtain valuable research data, an interactive stability alert and monitoring system was designed and maintained for each structure.
This paper documents the equipment, method of operation and general response of the geotechnical instrumentation package of the SSDC system at Uviluk and Kogyuk.
The Koqyuk N-67 site lies approximately 75 km north of Tuktoyaktuk in the 29 meter water depths of the Canadian Beaufort Sea.
