Offshore Canada Research Articles

Geotechnical Sampling of the Sea Floor

Abstract Introduction A part of exploration for petroleum resources is the development of a regional geological model of a prospect area. Such a model requires ground truthing of geophysical data through sampling of bedrock sequences. This ground truthing is provided by borehole drilling and, in areas where the bedrock closely approaches the seafloor, can be achieved cheaply and effectively with remotely operated seafloor drilling systems. Development of offshore petroleum plays requires a knowledge of the engineering properties of the seafloor sediments at the location of the potential development. Offshore Canada, the need for detailed geotechnical knowledge of the seafloor sediments is increased by the requirement to provide protection for pipelines and production systems from impact of ice and icebergs. Information is required for foundation studies, excavation/trenching criteria and the more general consideration of effective loading on buried systems resulting from iceberg impact on the overlying seabed. Generally, geotechnical information of this kind has been obtained from borehole drilling and associated in-situ geotechnical measurements. In the offshore, this is expensive if carried out from a surface drilling vessel, particularly in water depths greater than 150 m, where the use of a dynamically positioned drill vessel becomes almost essential. As a result, increased emphasis has recently been placed on the refinement of geophysical tools and the development of remotely operated portable seafloor- mounted sampling and measurement systems for cost-effective preliminary assessment of extended areas. Acoustic "Sampling" Considerable work has been carried out on the relationship between acoustic velocity and impedance and other physical properties of marine sediments (e.g., Laughton, 1954; Hamilton et al., 1956; Hamilton, 1970; Sutton et al., 1957; Morris et al., 1978). This work was originally aimed at accurately predicting sound velocity in seafloor sediments from a knowledge of average grain size and sediment type. In recent years, the emphasis has' been on the remote measurement of acoustic properties for the estimation of the physical properties of the sediments (e.g., Li and Taylor Smith, 1969; Lee et al., 1977; Maclssac and Dunsiger, 1979). (figure in full paper) acoustic impedance mismatches resulting from changes in the physical properties of the sediment within a sedimentary sequence to map stratigraphic boundaries. Measurement of the reflected energy enables the acoustic impedance of the sedimentary interface to be calculated and thus changes in the physical properties of the sediments to be predicted. Advances in the design and manufacture of high-resolution marine seismic systems have led to the development of systems with well-defined and repeatable broadband output pulses. Sound sources such as the Huntec Deep Two Seismic System have enabled quantitative measurements of acoustic reflectivity and online graphic display of reflectivity indices to be included as part of the standard survey data acquisition (Parrott el al., 1979, 1980). The reflectivity data, coupled with the acoustic character of the seismic record has proved effective in defining sediments type and mapping the boundaries of geological units on the seafloor.

Offshore Drilling in Hostile Environments: Depth, Waves, Wind, Current, and Ice

Introduction The petroleum industry over the past 100 years has moved from past 100 years has moved from drilling offshore from fixed platforms extended from the platforms extended from the shoreline to floating drilling far offshore in water depths greater than 4,900 ft. The environment offshore has been more challenging and rewarding than man's trips to the moon. Offshore drilling has been challenged by water depth, currents, tides, waves, wind, icebergs, pack ice, ice floes, and distances. Not all places in the world have these environments in a hostile condition. Only those areas of hostile environmental conditions and their relationships with floating drilling will be discussed.Technology and equipment needed to drill a hole have been perfected for inland operations for perfected for inland operations for the past 70 years. The same technology and equipment for the past 25 years have been extended past 25 years have been extended offshore - first on small platforms, then on barges, jackups, platforms, then on barges, jackups, mobile platforms, floating ships, and semisubmersibles.Fig. 1 illustrates examples of hostile environments the petroleum industry has confronted. These include pack ice in the arctic; moving ice in Alaska's Cook Inlet; icebergs in Greenland; hurricane storm waves of 70 ft in Ireland; deep water to 4,900 ft offshore Canada; currents of 4 knots offshore Thailand; moving ocean floor sediments in South America; tides of 30 ft offshore Canada; and winds of 100 knots in the North Sea. Take as a specific example, the hostile environment of water depth. Deepwater Environment General The trend in offshore exploration clearly shows the industry is moving to deeper water environments to drill and explore for oil and gas (Fig. 2). Records show it took the industry 20 years to move floating drilling from 100-ft water depths to 1,300 ft. Yet in the past 8 years we have extended the water depth an additional 3,100 ft, from 1,300 to 4,400 ft. We now have drilled approximately 200 exploration wells in water depths of 600 to 4,400 ft. Operations are currently under way to drill a well in 4,900 ft of water. The future indicates we will be drilling in 8,000-ft water depths in the early 1980's.As water depths increased, floating drilling became necessary. This required three major technological extensions of know-how and innovation:motion; the rig no longer has bottom support;mooring; the floating unit must be held against the environment; andsubsea; the blowout preventer (BOP) is now below water. During 1953-70, the oil industry was developing the technology and equipment to work from floating vessels while drilling exploration wells. All of the above major technological extensions required many hours of engineering, new types of equipment, and years of operating experience to achieve confidence that exploration from floating drilling was safe and practical. These advances are now practical. These advances are now routine:motion we continue drilling operations in 40-ft waves and 60-knot winds;mooring we drill successfully in deepwater depths and stay on location in 92-ft waves and 100-knot winds; andsubsea safe, remote operations, subsea BOP control, etc., have been routine on the approximately 4,000 wells drilled offshore with subsea equipment. Offshore floating exploration drilling experience has moved from 200-ft water in 1958 to 4,900 ft in 1979. Our capability to drill in water depths has increased at a steady rate, and presently industry has the equipment and technology to drill in 8,000 ft of water. Extension of Technology for Deepwater Drilling The extensions of technology necessary to move floating drilling from 600- to 8,000-ft water depths are:mooring development of dynamic stationing to replace mooring systems;reentry development of acoustic/TV reentry to replace guideline systems;BOP controls development of electrohydraulic controls to replace hydraulic operations that were too slow; andriser development of couplings that can withstand high tensions and fatigue equivalent to pipe material and development of pipe material and development of reliable buoyancy materials to reduce riser tension requirements. Experience in Deep Water Dynamic Stationing Dynamic stationing utilizes thrust provided by propulsion units provided by propulsion units (thrusters) to hold a vessel on location and drill in water depths from 600 to 8,000 ft. JPT P. 111