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Difference between revisions of "Offshore Arctic operations"
Revision as of 17:11, 17 September 2013
Production operations in the offshore artic regions are within the reach of existing technology. Procedures used onshore and offshore in less hostile regions, however, must be modified to meet the challenges of the harsh climatic conditions in the remote locations.
In the last decade, the major area of industry interest has been the offshore region of Alaska and Canada. The environmental conditions vary significantly in each of these regions. Major factors that affect normal offshore operations are:
- Extremely cold temperatures
- Gusty winds
- Short open-water season
- Persistent ice
The specific production system that is selected must be tailored to each unique combination of these factors to ensure safe oilfield development.
Sea ice is the principal environmental factor in all of the offshore artic areas. The most abundant type of sea ice encountered offshore is less than 1 year old. This first-year ice begins to form during fall and grows to a thickness of 4 to 8 ft during the winter. Sheets of ice close to shore become landfast and remain locked in place throughout the winter. Beyond the landfast zone, the ice is kept in constant motion by wind, currents and, in some areas, the influence of the arctic polar pack. This dynamic movement causes shearing impacting between ice features that produce ridges of ice several miles in length. Ice ridges formed in this manner are called pressure ridges. Localized ridging around a grounded ice feature, the shoreline, or a structure is considered a rubble pile. In areas of extremely cold winter temperatures, the ice blocks within a ridge or rubble pile begin to refreeze into a contiguous feature. Depending on the conditions, the refrozen consolidated thickness could become several times larger than the first-year ice thickness.
The other major type of ice is not formed from the seawater but is freshwater ice from the glaciers of northern Canada. In the Arctic Ocean, the glacial fragments are called ice islands. These tabular-shaped features are several thousand feet in diameter and more than 200 ft deep.
Ice usually exerts the predominant forces on arctic offshore structures. Extensive laboratory and field tests have been conducted on small-and large-scale specimens to determine in-situ strength characteristics for design. From the results of these tests, the mechanical properties of ice are predicted. They consider salinity, temperature, crystallographic structure, and loading rate.
When ice is loaded at a very slow strain rate, it exhibits a plastic behavior. Loaded rapidly, it behaves as a brittle material. Empirical equations have been developed that relate the ice movement rate and shape of the structure or indenter to the strain in the ice feature.
The shape of the structure is also a primary factor in producing a crushing, buckling, or flexural failure of the ice feature. For narrow structures relative to the ice thickness, crushing is the predominant failure mode. As the width increases, a combination of crushing and buckling of the ice field around the platform results in the development of a rubble pile. The rubble pile will then shield the structure from direct impact of subsequent ice floes and ensure failure of the ice mantle away from the production facility. Sloping sided structures normally force a flexural ice failure. Because ice flexural strength is 20 to 40% of the crushing strength, an appreciable reduction in ice forces can be achieved when a bending failure is induced.
The wave conditions in the arctic are similar to other offshore areas, and the design of structures against wave loading is well established. Near-shore sea states can be defined by determination of the open-water area along the storm route or fetch and the water depth. In the Arctic Ocean, the presence of the sea ice and the polar pack limits the open-water fetch for storms to generate and consequently reduces the design wave height.
Permafrost is soil at a temperature below 32°F with partially or completely frozen pore water. Drilling and production operations in areas with permafrost have been well defined from experience of the Prudhoe Bay field. In most near shore areas of the artic, permafrost has been found at or near the mudline. These soils are very stiff and can make excavation for pipelines or driving of piling nearly impossible. Permafrost normally is soil bonded by ice and is very susceptible to change in temperature. This can result in significant changes in the soil characteristics and must be considered in the design.
Artificial islands already are used in many shallow-water areas throughout the world for permanent drilling and producing facilities. The islands that are currently being used for drilling in the arctic consist of either unretained or retained beach slope systems, as shown in Fig. 1a and Fig. 1b. Because of the short summer construction season and, in some areas, the lack of island fill material, the quantity of fill required for the island should be minimized. The minimum island working surface is determined by the area required for drilling and production operations. To reduce the quantity of island fill, the steepest side slopes that the mode of construction and fill material will allow should be provided. The minimum side slopes of unretained islands depend on whether the island is constructed by summer dredging or winter transport of onshore borrow material over the ice to the desired location. The side slopes for summer dredging are approximately 1:20 (vertical to horizontal), and for winter construction are 1:3. On completion, sandbags or concrete mats are placed on the exposed slopes of the island to prevent ice and wave erosion. Sandbags, stiffer soils for embankments, or caisson units are used on retained islands during construction to reduce the required volume of fill. The caisson units typically consist of vertical walled concrete or steel units. The caisson also provides easy access to the island as a dock for resupply and could be used for storage of consumables or oil.
Artificial islands must be designed to withstand the horizontal forces exerted by ice. The potential failure modes of the island consist of slope instability, bearing failure, or horizontal shearing of the island near the waterline. Each of these failure modes can be predicted by classic geotechnical analysis. The only variable in the analysis is the properties of the island fill material. During winter construction, the fill is delivered to the site at the cold ambient temperature and dumped into the sea. Ice forms on the granular material and inhibits consolidations. As the island surface thaws, considerable settlement may take place. To minimize the effects of thaw settlements, thermal analysis of freezing and thawing interface should be conducted to determine the proper graduations of fill material.
The design of production facilities placed on an island is similar to that on land. Equipment foundations must be designed and insulated to reduce the potential for frost heaving, pile jacking, and thaw settlements from seasonal thawing and freezing of the island surfaces and, in some areas, subsea permafrost. To prevent thaw settlement, an artificial refrigeration system for the fill material could be installed. Placement of equipment and accommodation modules should account for predominant wind, ice movement, and wave directions to ensure safe year-round operations.
Various types of gravity structures are being proposed for use in the arctic. Many of the conventional gravity structures that are used in the North Sea are being adapted for the deepwater and moderate-ice-concentration areas. In the more hostile areas of the high arctic, vertical- and sloping-sided gravity structures are being proposed. These structures provide the large deck load and space requirements, protection of the wells within tower shafts, and storage of oil. Because of the extreme winter ice conditions in many areas, the production facilities will have to operate nine months without major resupply.
The vertical-sided structures (Fig. 2) are proposed for the shallow, near-shore areas in the arctic. These structures typically are rectangular or hexagonal and are capable of being installed directly on the seabed or subsea berm. Production equipment can be placed directly on the working surface of the top slab or integrated into the hull of the structure. Wells are drilled and produced directly from the deck of the structure. Because of the large width of this concept, the structural integrity of the system is not sensitive to local discontinuities in the seabed from ice gouges or settlements in the foundation from local degradation of permafrost.
Conical, sloping-sided structures (Fig. 3) are being proposed for the deeper-water, dynamic-ice-movement areas. This geometry induces flexural failure of the ice features and is relatively transparent to pack-ice movements. The deck is fully outfitted with processing equipment before it is mated with the structure. Wells are confined to a central moon-pool area in the cylindrical throat. Consumables and oil can be stored in the base.
Piled steel structures have been developed primarily for the Bering Sea area of offshore Alaska. These structures are similar to conventional template or jacket concepts but must be modified to resist annual sheet-ice loading. A typical geometry is shown in Fig. 4. The platform concept consists of four or eight main pile legs with intermediate bracing of the legs omitted in the ice-loading zone near the waterline. Well conductors and oil-transport lines are positioned within the legs of the platform for protection from ice loading. This requires close spacing of the wells and, in some cases, completion of the wells at different levels of the deck. Diver access tubes may also be located in the legs to facilitate the repair and inspection of subsea components of the platform during complete ice coverage.
In most other arctic areas, pile structures are not practical. Subsea permafrost makes pile installation nearly impossible. The short construction season also does not accommodate the installation, pile driving, and placement of the topsides modules in one season. Also, the hookup and commissioning of the production equipment modules would be very expensive in these remote areas.
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