The most recent wave of oil exploration in Alaska’s Beaufort and Chukchi seas has hardly left the starting block. But the question of how to develop an offshore oil or gas discovery must figure high in any offshore exploration decision. Finding a viable and environmentally sound means of extracting product from a field located under ice-infested water miles offshore could make or break a development project.
In early 2008 the U.S. Minerals Management Service published a 365-page report on Arctic offshore development options. Prepared by IMV Projects Atlantic, the report reviews the technical feasibility of all of the various types of platform, artificial island and subsea completion technologies that might work in the Arctic waters of the U.S. Outer Continental Shelf.
Previous work on this topic dates back to the 1980s and MMS commissioned the study to obtain an up-to-date view of potential offshore technologies, MMS petroleum engineer Kyle Monkelien told Petroleum News July 23, 2008.
“We thought that it would be a good time to go back and see what’s changed, what lessons have been learned from the activities and the construction that has gone on between then and now,” Monkelien said.
A current perspective of feasible technologies will help MMS evaluate potential development scenarios in the OCS, especially in areas where there is oil and gas leasing interest, he said.
However, Monkelien emphasized that were a company to propose a specific offshore development MMS would initiate a rigorous review of whatever technology the company planned to use, including a review under the National Environmental Policy Act.
Past experience
In assessing the various development options, the study researchers reviewed the experience of the Northstar, Oooguruk and PanArctic Drake developments in the Alaska and Canadian Beaufort Sea. The researchers also considered offshore developments off the east coast of Canada, off Sakhalin Island, in the Caspian Sea and in the Barents Sea.
The researchers recognized that the principal factors affecting the design of Arctic offshore facilities include ice loads; the requirement for a platform or island structure to store sufficient consumable supplies for operation in an isolated location; the availability of an adequate foundation or mooring capability; the capability to protect pipelines and subsea equipment; and the practicality of transporting produced hydrocarbons from the offshore site.
And in the Beaufort and Chukchi seas ice loads are an especially important consideration in designing exploration and production structures, the report says. However, research into ice loads has indicated that these loads are likely to be significantly lower than was assessed a few decades ago.
“At the same time, advancements in structural steel research combined with these more realistic ice-load predictions led to improvements in the economic feasibility of Arctic offshore facilities,” the report says.
Gravity-based structures
But massive gravity-based structures sitting on the seafloor remain the most likely option for oil and gas production in the challenging ice conditions of the Beaufort and Chukchi seas. And the study says that in Arctic conditions steel is likely to prove to be a more suitable construction material than concrete.
A variety of structure designs is possible, ranging from massive vertical cylinders to more tapered profiles. The tapered structures would likely have bases hundreds of feet across stepping up into much smaller topsides.
The researchers found that in areas of multi-year ice, water depths of about 250 feet would likely become an upper limit for the technical feasibility of installing these structures, but that limit would go down to 200 feet where the seafloor foundation properties are weak.
“There are no known bottom-founded platform design solutions for water depths greater than 330 feet that could be deemed workable or proven for multi-year ice areas,” the report says.
In more southerly area where multi-year ice is absent, bottom-founded structures in water depths up to 500 feet might be possible.
Jacket platforms
The lighter-weight ice reinforced jacket platforms of the type deployed in the offshore fields of the upper Cook Inlet might be suitable for use in areas of the Bering Sea where there is light first-year ice and water depths are less than 200 feet, the report says.
“(And) developments in jack-up technology and the advancement of ice maintenance programs indicate that the operating range and season of jack-up exploration could potentially be extended in the Bering Sea,” the report says.
Grounded ice islands have also been used successfully for exploration drilling in the nearshore waters of the Beaufort Sea, the report says. Research has found that ice islands might work for drilling in water depths up to 30 feet, or perhaps 40 feet. But unstable or unreliable landfast ice would render ice islands infeasible in the Chukchi Sea, the report says.
The use of gravel islands is another possibility in shallow water.
“Gravel islands have successfully been used in the Beaufort Sea for decades and continue to be viewed as a candidate structure for exploration and/or production in this area of the Alaska OCS,” report says.
However, because no one has ever used a gravel island in the Chukchi Sea, someone would need to investigate issues relating to dynamic sea ice conditions to determine gravel island feasibility in that region. And high waves and large wave loads would be a consideration for gravel island use in the Bering Sea, the report said.
Floating structures
Although various types of floating structure such as drillships and semi-submersible platforms can be used during the summer open water season in the Arctic outer continental shelf, the only region in which a floating structure might stay on location year round might be the Bering Sea in light ice conditions. A semi-rigid floater concept, with a floating platform moored in place under tension, might operate year-round in first-year ice conditions but would need to be able to disconnect to move away in the event of high ice loads, the report said.
“Floating production systems for the Beaufort Sea, Chukchi Sea and North Bering Sea are not considered to be technically feasible, even with continuous ice management,” the report said. “No floating production structures could be economically designed to stay on station with multi-year ice loads in the Beaufort and Chukchi Seas, and possibly northern Bering Sea, depending on ice conditions. Floating systems may have some merit in southern OCS areas, however.”
Subsea completions
Subsea completions, in which wellheads lie at or below the seafloor and are connected to subsea pipelines, are another possibility.
“Improvements in the area of subsea facilities and processing have been made in recent years in the pursuit of resources in harsh and remote environments,” the report says. “As a result of these improvements, fields requiring longer, deeper subsea tiebacks are now becoming much more technically and economically feasible. Gas tiebacks have reached 105 miles and oil tiebacks have reached 40 miles.”
Depending on whether the water depth exceeds the maximum ice keel depths, the well heads may or may not need to be protected in “glory holes” in the seafloor. However, the possibility of an ice keel scouring or hitting the seafloor at the location of a well would pose limitations on what technology could be used.
And there are also some significant technical issues associated with the construction and operation of subsea pipelines in the Arctic. But pipeline designs could accommodate factors such as strudel scour, the settlement of thawing permafrost, upheaval buckling and ice gouging, the report says.
“However, pipeline burial for protection in water depths from approximately 65 to 130 feet will be a challenge given the more severe gouging in these water depths and the fact that the pipeline can likely not be installed from the ice in winter,” the report says.
The report also notes that advance drilling technologies, such as extended reach drilling, present some other possible options for Arctic offshore oil and gas development. BP is in the process of developing the Liberty field in the Beaufort Sea outer continental shelf using extended reach drilling from the Endicott satellite island. (See article this issue.)
