№7 July - August 2011Table of contents Issue Archive
№7 July - August 2011Table of contents Issue Archive
№ 6 (June 2011)
In previous articles we covered the necessity and inevitability of developing the Russian Arctic shelf and the state’s role in implementing such a program. Today the talk will be about the hardware required for implementing upstream projects in the Russian Arctic seas.
By Timofei Krylov
First, consider the ice-resistant oil platforms. In earlier article (OGE 04/2011) the author has already mentioned the only ice-resistant production platform currently available for difficult ice conditions – Hibernia platform. To date, of all existing platforms, only the Hibernia can withstand the really difficult ice conditions and icebergs. Ice conditions similar to those in the Okhotsk Sea on Sakhalin-1, Sakhalin-2 and Sakhalin-3 projects, when the ice thickness is below one or even 1.5 meters, can’t be seen as really difficult. In the case of moving ice mass, such ice can be easily broken by gravity bases (the so-called “legs”) of the platform.
The Hibernia, at the same time, can withstand greater impact of ice mass, including even icebergs. The platform is installed at a depth of 85 meters, its gravity base weighs 450 tons. Four “legs” – four gravity bases of Hibernia – are shackled and protected by the gravitational saw-edged spud can (diameter 106 meters), made of high-tensile prestressed concrete with steel rod reinforcement and prestressed reinforcement elements. External 1.4-meter thick anti-ice wall is star-shaped, with 16 legs used for breaking the coming ice mass. Inner anti-ice wall is 15 meters thick. The platform is designed to withstand the impact of up to 1 million tons iceberg without damage (the probability of meeting such ice mass is estimated as one in 500 years). The platform can withstand up to 6 million tons iceberg, too – but in this case, the platform will sustain repairable damage. The probability of such meeting is estimated as 1 in 10,000 years.
Hibernia platform was built in Canada. However, Russian scientists, in particular designers of Rubin design office, also offer ice-resistant offshore platforms, though of somewhat different design. Hibernia is generally cylindrical, with cylinder height 111 meters and diameter 106 meters; the Russian engineers prefer the conical shape. Based on the experience of ancient developers, the pyramid is the most durable form of construction. Russian platforms use wide and heavy gravity base with walls rising at almost 45 degrees to the sea surface; at the top, on the “peak” above the water, there is a lightweight and durable cribrate metal deck that houses the necessary drilling and production equipment.
In addition to reinforced concrete Hibernia-type structure of ice-resistant oil rigs there are other models, such as steel-based ice-resistant design used in construction of Molikpaq platform offshore Sakhalin and similar Prirazlomnaya platform built by Gazprom in the Pechora Sea. This design is less resistant to the icebergs but is fully capable of withstanding thick old ice. This construction is based on a steel cube, reinforced and weighted by concrete structures with decreased edges at the top and increased edges at the bottom, the base of the platform. For example, Molikpaq base measures 111 x 111 meters, which trims down to 87 x 87 meters at the upper-deck level. Platform substructures weigh 37,520 tons. The platform is filled with 278,000 cubic meters of sand ballast.
At Molikpaq, the produced crude is initially directed into the storage tanker; then the oil is loaded to transport tankers using a floating buoy. In ice period, company stops production on the platform, ships out the storage tanker and temporarily sinks the offloading buoy.
The Prirazlomnaya platform is even bigger than Molikpaq. Overall Prirazlomnaya dimensions are 126 x 126 meters at the spud can bottom. The platform weighs 110,000 tons without solid ballast (506,000 tons with ballast). The spud can volume for ballast filling is almost 160,000 cubic meters. The platform operates 40 bores, with total capacity up to 20,000 cubic meters of production per day. For product shipment the platform uses two shipping units. Depending on the direction of ice drift production can be shipped via the shipping unit located on the opposite to ice side of the platform. In this way the mass of platform substructures decreases ice impact on a tanker being loaded at the platform at a given time.
The mass of the Prirazlomnaya platform is greater than Molikpaq’s, mainly due to more difficult ice conditions in the Pechora Sea. Gravity platforms built in the Laptev, Kara and East Siberian seas are likely to be significantly larger and capital intensive.
Despite the complexity and danger of Arctic ice conditions, in parallel to gravity platforms the companies could use some types of floating production platforms, which is slightly cheaper, compared to the usage of gravity structures. Still, there are some technical difficulties. Of all currently globally available floating production rigs, only SPAR platforms can be used in heavy ice conditions. These are the floating platforms designed so that the body of the rig has much bigger draft compared to rig’s width. Height of the platform body is about 200 meters; water depth at the platform location must be at least several hundred meters. In stormy weather SPAR platform behaves just like a vertically-oriented cork float. Its stability is ensured by a low center of gravity of the platform which is always located below the buoyancy center, and also by the maximum tautness of the mooring that almost staple the platform to the seabed. Maximum tautness of the mooring and relatively small diagonal diameter of the platform ensures high rigidity and independence from the waves, ice and other lateral physical impact. For even higher stability, the underwater part of the SPAR platform equipped with a slanted helical “blades” which cut off the side streams and ice arrays, directing them downward or upward, parallel to the platform body. Yet despite the potential for usage in ice conditions, SPAR type platform cannot withstand the icebergs. To counter the icebergs, this type of platform must house a system for emergency riser disconnection (in the event of a major iceberg hazard) and a system of active positioning and holdup (this will help counter the Arctic storms and quickly return the platform online after emergency stop of the production).
In parallel to expensive and difficult to build offshore oil facilities, Russian Arctic also means quite challenging seismic surveys and exploration work. It is no secret that the entire volume of geophysical data currently held by Russian oil companies about the potential sites and structures in the Arctic dates back almost to the Stalinist era, when the Soviet icebreakers plowed then seemingly endless Arctic seas, running rather primitive geophysical surveys matching then existing levels of science and technology. To date, the results of those studies are open to geologists of the oil companies in the archives of AARI and other government agencies. However, for development of the Arctic shelf more research is needed. Today, scientists can use such methods as 2D and 3D seismic surveys. It should be understood that during processing a prospective site and collecting 2D seismic data, the research vessel carries a seismic cable up to 12 kilometers long. For 3D surveys, the vessel would carry between four and 20 such cables at a time. Conducting such studies under the Arctic conditions, when the seas almost year-round covered with ice, is very difficult. Even if the sea is generally clean of ice, large and small drifting ice floes and ice bars physically impede with the work of seismic ships, creating interference for recording equipment.
To date, the so-called seismic grid, for example, in the North Sea and near the coast of Norway, is about 250 meters, which gives almost full accuracy about the shelf natural resources. For the Russian Arctic seas this indicator runs for a few dozens of kilometers at best; in some cases there is not even a trace of “seismic grid”. In particular, extremely little exploration work has been done in the East Siberian Sea.
Finally, many technical and technological problems of developing the Arctic shelf can be avoided if we use drilling submarines for remote Arctic regions. To date, drilling submarine concept design has been developed by Lazurit engineering design company. Drilling submarine carries a set of drilling equipment and a small supply of consumables required for drilling of one vertical hole up to 4,000 meters deep. Additional consumables can be supplied from the surface by underwater cargo transport shuttle. The submarine is equipped with wellhead blowout equipment set; this ensures sustainable earth-level atmosphere throughout the drilling process in all submarine compartments. Underwater operations are carried out by robots, passenger traffic and rescue work is done by surface-based supply and rescue undersea carriers.
Apart from drilling submarine, another interesting concept is a hoverborne drilling rig. The advantages of such design are high mobility and freedom from the need to physically resist the ice masses at the drilling point. On reaching the drilling point, the hoverborne rig lowers itself directly onto the ice layer drills through the ice. Upon completion of drilling a well is linked via underwater wellhead set to an underwater hydrocarbon collection system, preventing technological structures from rising above the sea level and consequently from being hit by the ice masses.
At small depths of up to 30 meters drilling and production equipment can be placed at man-made grit islands or at an artificial icy frosting. The later structure is a thick layer of ice, enfreezed on the sea surface using water cannons running on the seawater. As the freezing goes on, the ice layer thickness increases. The freezing continues until lower part of the ice layer reaches the seabed and the upper part rises to a safe height above sea surface; subsequently the “ice island” is equipped with drilling and production equipment.
In conclusion, it should be noted that even with the full support of the state and granting oil companies the means for implementing the Arctic shelf development program, shelf development remains extremely complex and knowledge-intensive task from the technical viewpoint. More than one generation of geologists, engineers and technologists will have to work hard to materialize hydrocarbon production at the Russian Arctic shelf.