The arctic spar is essentially a classic spar with an ice breaking cone at the water line. (Images courtesy of FloaTEC)

Recent studies, including one by the US Geological Survey, suggest that regions in the Arctic hold upwards of 25% of all known remaining hydrocarbon reserves. Some studies suggest that as much as 400 billion bbl of oil are there to be recovered.

Arctic developments have to contend not only with wind, waves, and currents similar to those in the Gulf of Mexico, but also with long distances from infrastructure and frigid temperatures. In this climate, normally accepted design practices are challenged. Common steel materials can fail at low temperatures, which means construction materials are a concern. And wind-driven freezing spray from waves and green water ingress cause ice accretion on topsides and other exposed structures. Facilities and equipment must be winterized, which usually requires enclosures and other forms of climate control.

But by far the biggest challenge is ice.

Defining the threat

Ice is generally classified as sea ice, which is frozen seawater, or glacial ice, which is ice that forms on land and is calved from glaciers and ice shelves in the Arctic and Antarctic. Both types of ice have complex material properties. The strength of sea ice is dependent on a number of factors such as grain size, temperature, porosity, brine content, the rate at which it is loaded (strain rate), and its failure mode. The age of sea ice is also a factor affecting its strength. Over time, brine leaches from sea ice, which increases its strength. Multi-year ice, sea ice that has survived multiple years, normally exhibits higher strength properties than first-year ice.

Ice sheets are eventually broken up into pieces called floes that are driven by wind and currents. Ice floes can be pushed together to form compression ridges or shear ridges, depending on the interaction between the floes.

Glacial ice in icebergs is typically stronger than sea ice. Glacial ice properties are dependent on parameters, that include strain rate and temperature. Icebergs are classified based on size. The environmental force (i.e., waves or currents) that most influences their motion depends on the iceberg’s size.

In general, larger bergs are driven by currents and wind. Even bergs of several million tons in mass can attain velocities of 2.0 knots. Smaller bergs are influenced mainly by wave loads. Though they are smaller, they can cause considerable damage when waves thrust them at high velocities against a hull structure.

Various types of ice management are employed to protect floating production systems from contact with ice that could cause hull damage. Sometimes bergs are towed to alter their trajectory. In other cases, icebreakers open the ice cover and move ice floes to reduce pressure around the hull. Hulls can also move to a limited degree by means of their moorings.

Designing for ice

Structures can be strengthened locally to withstand ice impact, but in the presence of very large bergs, a floating production system has to be able to move out of harm’s way, which means it needs to be disconnected from its mooring and riser systems. The two most prominent floater concepts for use in the Arctic are the turret-moored floating production unit (FPU) and the spar.

The FPU has proven itself as a viable system in the harsh conditions of the Grand Banks offshore Newfoundland on the Terra Nova and White Rose fields.

The spar, which has yet to be deployed in arctic conditions, is an inherently stable, field-proven, rough weather concept. Fifteen spars have been deployed in the Gulf of Mexico over the past 11 years.

Both the spar and FPUs follow the same principles of operation and ice management. They both are designed to withstand ice loading up to a particular limit, and once the limit is reached, they disconnect. Both designs support the riser systems by means of a detachable buoy. When the production system has to disconnect, the buoy drops away from the hull. During reconnection, the buoy is raised back into position. On an FPU, the mooring system is an integral part of the buoy, but the spar allows mooring lines to be released on individual floats that can be recovered and brought back to the spar for reconnection.

An advantage of the spar’s disconnectable riser buoy is that it is not contained in a weathervaneing turret like that of the FPU. Because the spar’s reaction is omni-directional to the weather, the buoy supporting the risers at the keel can be much simpler that the FPU buoy. Also, since the ice loads are transferred to the mooring system, the practical solution to increasing the floater’s ability to withstand large loads is to increase the size and number of mooring lines. The spar is more adaptable to this approach because the lines do not have to be attached to a weathervaneing turret. The mooring lines for the FPU are attached to the turret over a smaller area than those on a spar, and the flow lines passing through it are required to swivel as well.

Design dictates the size of the load the spar can withstand, and from that, the circumstances under which the spar should be moved off-station can be determined.

A basic Arctic spar design is shown in Figure 1. Apart from the Arctic winterization of the various parts of the hull, the Arctic spar is essentially a classic spar with an ice breaking cone at the water line.

Since the late 1980s, floater design concepts, including those for spars, have incorporated stepped and conical shapes at the waterline to break ice and reduce ice loads. The function of the cone is to cause the ice to fail in bending since the flexural strength of ice is about 30% to 40% its compressive strength. When the ice fails in this mode, the resulting force against the hull is significantly less than what would be created if the ice had failed by crushing against a vertical cylinder.

Figure 2 illustrates the effectiveness of the cone in reducing the loads on the conical hull compared to the vertical cylindrical used in standard spar designs. The global ice load is transferred to the mooring system as shown in the graph.

In addition to level ice cover, the cone is designed to withstand loads from ice ridges and small icebergs that can come into contact near the waterline. The cone can be optimized to suit the design ice load conditions by altering the slope of the cone surface, the height of the necked section, and the neck’s diameter. Both the mooring and riser systems can be disconnected in the event of large icebergs or ice loads that exceed the mooring design limits.

The Arctic is the next frontier for oil and gas production, and it will offer new and exciting challenges for engineers, who must design safe and efficient systems to operate under extreme conditions.

Eventually several designs will be proven suitable for this frigid environment, and the Arctic spar will be among them.