Polar E&P: the iceman cometh

Offshore-Technology.com

March 23, 2012

By Nigel Ash

Gazprom's mighty Prirazlomnoye platform in the intensely inhospitable Pechora Sea is due to begin production this year and has been designed to withstand 5m-thick ice movements. At 117,000t (unballasted), this platform sits in 20m of water and is expected to operate through the 230 days when the area is ice-bound and temperatures drop to -50°C. Winds can reach 40m/s and waves up to 12m high.

The Russians describe the Prirazlomnoye as 'the first ice-proof marine structure'. The oil it produces will be taken away by two specially built ice-breaking tankers, prompting platform designer Morneftegazprojekt to carefully consider the potential impact of the tankers' movements on the surrounding ice.

More than three decades earlier, the drilling and oil production platform Molikpaq, now in service off Sakhalin, taught scientists a great deal about the influence of ice. The platform had previously been employed in the Canadian Arctic.

"The 1986 ice event that the Molikpaq structure met was 8-10m thick," says Dr Erland Schulson, director of the Ice Research Laboratory at Dartmouth, "and the platform was extensively instrumented for the structural responses due to ice loads and other environmental forces. So this event led to the generation of an awful lot of information on ice forces against structures. That information has been mined quite seriously and used quite well in subsequent designs."

Physics of fracture

The growing body of ice research is reviewed by Schulson with his French colleague Paul Duval in their book entitled 'Creep and Fracture of Ice', which examines work done both in the field by the likes of Sanderson and Exxon as well as in the laboratory, starting with the seminal paper on fracture mechanics by AA Griffith, which established that the fracture pressure scales as one over the square root of the crack's size.

All the information within their book was both hard won and useful, insists Schulson, but "the data is not necessarily well understood from the point of view of the physics and the fracture - the process that is going on within the ice." This is where his work is focused.

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