Cyclic Ice Loading on the Molikpaq

Sarah Mapplebeck

Abstract


The Beaufort Sea is located north of Northwest Territories, the Yukon and Alaska, and west of Banks Island. The centre and northern part of this sea is frozen solid all year round, and the sea is exposed to considerable ice scour. Petroleum exploration in the ice packed Beaufort Sea in the 1960s led companies to develop offshore structures that would withstand extreme ice loading. In 1984 Gulf Canada Resources Ltd. deployed a mobile arctic caisson into the Beaufort Sea called the Molikpaq. The Molikpaq was installed on an artificially constructed berm, where it completed exploratory drilling during four subsequent winters.
At the time that the Molikpaq was designed, ice loading on structures was a little known matter. To alleviate this knowledge gap, hundreds of sensors were installed on the Molikpaq to observe and attempt to predict behaviour due to loading.
In 1986 the Molikpaq platform was brought to within minutes of failure due to cyclic loading. In this particular event the ice loading occurred at such a frequency that vibration occurred, causing liquefaction of the inner sand core. The sensor readings and detailed observations of this event and the various other loading encounters on the Molikpaq have provided a considerable amount of information for future projects.
This paper compiles information on the design and installation of the Molikpaq in the Beaufort Sea. This drilling structure had various successes in regards to resisting ice loading, but eventually came near failure due to the unfamiliar characteristics of cyclic loading. This event will be explored, noting the lessons learned, and how this has affected and will continue to affect other offshore structures deployed in ice-filled waters.

Keywords


Coastal and Ocean Engineering, 8751; Case Study; Civil; Ice Loading; Cyclic Loading; Liquefaction; Molikpaq, Beaufort Sea

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References


J.C. Bruce, A.G. Harrington, “Design Aspects of a Mobile Arctic Caisson”, in Offshore Technology Conference, Houston, Texas, 1982

G.W. Timco, M. Johnston. “Ice Loads on caisson structures in the Canadian Beaufort Sea.” Cold Regions Science and Technology 2-3 (2004): 185-209. Web. Feb 2013.

T.G. Gijzel, et al. “Installation of the Mobile Arctic Caisson Molikpaq,” in Offshore Technology Conference, Houston, Texas, 1985.

G.W. Timco, M. Johnston. “Ice Loads on the Molikpaq in the Canadian Beaufort Sea.” Cold Regions Science and Technology 37 (2003): 51-68. Web. Feb 2013.

R.E. Gagnon. “An explanation for the Molikpaq May 12, 1986 event.” Cold Regions Science and Technology 82 (2012): 75-93. Web. Feb 2013.

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Sakhalin Energy. (n.d.). Molikpaq Platform (PA-A) [Online]. Available at http://www.sakhalinenergy.ru/docs/FactLists/English/MOLIKPAQ.pdf


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