In order to meet the demand for offshore oil development in the Sub-arctic and Arctic, Mitsui Engineering & Shipbuilding Co., Ltd. has conducted research to establish technology to predict the ice loads applied on various shapes of ice resistant structures as a basis for designing optimum structures. The ice force measuring experiments have included laboratory tests performed by Arctec Inc., U.S.A., outdoor tests using medium size models in a lagoon and field tests on a large size tower specially installed in the Okhotsk Sea.
This large tower was built in August 1976, 600 m off Mombetsu in Hokkaido at a water depth of 6.5 m and had a cylindrical shape of 9.5 m in height and 2.5 m in diameter. This tower was converted the next year to a conically shaped structure of 11 m in height and 5.5 m diameter at the water line. In winter the site was covered with pack ice ranging from 30 to 60 cm in thickness. The ice loads were sensed by measuring bending strains at the tower root and transmitted through sub-sea cable to land, and observations of ice behavior around the tower were conducted.
Model tests of a mono-cone, mono-pod, four-legged conical structures, and four-legged cylindrical structures were conducted in an ice model basin of Arctec Inc. These were tested in unconsolidated and semi-consolidated first-year pressure ridges and rafted ice.
Also, outdoor tests have been conducted at the Saroma Lagoon near Mombetsu since 1976 using medium sized models on various types of ice-resistant structures. The phenomena of vibratory crushing were also investigated.
In this paper the outline of these tests including the correlation of ice loads on cone-shaped structure between the different tests' is presented as well as some design consideration for ice-resistant structures
In recent years, exploration for further offshore resources to meet growing energy demand has been expanded to activities in the ice covered seas including the Arctic frontier. Shortage of oil and sharp increase of oil price have accelerated the necessity of the development of the oil production system in the Sub-arctic and Arctic. However, the severe environmental conditions of the Arctic Sea create technical and economical problems.
In 1975, Mitsui Engineering & Shipbuilding Co., Ltd. started the ice engineering project in order to meet the demand for oil and gas development in the ice covered sea in 1980s. The research project covers drilling system, production platform, supply and transportation systems including ice breaking tankers.
In this paper, the results of researches conducted at Mitsui for the estimation of the ice force applied on various shaped structures in a variety of ice conditions and the development of economical and safe offshore structure designs suitable for the environmental condition are presented. Ice force against a structure depends on the structure's shape itself, properties of sea ice, ice thickness, ice movement and ice failure mode. A number of tests of model structures and ice properties have been performed to investigate ice forces exerted on ice resistant structures.
