Ice/Berm Interaction Study Using Rotary Sidescan Sonar and Acoustic Profiling Systems

Abstract

ABSTRACT Tarsiut Island, in the Canadian Beaufort Sea, was the first dredged caisson retained island built for exploration drilling operations in the Arctic offshore. Due to the island is configuration location, a large first-year ice rubble pile would result from the ice/structure interaction. This paper outlines how a rotary side-scan sonar and a mechanically scanning, narrow-beam acoustic profiling system were used to determine the geometry and the contact area of the underside of heavily rubbled first-year ice. The results of this study are to be used to further the understanding of the nature and mechanism of the ice/structure interaction in Arctic offshore structures. The sonar transducers were deployed through augered holes in the ice thus permitting horizontal rotation. The systems were also configurated to allow an indirect remote means of data acquisition of the underside of the ice as well as the beam/seabed surrounding the caisson retained island. Of main interest, was the geometry of the deep draft grounded ice at the edge of the ice rubble annulus, knowing the geometry and the contact area of the grounded rubble was required for use in understanding the ice load transfer mechanism between the rubble field and the island beam. Thirty-six under-ice profiles were taken from each of 14 locations in conjunction with a 360 degree side-scan sonograph acquired from each hole so as to encompass the circumference of the island. Aerial photographs were used to provide the above water geometry of the rubble field. Sonar profiles were ground truthed utilizing manually drilled holes to establish the level of accuracy, and to allow necessary calibration corrections. The combination of these two systems utilizing remote indirect methods in this unique application provided Gulf Canada Resources Inc. and the participants in APOA Project No. 198 with both cost effective, quantative and qualitive engineering data. The definitive resolution and detail of the data over such a large extent had not been previously attainable in so little time. INTRODUCTION To ensure safe stable platforms for offshore exploration, artificial islands are commonly being wilt in the Beaufort Sea. Two main types of artificial islands have beenused to date; sand with long-sloped beaches, in up to 20 m of water, and sand-supported steel or concrete caissons. Tarsiut is a caisson retained island on a sand beam designed to reduce the quantities of fill required and the time needed for construction. Although many artificial islands have been wilt in the Canadian Beaufort Sea (Figure 1), the Tarsiut structure represented a significant new extension to the Arctic drilling technology. The island (Figure 2) was designed and constructed under contract to Gulf Canada Resources Inc. by Canadian Marine Drilling Ltd., and was the first caisson retained island used for exploratory drilling in ice-infested waters (Fitzpatrick and Stenning, 1983)3.