Movement And Placement Of Large Offshore Drilling Structures In The Arctic
Authors: Walsh M.R. , Drolet J.J.P. , Eddy P.R. 
Published: October 2008

Large bottom-founded offshore structures positioned within tight tolerances are usually placed with the aid of extensive ground gear and winch equipment. In ice-infested waters, a dynamic placement procedure that requires no seabed equipment is preferable.

One of these exploration structures, the Canmar SSDC (90 000 dwt), has been relocated six times in the Beaufort and Bering Seas using a "fly-in" method of dynamic placement.

This Paper presents the experience gained and the procedures developed with this unit, which will have direct application to the design process for the next generation of Arctic exploration and production structures.

INTRODUCTION

Large offshore structures are usually placed with the aid of extensive ground gear and support equipment. For permanent structures, this is a one-time expenditure of effort and equipment, and the cost is not a large part of the overall project costs.

Large exploration structures, such as those used in the Arctic to resist environmental loading are, however, relocated on an annual basis in an ice-infested environment, This increases the risk of losing the ground gear systems, as well as increases the size and number of the support vessels required to manage these systems. Thus, resulting annual costs soon become considerable. One such exploration structure, the Canmar SSDC, has been successfully relocated six times without ground gear in the Beaufort and Bering Seas using a "fly-in" or "glide-path" method of dynamic placement (Fig. 1-Cutaway view of the SSDC on location).

This Paper presents the placement technique developed from experience gained over six seasons of deployment in the Arctic. Details of setdown site selection, ice management, tow considerations, and ballasting procedures are also presented. It is hoped that the experience gained may be directly applied to the design process for the next generation of Arctic exploration and production structures.

BACKGROUND

The Arctic environment presents unique challenges to offshore exploration structures. A relatively short (90 to 100 day) open water season is exploitable using ice-strengthened moored drillship which can resist minor incursions of seasonal ice with the aid of icebreaking support vessels. These systems have been in use since 1976, and have drilled approximately 80% of all the exploratory wells on the North American Arctic Shelf.

In the early 1980s, year-round drilling was conducted from dredged sand and gravel islands, generally in the areas inshore of the 8 m (26 feet) isobath. Costs associated with dredging volumes for fill islands in deeper waters spurred the development of several self-contained systems which could be deployed on dredged submarine berms to depths of 30 metres (98 feet). One such system was the SSDC, an ice-strengthened, single caisson system derived from a surplus VLCC (very large crude carrier). It incorporates multiple well drilling capability with sufficient material storage to eliminate the need for costly mid-season re-supply. After its second deployment on a sand berm, it became apparent that costs could be considerably reduced by eliminating the dredged berm portion of the system, and substituting for it a reusable "steel berm".