Abstract

The paper describes the design, fabrication and installation of a novel self-erecting platform in Cook Inlet, Alaska. The platform project was completed by Forest Oil Corporation at a fraction of the cost of a conventional platform, thereby allowing exploration and subsequent development of a hitherto noncommercial oil field discovery dating back to 1969. This was accomplished despite the fact that Cook Inlet is one of the most inhospitable offshore areas in the world; there are thirtyfoot tides, moving ice floes in the winter and there are no derrick barges. The platform was assembled in a floating floatover operation in a deep-water bay in lower Cook Inlet. The assembled platform, supported on a barge, was towed to location in upper Cook Inlet and held in place at high tide with several tugs. As the tide ran out the legs reached bottom and the barge was removed. Foundation piles were driven using the pre-installed platform cranes.

Introduction

The Redoubt Shoal field in Cook Inlet, Alaska, see Figures 1 and 2, was discovered by Amoco (then PanAm) in 1969, but was not deemed commercial at the time. Forest Oil Corporation (then Forcenergy) took over the acreage in 1996 and, following a 3D seismic survey, concluded that the field could be commercial. To confirm this, however, required the drilling of at least two and possibly four appraisal wells.

Confirming the commerciality of an offshore prospect is usually done by drilling one or more expendable appraisal wells with a mobile drilling unit. For remote Cook Inlet this method is cost prohibitive, because the equipment has to be mobilized from out-of-state and a single appraisal well was estimated to cost some $25 to 30 million in 1997. Installing a platform in Cook Inlet the "conventional" way is also cost prohibitive, because there is no longer any heavy lifting equipment available on the West Coast and such a platform was estimated to cost $ 60 to 80 million.

As an alternative Forest decided to investigate the possibility of installing an "exploratory" drilling structure. Requirements for a minimum structure were:

  1. capability to support a 20,000-foot modular drilling rig,

  2. installation without heavy lifting equipment,

  3. feasibility to be converted into a permanent platform,

  4. easy removal in case the prospect did not pan out, and

  5. relocatable to another Cook Inlet location.

A number of alternative platform concepts were developed by Redondo Beach, California consultant Belmar Engineering and further investigated for feasibility by Brown & Root in Houston. The selected concept requires no heavy lifting equipment and uses the tide to install the platform. To keep costs down, the platform, which was named Osprey, is located in 45 feet MLLW about two miles from shore rather than in much deeper water, four miles from shore, where the Redoubt Shoal field is located. The lower platform and future pipeline costs offset the increased extended reach drilling costs.

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