Summary
The paper describes two minimum-cost, self-erecting platform designs and one
caisson structure design that can be installed in Cook Inlet, Alaska, without
the need for heavy lifting equipment. The Cook Inlet environment is
characterized by 30-ft tides and during the winter months by fast-moving ice
floes. The area is remote from other offshore operating areas, and there is no
heavy lifting equipment. One of the designs has been built: Platform Osprey was
installed in Cook Inlet in 2000. The paper will describe how it was designed,
constructed, and installed. A second, different design concept allows platform
installation in deeper water, again without the need for heavy lifting
equipment. The third design concept is an outrigger caisson design that can be
installed with a jackup drilling unit. It would be applicable as a satellite
platform structure.
Use of these minimum-cost platform and caisson designs would enable development
of marginal fields in Cook Inlet that heretofore could not be developed because
the high cost of mobilizing and demobilizing heavy lifting equipment to and
from Cook Inlet would have made development uneconomical.
Introduction
In the period from 1964 to 1968, four oil fields and one gas field were
developed in Cook Inlet, Alaska, from 14 platforms (Visser 1992). During this
period of high activity, there were several mobile drilling units, derrick
barges, and pipeline lay barges in the Inlet. At the completion of development,
however, the equipment departed, and Cook Inlet became a backwater as far as
offshore oil activities were concerned. Yet there were still undeveloped
discoveries and identified prospects. However, the high cost of mobilizing and
demobilizing mobile drilling rigs and heavy lift equipment discouraged
operators from pursuing these potential opportunities.
One such undeveloped discovery, the Redoubt Shoal field in Cook Inlet,
Alaska (Figs. 1 and 2), was found by Amoco (now BP, then PanAm) in 1969
but 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, would require the
drilling of at least two and possibly four appraisal wells. This is usually
done by drilling one or more expendable appraisal wells with a mobile drilling
unit. For remote Cook Inlet, the cost of a single appraisal well was estimated
to be USD 25 to 30 million including the cost of mobilization and
demobilization. Installing a platform in Cook Inlet the “conventional” way was
also cost-prohibitive without the ready availability of heavy lifting
equipment, and the cost of such a platform was estimated to be USD 60 to 80
million.
As an alternative, Forest decided to investigate the possibility of
installing an “exploratory” drilling structure. Requirements for such a minimum
structure were:
- Capability to support a 20,000-ft modular drilling rig
- Installation without heavy lifting equipment
- Feasibility of conversion into a permanent platform
- Easy removal in case the prospect did not pan out
- Possibility of moving the structure to another Cook Inlet location.
Several platform concepts were developed by a Redondo Beach, California,
consultant, Belmar Engineering (Visser and Carlson 2002). The selected concept
requires no heavy lifting equipment and uses the tide to install the
platform.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
19 January 2005
- Manuscript approved:
10 January 2007
- Version of record:
20 September 2007
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