Sea Tank Prestressed Concrete Underwater Oil Storage for Deep Water
Authors: Pliskin Lucien , LaCroix Roger , Frankhouser H.S. 
Published: October 2008

The concept of a large concrete tank for underwater oil storage, which can resist predicted external forces by reason of its own density, is an answer to one of the problems involved in the exploitation of deep water oil fields.

The concrete "Sea Tank", post-tensioned in three directions, has been developed by The Sea Tank Company of France, and the design is the result of extensive studies, begun in 1967, following this basic concept.

The design and subsequent model tests have produced a viable and practical facility, completely stable on the sea floor. It is capable of resisting the predicted horizontal and uplift forces without the use of piling or other anchorage, and can be constructed using techniques materials, and equipment readily available today.

Comprehensive theoretical studies have been carried out using mathematical programs to establish the resistance of the structure to the various hydrodynamical forces acting against it. The studies and verifying model basin tests considered the forces acting on the structure in conjunction with the performance of the tank during towing, immersion, and operation. The spectrum of studies and tests covered a range varying from calm seas to the conditions imposed during the " 100 year storm" in the North Sea.

INTRODUCTION

The world's rapidly increasing demand for energy is creating an urgent need for the petroleum industry to exploit known offshore fields no matter how remote.

The exploitation of these remote offshore areas along with the technical and economical problems involved with production and transportation of the oil is creating a demand for offshore storage terminals located in the proximity of the field.

The use of underwater concrete tanks for storage of oil provides a solution to many of the problems of exploitation of offshore fields, particularly where the conventional method of pipe lining the oil to shore proves too costly. Offshore storage, of course, also involves a loading facility, which means that the percentage of utilization (tanker loading) is very closely tied to meteorlogical conditions at the site.

Whenever the severity of any of the following elements is a factor to be considered, under water storage constitutes a most valid solution: for getting the field "on-stream".time involved in requiring production is short.the financial risk is relatively slight in the event production output will be less than forecast. for development phase.the investment is progressive and parallels the requirements.

The advantage of being able to put the tank to work early in the development, as opposed to the low level of investment return on a pipeline, permits the consideration of the amortization of one tank, for example, over 3 years or more. This provides an effective economic solution for start-up of production of a field.

Moreover, consideration can be given to the retrieval of the tank at the conclusion of this first phase and its reuse in another field.

BACKGROUND

The Sea Tank Company, emenating from several French engineering, manufacturing, and construction companies was organized in 1967 to study, design, and put into place, marine, and submarine structures of all types.