Prestressed Concrete Underwater Oil Storage System
Authors: Gerwick Ben C. 
Published: October 2008

A system has been developed which meets the functional, structural, and economic requirements for large-volume underwater oil storage. This system utilizes twin-barreled tanks of prestressed concrete and the saltwater displacement system. Prestressed concrete was selected for its inherent strength, ruggedness, durability, economy, and for its high factor of safety against catastrophic rupture.

The configuration chosen permits a high rate of discharge without interface mixing. Storage may continue from production even while discharging.

Provision is made for separation of oil from displaced water to prevent any possible contamination; also for prevention of clogging from sand or marine growth, for venting of gas, and for discharge of sludge.

Construction of these huge underwater storage vessels has been carefully detailed and programmed in order to insure economy, quality, and ability to meet overall schedule requirements. The vessels are to be constructed either partially or completely in a basin, and towed to the site.

Model tests have been utilized to develop methods of sinking and installation for depths up to 300 feet.

Stability under current and normal wave forces is provided by dead weight. Stability under storm wave is extremely critical in depths less than 200 feet or so. Model tests showed lateral forces developed are greater than those given by conventional analysis. Techniques for providing additional stability when needed have been developed. These include modification of the configuration and increase in the effective weight.

The operating system has also a been tested and verified with models.

Cost analyses show the system to be economical for locations over 10 miles from shore if the stored oil must subsequently be shipped by tanker. While each site and storage requirement requires an individual design for that specific installation, the basic concepts and principles of the prestressed concrete system can be adapted economically and practically to meet a wide range of criteria.

Introduction

The growing production of oil from offshore fields and the increasing distances of these fields from suitable onshore bases necessitate the development of more economical means of storage. Onshore storage becomes less economic and les s practicable as distance to the offshore field increases. This is true, also, in areas where onshore sites require extensive development, as in the tropical coastal swamps of Nigeria, and in areas where the shores are highly developed, such as the Santa Barbara Channel. The most significant factor of all, however, is how the oil is to be subsequently shipped from storage. If it must move away by tanker, then a second pipeline may be required to move the oil back out to a d l3ep water terminal.

One alternative is floating storage, such as captive tankers or even larger, specially-designed hulls. These have the advantage of mobility, but are relatively expensive to operate. The greatest problem is that of mooring the vessels against design storm conditions. This last feature has limited the general use of floating storage to relatively protected bodies of water. Various modifications of floating storage are in the conceptual stage, including submerged and column-stabilized schemes.