Submerged Concrete Structures

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ABSTRACT An overview of the existing technology for submerged, pressure resistant, concrete structures is presented for the offshore engineer. Empirical equations allow predictions of the implosion strength of spherical and cylindrical structures. Other design aspects related to submerged concrete structures such as penetrations, reinforcement, permeability and compressive strength, are also discussed. INTRODUCTION The offshore industry is experiencing an upsurge of interest in ocean concrete structures. Many of the applications are for floating structures, such as LNG carriers, for fixed seafloor structures, such as the Ekofisk oil storage tank and for submerged structures for oil production and storage systems. The interest in concrete is spurred by several factors. Notably the potential for significant cost savings; yet, this is the outcome of other supporting features. First, the resistance of concrete to attack by the seawater environment is good. By using a well designed concrete mix and thorough inspection procedures to insure high quality workmanship, concrete structures will exhibit low maintenance costs over a long life. An actual cost comparison has shown that steel barges required from 300 to 1100 percent greater maintenance costs over a six year period than comparable size prestressed concrete barges.l Second, positively or negatively buoyant structures can be designed of concrete by varying the material density and wall thickness. Large structures possess excessive buoyancy if fabricated of steel; hence, great quantities of ballast are required to submerge the structure. With concrete, the wall thickness is usually increased to overcome excess buoyancy so the "ballast" is used to enhance the strength of the structure. Third, the fabrication of concrete structures can be performed rapidly and in a world wide competitive market. Also complex structural shapes, such as spheres or toroids, are economically formed of concrete. Most existing submerged or partially submerged concrete structures are pressure-compensated. One exception is the submerged concrete transportation tunnels which are pressure resistant. Yet, even pressure-compensated structures should be evaluated for their pressure-resisting capability so that during emplacement of the structure to the seafloor the permissible pressure loading is known. The purpose of this paper is to present empirical design equations which predict the implosion strength of concrete spherical and cylindrical structures, and to discuss other major design aspects of submerged concrete structures. The technology exists to use concrete structures today with utmost safety to 1,000 ft. and within several years further testing and experience will permit concrete structures to operate at 3,000 ft. The information presented herein should enable an offshore engineer to assess whether concrete is applicable for his planned or contemplated submerged structure. SCOPE OF EXPERIMENTAL PROGRAM A total of 120 models of spherical and cylindrical pressure-resistant structures have been tested under hydrostatic loading. The majority of the specimens were spherical structures of sizes 16, 32, and 66 inches outside diameter and fabricated of plain and reinforced concrete. The cylindrical structures were 16 inches outside diameter and fabricated of plain concrete, with one exception.