Analysis Of Reinforced And Pretressed Concrete LNG Offshore Structures

ABSTRACT Results of a preliminary numerical study on the response of reinforced and prestressed concrete slabs to simulated accidental Liquefied Natural Gas (LNG) spillage conditions are presented and evaluated. The slabs analyzed were designed to serve as secondary barriers in a gravity type offshore LNG storage facility. Reinforced concrete one-way slabs with simple and continuous support conditions, and precast, simply supported, prestressed concrete slabs with concentric and eccentric straight tendon profiles are investigated. Environmental effects considered represent sudden, well stirred LNG spills on the top of the slab, the selfweight of the slab and various heads of LNG. The nonlinear heat transfer and thermalstress analysis methods utilized are briefly reviewed emphasizing their assumptions and limitations. Problems in formulating realistic mathematical models for materials during transition to cryogenic temperatures are discussed. Analytical results are presented indicating that the initial surface cracks which developed in the simply supported members considered tend to close with time and that displacements generally stabilize. However, axially restrained (continuous) members develop significant tensile membrane forces which adversely affect behavior. Recommendations for future experimental and analytical research are offered. INTRODUCTION The world's rapidly increasing demand for energy has created a need for marine structures for the production, storage and transport of LNG. Considerable success has been achieved in constructing land based LNG facilities using prestressed concrete (e.g. Refs. 1 and 2). Reinforced and prestressed concrete have also been used in a number of important offshore applications, primarily in the North Sea. 3, 4 With this background, it would be natural to extend the use of reinforced and prestressed concrete to the construction of LNG-related offshore structures. 5 However, before this can be done with confidence, a number of problems must be resolved. One of these problems relates to the structural behavior of reinforced and prestressed concrete members exposed to sudden changes in temperature that might occur in the event of accidental spillage of LNG. The large thermal gradients arising in such cases result in internal stresses that may cause concrete cracking and undesirable member deflections. 6 In restrained members, this tendency to deform can also give rise to moments, shears and axial forces which in turn may lead to cracking of the entire cross section. Such cracking has important implications for the design of secondary barriers in land based as well as offshore LNG facilities. However, special problems are encountered in offshore structures since secondary barriers would normally, be designed to be integral with the structural system and as such would carry significant service loads. Moreover, cracks formed during LNG exposure could adversely affect the fatigue life of a member an important consideration in marine structures. Current design guidelines7 recommend that thermally induced stresses be accounted for in design.