Earthquake Survivability of Concrete Platforms

Abstract

Watt, B.J., SPE, Brian Watt Assocs. Inc. Boaz, Irvin B., Shell Oil Co. Ruhl, J. A., Shell Oil Co. Shipley, Stanton A., Boeing Engineering and Construction Dowrick, David J., Ove Arup and Partners Ghose, Amitava, Keith, Feibusch Assocs., Engineers Nonlinear dynamic analyses were conducted to assess the survivability of concrete gravity platforms during extreme earthquakes. Structure, foundation, and earthquake characteristics were varied among 15 analysis cases. It was concluded that suitably designed platforms can survive ground-shaking conditions likely to be associated with rare intense earthquakes in the Gulf of Alaska. Introduction This paper describes the results of efforts to quantify the damage that would be sustained by a typical concrete gravity platform during an extreme earthquake. Earthquake-induced loads are rather different from those caused by waves. Among the most significant differences are the greater uncertainty associated with earthquakes and the ratios of the loads at the extreme (very low probability) and design levels for the two phenomena. Consequently, while the safety factors used in design usually will provide adequate protection against collapse from extreme winds and waves, this is not necessarily the case for earthquakes. It is not unreasonable to envisage rare intense earthquakes producing ground-shaking magnitudes much larger than those associated with a design-level event.The structural response to earthquakes is energy-controlled rather than load-controlled; as a consequence, structures can be designed to survive extreme earthquakes by providing them with adequate ductility. Therein lies the key to earthquake-resistant structural solutions that are both reasonably economical and safe. The concept of designing elastically for the design-level earthquake and inelastically for survivability, or rare intense earthquake, is not new. It forms the basis for the more progressive building design codes in seismically active regions and is the kernel for the most recent API RP 2A recommendations for steel-framed structures. Unfortunately, structural ductility is not a straightforward characteristic to define, nor is it easy to set ductility criteria that suit a wide range of structures. Experience, analysis, and experiment have been used to establish ductility requirements for certain classes of structures, such as framed reinforced concrete buildings, with and without shear walls, steel-framed buildings, and offshore jackets and towers.Concrete gravity platforms have not been subjected to the same close scrutiny. The purpose of the investigation described here was to find answers to these questions:Can typical concrete platforms survive the conditions of an extreme or maximum credible earthquake? andWhat would be the extent of the damage? Survivability in the present context means the ability to endure the event without collapse of the structure, foundation, or any primary structural component. It is implied that the system could be unserviceable after the event.Shell Oil Co. has examined several aspects of the seismic design problem in a series of studies, and some preliminary results were published in 1976. This paper describes the development of answers to the above questions and some of the other principal findings from these studies. JPT P. 1090