Based on the Multimechanism-Deformation constitutive framework, or model, that mathematically describes the mechanical behavior of salt, it is possible to establish a methodology for analysis of sparse laboratory creep databases. When this analysis is applied to the sparse databases of the domal salt materials, it yields specific values for the relevant material parameters of the model. Although the model framework remains the same for all salt materials, the specific parameter values depend upon each individual domal salt or region of domal salt. In general, the parameter values are shown to define the salt as having either a soft (low creep resistance) or a hard (high creep resistance) behavior. Variations in salt behavior suggest reasons for differences in the operational behavior of storage caverns and in the internal structure of salt domes. More importantly, this methodology potentially makes it possible to analyze a number of sparse proprietary databases, which could provide valuable insight into material behavior and relative creep strengths of developmental or operational sites; moreover, the constitutive description could provide numerical evaluation calculations to further define facility response.
1.0 INTRODUCTION
The need for constitutive descriptions for analysis of Gulf Coast salt domes and offshore structures has grown significantly in the last several decades. Offshore developments relate specifically to the expansion of oil recovery efforts in the deeper waters of the Gulf. In the salt domes of the Gulf Coast, the development of cavern storage facilities continues to progress. Among the largest developers of storage caverns along the Gulf Coast is the Strategic Petroleum Reserve (SPR), which has purchased or constructed 62 crude oil storage caverns in four storage sites (domes). Although SPR and commercial caverns have been operated economically for many years, it is still critical to better understand important mechanical responses, such as, creep closure volume loss and hanging string damage from salt falls. This same need for understanding is increasingly of interest in the offshore developments. Since critical construction and operational situations arise from deformational aspects of the salt behavior, the correlation to mechanical creep behavior as determined in the laboratory is of considerable value.
Much of our understanding of the constitutive description of salt originated from the Waste Isolation Pilot Plant (WIPP) study of a layered, bedded salt. This program provided a large and comprehensive database of the type not usually available from geomechanics programs to establish the Multimechanism-Deformation (M-D) model [1]. Additionally, the knowledge gained provided the methodology for analysis of sparse databases.
In an initial study the available data for domal salts used a simple graphical analysis to establish a bound to steady state creep of a given salt [2]. This permitted analysis of sparse creep databases for a number of domal salts; however, this method obscures some critical differences of the salt material behavior. Consequently, an analysis method was developed based on integration of the M-D creep constitutive model to obtain fits to the transient response while incorporating the steady state behavior [3]. This integration process permits definition of all the material sensitive parameters of the model, while those parameters that are constants or material insensitive parameters are fixed independently.
