Analysis of major risks associated with hydrocarbon storage caverns in bedded salt rock

Abstract

Salt rock is internationally accepted as an ideal medium for energy storage. As an energy storage structure, the safety of hydrocarbon storage caverns in salt rock is related to the national economy and to social public security. Risk analysis is an important method of engineering safety evaluation. In this paper the major risks associated with hydrocarbon storage caverns in bedded salt rock are defined. The major risks are classified under the headings of ‘oil and gas leakage’, ‘ground subsidence’, and ‘cavern failure’, and are discussed under these topical titles. The factors leading to the major risks associated with storage caverns are identified by reviewing descriptions of major accidents of salt storage caverns around the world. Fault tree models for the three major risks are established and analyzed. Basic paths of the risk and their occurrence probability ranking are derived. The risk factors which contribute greatly to the risk are identified by calculating the importance degree of all the basic events. Finally, a comprehensive evaluation methodology for major risk loss is generated based on the analytic hierarchy process. This provides a theoretical foundation for the evaluation and prevention of major risks in the construction and operation of storage caverns in bedded salt rock.

Introduction

Salt rock is seen as a perfect medium for energy storage because of its low permeability, self-repairing of damage and good creep characteristics. It is widely accepted to store energy and resources in deep salt caverns. The construction of hydrocarbon storage cavern groups in salt rock has started recently in China [1]. Although energy storage caverns in salt rock are more secure compared with those in other host rocks, some accidents, such as oil or gas leakage, ground subsidence, cavern failure have happened in storage cavern groups. These accidents, frequently abrupt and destructive, can have a disastrous impact on safety and on the environment. As the geologic environment of bedded salt rock in China is very different from that of the huge salt domes frequently used abroad, the energy storage caverns in bedded salt rock carry a higher risk. Construction of underground energy storage caverns has started only a short time ago in China and construction and operational experience remains limited. In addition, compared with the huge salt domes with deep embedding depths in other countries, the salt rock in China has the characteristics of shallow depth, layered structure, and complicated geological conditions. The presence of thin interlayers in between the salt beds adds potential flowpaths for oil or gas leakage. Shallow depth intensifies ground subsidence, and dense distribution of energy storage caverns increases the possibility of accident-chains resulting from the failure of one cavern or a pillar between caverns. Additionally, the storage caverns in China are located near areas with dense population and developed economy, so not only the safety of storage caverns but also people’s life and property will be seriously affected if a major accident were to happen. Carrying out risk analysis for construction and operation of oil and gas storage caverns in bedded salt rock can provide a scientific basis for disaster prevention and risk reduction for the responsible government department, construction units and operating units.

In the 1970s, risk analysis theory began to be used in the field of underground projects, such as tunnels, foundation pits, underground caverns, etc. Einstein has written multiple valuable papers on the risk analysis of tunnel engineering and pointed out the characteristics and ideas of risk analysis [2], [3], [4]; Stuzk et al. [5] applied risk analysis methods to Stockholm ring road tunnels and derived some general conclusions; Arends et al. [6] proposed a method for the evaluation of tunnel safety using probabilistic risk assessment, including three criteria: personal-, societal- and economic risk; Diamantidis et al. [7] analyzed the state-of-practice related to risk analysis of long railway tunnels; Fan [8] proposed the concept of anti-risk design of underground structures using an improved analytic hierarchy process; Li [9] established a stability risk assessment system for a large-scale underground cavern group. Cagno et al. [10] provided an integrated risk analysis approach for vulnerability and resilience analysis of underground infrastructures, which breaks down the analysis in sub-areas and assesses the dependencies both in terms of interoperability and damage propagation of critical infrastructures.

The research on risk analysis of energy storage caverns in salt rock abroad is mainly focused on accident cause analysis. Evans [11], of the UK Geology Research Association, briefly described and analyzed many accidents that have happened in underground energy storage facilities (including storage caverns in salt rock); Bérest and Brouard [12], Thoms and Gehle [13], Ehgartner et al. [14], Ginn et al. [15] and Cowley [16] analyzed fire and explosion accidents of storage caverns in salt rock, emphasizing accident causes on which agreement had been reached. They also pointed out that the safety of storage caverns in salt rock is much higher than the safety of natural gas structures on the surface, but that accidents could not always be avoided. Until now there is very little research of risk analysis of energy storage caverns in salt rock in China. Yang et al., Zhang et al., and Jia et al. [17], [18], [19], [20], [21], [22] are carrying out special research on disaster mechanisms and risk analysis of energy storage caverns in bedded salt rock, and have accomplished some achievements on the reliability analysis and risk mechanism. But up to now the research on risk analysis of energy storage caverns is not very deep and systematic, and still rests on qualitative analysis. But lots of achievements on the study of the mechanical properties of salt rock and interlayers [1], [23], [24], [25], [26], [27], [28], [29], constitutive modeling of salt rock [1], [30], [31], [32], availability [33] and stability [1], [34], [35] of energy storage caverns underground have been generated. These provide an extensive foundation for risk analysis of energy storage caverns in bedded salt rock. At present the mechanical mechanism analysis, occurrence probability prediction and quantitative loss evaluation are the key problems remaining in risk analysis of energy storage caverns in salt rock.