ABSTRACT

The performance of bolts in drilling and production systems is determined by many parameters, such as the bolting material structure and properties, metallic and non-metallic coatings, installation factors (e.g., torque), and operational factors (cathodic polarization, etc.). The acceptance of bolts is also determined by test method. A variety of test methods are used ranging from bend tests to fracture mechanics-based testing. These factors have attendant data and knowledge uncertainties. Thus, a probabilistic assessment is appropriate. This paper examines the effect of the combined uncertainties on the performance of these bolts using a Bayesian probabilistic approach. The paper also discussed conceptual approaches to managing the performance of bolts.

INTRODUCTION

There are numerous flanged joints in offshore oil and gas production requiring bolts of various sizes and materials. It is estimated that in offshore Gulf of Mexico there are approximately 60,000 critical bolts used in various sub-sea drilling and production equipment.1 In the last 15 years, there have been several incidents of bolt failures or near failures in sub-sea components. 2-4 One company reported that 9 percent of the failure analysis cases from oil and gas production was related to failure of fasteners, 65 percent of which originated through brittle fracture, including hydrogen embrittlement.5 Although the frequency of bolt failures is relatively low (of the order of 10-4 per year), the consequences of bolt failure in certain critical locations, such as those connecting the blow out preventer (BOP) stack to the well-head, can be high. This has led to an intense industry push to identify ways to prevent these failures through improved materials specifications, testing, and quality control practices. 6-8 These efforts to date predominantly focus on deterministic approaches, such as hardness specifications, limiting certain grades of steels from being used, thermal treatments, and plating specifications. Although deterministic approaches are easy to follow and are generally conservative, they also have some significant disadvantages:

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