A Reliability-Based Approach for Survival Design in Deepwater and High-Pressure/High-Temperature Wells

This paper presents the application of reliability-based approaches to the survival design of critical wells, in particular deepwater and high-pressure/high-temperature (HPHT) wells. First, the concept of survival design is discussed. As in other structural design disciplines, a distinction is made between operating (service) loads and survival loads. In essence, survival loads are extreme magnitude loads with low probability of occurrence, but with potentially severe consequences if failure occurs. Survival scenarios falling into this category in critical wells are presented. It is shown that the current practice of using standard working stress design (WSD) approaches for survival scenarios, even with reduced design factors, fails to quantify the risk of failure and can lead to design practices and outcomes that are not risk consistent or optimal. Reliability-based design (RBD) explicitly quantifies the risk of failure of a given design. This paper describes RBD and the prevalence of its use in other structural design codes and shows how it can be used for survival design in critical wells. It is argued that a probabilistic approach in which a deterministic load at its extreme survival magnitude is compared with stochastic strength (from data on strength parameters) is a rational approach to survival load design. Regardless of how low the probability of occurrence of the load is at its survival magnitude, well integrity is demonstrated by assuming that such a load occurs. The method can be implemented by constructing resistance distributions using limit state equations such as the Klever-Stewart rupture limit, and the Klever-Tamano collapse limit equations (API TR 5C3/ISO/TR 10400). Statistical strength parameter data can be obtained from API TR 5C3 (ISO/TR 10400), manufacturer reports, or direct material and dimensional measurements. Statistical approaches to constructing such distributions are presented. The deterministic survival load is then compared with this resistance distribution, and a probability of failure is calculated. This probability of failure then becomes the basis for design. The goal in survival design is to demonstrate survival rather than continued operability. On the basis of this, acceptable probabilities of failure for typical survival loads are recommended and contextualized with other design codes. Particular attention is given to worst case discharge (WCD) and well containment loads, which have become design-dictating survival loads in many deepwater well designs and are driving design choices of tubulars and connections. The applicability of this approach to connection selection and brittle failure is also demonstrated. A deepwater well example is presented to illustrate using the approach. It is shown that designing to an acceptable probability of failure leads to more robust and risk-consistent designs in critical wells. Furthermore, such an approach allows designers to focus on the specific design or well construction changes that enhance survival. It is noted that the approach is applicable in its entirety to HPHT wells, where similar challenges are present. The approach described in this paper provides a quantitative basis to examine design adequacy of wells under survival scenarios. The approach is in keeping with the traditional practice of allowing using all available strength in designing to survival loads. Using stochastic strength data rather than deterministic strength estimates provides a probabilistic basis for design, thus quantifying risk. The authors believe that this is a needed rational and quantitative approach to optimize design of critical wells under increasingly demanding loads.