Application of Connection Productline Evaluation

Abstract

Premium threaded casing and tubing connections are expected to maintain structural integrity and sealability performance throughout the life of the well. Industry and various company standards for evaluating connection performance via physical testing on specific size/weight/grade combinations have been optimized. The industry now seeks methods to infer performance for combinations that have not been tested, based on completed tests. Time and cost savings are realized by extending test results within a family compared with multiple individual evaluation programs. One operator has implemented an approach to evaluating a family of premium connection designs across a range of size/weight/grade combinations. This approach includes a comprehensive review of the threading specifications within the family to provide a consistent design approach, finite element analysis of all of the family members within the range under extreme tolerance and load conditions to evaluate the consistency of performance, and rigorous physical testing of combinations representing the boundaries of the range or worst cases within the range. The operator has confidently assigned performance envelopes for several premium connection design families. Physical testing and finite element analysis evaluations assessed design and performance consistency, and established specific design-family-specific performance criteria. This paper will describe the process implemented to provide appropriate performance assessment of a connection product family for which all size/weight/grade combinations were not tested. Connection Performance Performance is just one of many factors that must be considered when selecting a specific connection thread design for a well casing or tubing application. Some well designers seek connections that are as strong as the pipe body; however, some connection designs remove enough material during threading that the assembled connection does not provide structural integrity equal to that of the pipe body on which it is threaded. Also, connections represent a potential leak path at the ends of each joint. ExxonMobil affiliates ("ExxonMobil") operating with many different well types and applications worldwide, have taken the approach of determining an envelope for each threaded connection based on structural integrity and sealability performance limits under combined load conditions, and ensuring that the anticipated loads from the well application are within the envelope. Fig. 1 is an example of a generic connection envelope that describes the structural and sealability performance of a specific connection. The sizes of the different sealability regions are determined by evaluating the connection's gas or liquid seal integrity for a load combination (i.e., axial and net pressure). The well designer determines design scenarios anticipated over the well's life and calculates the corresponding loads. These loads are plotted on the seal envelope corresponding to the fluid type for that design scenario. If all the loads fit within the appropriate envelopes, that connection design is suitable for the application. A challenge for the well designer and the industry is to determine accurate performance limits for a connection as economically as possible. A typical approach is to create geometries (specimen threads or an analysis model) representing extreme tolerance combinations within the connection design and then subjecting the geometries to combined load conditions in the test lab or in the analysis that exceed anticipated downhole conditions. The true seal integrity performance envelope of a connection may extend past the connection manufacturer's uniaxial ratings or even past the triaxial yield ellipse of the pipe body, although this latter performance may be difficult to validate during physical tests. The well designer wants to know the absolute limits for the connection even if other design constraints will keep the loads from approaching those limits.