Collapse and Post Collapse Behaviour of Tubulars: a simple approach

Abstract

SPE Members Abstract This paper describes a simple method of evaluating the collapse strength and post collapse behaviour of oil field tubulars subjected to external pressure. The method is based on elastic ovalisation and subsequent plastic collapse by formation of a four-hinge mechanism. The resulting predictions of collapse strength are compared with available collapse test data, and other existing collapse equations. For post collapse behaviour, model predictions are compared with available experimental and numerical data. Applications of the method to collapse of perforated and slotted liners, prepacked screens and collapse under non-uniform loading are described. Introduction Collapse due to excessive external pressure is one of several failure modes against which oil field tubulars are designed. Due to long lengths of tubulars used in a well, unduly conservative design can carry an undesirable penalty on well cost. On the other hand under-design increases risk of failure and the associated trouble cost. Collapse of tubulars has therefore been studied by numerous investigators and various collapse strength equations have been proposed. Present design guidelines such as those published by API for design of intact tubulars against uniform collapse loading are used extensively by the oil Industry. These guidelines are semi-empirical based in part on full scale collapse test data and therefore incorporate various assumptions and restrictions on degree of ovality, material yield stress range, diameter to thickness ratio (D/t), thickness tolerance, nature of pressure loading, amongst others. However in some special circumstances these design guidelines can no longer be readily applied. Examples of these are perforated and slotted production liners, pre-packed screens, tubes with known excessive initial ovality, and casing under non-uniform pressure loading. These situations are usually addressed by numerical analysis or through full scale test programmes. Both involve high cost and as such are more suitable for specific design cases rather than for the generation of design guidelines. This paper describes a simple method of calculating collapse and post collapse strengths of tubulars subjected to external pressure. The method is based on pre-collapse elastic ovalisation and subsequent plastic collapse through the formation of four plastic hinges. The calculation approach is readily applicable to collapse analysis of perforated and slotted liners and also to casings under non-uniform pressure. In this paper collapse strength predictions are compared with a set of full-scale collapse test data, API design guidelines and a number of other existing collapse equations. For post collapse behaviour, model predictions are compared with available numerical data. Application to collapse assessment of perforated and slotted liners, prepacked screens and to collapse under nonuniform external loading are discussed. P. 149