Collapse Capacities of Casings With Internal Wear Based on a Structural Reliability Approach
Authors: Hørte Torfinn , Liu Ting , Bjørset Arve , Kjørholt Halvor 
Published: August 2024

Abstract
Casing collapse capacity was identified by Equinor as a critical operational parameter on one of its fields in production. This initiated a re-evaluation of the overall well design, specifically the production casing’s collapse capacity. Studies were performed to analyze and objectively increase the documented casing collapse capacity, while maintaining the safety level.
Initially, the casing collapse capacity was evaluated using API TR 5C3 / ISO 10400, with insufficient capacity being documented. To investigate further, physical material testing and collapse testing of pipes were performed. Two kinds of collapse tests were carried out: i) tests of unsupported pipe and ii) test of pipes with external support to simulate the cement and formation surrounding the pipe. Detailed finite element analyses were carried out to compare calculated casing collapse capacity with the test results. The comparison was used to assess the model uncertainty in the FE analysis. Papers from 2018 (OMAE2018-78767) and 2020 (OMAE2020-18887) show results from pipes without and with external support respectively. Generally good correspondence between FE analyses and tests was obtained, providing confidence that FE simulations can be used to predict the collapse capacity of pipes with and without external support quite accurately.
Additional FE collapse analyses have been performed for different pipe sizes and materials. Systematic sensitivity analyses with variations in the identified critical parameters comprising axial load, casing ovality, casing wear from drilling, pipe wall thickness and temperature-dependent material properties were performed. Both unsupported pipe and fully supported pipe from surrounding cement and formation, were analysed. A multi-dimensional response surface was fitted to the FE simulation results for each pipe. Probability distributions for the identified input parameters were assessed, based on measurements and various logging results. The uncertainty input together with the response surface were applied in a structural reliability analysis (SRA) analysis where the probability of casing collapse was calculated for different deterministic values of the differential pressure. The importance of each parameter was mapped, and particular attention was made towards the results for different mean values of the internal wear due to drilling. Considerations were also made for length effects from the exposed section of the casings.
The results provide characteristic collapse capacity at a chosen probability level for a range of casing properties and measured internal wear which is not readily available by simplified formulae in e.g., API TR 5C3 / ISO 10400. The SRA is used to quantify and benefit from improved knowledge from testing and measurements to reduce uncertainty and correspondingly obtain higher collapse capacity at a selected probability level. By using SRA the potential over-conservatism in the conventional deterministic formula-based calculations is then avoided, while maintaining the required overall safety level. The SRA results are used as input to risk evaluations that in some cases might allow for continued production on existing wells that otherwise would be shut in.
This paper provides a brief summary of the methodology with focus on results for the different casings analysed, ranging from 9 5/8″ to 20″ casings. Additional testing and analyses are ongoing for smaller pipes and might be added in an extended version of this paper at OMAE in 2025.