In the spring of 1998, BJ Sevices and Total Oil Marine carried out an "HP Intervention Feasibility Study" for the reliable entry of a high pressure gas well using 1 ¾" 100 ksi Coiled Tubing. The research was prompted by the requirement to use Coiled Tubing to enter Triassic wells on the Alwyn North platform in the North Sea for post frac cleanout operations among other service requirements. Potential wellhead pressures of 7,500 psi and operating depths of 6,000 metres meant that a better understanding of Coiled Tubing collapse mechanisms and an improved method of predicting collapse would be vital to optimize operations on Triassic wells. The first phase of this study was based on experimental fitness-for-purpose investigations involving a 3060 ft long test string at an on-shore test well in Aberdeen, Scotland. Recognising the conservatism of collapse resistance predicted by existing theoretical methods and appreciating that knowledge of actual collapse limits is more critical in High Temperature and High Pressure (HPHT) coiled tubing operations, this initiative culminated in a joint research venture between the operator and the coiled tubing service provider. The second phase of this research involved a laboratory collapse-testing program conducted in Edmonton, Alberta, that set out to obtain extensive experimental data on CT collapse and to investigate the effect of loading history on the residual collapse strength.
This paper is concerned primarily with the laboratory-testing program. The testing protocol is described and the key parameters that influence collapse resistance are identified and discussed, as well as how each may be affected by plastic cycling, high temperature and/or high pressure operations. Some selected experimental results are presented. Although actual factors of safety against collapse requires knowledge of the individual test results, general trends for the degradation of collapse resistance due to prior service loading and the range in magnitude of the difference between actual and predicted collapse strength, are identified. Theoretical predictions of collapse resistance are based on existing formulations in which the ovality and axial loading appear as explicit parameters. Collapse predictions are also discussed in light of a newly proposed methodology submitted to the API under RP 5C7 as recommended practise for determining allowable collapse pressures on coiled tubing. A new methodology for accurate prediction of CT collapse that is based on measured stress-strain plots, is also discussed.