High Strength Coiled Tubing - How is Fatigue Life Affected by Slip Damage?

Abstract

Abstract Since the inception of quench and tempered coiled tubing, a question has come up numerous times. Do we need specialized handling equipment for this stronger and correspondingly harder pipe? Harder coiled tubing implies that blow out preventers (BOPs) must have sufficiently hard slips to grip the pipe as well as sufficiently hard shear blades to cut the pipe. Testing has shown that in most cases, existing BOP components have been sufficient to grip and cut quench and tempered coiled tubing. Once BOP slips have been set on the harder pipe and a real world ‘pipe heavy’ load applied, how significantly is the fatigue life of quench and tempered coiled tubing affected? This paper will highlight the findings of some recent testing of quench and tempered coiled tubing and will compare the results to conventional coiled tubing. In this study, 2-3/8″ coiled tubing was subjected to a tensile load of 44,500 daN (100,000 lbs) while being held in BOP slips. The coiled tubing was then tested on a bend fatigue machine with the slip marks at the ‘sweet spot’ to determine the number of cycles to failure. Four grades of quenched and tempered coil tubing were tested as well as a baseline sample of 100 grade coiled tubing. The aim of this paper is to demonstrate that modern BOPs can grip high strength coiled tubing and point out the effect on fatigue life of the resulting slip marks.