Coiled tubing is a continuous pipe that, having been coiled around a reel for storage, can be deployed and used as a pipeline or riser. During deployment as a riser, the coiled tubing is unspooled from the reel, run into the water, and connected to the wellhead. This process plastically strains the pipe, causing plastic (or low-cycle) fatigue damage. When the coiled tubing is connected to the wellhead, the environmental loading causes elastic-stress cycles, resulting in elastic (or high-cycle) fatigue damage.
Numerous methods are available to determine fatigue life from either plastic or elastic cycling; however, few data are available within the industry on how the fatigue damages from elastic and plastic cycles combine.
This paper presents the experimental work conducted to show the combined fatigue life of notched samples of flat steel used to manufacture coiled tubing that has been plastically and elastically cycled. The data show that the combined fatigue life can be lower than the total of the plastic and elastic fatigue damages by use of Miner's rule. Existing theory suggests that the combined fatigue life could be as low as 10% of the Miner's-rule fatigue damages; however, the experimental data indicate that a more appropriate value is closer to 75%.