This paper reviews a coiled tubing management system that has been in operation for over two years in European and West African operations. Complete tubing records for up to at any one time 20 coiled tubing units and in excess of 100 strings of coiled tubing have been recorded by the system. Data has been taken from the system, covering the total amount of tubing run over the period and has been broken down to show running footage between different pipe sizes, amount of failures and maintenance carried out on coiled tubing.

Corroboration of the fatigue predictions of the system is also discussed, with test results of tubing that had reached its safe working life under the CYCLE system being tested to destruction on a fatigue rig to evaluate the suitability of the fatigue algorithm used.

Fatigue and Plastic Work

As coiled tubing is spooled on/off a reel or gooseneck, it follows the radius of curvature of the equipment.

Excluding cases where small diameter (1" or 1.25" OD) tubing is used and the gooseneck has a very large radius, the tubing is plastically bent six times during every trip in and out of the well (Fig. 1). This cyclical plastic bending results in low cycle fatigue, normally considered to be of the order of up to 103 cycles to failure.

Internal pressure is frequently present in the tubing during the process, thereby super imposing additional circumferential, radial and longitudinal stresses. These additional stresses have tow effects. The first is a reduction in the fatigue life simply due to the introduction of a three dimensional stress system and the second is the introduction of an additional failure mechanism. Each time the tubing is bent, it dilates perpendicular to the plane of bending. The dilation is constrained by the material remaining elastic along either side of the neutral axis and is funded by a thinning of the wall at the inner and outer surfaces on the bend. Tubing dilation can in itself render the tubing redundant since there is a +0.05" maximum tolerance at the stuffing box. As the internal pressure is increased, the hoop stress becomes more dominant in the mode of failure. At burst pressure, the fatigue life will be zero and the tubing will rupture, shearing through the wall at the surface farthest from and parallel to the neutral axis.

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