Due to limited crane capability, some coiled tubing (CT) strings are transported in two or three sections to offshore locations, requiring the use of welding or mechanical connectors for reassembly. CT utilization subsequently can be hampered by welding issues as well as lack of integrity of the connectors. Field welding techniques and commercially available spoolable connectors can decrease the fatigue life of CT string to 35-70% of the base tube.
This paper describes the development of a mechanical spoolable connector with a fatigue life as good as that of the base pipe. The connector was designed using extensive nonlinear finite element analysis (FEA) design iterations. The connector pressure rating is equal to that of the pipe, and the connector maintains its sealing ability throughout the fatigue life of the pipe—even on very aggressive radius drums and tubing guides. The connector conforms very well when spooled to the drum without "sticking out." The connector assembly OD fits flush with the CT string, providing continuous gripper engagement. Single or multiple connectors can be used to join sections of coiled tubing in the yard or on location.
The new connector uses material certified for sour wells and acid jobs, and its seals are compatible with a variety of chemicals, acids, and sour service conditions.
In addition, the testing of the connector is discussed. The connector design performed very well according to several prototype tests on a fatigue machine and full-scale yard test. The results demonstrate that the new spoolable connector can increase the utilization of CT with various configurations and enable operations previously not deemed feasible due to various limitations imposed by other joining techniques.
Introduction
As the offshore industry has moved to accessing deeper reserves, the coiled tubing (CT) work strings required for intervention operations have become longer and heavier. By far, the largest and heaviest component of a CT unit is the coiled tubing itself and the reel on which it is handled. Often, due to the weight of a CT reel and the inability of a crane or other device on the offshore location to lift it, the CT must be placed on multiple reels before being transported to the job. Because coiled tubing work is based on a continuous string of tubing, the CT work string must be assembled before the job is initiated. For onshore jobs, the weight of the CT reel usually is not a significant problem since cranes able to handle single high-capacity reels are typically available. Offshore, weight handling presents a bigger challenge as many platforms are not designed to lift the heavy CT reels off the workboats and onto the platform.
Once the coiled tubing is delivered to the platform on multiple reels, another major challenge is joining the CT sections together into a continuous string. Until recently, the preferred method has been to connect the CT sections by butt-welding the ends of the CT; however, this can be an expensive and time-consuming proposition. First, welding specialists are required to set up the operation and, in most situations, a special work permit is required and a discrete area on the rig must be used. Second, in many instances the well(s) must be shut in for the duration of the welding job for safety reasons. Finally, the welds have to be X-rayed by other specialists with more equipment.
Ideally, a mechanical, spoolable connector should have the same fatigue life as the base tubing, while maintaining mechanical strength similar to that of the base tubing. The best results published thus far in fatigue life tests are 50 to 60% of the base tubing (Luft, Laun, and Thov 2004). This paper discusses the design and testing of a novel mechanical spoolable connector having a fatigue life equal to that of the base tubing.
