A Tapered OD Coiled Tubing System (TODCTS) enabling the deployment of multiple outer diameter (OD) coiled tubing sections in a single string is being developed.1 By employing larger OD tubing sections near the top of the string and smaller OD sections near the bottom of the string, the tension along the string length is reduced while maintaining sufficient flow capacity for well intervention operations. The TODCTS can enable intervention service in deeper wells or installation of multiple OD velocity strings with the diameters optimized based on well production requirements.
This paper describes the development plan, system and component design, and the results of analyses and tests conducted to verify that the the tapered connector and surface equipment meets the requirements necessary to deploy the connector and adjoining strings of different ODs.
The advantages of multiple or variable OD coiled tubing in well servicing and velocity strings applications, resulting from a lighter weight string and improved flow characteristics are well documented.2 By employing a multi-company development team approach, a system for deployment of coiled tubing strings with multiple ODs is becoming a reality.
The TODCTS will extend the current depth capability of high pressure coiled tubing operations in deepwater/deep measured depth (MD) wells. The design is targeted to service offshore wells with depths in the range of 32,000 ft. MD and be compatible with 15,000 psi wellhead pressure control equipment. Velocity or coiled tubing production strings with ODs that are smaller near the bottom of the well and larger near the top can be deployed to maximize produced fluid rates.
The project is the team effort of an operating company, service company and equipment suppliers that are working together successfully to develop and deploy this new technology.
The main issues addressed in the development project are the manufacture and welding of the tapered connector for transitioning between constant OD coiled tubing sections and the interface of the transition with the coiled tubing surface equipment. The diameter transition, accomplished by a tapered connector, must be able to be spooled off the reel, over the gooseneck, through the injector, stripper and blowout preventer (BOP) stack while maintaining tension loads and a wellhead pressure seal without damaging the coiled tubing string or connector. As part of the project, tools are being developed to accelerate the design and fabrication of different tapered connector sizes and surface equipment for the diameter ranges of interest. In addition, the coiled tubing management software is being modified to design and maintain strings with multiple ODs.
In the first phase of the project, a system is being developed for deployment onshore in a lower risk environment. With successful land deployment, a larger diameter transition will progress, enabling intervention in deep offshore wells. Other diameter transitions would be developed based on market demand. The following target requirements were established cooperatively between the operating company and service company:
The tapered connector should be the minimum length and shape to minimize stress concentrations in the tubing transition and provide adequate interface with the surface equipment. The connector and surface equipment had to be designed/modified iteratively to meet the system requirements.
Initial sizes: Phase I - 2-in. OD × 1.75-in. OD, 0.156-in. wall thickness; Phase II - 2.375-in. OD × 2.00-in. OD, 0.203-in. wall thickness.
The tapered connector will be installed with standard butt welds between adjoining coiled tubing sections.
120-ksi yield strength steel coiled tubing material.3
The tensile strength (yield and ultimate) of the tapered connector, including the welds, shall be at least 100% of the smaller OD tubing adjoining the tapered connector, including the weld.