Coiled tubing is being widely used as temporary or permanent production strings. The installation of a continuous string into a well reduces or eliminates threaded connections thereby minimising potential for leaks. It can be installed in live wells and the insertion or retrieval is generally faster than with jointed pipe.
Future applications of flush, continuous, single-flow pipe in for example slender well designs, all require a good field method of joining pipe sections, which will not compromise material properties or string integrity. The Big Loop method will also overcome the transport weight limitations associated with long and/or large diameter coiled tubing reels.
Considering that the entire pipe handling, connection and well insertion process can be conducted without the use of a rig, there are considerable incentives to use the Big Loop system on offshore installations.
This paper deals with a new concept where a complete continuous string, including subassemblies, can be pre-manufactured on site, tested and subsequently installed once the well becomes available. The connection time is taken out of the critical path of the construction or intervention process of the well.
The concept applies civil engineering pipe upto 7" body OD, which allows the selection of the optimum pipe size required for a particular well.
The pipe is orbital welded and spooled into a free-standing spiral using a bending machine. Plastic deformation is generally less than with general coiled tubing strings on reels. During installation of the string a straightener in combination with conventional coiled tubing or snubbing equipment is used.
To date, two strings have been successfully pre-manufactured, tested and installed. Further developments are aiming at corrosion resistant alloys, cladded pipe and continuous pumping capabilities during insertion.
Installing casing or tubing strings with leak-free and non-upset connections for a range of materials will increase flow potential and lower well costs on many wells. The absence of couplings, which are normally larger in diameter than the pipe itself, allows larger strings to be installed into conventional wells or the placement of conventional strings into smaller wellbores.
About a year ago we began to investigate and analyse the different field methods for the pre-manufacturing and installation of welded tubing strings. This resulted in the development of a technique where modern welding technology is combined with common heavy industry bending techniques
The string is prepared by welding 12 to 18 m joints together. The pipe is fed into a bending machine (fig. 1), which bends the pipe into a spiral. This spiral is suspended on a spiral support structure, see fig. 2. Two driven bottom rolls support the free-standing spiral which has no elasticity contained in the pipe. A top roll is used to provide stability to the spiral. The radius of the spiral is dependent on the maximum allowable theoretical strain. Generally it is kept at 1% or less, resulting in much lower fatigue and residual stress levels than seen with conventional coiled tubing strings.
For onshore wells, the above described pre-fabrication could take place at a safe distance from the wellhead area. Furthermore, the welding container can be completely sealed to comply with Zone I or Zone II regulations as required on most offshore platforms. In this way, the connection time is taken out of the critical time path; i.e. the string can be prepared while the well is being drilled, constructed or even produced.