Special care must be taken to maintain the integrity of the coiled tubing when performing subsea well intervention from a light vessel without the use of a riser. In these cases the coiled tubing will act as a riser with respect to withstanding the external loads from waves, current and vessel motions. To avoid local buckling of the coiled tubing as it enters the subsea lubricator it is important to keep the vessel positioned such that the bending of the coiled tubing is kept as small as possible.
This paper will outline a method for estimating the optimal set point for the vessel position during riser less coiled tubing operations. The method is based on a combination of measurements and analytical methods for description of the structural behaviour of coiled tubing and the subsea stack. The analytical method developed is compared to simulations using commercial riser analyses software.
With the increasing number of subsea wells, there is a need for more cost-effective intervention methods. While platform wells are commonly intervened as often as 2–4 times a year, subsea wells are in general only intervened if absolutely necessary, i.e. in case of barrier failure or dramatically reduced production. The main reason for this is the cost and difficulty associated with subsea well intervention.
To significantly reduce the subsea well intervention cost, systems that can be used efficiently from a dynamically positioned (DP) monohull vessel is required. Riserless wireline intervention tequniques has been developed over a number of years [1–3], and can now be considered mature technology. Subsea wireline systems are used on a routine basis, at least in the North Sea, and several rental systems are available in the market.
While wireline is suitable for well diagnostics such as production logging or sampling tools, remedial operations such as running plugs, straddles, stimulation and perforation are more efficiently performed with coiled tubing, especially in highly deviated and horizontal wells. This is the motivation for development of a riserless coiled tubing system.
One of the challenges in developing a riserless coiled tubing system is controlling the behaviour of the coiled tubing. The coiled tubing will act as a riser with respect to withstanding the external loads from waves, current and vessel motions. To avoid buckling and excessive stress on the coiled tubing it is important to keep the vessel positioned such that the bending of the coiled tubing is kept as small as possible, i.e. at surface and subsea where the coiled tubing enters the lubricator system.
A riserless coiled tubing system is described in . The two novel features of this system compared to previous solutions  is the use of tensioned coiled tubing between the vessel and the wellhead and a new lubrication system where the lubricator is placed above the coiled tubing injector, Figure 1.
The Well Barrier Packages and X-mas tree adapter shown in the figure are conventional. By mounting the lubricator above the injector package, the bending moment on the X-mas tree adapter is considerably reduced, and tool string deployment and retrieval is considerably simplified compared to a surface type coiled tubing system where the injector needs to be lifted off when changing toolstrings.