Riserless Coiled-Tubing Well Intervention
Authors: Håheim Svein , Hoen Christopher , Heigre Birger , Heim Edgar , Windsland Øystein 
Published: October 2008

Abstract
This paper describes a concept for subsea coiled tubing well intervention, without the use of a workover riser system. Coiled tubing analyses and design, surface and subsea equipment design, vessel and operational requirements and limitations are addressed.
The novel steps in the development are the lubricator system, where the lubricator resides on top of the injector, and the use of a tensioned coiled tubing configuration with a heave compensation system and a second coiled tubing injector at surface.
A comparative risk assessment has been performed, and concluded that the overall personell and environmental risk is the same or reduced compared to a workover riser based system.

Introduction
With the increasing number of subsea wells, there is a need for more cost-effective intervention methods. For offshore platform wells, typical recovery factor (for the North Sea) is in the 50 - 60 % range, whereas for subsea wells, this number is significantly lower, typically in the 30% range. One of the main reasons for this is the low intervention frequency in subsea wells. 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.
Intervention in subsea wells traditionally involves mobilizing a drilling rig and running a workover riser system [1]. With this approach several days of operation is required before any productive work in the well can commence due to the time required for positioning and anchoring of the rig and installing the workover riser system. While the use of dynamically positioned (DP) vessels has eliminated the time required for anchor handling, running of jointed workover riser (WOR) is still a time consuming operation [2].
Operators also tend to use contracted drilling rigs for drilling and completing wells rather than intervention operations, where the outcome is in general more uncertain. While production logging and well test data is usually available for platform wells, this type of information is in general not available for subsea wells, making planning of the intervention operation difficult. Usually diagnostic runs, i.e. production logs are included in the intervention campaign, as well as mobilization of equipment for various outcomes of the diagnoses. For a platform or onshore well, the intervention campaign often continues at another well while i.e. production logs are being analysed and additional equipment is mobilized. This is not practical for subsea wells, with the possible exception of templates or other configurations where the wells are in close proximity to each other so the rig may be moved between wells without pulling the workover riser.
Experience from platform wells indicates that well intervention has higher risk of incidents and accidents than drilling and completion operations. When the risk of operating with a workover riser with full well pressure up to the vessel is added to the equation, the risk of incidents and accidents is above operators' comfort level, especially in harsh weather areas such as the North Sea.