A study of wireline and downhole position control for riserless light well intervention in deep water

Master's thesis in Offshore TechnologyThe light well intervention for subsea wells using riserless method has been carried out extensively over the last 10 years to increase the oil and gas recovery from depleted wells. Riserless Light Well Intervention (RLWI) is a relatively new technology that can be used to replace the majority of the well interventions previously done from semisubmersible rigs. RLWI performs subsea well intervention without using a drilling riser package and a subsea BOP. Alternatively, the Well Control Package (WCP) will be installed on top of subsea Xmas Tree as a barrier replacing BOP and then downhole tools can be deployed into the subsea well utilizing wireline from intervention vessel. The RLWI method is expected to reduce the cost and time in performing well intervention compared to conventional method. But when it comes to lowering down the downhole tool, it has a major issue regarding the accuracy in determining the operational well depth for “in well” work. During RLWI operation in deep water, the environmental conditions will have much influence to the wireline part that is suspended in open water. It is found very difficult to measure part of the cable length that has been submerged into the well since it depends on the “deflection shape” of the cable after it get exposed to the varying sea current from surface to sea bottom. The environmental load from sea current acting along the cable length will be transformed into uplift force on the other end of the cable inside the well. The combination of vertical displacement in upward direction due to uplift force and in downward direction due to cable stretch will affect the accuracy of the well depth measurement and position control of the downhole tool. Depth accuracy is very crucial in performing work inside the well. This project will study the cable behavior under prevailing dynamic sea environment for RLWI wireline operation in North Sea and will present one approach to correct inaccuracy in determining the exact location of the downhole tool inside the well from marine perspective. The study case was taken from Aasta Hansteen field with 1300 meters water depth in particular. OrcaFlex software will be utilized to perform static and dynamic analysis of the wireline cable under sea current and wave effect. OrcaFlex was developed by Orcina to support a wide range of hydrodynamic and marine structural analysis, including modeling the cable under sea current. During the analysis, the wireline behavior under currents will be observed for different operational well depth cases. The analysis results obtained from OrcaFlex simulations are vertical displacements and forces acting upon the wireline. Then recommended measures are discussed in this thesis to maximize the accuracy during “in well” work with different types of downhole tool weight. In addition, this thesis will also discuss the latest technologies of light well intervention, such as well control equipments used nowadays in RLWI operations