A novel solution for plugging of subsea wells
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In recent years, Statoil and other operators on the Norwegian continental shelf (NCS) have publicly announced the need for innovation and new solutions that can improve the efficiency, and reduce the cost of plugging and abandonment (P&A) operations. The industry wants to use cheaper light well intervention vessels (LWIV) for plugging operations, but the technology that exists today is limited to riser capable vessels only. The aim for this thesis is to investigate new concepts for rigless P&A, and propose adaptations to further improve already existing P&A technologies. A thorough investigation of challenges on how to adapt existing technology for riserless P&A operations, and the development of new concepts has been conducted. The main challenges with riserless P&A, is the limited circulation possibilities, incapability of removing tubing, and placement and verification of cement plugs. This thesis presents several concepts to solve some of the challenges regarding P&A operations using a LWIV. Several concepts for tools capable of riserless P&A have been developed and evaluated. Additionally, evaluation of two concepts for riserless CT systems have been conducted. The evaluations are based on feasibility, complexity, repeatability, reliability and efficiency. The conclusion from these evaluations were that it would be faster to develop riserless CT systems, than to design, test and verify completely new plugging concepts. It s also performed calculations indicating that the efficient and proven P&A method, perforate wash and cement (PWC), can be adapted to coiled tubing. These calculations substantiate the conclusion mentioned above, since LWIVs can utilize this method when a riserless CT system is implemented. Further research should focus on continued development of the riserless CT concept. At the same time, the PWC method should be further developed and tested on coiled tubing system through a riser on a rig to verify the functionality, while the riserless CT concept is being developed. Additional proposals would be to research and further develop the wireline tools presented in this thesis.