Analysis of Methods for Handling of Cemented Casing in Subsea Wells

Abstract

Plug and abandonment (P\&A) of subsea wells often involves removal of steel casing. In some parts of the well the casing is cemented in place making the operation of retrieving the casing more complex. The conventional methods for retrieving casing are unpredictable in terms of time consumption and consequently cost.This thesis has investigated the current guidelines and regulations that apply for P\&A. Further, the conventional methods for removing cemented casing have been analyzed and compared to alternative approaches that have not yet been field proven. A case study on a Cut and Pull operation performed by Statoil was assessed to identify key operational issues. The case study suggested that there is a need for new approaches for freeing cemented casing.An alternative method for removal of cemented casing has been proposed, namely the Spiral Cut Approach (SCA). Laboratory testing of small scale spiral cut specimens were performed, and it was found that the spiral cut allows the cement to debond successively along the surface of the specimen exposed to torsional loading, thus freeing the casing. FE-analysis were performed to investigate the criticality of the spiral cut pitch and the required torque for a fullscale casing.The method is based on debonding one joint of casing from the cement before moving on to the next casing joint. After the spiral cut has been made between the casing couplings, each casing joint is exposed to sufficient torque to overcome the cement bond between each casing joint (approximately 12m). Finally, a drillstring deployed pulling tool is connected to the upper section of the casing string and the casing is retrieved in one go.Operational steps and tools necessary to perform the SCA have been assessed. The laboratory testing, FE-analysis and operational assessment indicated that a significant higher number of casing joints can be pulled in one go using the SCA. A time comparison based on the case study and an assessment of the SCA, indicated that a total time saving of 2 days can be made by using the SCA to retrieve 100m of cemented casing. However, more work will have to be carried out for more detailed verification of the SCA. This includes development of a effective tool for spiral cutting of casing, a combined tool for applying torque and pull for retrieval of cemented casing and lastly an assessment on the effects of using extended diameter couplings.