Subsea live well intervention using a riserless/open water coiled tubing system - System overview and operational weather limitations

Abstract

This thesis gives an introduction both to a conventional CT operation setup and to an open water coiled tubing (OWCT) system and focuses on the main differences. An overview of proposed OWCT concepts is briefly presented. To assess operating limitations of an OWCT system consisting of several components, it is important to identify the weaker parts of the system. A specific OWCT system was modeled and analyzed using the Finite Element Analysis (FEA) software Orcaflex. Orcaflex provides system response loads which are post-processed for code check analysis. The analysis has been supported and verified by hand calculations. Based on the results from this analysis it is evident that the CT string is the weak link in this specific modeled OWCT system. This is not a general conclusion, but is a general recommendation based on the presented work. A CT string with a larger structural capacity or a subsea stack component with reduced structural capacity could change this conclusion for a different OWCT setup. Applied top tension plays a significant role for operating limits in an OWCT system. Both excessive and inadequate tension has a negative effect on the CT string. However, no guidelines exist to properly set the value for top tension in the CT string. Based on the present work, the applied top tension during an operation should be as low as possible, but sufficient to ensure positive effective tension in the entire CT string. A general procedure for determination of applied top tension in a tensioned heave configured OWCT system has been established and is presented as a recommendation.