Application of an Instrumented Pull-in Head during the retrieval of a flowlinecontaining a hydrate plug and oil inventory

a case study of the MLL-20 GL lineat FPSO Cidade de Niterói

Abstract

The decommissioning of subsea flexible flowlines containing hydrate plugs and residual oil inventory presents a critical technical and economic challenge in deepwater and ultra-deepwater environments. This paper introduces an innovative methodology for the safe retrieval of lines with complex hydrate blockages. This methodology was successfully developed and implemented during the decommissioning of the flowline GL from well 3-MLL-20D-RJS (FPSO Cidade de Niterói). Conventional methods, such as the widely used N₂ lift, had previously failed to clear a long blockage, incurring substantial costs. To overcome this persistent challenge, a new protocol was centered on the use of an Instrumented Pulling Head (IPH). This paradigm shift leverages the natural thermodynamic changes occurring during the retrieval operations specifically, the simultaneous pressure reduction and temperature increase to promote the safe and controlled dissociation of the hydrate. The instrumented pulling head provides real-time monitoring of the line's internal pressure and the applied pull tension. This capability enables active process management, effectively mitigating the critical risks of overpressure within the line and subsequent mechanical failure. The application of this novel protocol, which is 100% diverless and guided by controlled phases, resulted in the complete success of the operation, ensuring the full retrieval of the line and the safe removal of the residual oil inventory. This approach achieved final cost savings exceeding 95% compared to the previous failed attempts. These results unequivocally demonstrate the effectiveness of the proposed methodology, establishing a new benchmark for safety and efficiency in the retrieval of subsea lines affected by complex blockages.