Operator Reviews Decommissioning Options for Deepwater Flowlines and Risers
Authors: Carpenter Chris 
Published: June 2021

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30397, “Deepwater Flowlines and Risers Decommissioning,” by Azam Syah Jaafar, Ahmad Taqiyuddin Obaidellah, and Khairul Anuar Karim, Petronas, prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 2–6 November. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission.
The complete paper reviews an operator’s deepwater field project offshore Mauritania, for which several techniques were considered with respect to decommissioning a subsea pipeline system at water depths of 700 to 960 m. Options included preservation for potential future use, leaving in situ, and full recovery. The paper covers only the operator’s deepwater subsea flowline and riser decommissioning experiences and reviews activities from planning and concept to operational activities such as pigging, flushing, cleaning, disconnection, and retrieval.

Case Study
The operator’s deepwater field is 80 km from the Mauritanian coastline. It has been in production since 2006 but has become uneconomical. The operator decided to cease production and subsequently conducted a field abandonment and decommissioning (A&D) study.
The field was developed using subsea wells, manifolds, umbilicals, flexible flowlines, and flexible risers tied back to a permanently moored floating production, storage, and offloading (FPSO) vessel at a depth of approximately 800 m. The FPSO was moored by an external turret connected to three clusters of mooring lines attached to piles in the seabed. All subsea wells were connected to the FPSO by five flexible pipeline systems (including dynamic flexible risers) and one umbilical system. All flexible pipeline systems were meant for the transportation of production fluids, water injection, gas injection, and gas-lift services.
Because the FPSO is a turret-moored type, in which the turret is externally positioned at the FPSO’s bow, all flexible risers were designed to decouple the load and motion at hangoff from touch-down point with a lazy-wave configuration. The lazy-wave configuration was obtained by clamping sets of buoyancy modules on the risers.
The execution of abandonment and decommissioning work has been divided into two main scopes: restoration of well integrity by installation of deep and shallow set plugs and FPSO disconnection and demobilization, which covers subsea facilities, risers, umbilical and flowline flushing and cleaning, topside process plant flushing and cleaning, pipeline systems and mooring-lines disconnection and retrieval, and FPSO sailaway. A plan for disposal of all resulting waste also was incorporated.

Guidelines, Rules, and Regulations
No universal statutory requirement, standard, or recommended practice existed to fully address pipeline decommissioning. Nevertheless, countries with more-established offshore industries (e.g., Norway, United Kingdom, US) have developed local legislation and practices that have been implemented in these respective jurisdictions.