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Subsea DeCommissioning Philosophy

Published on 20 Mar 2018 by Emma Leoni


This document is to detail the requirements for decommissioning of the subsea systems, flowlines and wells.

Wells, subsea and flowline systems will be decommissioned following the end of their operating life, in accordance with the plan of abandonment that will be submitted to the Ministry of Petroleum (of the applicable country) for approval prior to a predetermined percentage of the recoverable reserves have been produced.

The decommissioning requirements are as follows:

  • The subsea infrastructure and wells must be designed to allow for well plug and abandon with monitoring for integrity management requirements based on Risk Base inspection and Review assessment
  • Decommissioning activities for subsea equipment must be in accordance with Legislative and Regulatory Approval Compliance according to the host country
  • Subsea wells must be plugged and abandoned in accordance with the Drilling and Completions Well Barriers Procedure (NORSOK Standard D-010 or required Company Standard)
  • All subsea equipment (e.g. Christmas Trees, wellheads) must be capable of being removed from its location in the field
  • Subsea equipment including, but not limited to the tie-in spools, flowilnes, FLETs and umbilicals must be able to be flushed with fluids and returned to the FPSO production system to an ALARP residual oil in water content
  • To facilitate FPSO End of Field Life sail away the risers will require disconnection and laying on the seabed for in-situ decommissioning or future phase recovery. Good industry practice is to remove buoyancy modules from risers at the time of disconnection to mitigate uncontrolled release
  • The system should be designed to minimize the monitoring required to demonstrate facility integrity post completion of operations and until decommissioning
  • Subsea wells should be designed to facilitate the abandonment requirements described in the Drilling and Completions Well Barriers Procedure (NORSOK Standard D-010 or required Company Standard)
  • The design of well jumper / spools, flowlines subsea umbilicals, HFLs / EFLs should facilitate decommissioning activities and designed to be capable of being removed from the seabed
  • Wellhead design should facilitate decommissioning activities
  • The subsea system design should be designed to allow flushing with sea water or other environmentally accepted fluid which may be displaced to the environment during decommissioning activities
  • Dead legs in the subsea system should be avoided to allow ease of hydrocarbon removal
  • Flushing fluids should be returned to hydrocarbon separation facilities for treatment
  • Adequate isolations should be provided so that subsea system hydrocarbon removal operations and riser disconnection method can be performed accordingly
  • Isolations should be provided so that subsea system flushing and cleaning operations can be performed accordingly
  • Gas Injection wells should be capable of being flushed from subsea to topsides process to an ALARP residual oil in water content



Tags: subsea flowline decommissioning

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