10. Decommissioning

Decommissioning of offshore structures is still a major challenge for health and safety issues. It also forms a significant risk to the environment, if done incorrectly. The incentive to save money, by putting less emphasis on decommissioning strategies is large. This view can be short-sighted because of two reasons.

The initial investment has a positive effect on the cost reduction during abandonment. In order to make plugging, abandoning and decommissioning cost-effective, requirements for them are already taken into account during the design phase of the structure.

The responsibility for post-operational accidents and structures remains with the operator. A thorough research and investment in decommissioning is therefore recommended.

Regulatory Requirements

Decommissioning of any structural elements shall be done conform the Guidelines and Standards of the International Maritime Organisation (IbaƱez, 2011). This means that every structure installed after 1998 must be completely removed after its abandonment. Where requirements of the Vietnamese Government are more stringent, the IMO regulations are supplemented. Moreover, it is expected that by the time of decommissioning the requirement will be more demanding; thus decommissioning should be considered in detail.

Elements to be decommissioned

  • Sub surface
  • Subsea Well Tubing
  • Subsea Equipment
  • Pipelines
  • Surface
  • Topside
  • Connecting buoy with risers and mooring lines

Subsea well tubing
Wells will be cleaned after operation. Special attention will be given to the subsea well tubing because corrosion can undermine the integrity during sealing operations. Then, the well tubing and well bore will be plugged with cement.

Subsea equipment
All pipelines and subsea equipment that are located on the surface of the sea floor will be removed after cleaning residual hydrocarbons that might still be in the tube. Equipment on the surface of the seabed will be removed using standard work over vessels. After that, the wellhead can be cut off below the mud line and removed.

The removal of pipelines is still not as clearly regulated as other structural elements. This means that there are several possible scenarios for the abandonment. Amongst them: leave in-situ and undertake remedial action; trench and bury; recover and take to shore. Interesting tools are developing for this operation and the option by the time of decommissioning will be much more.

The Floater
The decommissioning of the FPSO does not pose any substantial problem. After the field life the FPSO will simply sail to a designated shipyard, where it will be dismantled or converted. Given the large fleet of vessels of this size worldwide, there are numerous shipyards to choose from, and they are also capable of dealing with the topside facilities.

Risers and mooring lines
After disconnecting the FPSO, the mooring lines and risers will have to be removed. Risers will be cleaned and disconnected from the subsea structures. Anchors and mooring lines will be removed using anchor handling tug vessels.

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