More and more offshore structures and facilities are coming to the end of their life cycle, and therefore decommissioning has become a critical phase of the industry. These facilities have to be decommissioned according to the local regulations, environmental and stakeholders’ requirements. As part of the effort, the International Maritime Organisation (IMO) Guidelines and Standards for the Removal of Offshore Installations and Structures on the Continental Shelf and in the Exclusive Economic Zone, adopted by IMO Assembly on 19 October 1989, (Resolution A.672 (16)), set out the international standards for the offshore decommissioning activities. This guideline is often used alongside with the local regulations, for example in the North Sea, the offshore decommissioning activities are governed by the Pollution Control Act and Norwegian Petroleum Act in the Norwegian sector, and Petroleum Act 1998 and Energy Act 2008 in the UK sector.

Detailed studies and planning should be carried out as the decommissioning phase is as critical as the initial installation phase. In most cases, we need to find the right balance between the technical feasibility, EHS (Environmental, Health & Safety) concerns, and the cost involved for the whole program.

As a minimum on EHS requirement, an Environmental Aspect and Impact Assessment (EAIA) should be prepared to identify and evaluate the technical and environmental issues associated with the nature of decommissioning operations.  Also, it is critical to implement the Hazard Identification and Risk Assessments (HIRA) process prior to executing the decommissioning operation, to ensure that operation hazards are identified so that appropriate controls can be implemented to reduce the potential risk of loss to personnel, operations and the environment. These processes are critical and should not be bypasses in any decommissioning operation. To name but a few the potential hazards and environmental impacts arising from the decommissioning operation,

  • Leak of hydrocarbon, drilling mud, chemicals to sea and handling onshore
  • Heavy metal such as lead or mercury that are present in coated structures which can be release during removal or flame cutting
  • Toxic or flammable gas release and explosion during hot work
  • Radioactive material
  • Loose components and potential dropped objects
  • Asbestos present in building materials

There are several technical challenges linked to decommissioning during the planning stage:

  • Acquiring up to date or accurate as-built reports, drawings and inspection records of asset condition
  • Condition and structural integrity of existing structures due to corrosion, wear and tear. Certain level of engineering assumption as per approved codes and standards , as well as best practices shall be applied
  • Added weight of structure due to marine growth
  • Uncertainty of weight and COG of structures
  • Seabed condition
  • Accessibility subsea for ROV using diverless option

Therefore steps as below shall be taken:

  • site survey and verification of integrity (eg. Non Destructive Testing, Flooded Member Detection),
  • site preparation, hook down and SIMOPS planning
  • Selection of correct equipment to suit the task (eg. diamond wire cutter, abrasive water jetting)
  • intensive trial testing to simulate the operations where possible

In general, our engineering analysis and procedure development will be based on the guidelines laid down by Recommended Practice DNV-RP-H102 (Marine Operation during Removal of Offshore Installations). The load analysis, structural analysis and capability check, equipment (including vessel) selection and operation method statement development will be performed as part of the decommissioning engineering preparation works.

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Some of the general guidelines for the offshore decommissioning work:

1. For decommissioning of subsea pipeline, prior to cutting and removal from seabed, the system must first be made hydrocarbon free. The pipeline shall be deoiled (oil) or degassed (gas) by running a series of pig trains. The oil can be collected into tanks or filtration system and followed by special gel slugs being run to further clean the pipeline. For gas pipeline, inert gas or Nitrogen can be used to ensure it is gas free. Once the system has been “make safe” only the removal can be implemented. The rigid pipeline can be recovered using few methods such as recovery and reverse S-Lay using pipelay vessel via stinger, underwater cutting and lift, or tow method. For flexible flowline, these can be removed by reverse spooling or recover on deck for cutting.

2. Subsea structures like tree, manifold, PLET and template can be lifted to the surface using crane vessel for decommissioning. Lighter structure can be recovered by light construction type vessel, however bigger structures like template or manifold will require Heavy Lift Vessel or even Semi Submersible Crane Vessel. Sometimes subsea cutting may be required to split the structure to allow for lighter weight.

3. For a more complex decommissioning work such as topside removal, detailed work packages need to be developed with the involvement of multi engineering disciplines. For example, we need to determine the connection/ break points to be removed so that the topside can be lifted in modular sections. Unique lifts such as derrick drill package and flare boom shall be studied in detail. Piping and pressure vessel system have to be cleaned, flushed or isolated. Electrical cables and equipment need to be de-energized and isolated. It can be said that the topside decommissioning operation is far more complex than their original construction.

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