Decommissioning: Caisson Removal versus Fixation in Jacket



Caissons and conductors can cause headaches in many decommissioning projects. Often the caissons are hanging from the topsides or standing on the seafloor and held in place by the guides or braces. They have been installed after the installation of the jacket. For the removal of the offshore facility, they need to be removed entirely or need to be separated from the topside and connected to the jacket. This is a difficult choice for decommissioning teams all over the world.

There are many variables determining the project approach. The associated costs form a big driver in the selection of the method, and safety is also of major concern. In this blog post, I would like to highlight some of the concerns and the conclusions from a study that was done at Conbit.

Integrity

The integrity of caissons and conductors is often unknown. From a distance, you will notice holes, cracks, corrosion, and brittle material that are of increasing concern. The project team understands that handling and storing the caissons and conductors require strong points. These strong points will be assessed by engineers, but will need dedicated mitigations to overcome integrity surprises.

Marine growth

Marine life is attached to the caissons and conductors. This will have two major project impacts. Firstly, the weight of the caissons and conductors is increased significantly and secondly, the caissons and conductors might not be able to pass the guides.

Cleaning of these items is difficult. There are special tools available that have limitations. Also, divers and ROV systems can be used. However, the caissons and conductors are installed inside the jacket and, therefore, hard to approach, especially in the splash zone.|

Hazardous substances

The existence of hazardous substances cannot be neglected. There is a reasonable probability of the presence of NORM, Chrome IV, or unwanted gases, for example. To prevent harm to people and nature, mitigating measures will need to be established and implemented.

These measures will be proposed after tests have been carried out. Prior to commencement of the works, NORM values need to be measured, paint samples (may) need to be taken and analyzed, and holes (may) need to be drilled to sniff for gas. The results of these tests will form the basis for mitigation.

Removing caissons will often require some sort of breathing apparatus and specialized workwear.

Access

The activities take place in harsh environments and with the required Personal Protective Equipment, the job is not becoming easier. Certain areas will be accessible by either scaffolding or rope access. Both methods have their pros and cons.

The spider deck is often required for accessing cutting locations. In many decom projects, we have seen the need to reinforce the spider deck to make it accessible again. Reinstatement of the spider deck can easily be combined with the works on the caissons and conductors.

Transport

The way the jacket will be transported after it has been removed will impact the choice for removing or fixing the caissons and conductors. If the jacket is lifted vertically and positioned horizontally on deck, many reinforcements to the connections of caissons and conductors are required to withstand the additional load cases. In that case, the operator might prefer to remove the caissons and conductors prior to the heavy-lift campaign.

Timing

The caissons and conductors can be removed or secured in the heavy lift preparation phase, or they can be removed during the heavy lift campaign itself. This consideration is impacted by the selected subcontracting strategy. If the topsides and the jacket are removed separately and subcontracted in one package, then it might make sense to let the heavy lift contractor remove the caissons and conductors in the same campaign.

If the owner of the asset considers a wider competitive field, they might decide to perform the work to the caissons and conductors in the heavy lift preparation phase. In that case, they will need to make sure that the actions taken result in a situation which can withstand a few seasons at sea.

Costs

The financial part of the decommissioning project is very important. At Conbit, we studied the difference in costs for removing the caissons and conductors compared to fixing them inside the jacket. Fixing the conductors and caissons and separating them from the topsides is 50% more cost-effective than removing the caissons and conductors and positioning them on the topsides.

Conclusion

The choice for either method depends on many project-specific factors. I would recommend creating a workshop to foster the collective brains of specialists of different market parties.

Some considerations:

  • If the jacket is to be transported horizontally, consider removing the caissons and conductors.
  • If large amounts of marine growth are found between the lowest guide and the one above, consider removing the top part of the caisson or conductor and connect the remaining part to the jacket.
  • Use rope access as much as possible during the work on the caissons and conductors. Scaffolding will obstruct the handling routes. The only exception is in the case of extensive welding scopes.

    Conbit can support you for the preparations of the caissons and conductors in a decommissioning project. We can provide engineering support and perform the full operation on a turnkey basis. We offer our time to moderate your workshop on this matter.

By Deniz Him

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