Decommissioning Caissons & Conductors: Removal vs. Jacket Fixation

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. The engineers will assess these vital points. However, they will need dedicated mitigations to overcome integrity surprises.

Marine Growth

Marine life is attached to the caissons and conductors, creating two major project impacts:

• The weight of the caissons and conductors is increased significantly. 
• The caissons and conductors might not be able to pass the guides.

Cleaning these is difficult. There are special tools available that have limitations. Another option is to use divers and ROV systems. However, the caissons and conductors are installed inside the jacket. Therefore, they are 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 gasses, for example. Mitigating measures will need to be established and implemented to avoid harming people and nature.

These measures will be proposed after tests have been carried out. Prior to the 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. With the required Personal Protective Equipment, the job is certainly not easy. 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 decommissioning 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 the campaign itself. The selected subcontracting strategy impacts this consideration. If the topsides and the jacket are removed separately and subcontracted in one package, letting the heavy-lift contractor remove the caissons and conductors in the same campaign will make sense.
If the asset owner 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 ensure that the actions taken will result in a situation that 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 caisson or conductor's top part and connecting 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 in preparing the caissons and conductors for decommissioning. 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