The 60-tonne caisson lifted from the quayside to the vessel to transport offshore
As mechanical handling contractor, we produced the lifting plans, engineered bespoke lifting frames/lifting points, and supervised the offshore implementation
The removal and installation of the caisson were completed within a three-week timeframe
Over the past few years, Altrad Sparrows has undertaken work for a major North Sea operator to ensure the integrity of caissons and conductors aboard two of its assets in the West of Shetland region.
The operator is a long-standing client of Altrad Sparrows, and by providing them with cost-effective solutions that are safer to execute and easier to implement, we can help them ensure their assets’ integrity while minimising risk to personnel and disruption to operations.
With comprehensive lifting plans in place, we were able to complete work relating to caissons and conductors in short time frames, maintaining our reputation for delivering fast, cost-effective solutions that minimise risk to personnel, the asset, and the surrounding marine environment.
The workscope included:
- Providing temporary solutions as winterisation measures
- Installing permanent, robust solutions that ensure continued operation of safety-critical equipment
- Completing processes in tight timeframes to ensure safety and maximise efficiency
- Working alongside the client’s subsea and projects departments, as well as several other vendors
- Identifying potential hazards and opportunities for failure in advance and having a plan to quickly and safely rectify these should they arise
Clamp Installations on Caissons and Conductors
Following an inspection campaign in 2019, crack-like indications were found on structural welds. Without repair, the caisson posed a serious safety risk as it could fall, causing damage to the asset and subsea pipelines below.
In 2020, we installed lateral restraint rigging to secure the caisson as a temporary solution for the section most exposed to the harsh weather elements and then returned to the platform in June 2022 to install a permanent clamp.
Within the same inspection campaign, defects were also found on structural welds between the jacket stub and a conductor slot guide barrel. Again, to temporarily guard the area above sea level against bad weather, PDO rigging was installed in 2020.
A conductor clamp was subsequently installed cross-haul under the cellar deck route at one of the conductor slots in July 2022, and the second conductor clamp is scheduled to be installed in 2024.
Caisson Removal
The operator also discovered that part of the outer pipe enclosing an open drains caisson had severed and fallen from the platform to the seabed. In August 2021, our team installed a new caisson, complete with two guide clamps and a DWS clamp.
The removal of the caisson requires three distinct stages. Firstly, the equipment encased by the caisson would need to be removed in two phases, followed by the removal of the caisson itself. With a distinct “point-of-no-return” moment and just three weeks to fully complete the caisson removal and installation of a new one, every part of the plan needed to be carefully considered. As safety is the most crucial aspect of the work for all parties, time was taken to troubleshoot the entire process and ensure we had a solution for any problem that may arise.
Lifting a 60-tonne caisson must be completed in a specific, sequential order to avoid risk to personnel and the asset. Therefore, the experience of lifting specialists was vital for this project, as they understand the components of an effective lift plan and can assess every stage of the lift while identifying worst-case scenarios and ways to mitigate these risks.
Planning and Executing the Caisson Removal and Install
To ensure a successful outcome, we collaborated and worked as ‘one team’ with the client and all relevant vendors, to deliver the best solution. With several members of the Altrad Sparrows team working together on this project, including engineering, design, fabrication, storage, shipping, and riggers, we provided an experienced team with a wide range of skill sets.
A ‘storyboard’ was mapped out, which laid out the lifting procedure step by step and provided the list of deliverables needed to ensure the project was a success. By continually returning to and reviewing this sequence of events, we were able to pin down the finer details of every stage, ensuring it would be possible to carry out the work plan aboard the asset.
Delivering the project in a manner where 90% of the work offshore is preparation, we can minimise required deck space and heavy rope access work. This reduces disruption to the platform, and with less construction and personnel required, the project becomes more environmentally friendly.
A rigging supervisor was stationed on the operator’s platform to ensure the lift was executed correctly and safely at each stage. With just 16 hours available to complete the process, our extensive planning came into play, and we could carry out the work methodically, safely, and quickly.
Due to mapping out all potential scenarios in our comprehensive plan, when we came across an issue with the slings, there was a plan in place to rectify it quickly, allowing the subsea contractor’s ROV to connect them to the caisson.
High Praise from the Operator and Vendors
Following the completion of this work, we have received positive feedback from the client and the vendors we worked with to make it all happen.
Specifically commenting on how the volume of the design lift plan was streamlined and the high-quality standard of our design lift plans, the feedback shows that we delivered the project in line with the client’s expectations and provided them with cost-effective solutions that minimise disruption while ensuring the long-term viability of their asset.
Work will continue across all the client’s assets to ensure the integrity of caissons and conductors for years to come. This includes installing another conductor clamp in 2024, with the potential to broaden the workscope in future.