A jacket is a steel substructure for an offshore installation. It is a complex construction, and designing one takes several months of engineering, 3D modelling, computations, design and method work.
Prefabrication and assembly are done at Kvaerner’s yard in Verdal, while components such as leg sections and bracings are subcontracted.
At the bottom end of each leg there is a cluster, through which the jacket is piled to the sea floor. Oftentimes, the piles will go 50 – 60 metres into the soil to secure proper fastening.
In order to fasten the jacket to the seabed, clusters have to be sturdy and heavy. Pictured above is the lifting of one of the Clair Ridge clusters, which weighs 847 tonnes. It was lifted 70 metres up in the air before being attached to the Clair Ridge jacket.
All jackets are prefabricated in parts and assembled horizontally. The aim is to do as much work as possible while the jacket rows are lying flat on the ground. This makes construction easier and safer, though it still involves work at height.
When no more work can be done while the rows are still in a horizontal position, a roll-up procedure is initiated. It involves many people, several cranes – and again lots of computations and engineering work to make sure the rows are raised safely.
When the rows have been rolled up, a lot of work has to be done by lifting in and installing equipment like risers, J-tubes, caissons and umbilicals. The last construction work is done, and the jacket is made ready for mechanical completion and client inspection.
Finally, it is loaded onto a barge that takes it to its final destination at sea. The transportation contractor generally does the installation at the field.
Usually, the jacket is installed at the field about a year before the topside is installed. Jackets for drilling platforms often have a drilling deck for pre-drilling until the topside is placed on top.
Some key facts:
• Weight from 3 000 up to 23 000 tonnes.
• Typical time of construction: Up to 24 months
The second roll-up of Clair Ridge DP jacket.
Article from Trønder-Avisa (in Norwegian) – January 2013.
Here you can read the article.