Jacket refers in the oil and gas exploration and production to the steel frame supporting the deck and the topsides in a fixed offshore platform.
There are multiple types of offshore platforms depending on the applications and the depth of the water.
Most of the platforms are used in the shallow waters of the continental shelf, so 95% of the offshore platforms in the world are jacket designed
In these areas and where the water depth does not exceed 500 meters, these platforms may be anchored directly to the seabed.
These platforms are fixed and their deck is supported by a steel tubular structure having its feet on the seabed.
This steel tubular structure is called the jacket.
To fix the jacket onto the seabed, the jacket is equipped with thick steel piles of 2 meters diameter that can penetrate the sea floor up to 100 meters deep to ensure the stability of the whole platform.
The jacket may be hundreds meters high and weight thousands tonnes.
The height of the jacket is defined by the water depth plus about 15 meters above the sea level.
The tubular structure of a jacket is designed to support multiple constraints:
– Weight of the processing equipment (topsides)
– Impact of the waves
– Pressure of the wind on the topsides
– Flow of the sea water streams and tides
– Fatigue effect
– Life cycle time
Acting as a cage, the jacket is protecting all the piping going through to the seabed.
This space tubular frame is also protecting these pipes from lateral load.
The deck structure is connected to the jacket by the deck legs transferring efforts both ways.
The waves having a period of 14 to 20 seconds, the jacket is designed with a natural period of 2.5 seconds in order to prevent vibrations amplification under the waves effect.
As a result of all the requirements imposed to the jacket, its costs may represent up to 40% of the total platform capital expenditure.