STEEL jacket structure


rig The steel jacket type platform on a pile foundation is by far the most common kind of offshore structure and they exist worldwide. The "substructure" or "jacket" is fabricated from steel welded pipes and is pinned to the sea floor with steel piles, which are driven through piles guides on the outer members of the jacket.

The piles are thick steel pipes of 1 to 2 metres diameter and can penetrate as much as 100 m into the sea bed. The jacket can weigh up to 20,000 tonnes.

To ensure that the installation will last for the required service life, maintenance must be carried out including the cathodic protection used to prevent corrosion.

Typical design

Many parameters influence the design of the jacket, such as required strength, fatigue, load and life cycle. The pile design results in a balanced combination of diameter, penetration, pile wall thickness, and spacing.
The design of the pile is very important in the design of the jacket structure itself and the cost of pile foundation and installation could be as much as 40 % of the total cost of the platform structure. For example, the typical design requirements for a steel jacket of 150 m would be as described below.
At the seabed the dimensions of the structure are 70 x 65 m and at the top 56 x 30 m (the top is about 15 m above sea level.
Such a structure weights about 18,500 tonnes and would support topsides of up to 21,000 tonnes. The jacket can resist forces of up to 50 MN in compression and 10 MN in tension as well as having large resistance to lateral loads.

Maximum design forces for steel jacket platform are as follow:
  • Vertical load: around 50 MN
  • Horizontal load: around 5 MN
  • Overturning moment: around 10 GN.m


These structures can withstand immense vertical loading and overturning moments as they are designed to be resistant to toppling from very large wave fronts. It can be assumed that many fixed offshore installations can withstand vertical loadings and the overturning moment imposed by many renewable energy devices. It is not advisable to impose any further lateral loading on the offshore installation as this may affect the overall strength of the platform which could create a potential safety hazard.
An individual structural analysis will however have to be carried out to determine the suitability of re-using each fixed installation as OREC.