Platform: Formed from a combination of steel and concrete this offshore oil rig structure sits on the seabed. This immobile platform is designed to help drill the wells further into the ground and also to help produce greater levels of hydrocarbons.
All these platforms are fixed platforms. A fixed platform may be described as consisting of two main components, the substructure and the superstructure. Superstructure:
also referred as the 'topsides' supported on a deck, which is fixed (mounted) on the jacket structure. These consist of a series of modules which house drilling equipment, production equipment including gas turbine, generating sets, pumps, compressors, a gas flare stack, revolving cranes, survival craft, helicopter pad and living quarters with hotel and catering facilities.
It can weigh up to 40,000 tonnes
Substructure: is either a steel tubular jacket or a prestressed concrete structure. Most fixed offshore oil and gas production platforms have a steel jacket although a small number of platforms have a concrete foundation.
The main types of platforms are
Gravity base platforms
Steel jacket platforms
Monopile
Shallow to medium water depths
· Made from steel tube, typically 4-6 m in diameter ,
· Installed using driving and/or drilling method,
· Transition piece grouted onto top of pile
Jacket
Medium to deep water depths
Made from steel tubes welded together, typically 0.5-1.5 m in diameter ,
· Anchored by driven or drilled piles, typically 0.8-2.5 m in diameter
Tripod
Medium to deep water depths
· Made from steel tubes welded together, typically 1.0-5.0 m in diameter ,
· Transition piece incorporated onto centre column,
· Anchored by driven or drilled piles, typically 0.8-2.5 m in diameter
Gravity base
Shallow to medium water depths
· Made from steel or concrete,
· Relies on weight of structure to resist overturning, extra weight can be added in the form of ballast in the base,
· Seabed may need some careful preparation,
· Susceptible to scour and undermining due to size,
Steel jacket platforms
Description
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
Economic considerations limit development of fixed (rigid) platforms to water depths no greater than 1,500 ft.
Conclusion
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.
