Offshore Platform Structure

One month ago my lecturer gave me some material about offshore platform. now i am trying to give explanation about these material. Offshore platform has been used in oil and gas industry as early as 1930 in Venezuela. The first offshore platform was made by wood material. The steel type platform was then used in 1947 in a 6 m depth of water in the Gulf of Mexico. Since that period, various types of offshore platform has been developed and used as main offshore facility in oil and gas industry. Offshore platform can be used in many ways. A full offshore oil and gas facility can have many platforms such as quarters platform, wellhead platform, process platform, compressor platform, etc.
Type of structure and the support configuration of offshore platform are in consideration with depth and difficulties in erecting the platform. Therefore, there are many types of offshore platform. Based on the type of structure, offshore platform can be divided as :

• Concrete gravity platform
• Fixed steel platform
• Semi submersible platform
• Tension leg platform
• Guyed tower platform

A ‘Statfjord’ gravity based structure under construction in Norway.
1, 2) conventional fixed platforms; 3) compliant tower; 4, 5) vertically moored tension leg and mini-tension leg platform; 6) Spar ; 7,8) Semi-submersibles ; 9) Floating production, storage, and offloading facility; 10) sub-sea completion and tie-back to host facility.

In addition to that classification, depending on the number of legs & jacket type, fixed offshore platform can be divided into :

• Monopod platform
• Bipod platform
• Tripod platform
• 4, 6, or 8 pile platform

A common offshore platform is consisted of platform deck, jacket, and pile. Deck is the place where all production equipments are placed. Jacket is a tubular steel structure that serves as a lateral load support for the platform while pile serves as a vertical load support. For come cases, jacket is not used because the lateral load of platform is neglect able.
In a common Offshore platform, deck, jacket, and pile are fabricated onshore. The first stage of platform erection is transporting the jacket and putting it in the designated place. After the jacket is fixed in the right place, pile is driven via the jacket hollow tube to the sea bed. Finally, the deck part is placed and connected at the top of support structure.
For safety and ergonomic reasons, platform layout should be carefully designed. Platform north should be facing to deeper water. This north part is also used as the area for jack up drilling rig and foundations spud can or mat. The south side platform is used for boat berthing, pipeline riser and subsea pipeline. Layout of platform should also be designed by considering drilling sequence, prevailing wind, and other utility such as helipad.
Load acting on platform is vital in a platform’s design stage. In designing a platform, load can be divided to load at deck structure and load at jacket structure. In general, the working load in an offshore platform consisted of the following load.
Load at Deck structure:

• Topside load : Deck structure, piping, equipment, life load, crane load, helicopter load, drilling work over load.
• Lateral load : Wind and earthquake.

Load at Jacket structure:

• Vertical load : Deck structure and buoyancy
• Lateral load : Wind, wave, current, earthquake, and added hydrodynamic mass.

Some of applicable engineering standards for offshore platform design are:

• API RP2A Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms
• AISC Specifications for Design, Fabrication and Erection of Structural Steel Buildings
• API Spec 2B, Fabricated Structural Pipes
• AWS D 1.1, section 8 and 10
• ISO 19902 Petroleum and Natural Gas Industries – Fixed Steel Offshore Structures
• ISO 19901-2 Petroleum and Natural Gas Industries – Specific requirements for offshore structures Part 2 Seismic Design Procedures and Criteria