Offshore Steel Categories and Properties
Similar to the common steel structure, offshore steel platform use steel property as the main structural members. There are some profiles which can be assembled for those members on the topside, jacket, and pile. Based on the importance and possible failure of each element during fabrication, installation, and production, structural members can be classified into three categories. They are special category (failure member would cause total or partial disfiguration of platform), first category (failure member would incur total or partial shutdown), and second category (those members that is not included in special and first category). All steel components cannot be repaired in special category while it can be only repaired during shutdown for first category. In addition, supplementary member parts which are not partaking in the overall strength and integrity of platform structures (handrails, ladders, stairs, grating, etc) are defined as non structural members. Normally, 250 MPa (36 ksi) or 350 MPa (50 ksi) of yield stress (fy) is often used for different categories. However, design basis or general specification from the client or particular guidelines should be followed.
Table below shows the classification of structural member categories on the topside, jacket and pile.
|Description||Special Category||First Category||Second Category|
|Deck||Main Girder||Crane Boom Rest||Stiffener|
|Padeyes||Rolled Section and WPG||Secondary Girders|
|Crane Pedestals||Vent Booms||Hatch Covers|
|Jacket||Piles||Boatlanding support||Riser Protector|
|Jakcet Legs||Risres||Boat Landings|
|Conductor Pipes||Conductor Guide||Mudmats|
|Buoyancy Tank||Shock Cells|
|Cans of Bracing|
Wide Flange, Tubular, Box, Channel, Double Channel, Angle, Double Angle, Tee, and Plate Girder are the structural product which can be put on for offshore platform. Generally, main and secondary beam on deck can be designed by Plate Girder or Wide Flange. For small deck (access platform, mezzanine deck, or sump deck) purposes, Channel can also be utilized. Tubular profile with different outside diameter and wall thickness are recommended for designing deck leg, deck bracing, jacket leg, jacket bracing, jacket walkway, pile, boat landing, etc. AISC, that is code and standard for steel structure behavior, provide different product of steels with different sizes. Dimension, area, thickness, unit weight, section modulus, inertia, radius gyration, etc are given for engineer/company user, as well. Nevertheless, another steel profile are also available as project specification such as JIS and UK standard.