What are the requirements for steel used in offshore platforms

In order to ensure the safe and reliable operation of the offshore platform, higher requirements are placed on the steel used for the offshore platform than the steel used for the hull structure.

1. Strength

Steel for offshore platform structure is divided into Class I steel (general strength structural steel), Class II steel (medium strength steel) and Class III steel (high strength quenched and tempered steel). Generally, the first two types of steel are equivalent to the general strength high quality carbon steel and low alloy high strength steel in the hull structural steel, accounting for about 90% of the steel used for the platform structure; the type III steel is mainly used for the important components of the platform structure that bear the main load. , such as platform deck, jack up platform leg, conduit of jacket platform, etc., accounting for about 10% of the steel used in the platform structure.

2. impact toughness

The offshore platform works under severe environmental conditions, especially the joints of the pipe structure, which belong to the complex joint form. In addition to the stress concentration, there are also welding residual stress and uneven structure of welding defect joints, especially in cold sea areas. When the impact toughness of steel is reduced, brittle failure is easy to occur, so platform steel is required to have high low temperature toughness.

3. Solderability

Offshore oil platforms are mostly made of low alloy high strength steel, among which jackets are made of thick steel plates rolled into a cylindrical shape and welded. Therefore, the hardening and restraint of the heat affected zone during welding are very large, and the stress is high. Concentration, coupled with the absorption of diffusible hydrogen by the weld metal and the effects of post weld cooling rates, make the weld heat affected zone particularly prone to low temperature cracking and laminar tearing.

4. Laminar tear resistance

There are many welded joints with a large amount of deposition and a large constraint in the plate thickness direction at the pipe joints of the offshore platform, which are particularly prone to laminar tearing. Therefore, the steel requirements for platform nodes are very strict, and Z direction steel is generally used.

5. Corrosion fatigue resistance

The offshore platform works in the environment of seawater corrosion and repeated impact of wind and waves, and is particularly prone to corrosion fatigue damage, especially in the pipe joint area. Corrosion fatigue is not exactly the same as general fatigue. It is characterized by not only fatigue but also corrosion. In order to avoid corrosion fatigue damage of welded joints, in addition to reducing stress in design, appropriately reinforced to ease stress concentration, and adopting tungsten inert gas shielded welding in construction, and trimming the shape of the weld with grinding wheels; considering materials, Steel should be selected from the perspective of preventing brittle failure.