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offshore platform steel plate

EN10225

To use S355 steel plate for offshore applications, a full qualification is needed according to the appropriate standard EN 10225:2009: Weldable structural steels for fixed offshore structures – Technical delivery conditions.
Technical Requirements & Additional Services:

  • Z TEST (Z15,Z25,Z35)
  • Low temperature impacting test
  • Ultrasonic test under EN 10160,ASTM A435,A577,A578
  • Issued Orginal Mill test certificate under EN 10204 FORMAT 3.1/3.2
  • Shot blasting and Painting,Cutting and welding as per end user’s demands

This standard specifies a whole host of investigations to be performed, from welding with pre-defined heat input levels – using submerged arc welding (SAW) and flux cored arc welding (FCAW) – and full testing of the welds to specific tests to assess the hardenability and cold cracking sensitivity.

EN 10225 Weldable structural steels for fixed offshore structures.

This European Standard specifies requirements for weldable structural steels to be used in the fabrication of fixed offshore structures in the form of plates up to and including 150 mm thick. It also specifies sections up to 63 mm thick except for sections delivered in the as-rolled condition which are permitted up to 25 mm thick only. Seamless hollow sections up to and including 40 mm thick and high frequency electric resistance welded hollow sections up to and including 20 mm thick are specified. Greater thicknesses for sections and hollow sections may be agreed, provided the technical requirements of this European Standard are maintained. For plates the thickness limitations are:

  • S355G2+N, S355G5+M, – up to and including 20 mm
  • S355G3+N, S355G6+M – up to and including 40 mm
  • S355G7+N, S355G8+N, S355G9+N, S355G10+N – up to and including 150 mm
  • S355G7+M, S355G8+M, S355G9+M, S355G10+M – up to and including 100 mm
  • S420G1+Q, S420G1+M, S420G2+Q, S420G2+M – up to and including 100 mm
  • S460G1+Q, S460G1+M, S460G2+Q, S460G2+M – up to and including 100 mm

The standard is applicable to steels for offshore structures, designed to operate in the offshore sector but not to steels supplied for the fabrication of subsea pipelines, risers, process equipment, process piping, and other utilities. It is primarily applicable to the North Sea Sector, but may also be applicable in other areas provided that due consideration is given to local conditions e.g. temperature. In the case of hollow sections formed from plate with the seam fusion welded, this European standard covers only the requirements of the plate material. Minimum yield strengths up to 460 MPa are specified together with low temperature impact properties at temperatures down to –40 °C. This European standard applies to material supplied ex-mill or from merchant’s stock.

An oil platform is a large structure used to drill and produce oil and/or natural gas from the seas or oceans. Depending on the circumstances, the platform may be floating or an artificial island or attached to the ocean floor. A typical platform may have around thirty well heads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 8 kilometres from the platform.

Oil and gas platformPlatform types

Larger lake and sea-based oil platforms and oil rigs are some of the largest moveable man-made structures in the world. There are several distinct types of platforms and rigs:

  1. Fixed Platforms, built on concrete and/or steel legs anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are, by virtue of their immobility, designed for very long term use (for instance the Hiberniaplat form). Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed. Concrete caisson structures, pioneered by the Condeep concept, often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore (Norwegian fjords and Scottish firths are popular because they are sheltered and deep enough) and then floated to their final position where they are sunk to the seabed. Fixed platforms are economically feasible for installation in water depths up to about 1,700 feet.
  2. Compliant Towers platforms, consist of narrow, flexible towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 400 and 900 meters.
  3. Semi-submersible Platforms having legs of sufficient buoyancy to cause the structure to float, but of weight sufficient to keep the structure upright. Semi-submersible rigs can be moved from place to place; and can be lowered into or raised by altering the amount of flooding in buoyancy tanks; they are generally anchored by cable anchors during drilling operations, though they can also be kept in place by the use of steerable thrusters. Semi-submersible can be used in depths from 200 to 1800 meters.
  4. Jack-up Platforms, as the name suggests, are platforms that can be jacked up above the sea, by dint of legs than can be lowered like jacks. These platforms, used in relatively low depths, are designed to move from place to place, and then anchor themselves by deploying the jack-like legs.
  5. Ship-board Rigs platforms. Active steering of ships, especially based on Global Positioning System measurements, enables certain drilling operations to be conducted from a ship which holds its position relative to the drilling point, within the parameters for movement acceptable in a given circumstance — i.e. within the point at which movement of the ship would cause the drill string to break.
  6. Floating production systems are large ships equipped with processing facilities and moored to a location for a long period. The main types of floating production systems are FPSO (floating production, storage, and offloading system), FSO (floating storage and offloading system), and FSU (floating storage unit).
  7. Tension-leg Platforms, consist of floating rigs tethered to the seabed in a manner that eliminates most vertical movement of the structure.  Tension Leg Platforms are used in water depths up to about 1500 meters.
  8. Seastars platforms are mini Tension Leg Platforms of relatively low cost, used in water depths between 200 and 1000 meters. They can also be used as utility, satellite or early production platforms for larger deep water discoveries.
  9. Spar Platforms, moored to the seabed like the  Tension Leg Platforms , but whereas the  Tension Leg Platforms has vertical tension tethers the Spar has more conventional mooring lines. Spars have been designed in three configurations: the “conventional” one-piece cylindrical hull, the “truss spar” where the midsection is composed of truss elements connecting the upper buoyant hull (called a hard tank) with the bottom soft tank containing permanent ballast, and the “cell spar” which is built from multiple vertical cylinders. The Spar may be more economical to build for small and medium sized rigs than the  Tension Leg Platforms , and has more inherent stability than a  Tension Leg Platform since it has a large counterweight at the bottom and does not depend on the mooring to hold it upright.
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