The Tension Leg Platform (TLP) is a relatively new type of oil production in which a floating structure is tethered by vertical tubular legs to foundation structures on the sea bed. From a reliability point of view, heavy wall forged tubulars were considered the most attractive for construction of critical parts of the Snorre TLP. High strength weldable forgings with minimum yield strengths of 460 MPa and 500 MPa have not been previously used for North Sea installations, mainly because of the lack of established engineering data, especially elasticplastic fracture toughness, corrosion fatigue and weldability data. To cope with these problems, heavy thick forged steels and their weldments were subjected to such mechanical tests as fracture toughness and fatigue tests. The test results obtained showed that the forged steel has good performance and is sufficiently weldable. Consequently, the high quality forged steel tubulars for tether component of TLP, have been successfully fabricated by welding. This paper describes the development work production data and fabrication aspects associated in applying these forgings.
The Snorre Tension Leg Platform (TLP) is the first of this type of structure to be developed for the Norwegian Continental Shelf in the North Sea(l). The similar TLP has been successfully installed in the CONOCO Hutton field of UK Continental Shelf(2) (3)(4) The construction of Hutton TLP in the 150 m deep sea where is shallower than Snorre field (310 m deep), included the purpose of feasibility study on economics, pract1cability and safe operation and promoted the subsequent adoption of TLP des1gn for oil and gas production. Such high strength weldable forgings as w1th a yield strength of 500 MFa has never been used for North Sea installations, mainly because of the lack of established engineering data, especially fracture toughness, corrosion fatigue and weldab11ity data(5).