Metal deforming – Process – Tube making or reshaping
Patent
1986-05-20
1988-12-13
Larson, Lowell A.
Metal deforming
Process
Tube making or reshaping
72377, B21K 114
Patent
active
047901721
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a new solution for construction of tubular nodes in framework truss structures of the kind consisting of interconnected legs and braces made of steel tubes. Such structures are being used to large extent in stationary and mobile offshore platforms for oil drilling and production activities.
In conventional construction approaches for such structure the braces are directly welded to the legs to form the tubular nodes, i.e. the joints between the legs and bracings. Such nodes may consist of a single brace and leg or of multiple bracings joining the leg in one or multi planes. Although direct welding of the braces to the leg is a simple solution from point of view construction, it is fully realized that it causes many serious problems and detriments. Firstly, it requires careful and costly preparation and adaption of the surfaces to be connected by welding, particularly if the braces are joined in an oblique angle. The welding is also complicated by the fact that the wall thickness of the connected tubes is, as a rule, different. Further, the subsequent control of the weld quality, for instance by means of ultra sound or X-rays of such welds is often very complicated because of limited accessibility. Experience has shown that because of this, one often can not ensure the quality of the welding work. These problems are amplified, in the case of angular and multibrace nodal joints.
In order to increase the stiffness and strength of the nodes it is often necessary to increase locally the wall thickness of the leg at the node or also the wall thickness of the braces. Such measures contribute however further to the welding problems and lead also to increased weights and higher costs of the platform.
Also from structural-strength point of view, the conventional solution is far from ideal. The junctions brace/legs represent geometrical discontinuities which give rise to very high material stress peaks and stress concentrations at these locations. The fact that in traditional solutions the connecting weld is placed at these very locations of extremely high stress concentration will necessarily result in a very serious further detriment to the strength and safety of the node. This because the environment wave loads are dynamic in theis nature, and the latter together with the high stress concentrations at the weld toes of conventional nodes give rise to a severe fatigue strength problem which may result in development of early cracks at the weld toes and subsequently failure of the connection arising from these locations.
Serious failures in platform rig structures during the past years, especially with those used in severe environments such as the North Sea, have proven that in the design of such offshore platforms both the static and the fatigue strength of the structure must be thoroughly considered and its adequacy must be satisfactory verified. This implies first of all that the node and the members between the nodes must possess sufficient strength. This implies that the structures should show sufficiently low probability of failure with respect to the following failure modes:
1. Static strength of the node, i.e. a stability to sustain extreme loads without undue permanent deformation, collapse, or even crunching shear rupture.
2. Buckling/yielding of the members between the nodes.
3. Fatigue failure due to crack initiation and crack propagation.
In order to create a stronger and more durable joint between legs and bracings in frame work structures of this kind it has been proposed to avoid the welding at the junction of the tubulars where very high stress concentration occur and instead to form brace stubs in one-piece node which is prefacricated and is welded in place as section of the structure directly to the legs and bracings. Such solutions have, however, hitherto not been adapted by the platform rig industry and has as far as is known not been tried out in practice. The main objection has been that in connection with casting or moulding there will always be present an inev
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Eide Gunnar H.
Simensen Per A.
Larson Lowell A.
Saga Petroleum A.S.
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