Cast Steel Nodes - Their Manufacture and Advantages to Offshore Structures

Abstract

Summary This paper outlines an extensive development program designed to prove the use of cast steel nodes program designed to prove the use of cast steel nodes as alternatives for the welded nodes now used in the fabrication of oil production platforms. The manufacturing procedures and properties obtained are discussed, and the major advantages-increased fatigue life and design flexibility - are highlighted. Introduction The exploitation of offshore oil reserves has fostered the development and growth of industries associated with the design and manufacture of very large steel or reinforced concrete structures. Forecasts indicate that the fixed production platforms of the future are more likely to be built as piled steel structures or steel gravity platforms.Steel jacket structures for use in the North Sea currently are built to comply with draft standards or the rules of a certification authority. These instructions are based largely on extrapolation of experience with smaller structures located in less hostile waters, and the final specifications for new platforms often include substantial changes based on platforms often include substantial changes based on the availability of new data from current research programs. An analysis of these research activities programs. An analysis of these research activities shows that the most uncertainty concerning the manufacture and use of welded steel structures is related to (1) the quality and integrity of the welded joint, (2) the fatigue and corrosion fatigue resistance of the fabricated node, and (3) node design parameters and, in particular, the analysis of the high parameters and, in particular, the analysis of the high hot-spot stress concentration.Oil exploitation in deeper water requires the design and fabrication of larger structures. In these circumstances, plate thicknesses increase dramatically to provide the necessary stiffness, or complex joints with internal stiffeners are required. Either alternative creates problems during fabrication, making it essential to stress-relieve the node joint.British Steel Corp., recognizing the difficulties encountered during the manufacture and use of large complex node joints for fixed production platforms, pursued the development of cast steel nodes suitable pursued the development of cast steel nodes suitable for use either as direct replacements for the conventional welded node or, where circumstances permit, as part of a total structure designed to permit, as part of a total structure designed to optimize the use of castings. This development was supported by a grant from the U.K. DOE.This paper presents details of the development program, manufacturing principles, and, wherever program, manufacturing principles, and, wherever possible in quantitative terms, the relative merits of a possible in quantitative terms, the relative merits of a cast and weld fabricated structure. Material Development It is unrealistic to expect the offshore design engineer - with his wealth of experience and, therefore, confidence in the design of welded offshore structures - to be immediately enthusiastic about the benefits offered by cast steel nodes. The advantages offered in terms of improved fatigue resistance, cost, and possible weight savings have to be shown to far outweigh the potential risks involved. Because of this, it is likely that offshore operators initially will consider only the limited use of cast nodes, accepting a minimum departure from conventional methods and standards.For these reasons, it was decided to concentrate on the development of cast nodes that could be substituted simply in a structure designed as a welded fabrication. Such a substitute node would be recontoured to take advantage of the benefits of castings, achieving a significant reduction in stress concentration factor and improved fatigue performance, hopefully without a significant increase in performance, hopefully without a significant increase in node weight. JPT P. 1999