The Attraction Of Cast Steel Nodes For Offshore Structures
Authors: Walker E.F. , Webster S.E. , Wood A.M. 
Published: October 2008

Recent developments in both steelmaking and foundry technology have provided the British Steel Corporation with a unique opportunity to develop and manufacture a range of high integrity cast steel products. Amongst these, cast steel node joints offer many important advantages when compared with the normal welded fabrication. Besides the overall improved fatigue performance of the casting, special attention to the design of the fillet regions results in a highly significant reduction in the hot spot stress concentration factor and the absence of fillst welds results in a considerable reduction in the level of residual stresses. These factors, together with improved node design through the better location and geometry of overlapping braces, significantly improve the performance of the structure and at the same time offer significant cost advantages. For example, on a platform requiring over 100 major nodes, the subject of a detailed study, there could be :-a minimum 4-fold improvement in fatigue life.a cost saving of the order of 20%.no additional weight penalty.

The study has also shown that the fatigue lives of fabricated nodes in this platform example could fall as low as 8.5 yrs. when a more severe N.Sea wave spectrum is applied (i.e. Edelstein). High quality steel castings are essentially isotropic and offer excellent mechanical properties and superb fracture resistance in all stressing directions. The product development programme has been strongly supported by an extensive research programme. This work has included not only the development of a highly suitable cast steel but a thorough examination of its vital properties, fracture toughness, fatigue, corrosion fatigue and stress corrosion data are presented in the paper, together with a study of the weldability of these castings. The works-based development programme has been supported by the trial production of nodes weighing 23 tonnes, one of which has been subjected to an exhaustive non-destructive examination, followed by sectioning and further destructive testing. This work, together with the results of node design and stress analysis studies, forms the basis of the manufacturing procedure, quality standards and product specifications.

1. 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 are currently built to comply with draft standards (1) Dr the rules of a certification authority (2). These instructions are largely based upon the extrapolation of experience with smaller structures located in less hostile waters and the final specifications for new platforms often include substantial changes which are based on the availability of new data from current research programmes. An analysis of these research activities shows that the most uncertainty concerning the manufacture and use of welded steel structures is related to:The quality and integrity of the welded joint.The fatigue and corrosion fatigue resistance of the fabricated node.Node design parameters and in particular the analysis of the high hot spot stress concentration.