Material Properties and Internal Soundness of a Huge Cast Steel Node Joint
Authors: Kozuki M. , Sakamaki S. , An B. , Sakamoto K. , Sakakibara H. 
Published: October 2008

The main advantage of substituting cast steel nodes for conventional weld fabricated nodes is the capability of the cast steel node to avoid extreme stress concentration. It is important, however, when utilizing the cast steel node in practical situations, that the mechanical properties and internal soundness of the castings, generally considered inferior to steel plate, should be fully clarified. In this study, mechanical properties and possible casting defects of special steel cast pieces and a trial cast steel node were examined. Satisfactory results-were obtained in order to apply the cast steel node-to actual use on offshore structures.

INTRODUCTION

The use of castings for tubular joints has been studied for many years; and a few cast nodes have been recently examined in practical platform use. In comparison with weld fabricated nodes, cast steel nodes have the following advantages:Lower-stress concentration because of the smooth transition at the fillet, without notches such as weld toes.Optimum configuration, because the overlapped braces are more structurally suitable for nodes, and castings can easily be made in overlapping joints.

On the other hand, castings are, in general, more likely to have defects such as blow holes, inclusions, shrinkage cavitites, etc. which could cause higher manufacturing and maintenance costs as well as longer delivery-times.

The purpose of this study, therefore, was to confirm the practical use and establish the manufacturing technology of cast steel nodes through the experimental manufacturing of as large a node as possible.

BASIC DESIGN PHILOSOPHY

Although the lower stress concentration of the cast steel node is more effective in preventing failures during operation, even when there are minor defects in the casting, it is important to consider the following:Material with superior toughness and weldability. Since platforms operate in severe conditions such as in the North Sea, the material must have excellent toughness properties at low temperatures. At the same time, the material must be weld able without preheating in a fabrication yard.Configuration convenient for casting and inspection. In the case of complicated node joints, it is better to divide the node longitudinally into several sections consisting of braces and chord segments, and to assemble the separate castings by weld fabrication. In this way, casting design can be simplified and inspection carried out more easily.Reasonable structural configuration. The punching force on the nodes can be considerably reduced by suitable overlapping of the braces. The optimum overlapping dimensions can be determined by calculations using a newly developed structural analysis programme. Overlapping is also effective in reducing distortion during casting and, consequently, the node's weight as a whole.

PROPERTIES OF THICK CAST PIECES

The wall thickness of the intersection of the chord and brace or the brace and brace in the cast steel node is greater than that in the weld fabricated node due to the smoother transition at the fillet in the castings.