Recent Advances in the Technology of Cast Steel Nodes for Use in Offshore Structures

Abstract

Abstract The development of cast steel nodes has now reached the stage where the research programmed is largely complete. In addition to the numerous prototypes funded by the manufacturers, four have been purchased by operating companies for destructive tests to prove the quality of the product. A number of nodes have already been produced for offshore service in the N. Sea and the classification authorities are now taking an enthusiastic stance on their use. Some challenging new projects are on the horizon which will benefit greatly from the use of cast steel nodes. The paper summarizes the advantages of cast steel nodes, namely the opportunities for weight saving, reduced stress concentrations and greatly improved fatigue life. The stages in design optimization for achieving these are considered. The latest results of fatigue tests on both cast cruciform joints and full size cast nodes are also presented. The Metallurgical properties being achieved in large? and small prototype castings are discussed, and results from weld ability trials for simulated fabrication welds are also presented, demonstrating that cast nodes are readily wieldable. Dimensional accuracy and non destructive testing is also considered. 1. Introduction Fatigue damage problems with weld fabricated nodes are known to be on the increase, but cast steel nodes offer a solution to the problem. Indeed for future deep water structures cast steel nodes are likely to be essential to ensure the satisfactory fatigue performance of many of the numerous designs proposed to date. Through their recently formed joint Company OSCAL, the steel foundries of both British Steel Corporation and Estel Hoesch in Germany are demonstrating that massive cast steel nodes can be produced to the required quality levels and delivery schedules, and dozens of prototypes have been produced with weights varying from less than a tone to over 60 tones. 2.Production Experience To Date The first cast steel nodes to be incorporated in a N. Sea production platform are the 48 Column/ Pontoon Node Transition Pieces being incorporated in the Conoco Hutton T L P (See fig. 1). Each 2 tone node segment was designed with generous radii to minimize a difficult stress concentration problem and stress analysis of the cast joint shape showed that it had many times the fatigue life of the equivalent welded junction. Following the manufacture and destructive testing of prototypes to demonstrate internal integrity and through thickness mechanical properties, the production nodes together with weld fabrication procedure test plates were delivered at a rate of up to 8/week after a manufacturing period of 15 weeks. Two large prototype cast nodes have so far been purchased by operating companies for assessment purposes, one being a 15 tone brace node currently being manufactured and the other, a 68 tone launch brace node (fig. 2) now delivered and undergoing independent evaluation by BNOC. OSCAL have produced three 23 tone brace node prototypes and more than thirty nodes of around 11 tonnes for cut-up and welding trials and for large scale stress analysis and fatigue testing. Both OSCAL foundries are able to produce to the designs and specification of the partner to ensure delivery reliability.