A Comparative Study of the Design Basis for Complex Tubular Joints

ABSTRACT This paper describes a comparative study of the design basis for ring-stiffened, cast steel and internal grouted tubular joints. The current levels of knowledge, design code provisions and future development requirements for these complex joints are identified. Considerations are given to structural performance, fabrication, in-service inspection, the level of confidence in design and the long term behavior of different joint types. Results indicate that the behavior of the complex joints is radically different from that of the simple unstiffened welded joints. Anomalies in the existing design guidance for complex joints, which has been developed primarily from experience gained from simple joints, are identified. Tubular joints design has been a subject of considerable interest. Enhancement schemes are often introduced to meet functional requirements or as a means of saving materials and construction costs of platforms. Despite the fact that complex joints have been installed on many offshore structures, design codes provide little guidance for these joints. The results of this study should greatly assist in the development of rational design and inspection procedures for complex joints in offshore use. This paper also provides valuable information to assist designers either in the selection of the appropriate scheme for the design of new platforms or in reassessment of existing structures which were installed with one of the enhancement techniques. INTRODUCTION The design of steel offshore fixed structures is largely governed by the static strength and fatigue resistance of their joints formed at the intersection of the tubular members. The conventional method of constructing tubular joints is to join the contoured end of one tubular onto the undisturbed outside of the other by full penetration welds. The weld contour and weld toe at the brace/chord intersection of these joints produce high stress concentrations and these become a site of weakness. Enhancement schemes are often introduced to meet functional requirements or as a means of saving materials and construction costs of the platforms. This paper describes a comparative study of the design basis for joints with three different types of enhancement scheme, which are:use of internal stiffenerssteel joints manufactured by using a casting processcomposite joints formed by filling tubular members and the annuli between piles and platform legs with cement or grout. These methods improve both the static and fatigue performance of conventional welded joints; and the behavior of these joints is radically different from that of welded joints. In this paper, the background to the philosophy of these enhancement concepts and the structural behavior of these joints are described. All design recommendations for tubular joints are based on data from laboratory tests and service experience. New or modified design rules and provisions are being constantly introduced as additional data become available. Recent trends in design codes including methods of gaining acceptance have been to expose the data on which the Code is based for assessment.