ABSTRACT

This paper describes and presents the results of tests carried out on two cast X-joints which are 3/4-scale replicas of those installed on Conoco?s Victor JD platform in the Southern North Sea. Elastic tests have been carried out to determine the distribution of stresses within the casting for various load combinations. Fatigue tests under representative conditions have been carried out and the mode of failure of these cast X-joints determined. To supplement the tests, a 3D finite element analysis was also carried out for various load conditions using conventional h-version and higher order p-version elements. Other specialist monitoring techniques such as acoustic emission, non-destructive residual stress measurement, and thermoelastic stress measurement have been used to obtain a better understanding of the behavior of cast X-joints. Test data and the results of the analyses are given. The results of the finite element analyses are compared with the results obtained from the laboratory tests. Similarly the development histories of fatigue cracks within the test specimens are compared to the results from acoustic emission monitoring. An assessment is made as to the performance of non-destructive residual stress measurement and thermoelastic measurement techniques on cast joints. It is considered that the results of this research pragramme will greatly assist in the development of rational design and inspection procedures for cast joints in offshore use.

INTRODUCTION

Offshore exploration is beginning to move into deep water and platform design will become more fatigue-sensitive. The use of cast steel tubular joints is seen as a promising and economical method of providing resistance to fatigue stresses on tall jacket structures. Cast steel joints can also be used economically in shallower structures as in the Gas Fields of the Southern North Sea if good use is made of repetition in mould use. Conoco (UK) Ltd decided to install four cast X-joints in their Victor field jacket to obtain a working knowledge on the use of cast joints as a prelude to more extensive future use in deep water structures. This paper reports some of the results obtained from the research programme designed by Conoco to supplement the practical knowledge gained with the Victor joints by laboratory testing to obtain elastic properties and fatigue life two representative-sized joints.

JOINT GEOMETRY

The tubular joints tested were ¾ - scale replicas of the four cast X-joints installed at an elevation of -2.5m LAT on the Victor JD Platform. The locations of the joints are illustrated in Figure 1. The cast joints for both the Victor jacket and the test specimens were manufactured by River Don Castings Limited (formally Sheffield Forgemasters) to designs initiated by Conoco. The nominal dimensions of the test specimens were slightly reduced from ¾ scale to allow failure of the specimens within a reasonable timescale and at a load level within the capabilities of available test equipment. The test specimens were designed to be accommodated in the Wimpey Offshore's 6,300kN test rig. Each specimen consisted of a cast steel X-joint with two tubular members welded to the brace stubs.

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