Practical Aspects of Design of Thick-Walled Nodes for Offshore Structures

Abstract

Platforms in deeper waters require longer and better performance especially with regard to fatigue life of nodal joints. Experience in the design of recent jackets combined with the results of extensive experimental work have provided a better of the behaviour of commonly used ring stiffened nodes. Evaluation of this type of nodes has highlighted the limitations of their design and fatigue performance. This has initiated the use of alternative designs with better performance. Therefore, there has been a growing interest in the use of thicker nodes, with thicknesses in excess of 10Omm, to provide a simpler, more efficient and cost effective design. In this paper practical aspects of the design of thick nodes are discussed with regard to fatigue, fabrication and cost. Relevant information from recent published data are collated and combined with parametric studies to assess the upper thickness limit for this type of nodes, and to provide some insight into factors controlling their design and fabrication. Areas where further information is required for more efficient design are also identified. INTRODUCTION Major factors governing the design of steel offshore structures are static and fatigue strength of tubular nodal joints. There are a number of design alternatives available to the designer to provide a solution which satisfies these requirementsUse of unstiffened nodesUse of ring stiffened nodesUse of cast nodesUse of composite nodes This paper concentrates only on the use of unstiffened nodes with varying wall thicknesses which may be as high as 150mm. For the purpose of this paper a node wall thickness in excess of some is classified as a thick wall node. PRELIMINARY DESIGN Usually at the concept stage there is a terrence towards optimisation of the structural performance and weight at the expense of the wider range of considerations especially fabrication complexities and costs. Efficient structural performance and light weight should not be the only primary objectives at the conceptual design stage. These requirements must be put into perspective and consideration should be given to fabrication and long term maintenance and inspection costs. Costs can be expressed as time, material, labor or money which are more important during fabrication, installation and operation of the structure. More time spent at the beginning to consider long term implication of alternative designs for reduction of fabrication and inspection requirements, rather than weight optimization and material cost savings, would reduce the total cost of the structure considerably. Reduction or elimination of light but complex ring stiffened nodes and replacement with heavier thick walled nodes may prove to be more cost effective in the long term. Cast nodes are lighter and have superior fatigue performance compared with fabricated nodes, but there are limitations on their suitability (Ref. 1). Thick walled nodes, also require careful assessment to be beneficial and cost effective both in fabrication and performance. Therefore, early decisions regarding node design are of permanent importance to cost effectiveness of final structure, (Ref. 2).