Improved Finite Elements for Analysis of Welded Tubular Joints

ABSTRACT This paper describes the application of three-dimensional isoparametric elements to the analysis of welded tubular connections. The elements are shown to provide more accurate modeling of joints when compared to the commonly used flat plate shell elements because of their curved shapes and three dimensional characteristics. This three dimensional nature permits their use in modeling grouted joints and in predicting the complex state of stress in and near the welds in these connections. The paper discusses the features that are required in a practical computer program that can take advantage of the versatility of the elements. Results computed by the program PMBSHELL for a grouted and ungrouted K-Joint are presented and compared to ungrouted joint results, computed by a flat plate shell element program, KJOINT. The analyses for this case are shown to agree very well, although some differences are noted in the "hot spot" stress regions. These differences are discussed with regard to the modeling assumptions required when flat plate elements are used in shell analysis. It is concluded that the three-dimensional isoparametric elements, when implemented in an efficient and versatile computer program, can provide more accurate resolution of stress concentration factors than would be possible using flat plate finite elements. In addition, because of their three dimensional nature, they are appropriate for the analysis of grouted connections. 1. INTRODUCTION The state of stress in welded tubular joints has been a concern of offshore engineers for many years and a significant technology has developed as a result of this concern. This technology has been based on both analytical and experimental studies with many significant contributions made in the past few years. As larger offshore platforms are designed and installed in increasingly deep water, designers have been forced to develop innovative underwater connection techniques such as grouted .connections between skirt-piles and platform jackets, and more recently, between major sections of the platform itself. The behavior of grouted connections and the stresses in the associated tubular members and cement grout when subjected to the tremendous design loads prescribed for deepwater structures are often of critical importance to a designer. Besides questions of load capacity, the prediction of stress "hotspots" in and near the welds used in these complex connections has also been extremely difficult to evaluate. The calculation of these high local stresses, usually characterized in terms of stress concentration factors, has been especially important for fatigue considerations. A convenient and efficient method of stress analysis of such joints is the widely used finite element method [4,5J. Over the past few years, the finite element method has been implemented in special purpose computer programs for the analysis of simple K and T joints, and an extension of this method to the analysis of grouted and ungrouted tubular joints using new element formulations is the subject of this paper.