Stress concentration factors for internally ring stiffened multiplanar tubular DKT-joints of offshore structures

Hamid Ahmadi , Haleh Dasti

Abstract

Two-planar tubular DKT-joints are widely used in offshore jacket structures, where accurate assessment of stress concentration factors (SCFs) is vital for evaluating fatigue performance. However, SCFs in DKT-joints with internal ring stiffeners have not been thoroughly studied, and no specific design equations currently exist for determining SCFs under axial loading which is typically the dominant load in these joints. In practice, engineers often rely on SCF data from uniplanar KT-joints to estimate values for multi-planar joints. This approach, however, overlooks the influence of out-of-plane braces, potentially resulting in significant inaccuracies. This study addresses this gap by analyzing SCFs on the chord side of ring-stiffened two-planar tubular DKT-joints under axial brace loading. The investigation is based on 118 finite element (FE) models, validated through both experimental data and existing numerical results. A comprehensive parametric FE study was conducted, followed by nonlinear regression analyses to derive four new equations that accurately predict SCFs. These proposed formulas offer a reliable tool for fatigue design applications.

Key Words

fatigue; internal ring stiffener; offshore jacket structure; stress concentration factor (SCF); twoplanar tubular DKT-joint

Address

Hamid Ahmadi — National Centre for Maritime Engineering and Hydrodynamics, Australian Maritime College (AMC), University of Tasmania, Launceston, TAS 7248, Australia
Haleh Dasti — Faculty of Civil Engineering, University of Tabriz, Iran

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