W.D. Dover

Stress concentration factors for tubular Y- and T-joints

Abstract A systematic study of stresses in tubular Y- and T-joints has been conducted in which nearly 900 thin-shell finite-element analyses were performed. These cover a wide range of joint geometries under axial loading, in-plane bending and out-of-plane bending. For each mode of loading, and for both the chord and brace sides of the intersection, semi-empirical equations are derived which relate the stress concentration factors at selected locations to a parametric function of the joint geometry. Equations are also obtained for the angular location of the hot-spot stress site around the intersection. The accuracy of these parametric equations is then assessed by comparing the predicted values with results from steel model tests and also with the predictions of other previously published equations.

Parametric equations to predict stress distributions along the intersection of tubular X and DT-joints

Accurate information on stress distributions along the intersection is required for fatigue strength assessment of tubular joints. However, there is no parametric equation currently available in the open literature to predict this information for tubular X and DT-joints. Systematic thin shell finite element (FE) analyses have been conducted for 330 different tubular X and DT-joints, typical of those used in offshore structures, subjected to six different modes of loading. A novel two dimensional regression methodology was developed to curve-fit all of the FE results from this work. A set of parametric equations was derived to predict the stress distributions along both chord and brace toes in tubular X and DT-joints under each mode of loading. These equations were assessed by comparing the predictions with available experimental data. Validation results show that they can be used to predict stress concentration factor (SCF) distributions along the intersection but also provide an alternative method for calculation of hot spot SCF.

Fatigue analysis of offshore platforms subject to sea wave loadings

Abstract The fatigue damage calculation for random loading on offshore platforms takes the form of a rainflow analysis of the dynamic response of individual members to various sea states. This procedure is lengthu and consequently this paper attempts to provide a theoretical method for determining random load fatigue damage. This dynamic response for many joints leads to a broad band random loading but despite this, previous theoretical methods have simplified the loading to narrow band. This has not been done in the present case; instead, an analysis based on broad band random loading has been produced. This theoretical approach gives a fatigue life estimate which is slightly (6.6%) more conservative, for a typical example, than a rainflow analysis.

Prediction of stress distributions along the intersection of tubular Y and T-joints

Several sets of stress parametric equations have been derived for fatigue strength assessment of tubular Y and T-joints. Among them, the Hellier, Connolly and Dover (HCD) set is the only one which can provide the whole two dimensional (2D) stress information. However, the HCD characteristic stress distribution equations were derived from the limited number of typical finite element (FE) results and thus may not be able to capture the effects of all joint geometric parameters. As part of a large study including X and DT-joints, comprehensive thin shell FE analyses were carried out for 330 different tubular Y and T-joints and the whole FE result database was used to derive a new set of equations as a function of joint geometric parameters. These equations can be used to predict stress distributions along the intersection and also provide an alternative method for the calculation of the hot spot stress concentration factor (SCF). Furthermore, an improved methodology has been suggested for assessment of stress parametric equations.

A parametric study of the ratio of bending to membrane stress in tubular Y- and T-joints

The relative proportions of through-thickness bending and membrane stresses in tubular Y- and T-joints have been investigated by analysing a large number of thin-shell finite-element models. Nearly 900 finite-element runs were performed for a wide range of joint geometries and for each of axial loading, in-plane bending and out-of-plane bending. The validity of this approach is demonstrated by comparing the thin-shell finite-element results with data obtained from strain-gauged acrylic model tests and other finite-element analyses utilizing thick-shell or brick elements to model the intersection. For each mode of loading, the results are then used to construct semi-empirical equations which related the relative proportions of bending and membrane stresses to a parametric function of the joint geometry. Finally, the accuracy of the parametric equations is assessed by comparison both with the database from which they were derived and with the experimental results.

Stress concentration factor parametric equations for tubular X and DT joints

Abstract During the design stage, the peak stress is usually needed for estimating the fatigue life of offshore tubular welded joints by an SN approach. However, for fracture mechanics calculations of remaining life, on cracked joints in service, information is required on the magnitude and distribution of the stress acting in the anticipated crack path, not just the peak stress at one location. Fatigue crack propagation rates are important to reliability-based inspection scheduling: hence the need for this information is becoming more pressing. Parametric equations are available for Y and T joints in terms of peak stress, stress distribution and bending-to-membrane ratio. However, for X and DT joints, there are no parametric equations for stress variation through the thickness and around the intersection. Even for stress concentration factor (SCF), so far there is no full set of parametric equations especially for single-brace loading. Thin-shell finite element analyses have been conducted for 330 X and DT joints typical of those used in offshore structures, subject to six modes of loading. The results from this work have been used to produce a new set of parametric equations as a function of non-dimensional joint geometric ratios α, β, γ, τ and θ by carrying out regression analysis. These equations can be used to predict SCFs at the crown toe, saddle, crown heel and hot-spot positions for each mode of loading, for both chord and brace, as well as the angular location of the hot-spot stress site around the intersection. This set of SCF parametric equations has been assessed by comparing the predicted values with results from steel and acrylic model tests and also with the predictions from existing parametric formulae given in the literature. The degree of bending data, and stress distribution data, will be reported in other publications.

Stress analysis of drillstring threaded connections using the finite element method

Abstract Stress analysis of drillstring threaded connections under axial, bending and torsion loadings has been carried out using the finite element method in order to determine the regions of highest stress concentration. This information is required for fatigue and fracture mechanics analysis. For axial loading, two-dimensional axisymmetric models of the threaded joints have been used for the connector types, in standard form and with bore back and stress relief, and also in standard form with some slight modification to thread root geometry. Full three-dimensional models of the connectors, but ignoring the helix angle, were studied for bending and torsion loading. The peak stress concentration factors in all cases were at the thread root of the first loaded tooth in the pin and the last loaded tooth in the box. In comparison with axial loading, bending showed lower stress concentration factor values, but for torsion the values were very small. The study showed clear advantages in the use of bore back and stress relief in reducing stress concentration factor. It was found that a further decrease in stress concentration factor is possible with minor changes to the thread geometry. Sensitivity analysis on the effect of thread profile on stress concentration factor showed that the fatigue life could be increased by an improved profile thread design.

Prediction of the stress distribution in tubular Y- and T-joints

Abstract A comprehensive thin-shell finite-element study has been conducted of stresses in tubular Y- and T-joints with a wide range of geometric ratios and subject to axial, in-plane and out-of-plane loading. The results of this study are used to derive characteristic formulae for the stress distributions around the intersection. Parametric equations for the stress concentration factor as a function of joint geometry were previously fitted at key locations on both chord and brace sides of the intersection. By combining these two sets of equations the stress distribution for a Y- or T-joint may be predicted from its geometric parameters and the mode of loading. Comparisons are made of some predicted stress distribution curves with the finite-element data as well as results from strain-gauged acrylic and steel models.

Parametric equations for T-butt weld toe stress intensity factors

Abstract This paper describes the generation of parametric equations for weld toe stress intensity factors. The methodology employed used a two-dimensional finite element analysis to evaluate the ‘crack opening’ stress distribution in the uncracked plane of T-butt geometries. This was then used as input into a dedicated weight function solution for the determination of stress intensity factors. The final parametric equations describe the stress intensity factor distributions for tension and bending as a function of plate thickness, weld attachment width, weld angle, weld root radius, crack length and crack shape. The equations are compared and validated against a wide spectrum of published values and appear by comparison accurate and wide ranging. The validation exercise uncovered situations where present design guidance is unconservative.

Prediction of degree of bending in tubular X and DT joints

Abstract Analysis of the large scale fatigue testing results of offshore welded tubular joints showed that the fatigue life is not dependent on the hot spot stress alone, but is also significantly influenced by the through thickness stress distribution which can be characterised by degree of bending (DoB). Accurate prediction of DoB data in tubular joints is crucial for improving the accuracy of fatigue life prediction using stress-life (S–N) curve and particularly for predicting fatigue crack growth using the fracture mechanics method. However, there is no set of DoB parametric equations available for tubular X and DT joints. For this reason, comprehensive thin shell finite element (FE) analyses were carried out for 330 tubular X and DT joints typical of those used in offshore structures, under six different modes of loading. The results of these FE analyses were used to derive a set of parametric equations to predict the DoBs at the critical positions on both brace and chord toes in tubular X and DT-joints under each mode of loading.