Feargal P. Brennan

Predicting weld toe stress concentration factors for T and skewed T-joint plate connections

The results of linear elastic finite element analyses of stress concentration factors (SCFs) of 80 weld toe T-butt plate geometries are presented in parametric form for tension and pure bending loading. The closed form solutions describe the stress state of the two-dimensional plane stress models studied in terms of weldment angle, weld root radius, weld attachment width and plate thickness. SCFs are presented in full parametric form and also as simple reduced expressions quantifying the degree of error from raw data in each case. The paper also reports an investigation into the suitability of the SCF parametric equations to skewed T-joints.

The effect of rack/rib plate on the stress concentration factors in jack-up chords

It is well known that stiffener plates can have a large effect on the stress concentration factors found at the intersection of welded tubular joints. The strong relationship between the fatigue life of these tubular joints and the stresses experienced at the intersection make the accurate knowledge of the SCFs in this region of critical importance. Previous stress analysis work on SCFs in stiffened joints is briefly reviewed here. The results of almost 400 finite element models are presented in this paper, covering several popular jack-up chord designs. These results cover axial, in-plane bending and out-of-plane bending loading modes. This study has quantified the effect of rack/rib plates on SCF across a range of joint parameters representative of common SCF parametric equations for unstiffened tubular joints. The results of this FE study show that for every rack plate configuration investigated the hot spot SCF was reduced compared to the corresponding unstiffened joint. For axial loading, reductions of 50% of the unstiffened joint SCF were approached. The reduction under OPB was shown to be less but was still significant at around 20%. The peak SCF was unaffected under IPB loading as would be expected due to the different position of chord wall deformation. The mechanism by which the stiffener affects the changes in stress distribution has been investigated using evidence gained from applying various translational and rotational restraints to the chord surface of an unstiffened joint along the line where the central rack plate would intersect if it were present.

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.

Stress intensity factors for threaded connections

Abstract A generic stress intensity factor (SIF) solution for threaded connections is presented in this paper. This is based on multiple reference state weight function theory, making it applicable to any mode I crack emanating from a thread root. The reference SIFs required for the solution are a combination of published results representing geometrical features encountered in threaded components. These constitutive components of the reference solutions can easily be replaced in the event of more comprehensive or appropriate solutions becoming available. For this reason it is considered prudent to present the structure of the solution rather than a rigid set of parametric expressions so that specific solutions can be fashioned for particular applications. The new solution is rigorously examined, comparing it with published SIF solutions for threaded and related components.

Conceptual design of a floating support structure for an offshore vertical axis wind turbine: the lessons learnt

The design of floating support structures for wind turbines located offshore is a relatively new field. In contrast, the offshore oil and gas industry has been developing its technologies since the mid 1950s. However, the significantly and subtly different requirements of the offshore wind industry call for new methodologies. An Energy Technologies Institute (ETI) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 10–20 MW power range. The development of a case study for the NOVA project required a methodology to be developed to select the best configuration, based on the system dynamics. The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. The best ‘class’ or design solution (based on water plane area stability) has been selected for a more detailed analysis. Two configurations are considered: a barge and a semi-submersible. The iterations to optimise and compare these two options are presented here, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semi-submersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that the driving requirement is an acceptable response to wave action, not the ability to float or the ability to counteract the wind turbine overturning moment. A simple cost comparison is presented.

A new method for predicting stress intensity factors in cracked welded tubular joints

Abstract Fracture mechanics is an important tool for the analysis of cracked bodies. It has been established for use in a range of industries including the power generation and the aircraft industry. Fracture mechanics provides the basis for fatigue life prediction, steel selection and tolerance setting on allowable weld imperfections. It can also be used during the operational stage of a structure to make important decisions on inspection scheduling and repair strategies and as a tool for establishing limits on operational conditions. The stress intensity factor is a very important fracture mechanics parameter. Linear elastic fracture mechanics relies on the use of the stress intensity factor concept. Therefore the accuracy of any fracture mechanics model for the prediction of fatigue crack growth will depend very much on the accuracy of the stress intensity factor solution used. It is known that some factors are important in this process. One of these is the crack aspect ratio, which represents a major source of uncertainty in fatigue crack growth prediction. This paper presents a new approach for the prediction of stress intensity Y correction factors for welded tubular joints. The proposed method accounts for crack aspect ratio evolution during crack propagation. The method is based on a statistical model used to quantify and account for the deviation of experimental results from previous semi-empirical solutions and the modified Newman and Raju flat plate solution.

FloVAWT: Progress on the Development of a Coupled Model of Dynamics for Floating Offshore Vertical Axis Wind Turbines

In the present article, progress on the development of an aero-hydro-servo-elastic coupled model of dynamics for floating Vertical Axis Wind Turbines (VAWTs) is presented, called FloVAWT (Floating Vertical Axis Wind Turbine). Aerodynamics is based on Paraschivoiu’s Double-Multiple Streamtube (DMST) model [1] [2], relying on blade element momentum (BEM) theory, but also taking into account three-dimensional effects, dynamic stall, and unsteady wind profiles and platform motions. Hydrodynamics is modelled with a time domain seakeeping model [3], based on hydrodynamic coefficients estimated with a frequency analysis potential method. In this first phase of the research program, the system is considered a rigid body. The mooring system is represented through a user defined force-displacement relationship.Due to the lack of experimental data on offshore floating VAWTs, the model has initially been validated by taking each module separately and comparing it against known experimental data, showing good agreement. The capabilities of the program are illustrated through a case study, giving an insight on the relative importance of aerodynamics loads and gyroscopic effects with respect to hydrodynamic load effects.Copyright

The use of approximate strain-life fatigue crack initiation predictions

Abstract This paper describes the most commonly used approximate strain-life equations for prediction of fatigue crack initiation. Despite the development of advanced multiphase material memory models, the simple strain-life equations are useful ‘first-shot’ approximations of a components resistance to fatigue crack initiation. There are, however, certain precautions that should be observed when using these equations. Selection of appropriate cyclic material properties is of utmost importance. These properties must be internally consistent; however, many values given in published literature are not. A sensitivity analysis is also presented for predicted fatigue life with respect to percentage error in cyclic material properties.

Minimization of stress concentration factors in fatigue crack repairs

Abstract A numerical study is reported of a repair by flaw removal on a conventional welded joint. The repair profile is optimized with respect to the flaw and joint dimensions in order to minimize the resulting stress concentration factor ( SCF ). Two dimensional (edge repair) and three dimensional (surface repair) finite element analyses were made for the determination of SCF values and a graphical representation of results is presented. A relation between edge repair and surface repair is obtained and short and long repairs are defined. The weld geometry and repair orientation effects on SCF values are discussed. Finally, implications on using short and long repairs on fatigue initiation and inspection are presented.

Experimental determination of the overturning moment and net lateral force generated by a novel vertical axis wind turbine: Experiment design under load uncertainty

Recent developments in harnessing wind energy propose new, radically different designs to alleviate some of the difficulties associated with conventional wind turbines. New designs however require testing for a variety of reasons ranging from gaining confidence in the analytical models used in the design and development through to satisfaction of certification requirements. Medium-scale prototype testing of large-scale concepts, where parameters such as the response of the structure and the loading conditions are often highly uncertain demand special consideration. This article presents the design of a special test rig and calculation methodology for the experimental determination of the overturning moment and net force generated by the NOVA Vertical Axis Wind Turbine using a field experimental setup. The design of the experimental model involves dealing with modelling uncertainties as loads in operation and therefore the response of the structure are largely unknown before testing has been carried out. The variability in the wind speed and direction also need to be accommodated for.