The weld magnification factor method has been widely used in the determination of the stress intensity factor (SIF) for weld-toe cracks in welded structural components. Weld magnification factors Mk are normally derived from two-dimensional crack models with fillet weld profiles to take account of the effect of weld-notch stress concentration at the deepest point of the crack front. This paper presents a detailed three-dimensional analysis of weld-toe surface cracks in fillet welded T-butt joint models using the finite element method. Effects of the weld notch and the welded attachment stiffness on the SIFs of the weld-toe surface cracks have been studied quantitatively. Weld magnification factors applying to the whole surface crack fronts have been estimated. Numerical results show two contradictory effects; that the effect of weld notch increases S1F values throughout the shallow surface crack fronts which are in the region of notch stress concentration, while the effect of local structural constraint reduces the SIF values. The increase in the SIF values mainly depends upon the relative crack front depth and the decrease in the SIF values mainly depends upon the crack shape aspect ratio for a specific weld profile. Both effects on the weld magnification factors can be estimated separately. A simple approach for deriving the weld magnification factors for various weld-toe surface crack problems is proposed for engineering applications.

Fracture mechanics methods are used to assess the significance of cracking in any structure, but a tubular joint is very complex to analyse. However the improvements in numerical methods, such as stress analysis using finite elements, and extensive full-scale laboratory testing (UKOSRF', ECSC, etc.) have allowed various workers to propose fracture mechanics models of tubular joints. This paper presents a critical review and comparison of these models. It concludes that simple plate models can provide upper and lower bound stress intensity factor solutions for cracked tubular joints. Stress intensity factors from three dimensional finite element analysis of a tubular joint are in reasonable agreement with semi-empirical data. However, in the absence of a validated database the paper concludes that any analytical assessment of a crack in a joint should be viewed as tentative. NOMENCLATUREEi = polynominal coefficients G = strain energy release rate I = influence function K = stress intensity factor K, = mode 1 stress intensity K,, = mode 2 stress intensity K,,, = mode 3 stress intensity AK = stress intensity factor range K , = stress intensity factor for infinite length defect K,, = stress intensity factor for finite length defect K,, = stress intensity factor from weight function /qq, = distance between applied load (juq dA,) and a point q' on the crack front m = Paris crack growth law exponent M = correction to K for weld shape, etc. N = number of fatigue cycles P = applied load P, = applied load at point q q = point on crack surface q' =point on crack front Q = shape factor for semi-elliptic defect ds = length of crack front element t = brace wall thickness T = chord wall thickness w = plate width SCF = stress concentration factor W,, =weight function 483 FEMS 1415-A 484 J. HASWELL and P. HOPKINS x = distance through-wall of a joint Y = K/u;SCF. Jza or KlaJna Z =Young's modulus / I = d / D p4 = distance between applied load at q u = nominal stress in plate u, = nominal stress in brace v = Poisson's ratio and centre of ds T = t / T Subscripts b = bending component e = embedded defect exp = obtained from experiment m = membrane component s = surface defect u = uniform stress field v = variable stress field.

The United Kingdom Onshore Pipeline Operators Association (UKOPA) is developing supplements to the UK pipeline codes BSI PD 8010 and IGE/TD/1. These supplements will provide a standardized approach for the application of quantified risk assessment to pipelines. UKOPA has evaluated and recommended a methodology: this paper covers the background to, and justification of, this methodology. The most relevant damage mechanism which results in pipeline failure is external interference. Interference produces a gouge, dent or a dent-gouge. This paper describes the fracture mechanics model used to predict the probability failure of pipelines containing dent and gouge damage and contains predictions of failure frequency obtained using the gas industry failure frequency prediction methodologies FFREQ and operational failure data from the UKOPA fault database. The failure model and prediction methodology are explained and typical results are presented and discussed.

The United Kingdom Onshore Pipeline Operators Association (UKOPA) was formed by UK pipeline operators to provide a common forum for representing operators interests in the safe management of pipelines. This includes providing historical failure statistics for use in pipeline quantitative risk assessment and UKOPA maintain a database to record this data. The UKOPA database holds data on product loss failures of UK major accident hazard pipelines from 1962 onwards and currently has a total length of 22,370 km of pipelines reporting. Overall exposure from 1952 to 2010 is of over 785,000 km years of operating experience with a total of 184 product loss incidents during this period. The low number of failures means that the historical failure rate for pipelines of some specific diameters, wall thicknesses and material grades is zero or statistically insignificant. It is unreasonable to assume that the failure rate for these pipelines is actually zero. However, unlike the European Gas Incident data Group (EGIG) database, which also includes the UK gas transmission pipeline data, the UKOPA database contains extensive data on measured part wall damage that did not cause product loss. The data on damage to pipelines caused by external interference can be assessed to derive statistical distribution parameters describing the expected gouge length, gouge depth and dent depth resulting from an incident. Overall 3rd party interference incident rates for different class locations can also be determined. These distributions and incident rates can be used in structural reliability based techniques to predict the failure frequency due to 3rd party damage for a given set of pipeline parameters. The UKOPA recommended methodology for the assessment of pipeline failure frequency due to 3rd party damage is implemented in the FFREQ software. The distributions of 3rd party damage currently used in FFREQ date from the mid-1990s. This paper describes the work involved in updating the analysis of the damage database and presents the updated distribution parameters. A comparison of predictions using the old and new distributions is also presented.

Offshore structures are subjected to onerous environmental conditions. These conditions can cause cracking at tubular joints in these jacket structures and increasingly fracture mechanics is being applied to assess the cracks.There are a variety of fracture mechanics approaches to assessing cracks in tubular joints, all of which predict similar trends but give a wide variation of stress intensity factors. This variation is caused by the different models and inconsistent input parameters used.This paper presents a finite element study of a cracked tubular joint using several different structural models and consistent input parameters. The analysis includes both simple representations of a tubular joint, e.g. a plate model, and a full-scale analysis on the joint. A comparison of the full-scale analysis and the simple representations allow conclusions to be drawn on the applicability of simplifications to the problem and also allows errors to be quantified. NOMENCLATURE u = crack depth c = half crack length d = brace diameter D = chord diameter E = Young's modulus F = chord length J = J-integral K = stress intensity factor (SIF) L = plate length Q = shape factor for semi-elliptical defect t =brace wall thickness T = chord wall thickness or plate wall thickness W =plate width u = 2 L / D f l = d / D y = D / 2 T 5 = 2 / T D = stress u = Poisson's ratio Subscripts HS = hot spot TUB = tubular TOT = total.