Weld Defect Distributions in Offshore on Structural Reliability
Authors: Rogerson John H. , Wong W. Khean 
Published: October 2008

Failure by fracture is a serious possible fracture mode of steel offshore structures particularly in hostile, low temperature environments. Such fracture will almost certainly initiate from flaws in regions of stress concentration. For any probabilistic failure analysis to be credible it is necessary to have a good estimate of the flaw size and distribution. For fixed steel platforms the important flaw size distribution is the distribution of defect height in welds in node connections. Previous work has shown that different structures have very similar defect distributions which suggests that a function can be derived for the generality of such structures.

Data has been analyzed relating to &gt; 1000m of weld in one North Sea structure. From this we can see that a Waybill distribution is the appropriate function to use for defect height. Analysis of other, published, data confirms this. It is the best function because it accurately and conservatively describes the extreme values (the distribution of large defects) which are the most important. Conclusions can also be drawn about the required reliability of the inspection and repair methods and the importance of using even an imperfect inspection and repair technique to significantly reduce failure probability.

Introduction

Offshore structures need to have a high structural reliability, i.e. there must be a low probability of structural failure occurring during the working life of the structure. This is obvious when one takes into account the possible consequences of a structural failure.

Since the reliability is high, conventional reliability techniques are inappropriate and reliability must be estimated by probabilistic methods. Probabilistic methods, however elegant, rely for their credibility and usefulness on the quality of the data on which they base their estimates and, unfortunately, in many case the appropriate data is not available in sufficient accuracy or detail.

This paper presents the results of an extensive programmed of data collection and analysis on the distribution of weld defects in offshore structures constructed to current standard. This therefore provides reliable data for the estimation of structure failure probability when the failure mode is dependent on the presence of weld defects.

Relevant Failure Modes

Failure can occur as a result of fracture when the toughness is locally insufficient to tolerate a region of stress intensification under an applied load. Such fractures are frequently, but not always, a consequence of fatigue crack growth and is a failure mechanism inherent in the design and material properties for a given environment. This is an important possible failure mechanism whose probability can be controlled to a large extent at the design and manufacturing stage.

If a structure occurs in a welded structure it will often be from a defect in a weld. Consequent whatever probabilistic model is used to estimate the probability of failure due to fracture the distribution function for weld defects is necessary information.

Defect Information Which Is Required For Structural Reliability Estimation

Failure by fracture occurs when a defect exceeds a certain critical size such that it is energetically favorable for the defect to extend as a crack.