An Analysis of the Dimensions of Embedded Defects in the Welds of an Offshore Structure

Abstract

ABSTRACT This is one of a series of papers describing an analysis of the occurrence, size and type of defects in the welds of the lower hull of the Hutton Tension Leg platform found during a detailed survey of the structure in the late stages of fabrication. This paper gives the results of an analysis of the dimensions of embedded defects detected and sized by ultrasonic testing, and involving the examination of approximately 27 km of weld by the method. Analysis has been undertaken of characteristics such as defect width, defect length and aspect ratio. Sufficient data were available to investigate the influence of parameters such as the type of welded joint, welding process, welding procedure and defect type on the defect dimensions. Results are given in terms of the statistical distributions of the sizes of embedded defects for different types of welded joint and types of defect, and are based on a sample of approximately 8300 defects. INTRODUCTION This paper describes the conclusions reached during a detailed analysis of the dimensions of embedded defects in the welds of the lower hull of the Conoco Hutton Tension Leg Platform (TLP). The data that have been analysed are part of a large data bank compiled from the results of the non-destructive examination (NDE) of about 27 km of weld by ultrasonic testing (UT) and about 76 km of weld by magnetic particleinspection (MPI) - a total of 121,270 UT and MPI examinations. These data have provided considerable scope for analysis and a general overview of some of the results has already been published by Baker et al.1 A subsequent more detailed analysis of the frequency of occurrence of defects and the occurrence of multiple defects has been given by Kountouris and Baker.2 This paper is concerned with the distribution of the size of embedded defects and with the relative frequenty of occurrence of defects of different types. Information on the sizes of surface-breaking defects has been reported elsewhere.3 In addition, a number of detailed report a have been prepared which discuss various aspects of the analyses undertaken.4–6 Knowledge of the occurrence rate and size distributions of defects in completed structures and knowledge of the factors which influence them are useful in the planning of NDE procedures and also in the safety assessment of structures which could fail by fatigue and fracture, see e.g. Ref. 7. It must be stressed, however, that because the structural form of the Hutton TLP is radically different from that of typical jacket structures, having details more akin to those found in ships' hulls, care should be taken in extrapolating the conclusions from the TLP to other forms of offshore construction. BRIEF DESCRIPTION OF THE STRUCTURE AND NDE HISTORY The Hutton TLP is well known as the first tension leg structure and was installed in the North Sea in 1984, with oil production starting later than year.