An Insight Into the Design, Fabrication, Inspection, and Repair of Closure Welds in Offshore Structures

ABSTRACT This paper gives careful consideration to the fatigue and welding aspects of jacket construction at an early design stage from which cost savings can be achieved and potential downstream problems can be avoided. In particular factors affecting single-sided weld failures are discussed and the inadequacy of existing recommended SCF formulae are reviewed with recommendations for the use of more 'accurate' formulae. In addition, welding, repair and inspection techniques for single-sided welds are also presented with a view to improving the quality of such weld connections. Finally, guidelines are given for the improvement of design and weld quality of single-sided weld connections of fixed jacket type platforms. INTRODUCTION Offshore structures during their service lives are subjected to cyclic loading which is mainly due to wave action. As a result of this type of loading, fluctuating stress levels are induced within the structure which, when combined with stress concentration factors, can 1ead to very low fatigue lives at certain locations. A number of steel jacket structures, particularly in the North Sea, have experienced premature member cracking/failure in recent years. This is always as a result of either inadequate design or poor fabrication quality, or both, particularly at single-sided weld connections. Many of these structures have been subject to re-analysis and critical fatigue locations identified for subsea inspection. Unfortunately, because of their confidentiality, these failures are very rarely reported in the technical press. These types of failures could be obviated if detailed fatigue analysis had been carried out at the design stage and correct fabrication techniques, welding procedures and inspection procedures had been utilised during the construction phase. DESIGN ASPECTS When an offshore fixed platform is designed, a number of analyses have to be performed in a predetermined sequence. The sequence of analysis is determined in such a way that primary suits the project time schedule and secondly the analysis that dictates the S1ze of a substance part of the structure takes priority (e.g. static in-place before transportation and launching). As a result of this, fatigue analysis is performed at a late stage in the project and usually only major tubular joints are analysedfor fatigue using a suitable computer fatigue program. Fatigue analysis usually is the .most costly analysis during the design of a relativity big jacket e.g. for a jacket structure consisting of 650 joints and 1230 members the fatigue analysis computer cost could be double that of the static analysis. Nowadays, performing fatigue analysis in this traditional way is not adequate. The need for reducing the cost of a field development has resulted in looking for means of reducing steel weight and costs related to fabrication, inspection, installation, maintenance etc. Identifying seam welds along tubular braces which could be classified as single-sided closure welds during the design stage and prior to fabrication would lead to significant savings.