A Case Study Evaluation of Subsea Preferential-Weld Heat-Affected-Zone Corrosion
Authors: Tyson J.A.G. , Bell E.R.G. 
Published: December 2007

Annual subsea inspections of the Murchison field platform's major welded joints indicated that a significant proportion of the welds suffered from preferential corrosion of their heat-affected zones. Only a limited number of joints can be inspected economically each year, and the program is normally biased toward structurally important welds with relatively low predicted fatigue lives. The presence of corrosion grooves considerably modifies the weld stress concentration factors (SCF's) and thus impacts the fatigue rankings and choice of welds for inspection.

With only limited inspection data. there was a need to identify the extent of the corrosion problem. This paper discusses parameters studied in an attempt to correlate the spread and severity of the corrosion problem with variances in geometry, metallurgy, and cathodic protection. Correlations were found that enabled the successful prediction of location of further corrosion grooves and allowed a new optimum inspection program based on both fatigue prediction and weld corrosion history. The presence of the corrosion grooves in many cases gives spurious crack-like indications with magnetic particle inspection (MPI). This paper also discusses how alternating current potential difference (ACPD) inspection techniques together with profile-molding techniques were used to eliminate spurious readings and to characterize the grooving shape for detailed fracture-mechanics assessment.

Introduction

The Murchison field is located in U.K, North Sea Block 211/19. some 480 km [300 miles] northeast of Aberdeen. The jacket is a second-generation tubular steel structure that was installed in 156 m [512 ft] of water in Aug. 1979. There are about 180 major sub-sea nodes, with some 600 welded joints. Typical member diameters are in the range of 75 to 140 cm [30 to 55 in.], with major leg diameters increasing to 6 m [20 ft] at seabed. Member wall thicknesses range from 16 mm [0.63 in.] in conductor frame areas to as much as 90 mm [3.54 in.] at major nodes.

Corrosion protection is given by a hybrid impressed current and sacrificial-anode system comprising 216 impressed current anodes and 596 sacrificial anodes. The sacrificial anodes provide 25 % of the design current requirements of the jacket and complement the fully rated impressed current system.

Subsea monitoring and maintenance is undertaken annually with remote operated vehicles and divers. A major task is to check the structural integrity of the major welds and the effectiveness of the cathodic protection system. In particular, a selection of welds is chosen each year for detailed inspection. This involves cleaning to bright metal finish for close visual examination and includes, where specified, the use of MPI.

The Emerging Problem

Corrosion grooving was first reported in 1981. It was particularly noticeable on the sample of caisson welds that was inspected, but up to 30% of the weld toe circumference was affected on nearly one-half of the sample of structural welds. The caisson corrosion was investigated by grinding, and depths of grooves were reported to be 1 to 2 mm [0.04 to 0.08 in.].

The 1982 inspection concentrated on caisson welds, and in addition to further extensive corrosion grooving, cracks were found. A structural reanalysis was undertaken that confirmed low fatigue lives for caisson attachments, and strengthening programs were performed over the next few years. Further corrosion was found on more major structural welds, particularly at the first two subsea levels, 13 and 41 m [43 and 135 ft].

By 1984, 68 major structural welds had been inspected in detail, along with 66 appurtenance welds, and it was felt that enough data existed to try to establish the causes of what appeared to be quite extensive preferential corrosion (67 welds from 134).