Past Abstracts - 1999 part 1

Technical Presentations July 1999

1 "Guidelines for design and use of Titanium Heat Exchangers in marine and offshore applications", David Peacock (TIG)

It was emphasised that all the necessary data for heat exchanger design were available and examples of water velocity adjustment, support plate spacings and correction factors for heat transfer were given.

The use of Ti for heat exchangers can provide considerable cost savings, weight savings and improved power output and reliability.

2 "Examination of coating failure in marine environments using scanning acoustic microscopy", John Sykes (Oxford University)

Images and scans of coatings before immersion, after periods of immersion and after drying were shown. Features such as loss of adhesion, swelling of the coating and water-filled blisters were detected and the observations were correlated with results of lap shear tests. The effect of coating thickness was examined. A typical multi-component paint system for an automotive application was tested and no change in properties could be detected during the test period.

Initial studies on filiform corrosion of aluminium were also reported.

3 "Microbial corrosion of some stainless steels in marine mud", Roger Francis (Weir Materials)

4 "Corrosion monitoring - fact or fiction" Colin Britton

A definition of corrosion monitoring is given together with a brief review of the methods available to apply the technology. The distinction between monitoring and inspection of corrosion is noted although recent developments have 'bridged' the two technologies. Note that the corrosion monitoring under discussion here relates largely to internal monitoring in various process plant. The monitoring of cathodic protection systems is not considered.

A review of the early manufacturers and development of technology is presented for both the USA and UK. The links in some of the companies with the manufacture of corrosion inhibitors is described with the advantage provided to these companies in offering a corrosion monitoring option to their customers at that time.

The industries which are major users of the technology are the oil & gas industry, chemical and petrochemical manufacture, other process plant, power plant, and associated cooling water systems. The advantages for large process plant operating for extended periods between scheduled shutdown is emphasised. Other miscellaneous applications include shipping, aircraft and automobiles.

The contribution of the larger chemical companies in developing techniques is outlined. Other input has been provided by universities, the larger oil companies, various government sources including 'defence' industry requirements.

The literature available in the 1970's and contribution made by major conferences devoted to the subject are described together with 'standards' provided by NACE , I Corrosion and ASTM.

The results of a 'user survey' are reported which involved the data obtained and sensor unreliability. It was found that greater success was obtained when the design was carried out at the start of a project. The more successful programmes involved integrated systems using several techniques. A later survey indicated better reliability and a desire for automatic systems.

A number of case histories are described involving the chemical industry (prevention of a shut down due to corrosion), oil production (monitoring the dosage of corrosion inhibitor enabling the field to reduce the number of workovers), oil refining (use of automatic systems), power industry (boiler tubes), and in desalination plant (measuring corrosion rates for candidate construction materials). The successful application in ship cooling systems is described.

Case histories where problems occurred described include petrochemical production (corrosion of the probe sensor bodies), gas production (poor location caused erosion corrosion), and several failures of associated engineering involving attachment of the sensors in the plant.

Technical presentations March 1999 

1 “Corrosion issues with subsea electrical connectors”, Allan Nicholson (Tronic Ltd) 

In the last decade, significant advances have been made in the provision of reliable conductive, underwater mateable connectors for long term subsea operation. Reliability of such devices is now a pre-requisite for any subsea completion project, where vital power and communications are needed for the life of the field to maintain efficient and safe production operation. 

The art of connector design had progressed from early agricultural designs, to compact sophisticated units with high integrity sealing and electrical performance. Connectors can now be coupled remotely in deep water by Remotely Operated Vehicles (ROV’s) or by stabbing together as integral parts of the subsea modules during deployment, the design life of such connectors is typically 20-25 years but on some projects can extend to 40 years. For these rigorous applications a myriad of requirements is placed on the designer to ensure every possible failure mode is thoroughly assessed in the design process. 

Material selection and good design practices are essential to the performance of these products. The spectrum of materials and compatibility is, on its own, diverse and complex, requiring mechanical, electrical and chemical issues to be fully assessed. 

The presentation by J A Nicholson to the Marine Corrosion Club on 2 March illustrated some of the issues relating to corrosion issues with subsea electrical connectors, touching on post problems, selected material and design parameters. The presentation highlighted the complex nature of connector design from the design requirements, material selection criteria and corrosion protection philosophies adopted. Corrosion problems were identified on early ROV connectors which suffered pitting corrosion attack to Ferralium 255 yoke straps embedded in a polychloroprene rubber bushing. It was believed that this attack was due to free chlorine ions forming acidic regions in the crevices, producing pitting attack. Work was also progressing to determine the cause of calcareous deposits experienced inside connectors attached to CP systems through Glasgow University. 

2 “Corrosion and erosion-corrosion coatings of thermally sprayed Cermet coatings”, Trevor Hodgkiess (Glasgow University) 

The presentation commenced with a survey of erosion-corrosion problems encountered in marine industries involving components such as pumps and drill bits. Some comments were then made on erosion-corrosion of metallic materials to illustrate the complexity of the phenomenon and in particular the contributions to the overall material loss from mechanical erosion, pure electrochemical corrosion and corrosion-erosion synergy. 

The main part of the paper focused on the corrosion behaviour and erosion-corrosion of solid cermets with emphasis on Ni-Cr-Si-B and Wc-Co-Cr materials. Results were presented demonstrating that the static-corrosion behaviour of these cermets is much superior to carbon steel but inferior to the 31 6L grade of stainless steel. Moreover, the corrosion mechanisms of the cermets are complex involving attack by microgalvanic/microcrevice processes at interfaces between hard-phase particles and the metallic matrix. 

In relation to erosion-corrosion behaviour of the cermets, the influence of a range of factors such as solids loading and environmental parameters (e.g. temperature, salinity) were discussed. Results presented by the speaker substantiated that, in common with metallic materials, there are significant direct and indirect contributions from corrosion mechanisms to erosion-corrosion deterioration in cermets. 

The presentation finished with a listing of the important factors, both from a fundamental mechanistic and corrosion-engineering viewpoint, in erosion-corrosion performance of cermet materials. 

3 “West of Shetland — Some Experience”, Bill Murphy (BP-Amoco) 

The BP Amoco Foinaven field lies West of Shetland lies in approximately 500m of water and was the first UK “deepwater” field to be installed using diverless ROV technology. The project received a major setback when leaks were discovered during final commissioning of a subsea manifold. Cracks were discovered on two of the 25Cr super duplex forged pipeline connectors on the central manifold which were not associated with pre-existing forging or weld defects but were identified as being due to hydrogen embrittlement/hydrogen assisted stress corrosion cracking. A major investigation was undertaken to understand the reason for the failure to provide solutions and assurance of the long term fitness for service of the remaining 180 of these components used across the field. 

It is generally agreed that hydrogen embrittlement/cracking is the result of the combined action of stress and hydrogen acting on a susceptible microstructure. However it is clear that there are a large number of variables within the three basic elements which could influence the conditions under which cracking could occur and these are less well understood The talk outlined the work carried out by the investigation team which involved extensive laboratory material testing using industry accepted methods, full scale testing using actual components, hydrogen measurements, residual stress measurements, exhaustive finite element stress analysis for all possible installation and operational start-up/shut down cases. The information was used to establish a justifiable case for each connector in the field and allowed BP Amoco to maintain a viable project. 

A number of observations were drawn as a result of the investigation: 

1.         The failure would not have occurred in air but the combination of pipework configuration and fabrication tolerances resulted in high peak stress which exceeded the threshold for cracking at the -lO5OmV potentials achieved using aluminium anodes. This could have been influenced by the particular microstructure of the heavy forgings required for the pipeline connectors. 

2.         Stress concentrations were important and the failures were not caused by global yielding of the components. 

3.         Industry standard lab tests would not have identified the problem. 

4.         Testing seemed to indicate that plastic straining seems to facilitate crack initiation and could constitute a risk for pipelines subjected to thermal movements and buckles. 

5.         It is recognised that many duplex pipelines are operating satisfactorily but it seems that further work is required to understand the loss in ductility to determine whether existing design codes are appropriate. 

6.         Need to consider lessons in relation to 13Cr materials. 

4 “The effect of copper corrosion products on the crevice corrosion of stainless steels”, Roger Francis (Weir Materials) 

There have been numerous reports of crevice corrosion of superduplex stainless steel in screwed couplings. Some of these are due to incorrect material or unsuitable jointing compounds. However, other failures are not easily explained. The system originally had nickel aluminium bronze valves but these suffered severe galvanic corrosion and were replaced with duplex stainless steel. The results of tests with soluble copper products show that copper can precipitate outside stainless steel crevices. A mechanism is proposed in which the copper acts as a more efficient cathode than the stainless steel, stimulating dissolution within the crevice, which could lead to initiation of crevice corrosion at temperatures (200- 250C) which would normally not be of concern.

Technical presentations January 1999 

1. The BT Global Challenge Round the World Yacht Race, Alastair Hackett (BT Challenge Business)

The speaker first described the inception of the Round the World Yacht Race by Chay Blythe, the aim being to provide a significant event that was relatively easy to enter. Under initial British Steel sponsorship, 10 identical 60ft yachts had been launched in 1990, and a description was given of the construction of these vessels. They had been built up from a keel plate essentially following established practice with a 4mm thick steel hull, and type 316 stainless steel decking and cabling etc. The primary protection system was by epoxy paints; the entire interior was blasted to an SA 2½ finish and up to 7 coats applied, depending on the future access to different areas of the hull. Particular attention had been given to the weld between the hull and the stainless steel deck, the design being such as to ensure that the fusion boundary was well protected by the epoxy paint system.

Further vessels had been built, and two races held. The race rules required the vessel to be taken out of the water for complete examination and re-fit in New Zealand at the half way stage. In all cases, the general condition of the vessels was found to be excellent, although it was noted that severe pounding, primarily when rounding Cape Horn, had led to some plates being dented by up to 25mm, and in these areas filler and paint had detached leaving small areas of staining. A more significant problem was that 2 vessels had lost their masts following failure of 316 stainless steel wire stays. The precise mechanism of failure had not been defined, but available evidence indicated that some degree of crevice corrosion had occurred at end fittings, possibly in conjunction with fatigue loading leading to progressive failure of the wire strands.

It was finally reported that construction of new yachts was proceeding to a slightly modified design. A particular feature of these was the use of laser cutting and water-jet cutting for jigs and specific components, and the precision offered by these methods had been found to facilitate construction significantly.

2. Surface preparation and marine coating performance, Graham Boaler and Malcom Morris (W & J Leigh & Co.)

The paper outlines two research projects, which compare coating performancein relation to:

1. Method of surface preparation, including mechanical preparation, dry gritblasting and Ultra High Pressure (UHP) water blasting @ 35000 psi;

2. Degree of Substrate salt contamination.

The results display significant trends in coating performance, after 3 years on a wet/dry cyclic test regime; with both dry and wet blasted substrates (which offer significant reductions in substrate salt levels to below 3.5µg/cm2) vastly out performing mechanical preparation, which gives virtually no reduction in salt contamination.

The second project using a clear epoxy laquer on artificially contaminated substrates (from zero to 32µg/cm2) demonstrated that the level of soluble salts on a steel substrate is directly proportional to the rate of corrosion cell development, using an accelerated test method of immersion @ 30°C and 3 bar pressure.

3. Life cycle costing for offshore structures, Kjell Haughland (Jotun-Henry Clark Ltd)

The speaker pointed out that material selection to avoid corrosion could following one of three routes, viz.

1. a fully corrosion resistant material requiring no maintenance

2. a corrodible material that would be regularly replaced

3. a material offering some measure of corrosion resistance together with a suitable protection system.

In many cases, the third approach was the only practical option but it was necessary to consider lifecycle costs of any protection system that might be applied. He then considered the various factors that would influence selection of a protective paint system.

The first point made was the ease of maintenance: although a ship could be dry docked regularly for refurbishment this was much less possible with FPSOs and offshore structures. It was therefore necessary to give due attention to paint durability. The cost of paint per se would normally be less than 10% of the painting contract cost and less than 1% of the total project cost, but its failure could have significant financial consequences. Attention must be paid to the quality of application, and it was remarked that NORSOK N501 requires the applicator to be certified, as normally the case with welders for example, and it is further stipulated that some paints should be certified. When total costs are considered, it must be recognised that corrosion costs would increase at an increasing rate with time; however, the benefit of corrosion protection would be most marked by early action and careful specification and application.

When considering total costs, consideration was required of the expected design life, of the likely breakdown time of any paint system and associated maintenance frequency, and of the allowable end of life condition. Experience showed that most paint failures stemmed from poor workmanship during application, and that 80% of paint failures were reported in the first two years of service. The different paint systems available displayed various characteristics: although epoxy systems gave good corrosion protection, they would not necessarily be as "self-protective/healing" if broken as would be a zinc silicate type, while a heavy glass flake paint system could well give a longer life than others but would not necessarily represent the cheapest option for a given application.