Past Abstracts - 1995

Technical presentations October 1995 

1    "Gasket materials in the marine environment" - Mr Peter C Snitter, Klinger Ltd

The health hazards associated with using asbestos containing gasket materials is often overridden by the high performance and wide adaptability of these products.  Why has this material remained popular for so long?  What are the alternatives?  Do they perform as well as asbestos materials ? Are there problems associated with the alternatives?

These are some of the questions to be addressed today, whilst also covering manufacturing processes, and most importantly, the correct selection and installation of gaskets in general.

2   "Mixed metal systems for marine service" - Mr Brian Shone, Consultant

Over the past 30 years a wide range of materials have been used in marine environments.  These materials include cast iron, steel bronzes, brass and more recently various grades of stainless steel.  The development of materials has in general led to a reduction in the number of service failures and decreased maintenance demands.  Examples of corrosion problems and solutions to these problems in mixed metal systems in equipment such as pumps, valves and heat exchangers is described.

3    "Clad materials - An economic solution" - Dr Liane Smith, Intetech, Consultant to NiDI

Corrosion resistant alloy clad steel has been available in various forms for over 40 years and is being used increasingly in the oil and gas production industries.  Methods of manufacturing clad plate, pipe and fittings are highlighted along with welding details.

Life cycle costing of pipeline projects is discussed.  Generally clad pipe lines in comparison to carbon steel lines offered long term savings in spite of the higher initial costs.  A risk analysis approach to failure is also presented.  The probability of failure and estimated cost of that failure if it were to occur are assessed and probability limit curves produced.  It is possible to use this approach not only for material selection but also to assess the effect of changing parameters such as wall thickness.

4    "Investigation of marine corrosion failures" - Mr Ken Farrow, RTD UK

Investigation of failures from shipwrecks to stainless steel shaving blades are described.  The importance of investigations is discussed in order to prevent further similar failures.  Failure was generally found to have been caused by:

  1. inappropriate materials

  2. design faults

  3. unexpected service conditions

  4. extended service life beyond the original design life.

Technical presentations July 1995

1   "The corrosion performance of aluminium alloys in marine environments" - Mr Keith Stokes, DRA

Aluminium alloys are widely used in the marine environment, particularly where high strength to weight ratios are a requirement.  For many applications medium strength materials, based on the aluminium-magnesium or aluminium-magnesium-silicon series of alloys, are employed because they are weldable, easily formed and have superior corrosion resistance compared to other aluminium alloys.  They are, however, restricted in their use for deep diving submersibles, where increased mechanical properties are necessary to resist buckling.  This requirement led to the adoption of a high strength aluminium-zinc-magnesium-copper (7010) alloy but, unfortunately, the corrosion resistance is poor compared to the medium strength materials.

This prompted the marine corrosion evaluation of a number of alternative high strength contender alloys, including aluminium-lithium, another wrought aluminium-zinc-magnesium-copper alloy and cast aluminium-copper.  Corrosion tests were carried out under both static and flowing (0.3m/sec) conditions using natural seawater obtained from Portland Harbour and, in addition, corrosion coupons were exposed to synthetic salt spray and high humidity cycles.  For comparison purposes, trials were also undertaken on alloys currently used in marine applications, with emphasis being placed on their performance against that of the 7010 alloy.  Preliminary studies on hard anodised samples were also carried out.

Results clearly showed the superior corrosion resistance of the 5083 (Al-Mg) and 6082 (Al-Mg-Si) alloys under all conditions and highlighted the poor performance of the high copper containing alloys such as the cast and wrought aluminium-copper alloys and 7010 (Al-Zn-Mg-Cu).  The resistance of the aluminium-lithium alloys depended upon their chemical composition and heat treatment, with the higher copper containing samples exhibiting a greater propensity for pitting attack and higher susceptibility to crevice corrosion, particularly over shorter immersion periods.  The high strength commercial alloy, 7049 (HC) performed unexpectedly well in view of the high elemental additions.  Hard anodised coatings gave variable results, with poor film characteristics being noted on the cast alloys and lack of adhesion on the aluminium-lithium-magnesium-copper alloy.

Overall it was concluded that the high strength cast aluminium-copper alloys had inferior corrosion resistance to 7010, but that the aluminium-lithium alloys offered similar or improved resistance, depending upon its heat treated condition.  Surprisingly the 8% zinc containing alloy, 7049 (HC), was found to be less susceptible to localised corrosion that the lower zinc containing 7010 alloy.  It was also concluded that further research into hard anodising processes needs to be undertaken.

2    "Update on nickel base alloys in the marine environment" - Mr David Hopkins, Inco Alloys

Austenitic stainless steels are high nickel alloys with high enough Cr, Mo, W and N contents are very resistant to general, pitting and crevice corrosion in seawater – and Inco manufacture the bulk of relevant alloys starting from INCO alloy 25-6 Mo (alloy 926) through to INCONEL alloy 686.

In general :

Pickling corrosion resistance is proportional to pitting index, Cr%+3.3 (Mo%+½W%)+16N%

Crevice corrosion resistance is proportional to crevice corrosion index, Cr%+3.3 (M+½W%)+32N%

INCONEL alloy 25-6 Mo (alloy 926) has a pitting index of above the level of around 44 necessary to achieve reliable resistance to pitting and crevice corrosion in seawater up to around 45°C.  The other alloys G-3, 625. C-276, 622 and 686 have progressively higher pitting indices indicating resistance at progressively higher temperatures (or progressively lower pH's).

INCONEL alloy 686 has by far the highest pitting index value of any Ni-Cr-Mo alloy yet, and this is reflected in generally superior general, pitting and crevice corrosion resistance.

Welding products of the same composition have excellent corrosion resistance and one layer provides higher Cr and Mo+W retention (better corrosion resistance) and lower iron pick-up than two or more layers of other related alloys.  Potential applications in seawater are more economical weld overlays and superior seawater cooled heat exchangers.

3    "The cost of corrosion - An economic assessment" - Dr Bijan Kermani, BP International

The impact of corrosion on the oil industry can be viewed in terms of its effect on both capital and operational expenditures (CAPEX and OPEX) and health, safety and the environment (HSE).  Quantifying the cost of corrosion in these categories is not an easy task.  Nevertheless, a recent in-house survey on the cost of corrosion to the BP Group presents a fairly comprehensive examination of such costs; and in general terms provides a valuable insight into the relative breakdown of likely corrosion costs affecting operators within the industry.

It is widely recognised that corrosion is a costly problem.  These costs typically represent in any one year roughly 3% of Gross Domestic Product (GDP) for developed countries.  If this is translated directly on to a company's balance sheet, as a percentage of turnover, then it represents a major cost in any one year.

There are four main ways in which corrosion costs can be incurred, namely: Design Expenditure, Operating Expenditure, Replacement Expenditure and Lost Revenue.  It is also common to categorise corrosion costs in terms of "avoidable" and "unavoidable".  Savings under "avoidable" costs may be realised by better use of existing or currently-available technology together with greater corrosion awareness and education.  By contrast, there are other types of "unavoidable" costs which currently have to be incurred because of the realities of the world in which we live.

Reduction of costs which are currently "unavoidable", or even "potentially avoidable", will depend on advances in technology, and thus present challenges for research and development (R&D).

4    "Development of a new alloy for high strength Cu-Ni tubing" - Mr Keith Bendall, Langley Alloys

A new cupronickel tubing material has been developed based on the long established HIDURON 191 produced by Langley Alloys.  This tubing material offers high strength (over 400 N/mm2 proof stress) combined with corrosion resistance to a variety of media.  The development, properties and potential applications - such as offshore subsea hydraulics, heat exchangers and high pressure tubing on Naval vessels – were discussed.  Joining of the tubing can be readily achieved by compression coupling or welding.

Technical presentations January 1995

1.   “Nuclear Electric's experiences of corrosion in cooling water systems”, Mr Terry Parsons, Nuclear Electric

Nuclear Electric was formed in 1989 and is a government owned public limited company supplying 23% of electricity in England and Wales.  11 active power stations supply 8500MW with 10 utilising seawater as a cooling medium. Cooling water systems are divided between Main (cooling the turbines), Auxiliary (oil, gland steam, transformer coolers) and Reactor (pressure vessel cooling).  Safety is obviously very important and the latter system in particular must be reliable and diversified.  The systems generally have the same basic components and problems.

 Typical corrosion problems include:

Typical approaches to overcome these problems include correct materials selection, care during commissioning, coating of valves and flanges with plastic and use of modern materials such as GRP for waterboxes.  Future developments include trials of full length metal inserts for lining old condenser tubes, further use of plastics and GRP, selection of better trim materials and more use of advanced metallic and non-metatlic coatings.

2.   “The Possibilities For Screwed & Polymer Lined Pipelines”, Philip Jaques, Hunting Oilfield Services

Pipeline Connection Systems:  Traditional Pipeline welding techniques may be substituted by pipeline connection systems (PCS).  There are two basic types proposed depending on the size of the line:

The threaded connections proposed are specialised derivatives of well proven oil country tubular goods (OCTG) technology.   The snap together connections are derivatives of oil industry tubular structural technology.  We will describe these systems & their installation methods in detail later in this report.

The Background: The early oil industry used threaded pipe-line connection systems based on OCTG up until 1947.  After that more reliable welding technology was developed & the OCTG was discarded.  During the intervening 45 years OCTG development has moved on & the technology now exists to offer more reliable pipeline connection systems.

The Threaded Derivatives: A study of the loads provided by pipeline consultants for typical pipeline cases has led us to conclude that regular premium connections (as used downhole) are not well suited for use as pipelines.  This is because most downhole casing & tubing strings are designed to withstand low bending & high tension.  Most pipelines see high bending during laying & compression during thermal cyclic operation.  Fortunately Hunting Oilfield Services have a range of threaded connections that are specifically designed to withstand high compression & can easily be adapted for pipeline use.  These are known as the Seal Lock family of connections.

The Structural Derivatives: The Hunting Merlin connector is a hydraulic snap together connector with self energising metal to metal seals & very high preload.  It has seen adaptation for use in diverse structural applications & was first developed in 1980 as a J lay connection system for use from a semi-submersible drilling rig.  The connection system has seen extensive use in deep water riser & TLP tendon applications.  As no rotation is required during makeup, pipe handling is simplified & the makeup method incorporates the use of a hydraulic clamp system.

The Materials issues: The untreated fluids carried by infield lines may require expensive corrosion resistant alloys (CRAs) that are not easy to weld in the field.  Threaded connections promise a reliable lower cost method of installing these lines; al lowing the use of lower cost CRAs that are not weldable, such as 13% chrome or GRP lined carbon steel.  The savings made possible on raw materials are shown by the following cost proportions:

Carbon Steel      1.0;  GRP Lined Carbon Steel       2.2;  13% Chrome Steel       3.3;  22% Duplex Alloy        6.7;   25% Duplex Alloy        8.7.

One important issue to bear in mind is the lack of industry capacity to produce CRAs above 7" in diameter.  This is currently estimated at only 1000 mtons per month worldwide.  This will drive the industry to use polymer lined carbon steel for flowlines 7" & above.

Implementing The Technology:  I've described the systems & the materials that are available. The technology is made possible by quality manufacturing & installation methods: 

Offshore Assembly:  The key elements are:

The procedures are those adopted during the running of OCTG & in particular metal sealing "premium threads" as well as those for running snap together Merlin connectors.  The mechanical devices are the power tongs & clamps used with the above procedures.  The computerised electronic monitors are those used to control the power tongs by measuring the torque & turns during makeup & comparing them to a known standard.

Summary:

Types: Screwed & snap together connection technology offers rapid pipeline assembly.

Materials: Lower cost non-weldable materials can be used.

Manufacturing: Designs are derivatives of existing successful products & standard OCTG methods areapplicable.

Assembly: Standard tools & QC methods can be used.

3. “Mechanisms of Corrosion and SCC Resistance in Duplex Stainless Steels”, A. Barnes and R.C. Newman, UMIST, Corrosion and Protection Centre

Duplex stainless steels often resist localized corrosion better than austenitic stainless steels of similar molybdenum content, owing to their generally higher chromium content.  The austenite phase is attacked preferentially in oxidizing chloride solutions but is significantly protected by nitrogen alloying in many 25Cr duplex steels.  Normally duplex stainless steels are very resistant to chloride-induced SCC, since the austenite phase is below its "best" cracking potential.  When a high susceptibility to SCC is observed, it is associated with interfacial cracking or with an equalization of the tendency of the phases to crack in the local environment.  The latter can occur as a result of sulphide additions or oxidant additions to hot chloride solutions.  In solutions containing sulphide, pitting and SCC occur at very low potentials, around -300 to –400 mV (SCE), and the relative behaviour of the phases can vary according to the alloy, environment and welding procedure.  Critical sulphide levels for pitting or SCC are associated with local depletion of  H2S.

4. “Sacrificial Anode Protection of Firewater/Seawater Lift Pump Caissons”, Dave Glasgow, ACEL

ACBL have designed and supplied sacrificial cathodic protection systems to a number of North Sea Operators over the last 5 years, to protect Firewater/Seawater lift pumps.  The basic design of a lift pump comprises a corrosion resistant non-ferrous pump body and a protective ferrous (mild steel) caisson wall surround.  The pump body generally has no protective coating and the latter is normally coated internally.  The two components are traditionally connected on the topside section of the pump,

Once immersed in seawater as would be expected, galvanic electrochemical corrosion readily takes place on the internal surface of the caisson wall.  This is because the two dissimilar metals have a significant potential difference between them, and become electrically continuous within the electrolyte (seawater).

These are two other factors which have been found to significantly accelerate the corrosion rate.  The existence of a large Cathode/Anode surface area ratio due to coating defects on the internal surface of the caisson, and (vigorous cavitation of the seawater}, during the pumps operation, in which the effects of concentration polarisation are diminished.

Coating defects on the internal surface of the caisson have been identified using video inspection cameras.  Generally these were found where the separation between the caisson wall and the pump body was at a minimum, normally adjacent to the hinges and pump sections.  In such case it was believed that coating deterioration had occurred due to the vibratory motion of the flanges against the caisson wall and cavitation around the pump section during the pumps operation.  On some occasions, fatigue cracking has been found to initiate at sites of coating breakdown when accompanied by mechanical stresses.

The two main options which have been considered in order to reduce the effects of this type of corrosion are the removal of the mild steel caisson and replacement with a non-metallic material or alternatively the deployment of a sacrificial type cathodic protection system.  The first option has generally been rejected because of the high cost of installation and of additional structural supports.  Nevertheless, the second option could be a viable alternative, if considered at the initial design stage.

Cathodic protection has generally been adopted as the preferred option, however, the design has had to overcome several problems.  The potential throw of the anodes can be significantly reduced, owing to the enclosed environment between the caisson and the pump, hence spacing of the anodes is critical, in order to provide full protection of the caisson.  This factor also restricts the size and shape of the anodes that can be used.  Minimization of the size and complexity of the anodes is generally helpful in respect of future maintenance of the system.

Such designs make use of existing retrofit technology, whereby anodes are attached to the riser and pump sections.  In general, three types of anode are utilised.  Individual rod anodes, bolted to the riser flanges, bracelets to the pump body and sometimes additional hemispherical anodes attached to the base of the pump.

Once the system has been installed it is essential that electrical continuity is maintained between the caisson and the pump.  Additionally in order to extend the lifetime of the anodes, it is normally recommended that the pump itself is coated.

The chief advantages of the system lie in the fact that it is relatively easy to maintain such a system, install and reliability has been confirmed by several post installation surveys.  The savings as compared to full replacement of the caisson are of course very considerable.              Copyright  ACEL, December 1992

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