TOPSIDES BOLTING - CORROSION PROTECTION

IMPACT AND COSTS

BP Amoco No Corrosion Project - Technical Bulletin Rev. 1, February 1999

SUMMARY

Corrosion of topsides bolting: Threatens mechanical integrity Increases maintenance costs (e.g. nut seizures) Creates safety concerns (e.g. hot bolting operations, loss of integrity)BP Amocos North Sea experience over recent years has been of unsatisfactory per-formance of PTFE coated and electroplated low alloy steel bolting and that hot dippedspun galvanised bolting has provided better corrosion protection at reduced cost.This bulletin outlines the methods of corrosion protection and recommends the use ofspun galvanising for low alloy steel bolting in the majority of topsides flanged connec-tions.

Safety In the longer term, the poor corrosion protection offered by

PTFE and electroplated bolting will impact plant integrity. More immediately, personnel are subject to greater risk during

normal and hot bolting operations due to the difficulties andadditional time required to remove seized bolts.

CostThe cost impact of poor corrosion performance of coated/platedbolting arises in two areas:Preservation Costs:

Assets becoming aware of the impact of bolting corrosion haveinstigated programmes of retrospective bolt preservation usinginhibited wax. (N.B. Wax preservatives cannot be used at temper-atures >70C). Typical costs of preservation programmes are:

Bolting after five years exposure in a marineenvironment.Top: Electroplated ZincMiddle: Hot Dip Spun GalvanisedBottom: Sheradised

Maintenance Costs:

It is difficult to assess the overall cost impact on maintenance and repair times due to seized bolting. All Assetscontacted (Gyda, Ula, Harding, Miller, Andrew, Magnus, Thistle) reported that there was a regular need toremove seized bolts, which is expensive in time and causes delay, especially if specialist tools need to bebrought in. Repair and maintenance work takes longer, sometimes causing considerable additional downtimeand production losses. Assets report repairs which needed only minutes to carry out extending to hours by theneed to remove corroded bolting.

Asset Cost of Retrospective PreservationMillerSchiehallionBruceGyda

~ 250,000~ 160,000~ 150,000Cost not available (preservation + replacement with galvanised bolting)

This Bulletin has been sponsored by BP Amoco's "No Corrosion" Project to assist sharing of operations experience on corrosion andmaterials issues to New Projects, Partners and Business Units.

BACKGROUNDThe normal low alloy steel bolting material used on topsides process and pipeline connections is either ASTMA193 Gr B7/B7M & or A320 Gr L7/L7M. They are low alloy, quenched and tempered Cr-Mo steels. The corro-sion resistance of these materials is low and they require corrosion protection in an offshore environment.Although corrosion resistant alloys (e.g. stainless steel, duplex stainless steel, precipitation hardened cupro-nickel) are used in small quantities for topsides plant bolting, cost precludes their widespread use. Low alloysteel bolting with some form of corrosion protection make up the majority of topsides fasteners.Bolts can be supplied with a variety of surface treatments. The common options are:

OPERATIONAL EXPERIENCE

Type Standard Minimum ThicknessZinc and Cadmium electroplatePTFE coating + phosphatePTFE coating + electroplate (Zn or Cd)Sheradising (Barrelled in hot zinc dust)

BS 1706

BS4921 Class 1 / 2Spun galvanised (Dipped in molten zinc) BS 729

8m30m PTFE30m PTFE / 8m Zn/Cd30m/15m43m

Electroplated Zinc and Cadmium Bolting

Electroplated cadmium and zinc only protects bolting during storage prior to installation. In a marine environmentthe thin sacrificial coating is depleted within weeks.

The Miller platform suffered noticeable rust staining of corrosion resistant alloy process plant from electroplated zincbolting, leading to concern over long term bolting integrity.

The Ula platform experienced corrosion of cadmium plated bolting within 1-2 years of operation. Other operators have experienced similar poor corrosion performance with electroplated cadmium bolting. Concern over the toxicity of cadmium has led some operators to ban its use.

Hot Dip Spun Galvanised Bolting

The corrosion performance of hot dip spun galvanised bolting has been much better than the plain electroplated orPTFE coated equivalents.

BP Amoco Norway has been using galvanised bolts for almost ten years on carbon steel, 316 stainless steel, duplexstainless steel and cupro-nickel topsides systems on Ula. PTFE bolting is replaced with galvanised bolting duringany maintenance operations on Gyda.

Gyda has also used galvanised steel bolting on hot produced water valves to replace stainless steel bolts whichhave failed by stress corrosion cracking, when packing boxes have leaked.

The Forties Unity platform has used hot dip spun galvanised bolting for more than six years. During recent con-struction work, a tie-in flange was hot bolted with no problems of seizures of nuts, which could have been reused.

Amerada Hess has used galvanised bolting since the mid-eighties with proven success.

PTFE Coated Bolting

PTFE over phosphate has not been effective in the offshore environment. Damage to the PTFE during make-upexposes the phosphate layer which provides little protection. However, this system has worked well in the MiddleEast.

It was anticipated that PTFE over zinc or cadmium would offer significantly better performance than electroplatedcoatings. Experience on Gyda has been that PTFE coated electroplated bolts were corroded by the time the plantwas commissioned. In the longer term, nuts have seized and have had to be removed with a nut cracker or cut off.

This poor experience has been confirmed by the Forties, Andrew, ETAP, Bruce and Schiehallion Assets, wherePTFE coated bolts started corroding within weeks.

QUESTIONS

Why use hot dip spun galvanised bolting?Alternatives such as electroplated coatings and electroplated plus PTFE coatings have performed badly with failure occur-ring after only a few weeks on all of BP Amoco's new projects.The PTFE coating is inevitably damaged during make-up, corrosion prevention then relies on the electroplated coatingswhich are ~10m thick. A galvanised coating is much thicker at >43m. The life of a sacrificial coating is proportional tothickness. Once the PTFE coating is damaged, accelerated corrosion will occur at damaged areas, hence the rapid ratesof failure experienced.

Is liquid metal embrittlement (LME) of stainless steel by zinc from galvanised bolting a concern?LME of austenitic and duplex stainless steels can occur in the simultaneous presence of molten zinc, high tensile stress-es and a temperature >750C. The HSE issued a technical note 53/1 after the Flixborough incident alerting industry to therisks. Over the years this problem has assumed a significance out of all proportion to the real risk.In the event of a fire the mechanical properties of stainless steels will have been greatly reduced before the critical tem-perature for LME, such that in a fire scenario it will not be the primary failure mechanism. As the risk of LME by zinc is con-sidered to be very low, zinc coatings (electroplated and spun galvanised) are acceptable for use on bolting on stainlesssteel piping systems.

Are thread clearances an issue on galvanised bolting?Extra clearance must be provided on the threads to allow for the zinc coatingthickness. For hot dip galvanised bolts and nuts, it is normal for standard boltsfrom stock to be galvanised; the nuts are galvanised as blanks and thentapped oversize by an amount related to the coating thickness and threadtype (pitch). When assembled, the nut thread is protected by contact with thezinc coating on the bolt.Some galvanised bolting supplied to the Miller platform suffered a very slackfit between the nuts and bolts. While not inspiring confidence that the boltingwas fit for purpose, detailed measurement showed they were within toleranceand tensile testing demonstrated that the shaft of the bolt failed rather thanthe nut pulling from the bolt.Testing did not cover the case of galvanised nuts with oversize threadingbeing fitted to PTFE coated or electroplated bolts. Systems and training arenecessary so that galvanised nuts are NOT fitted to PTFE coated or electro-plated bolts.

Have any problems been found when hydratightening galvanised bolting?The recent Forties Unity Riser project used galvanised bolting per the original project. Nut seizure problems were experi-enced during hydratightening of the large diameter 31/4" flange bolting. This was found to be due to the hydratight compa-ny using a hydratight collar that was only suitable for PTFE coated bolting.The use of an undersized collar was compounded by a small ridge of galvanising along the threads of the large diameterbolting due to the cooling of the bolting when the operator has to lift them into the barrel of the centrifuge. This should notbe a problem but if the tensioning nut is found to be sticking there exists the real possibility of nut seizure. In this case oneend of the studbolt where the tensioning nut is installed should have the thread cleaned by thread chasers.

Does a PTFE coating help friction control during torque or tension tightening of bolting?Problems have occurred because there has been an assumption that an anti-seize compound is not required on PTFEcoated bolting. Torque tightening results in the coating being scrubbed off and friction control (essential for achieving cor-rect bolt load) is lost. Corrosion protection is also lost. Tension tightening also damages the coating (but not to the extentof torque tightening) and can mean that removal by de-tensioning is difficult or impossible.Is there accelerated loss of zinc when galvanised bolts are used on CRA plant and could consumption of thezinc on the mating faces of bolting cause bolt relaxation?Operational experience of topsides bolting has not shown accelerated loss of zinc. The zinc that is lost is only from theexposed surfaces, there is no significant loss from the mating faces.

What is the life expectancy of a hot dip zinc coating?BS 14713:1997 gives the average corrosion rate of zinc in a high salinity atmospheric environment as 4-8m/year. The min-imum zinc coating thickness when galvanising to BS 729 is 43m, but the actual galvanising thickness tends to be greater.BP Amoco Norway experience is that for nearly ten years galvanised bolting has provided satisfactory corrosion protection.Spun hot dip galvanised bolting of all sizes have been used on the Forties Unity platform for at least six years. During recentconstruction work, an existing tie-in flange on the manifold was hot bolted. There were no nut seizure problems on the gal-vanised bolting, which could have been re-used. Previous experience with electroplated and PTFE coated bolting requiredthe use of a nut cracker to remove seized nuts.

Cross section through galvanised nutand bolt.

How do the costs compare?The relative costs (August 1998) of a common size of studbolt are shown below. The hot dipped spun galvanised finishwas two-thirds the cost of the PTFE coated equivalent.

Does it take longer to supply galvanised bolting than PTFE coated bolting?BP Amocos current bolting supplier have in-house electroplating and PTFE coating facilities which enable them to pro-vide a turnaround of 1-2 days for urgent deliveries. They supply spun galvanised bolting using galvanisers in the Midlands.Delivery time for urgent orders is extended to 5-6 days.

HOT DIP GALVANIZINGThe Process: The terms spin galvanising or centrifuge galvanising are used to describe the process forhot dipping threaded components and other small parts. The procedure is, after chemical cleaning, toimmerse them into molten zinc (normally @ ~450C) in a perforated basket. After the coating has formed, thebasket is centrifuged at high speed so that the spinning action throws off the surplus zinc and ensures a cleanprofile.

The Coating: When nuts and bolts are immersed in the galvanisingbath, a series of zinc rich alloy layers are formed with a metallurgicalbond to the steel. As the adjacent microsection shows, there is notransition between the steel and the zinc, but a gradual transitionthrough a series of alloys which provide the metallurgical bond. Thesealloys are harder than a mild steel and are normally covered by anouter layer of comparatively soft zinc. This structure gives the coatinggood resistance to rough treatment.

How does it protect steel? Zinc protects steel by a process known as sacrificial protection, whereby thezinc corrodes preferentially protecting the steel. If the coating is damaged exposing a small area of steel, thezinc and the steel together with the moisture in the atmosphere form an electrolytic cell. The zinc (anode) cor-rodes and the cathode (steel) is protected. The corrosion products from the zinc are deposited on the steelre-sealing it from the atmosphere and stifling the corrosion reaction. Scars up to 5mm wide will be protected.

For more information contact:Paul Badelek (Corrosion Consultant)BP Amoco Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB21 7PB

Tel: 01224 834071 E-mail: badelekp@bp.com

Type Relative CostPlain SteelCadmium Electroplated / Zinc ElectroplatedPhosphate and OilHot Dip Spun Galvanised

11.61.62.3

Cadmium Electroplate + PTFE Coating 3.7Phosphate + PTFE Coating 3.7

Relative Costs of 5/8 x 4 ASTM A193 B7 Studbolts and Nuts

Zinc Coating. Galvanic cell protects steel and corro-sion products precipitate on steel coating to protect it.

Paint Coating. Rust creeps under paint film, which islifted. Corrosion continues.

01129468, February 1999

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