oatings for offshore struc-tures must withstand thecorrosive environment of seawater above and below thewater line and in the splashzone. However, on-site main-

tenance is very costly. Scaffolding alone on offshore structures can dou-ble the cost of coatings work. Difficult working conditions add to theexpense.

Therefore, protective coatings for offshore platforms should have aminimum life to first maintenance of 20 years. One way to achieve thislifetime is with a thermal spray metal coating.

Long-life metal coatings for corrosion protection are generally selectedfrom materials that are anodic to the iron or steel substrate in order toprevent interface corrosion resulting from penetration of the coatings bywater after years of exposure.

Materials most commonly used are zinc, aluminium, or their alloys incoatings with a thickness of 100200 microns. Aluminium is very effec-tive in salt water atmospheres and for total immersion service. Zinc pro-vides sacrificial protection for the structure in the event of any penetra-tion of the coating by an electrolyte. Alloys of these two materials areincreasingly used to maximise corrosion resistance for different condi-

PCE September 199928

By Massimo CornagoENI S.p.A. / Agip DivisionS. Donato Milanese, Italy C

Improving Performance

Using Thermal

SprayCoatings

for Offshore Structures

Improving Performance

Using Thermal

SprayCoatings

for Offshore Structures

Copyright 1999, Technology Publishing Company

tions and to enhance a coatingsmechanical properties. The life ofall sprayed coatings is extended(up to 25 years) by application ofsealants.

This article presents an experi-ence of ENI/Agip with the appli-cation of a thermal sprayed coat-ing (TSC) on the splash zone ofan offshore structure in theAdriatic Sea. Agip, a division ofENI S.p.A., an Italian energy com-pany, is engaged in the explo-ration, development, and produc-tion of oil and natural gas in vari-ous parts of the world. Followingis a description of the TSC systemused and how it was applied; adiscussion of health and safetyissues involved in the use of TSC;ENIs performance criteria for thisproject; and, finally, a brief costanalysis.

BackgroundA TSC system was applied by

the ENI/Agip Division for the firsttime on the splash zone of theRegina platforms jacket. Thestandard coating system forsplash zones is provided in ENIFunctional Specification 20000VAR.PAI.FUN. Rev. 0 February1995, which concerns protectivetreatments by galvanising.

Although a splash zone is diffi-cult to inspect in service, it wasselected for the application testbecause it is subject to damageand abrasion and because it is ina highly corrosive environment.

Literature reports of several testprogrammes were studied andevaluated. They indicated sprayedmetal coatings were found tohave a superior life to paint sys-tems provided that suitable sur-

tion 20004 VAR.PAI.FUN. Rev. 0May 1996, which concerns pro-tective coating systems by met-allisation, can be summarised asfollows: surface preparation by abrasiveblasting up to Sa 3 according toISO 8501-1; one coat of aluminium-magne-sium alloy (95/5) applied by elec-tric arc spray equipment (200microns); one coat of low-viscosity (3poise) aluminium epoxy primersealer applied by airless spray (60microns); and one coat of two-component flu-oropolymer-base enamel topcoatapplied by airless spray (30microns).

The main reasons for selectionof the aluminium-magnesiumalloy were its light weight, excel-

Topcoat10:30

31542043

Table 1: Environmental Conditions During Work Execution

HoursAir Temperature (C)Relative Humidity (%)Dew Point (C)Surface Temperature (C)Steel Profile after Blasting (m)

Blasting10:45

3154204395

TSA11:20

32522043

Sealant16:30

34442040

face preparation and applicationprocedures were performed.

Therefore, it was decided tocarry out a coating system appli-cation test with the intent to pro-vide up to 25 years of corrosionprotection before first mainte-nance. The application was com-pleted in 1996. Evaluation of thecoating system is still underway.

The procedure used in accor-dance with ENI/Agip Specifica-

lent resistance to polluted andmarine environments, excellentabrasion resistance, and its forma-tion of a protective oxide layer toreduce steel corrosion. Electric arcspray equipment was used toachieve good application and,therefore, to maximise the perfor-mance of the TSC.

The aluminium epoxy primersealer was selected because itpenetrates the pores of the metal-

PCE September 1999Copyright 1999, Technology Publishing Company

Copyright 1999, Technology Publishing Company

all suitable precautions be takento limit potential hazards duringall steps of metal spraying.Particular emphasis is given toreducing danger to personnel andprotecting the environment.These precautions can be sum-marised as follows.

Dust reductionThe combi-nation of metal particle size andits concentration in the immedi-ate atmosphere can be a hazard.Aluminium spray dust is particu-larly dangerous, since aluminiumis potentially explosive and flam-mable in this form. Therefore, anadditional dust extraction systemwas used. All extracted materialwas disposed into sealed contain-ers.

Protection of operatorsDarklens glasses were used to protectoperators from ultraviolet lightfrom the arc metal spray process.Respiratory protective equipmentwith air filter systems was usedto protect the workers from dust.Suitable masks and track suitswere used to protect against skinrash from metal particles.

Complete knowledge of theprocess and equipmentDuringapplication, metal spray equip-ment is supplied with high-volt-age power (380 volts). Althoughit is designed and built to besafe, potential danger ariseswhen it is not maintained ingood order or when the operatoris untrained in proper set-up orhandling of the equipment.

PCE September 199930

lic film, thereby reducing thetotal area of exposed metal andconsequently the rate of coatingdissolution. In addition, thesealant smoothes the surface tex-ture. It was selected to be chemi-cally compatible with the alu-minium-magnesium alloy TSCand the topcoat.

The two-component fluoropoly-mer-base enamel topcoat wasapplied for additional abrasionresistance, a longer life, and adecorative appearance.

Work Description Application

Before application of the TSC,the steel surface was blastcleaned to Sa 3 with a dry gritabrasive (0.251.50 mm) toobtain a profile of 95 microns(measured by replica tape) forgood mechanical bonding. Thesurface was dry and free fromgrease or any other contaminants.

The environmental conditionswere good during all workingsteps (Table 1).

The electric arc spray equip-

ment used to apply the alumini-um-magnesium alloy coating hadtwo consumable wire electrodes(insulated from each other) thatautomatically advanced to meetat a point in an atomising streamof compressed air. The result wasa continuous film with highadhesion (95 Kg/cm2) on thesteel surface.

The aluminium epoxy primersealer was formulated to flowover and be absorbed into thenatural pores of the TSC.Laboratory analysis was per-formed to assure compatibilityof the sealer with the TSC andthe topcoat.

Health and Safety Application of thermally

sprayed aluminium requires amore experienced and qualifiedcontractor than for application ofconventional coatings. Moreover,health and safety precautionsrequire particular care duringapplication. There are no specificrules for metal spraying.However, ENI/Agip requires that

Specifications&

Requirements

JobReferenceStandard

EquipmentSetup &

Operation

SurfacePreparation

Inspection&

AcceptanceTests

Pass

Fail

Acceptance

TSCApplication

SealantApplication

TopcoatApplication

Figure 1: Overview of the TSC application method

Copyright 1999, Technology Publishing CompanyPCE September 1999 31

Special trainingThis wasrequired for all people involvedin the work. It was based on abasic level of competence in theoperation of the safety systems ofthe process and more in-depthunderstanding of the specific haz-ards of metal spray processes(i.e., noise, high-pressure gassesand air, ultraviolet radiation, highvoltage, high temperature, etc.).

ENI Performance Criteria During all phases of the appli-

cation work, the TSC proceduressummarised in Figure 1 were fol-lowed. The ENI performance cri-teria applied to the process wereas follows.

1. Personnel must be knowl-edgeable of safety issues and pro-ficient in procedures for surfacepreparation and masking; equip-ment set-up, operation and fieldmaintenance; application of alu-minium and zinc or magnesiumthermal sprayed coatings; and in-process quality control.

2. Equipment for blasting, ther-mal spraying, and sealing/top-coating must be safe, effective,and productive.

3. The application process withthe specified equipment and ther-mal spray feedstock must pro-duce the intended TSC.

The TSC was applied in accor-dance with the equipment manu-facturers instructions. During allthe work, the equipment wasinspected to verify conformanceto specifications.

The ENI specification for theTSC detailed the coating systemscontract performance; the sched-ule requirements of the contrac-tors and subcontractors; and their

responsibilities for surface prepa-ration, thermal spraying, sealing,and topcoating.

Also within the specificationwere the acceptance/rejection cri-teria and the methods of mea-surement adopted for the inspec-tion tests. These criteria wereapplied to the TSC feedstockmaterial, surface preparation ofthe structure, and the materials,equipment, and methods used inthe application process.

A job reference standard forthe representative job sectionswas used as a pass/fail inspec-tion reference standard.

Dry film thickness was mea-sured in accordance withSSPCPA 2, Measurement of DryPaint Thickness with Magnetic

Application of Paints and RelatedProductsSurface PreparationMethodsPart 2: Abrasive Blast-Cleaning; ISO 2808Paint andVarnishesDetermination of FilmThickness; and ISO 4624Test Method for Pull-Off Test for Adhesion.

Cost Analysis As with most engineering deci-

sions, cost is a primary consider-ation when determining strategiesfor corrosion protection. Themain objective in selecting coat-ing systems is to minimise totalcost over the service life of thestructure.

Table 2 includes the averagecosts at construction and first

TSC Sealedand Finish(280 m)

110

Table 2: Cost of Typical Offshore Paint Systems ($/m2)

New ConstructionMaintenance

Solvent-free Epoxy 1 coat (1000 m)

35408293

Epoxy/AcrylicUrethane

4 coats (350 m)50

110125

Glass flake(1000 m)

66140160

Gauges, and inspections werecarried out in accordance withthe following specifications: ISO 8501-1Preparation ofSteel Substrates BeforeApplication of Paints and RelatedProductsVisual Assessment ofSurface CleanlinessPart 1: RustGrades and Preparation Grades ofUncoated Steel Substrates and ofSteel Substrates After OverallRemoval of Previous Coatings; ISO 8504-2Preparation ofSteel Substrates Before

Electric arc spray equipment(Photo courtesy of the author)

PCE September 1999Copyright 1999, Technology Publishing Company

maintenance of four differentpaint systems normally used onsplash zones of offshore struc-tures. It shows that the initial costof TSC is far ahead of the othercoating systems at new construc-tion.

However, if a TSC system lasts20 years before first maintenance,and the other systems typicallyare maintained every five or sixyears, it is easy to calculate thatthe total cost of TSC becomes themost economical over the life ofthe coating system.

(Two factors account for thehigh cost of maintenance work.First, the average cost of mainte-nance work is estimated to take120150% more time than thesame work for new constructionbecause of the added difficulty inaccessing parts of the structureonce it is in place. Second, thebiggest cost in offshore mainte-nance work is scaffolding. Forexample, it can cost as much as$30 to scaffold 1 m2, and the typi-cal splash zone for the eight legsof a platform is 108 m2 each or864 m2, which equals $25,920 inscaffolding costs.)

ConclusionsAlthough the use of TSC sys-

tems for offshore structures iswell established, this project wasENI/Agip Divisions first experi-ence with TSC, probably becausepreviously used conventionalcoating systems had producedvery few failures.

The system applied at the endof 1996 is still under evaluation,and the results of the TSC perfor-mance will be monitored for along time. However, the initial

Performance, December 1997. C.S. Baxter, Thermal SprayInspection Methods and TrainingRequirements, MaterialsPerformance, December 1997. E.C. Lohrey, Metallizing SteelBridges in the Field, JPCL, May1995, 3950. M. Cornago and R. Malfanti,The Paint & Coatings Costs forthe Corrosion Prevention in theConstruction and MaintenancePhases: The ENI/Agip Exper-ience, symposium on CorrosionControl by Coatings, CathodicProtection, and Inhibitors inSeawater, Dubrovnik 98. Systems and Specifications: SteelStructures Painting Manual,Volume 2 (Pittsburgh, PA, USA:Steel Structures Painting Council). SSPC CS-Guide 23.00, Guide forThermal Spray Metallic CoatingSystems (Pittsburgh, PA, USA:Steel Structures Painting Council,1 June 1991). Sprayed Coating for OffshoreProtection, Welding & JoiningEurope, October 1998. Tecno Service Italiana s.r.l.,Procedura TSI Arbatax 02,Trattamento Anticorrosivo JacketRegina. Agip Specification 20000VAR.PAI.FUN. Rev. 0 February1995. Agip Specification 20004VAR.PAI.FUN. Rev. 0 May 1996.

32

conclusions are positive in thefollowing areas: improvement of quality duringall TSC work, from the specialtraining programme for the work-ers involved to the high quality ofthe equipment used; very high adhesion value, giv-ing enhanced corrosion protectionpotential; reduction of the time frame forcoating application due to theshorter curing time required forTSC; and absence of maintenance costsover a service life of 20 yearsplus, thereby making the life-cycle cost for TSC lower thanother coatings traditionally used.

References T. Rosbrook, Thermal SprayAluminium: The Alternative toOrganic Coatings, in AchievingCost Effectiveness in CoatingsWork, The Proceedings of the PCE98 Conference, 13 April 1998,The Hague (Pittsburgh, PA, USA:Technology Publishing Co., 1998). R.A. Sulit, Thermal SprayCoatings for CorrosionProtection, tutorial presented atSSPC 98, 15-19 November 1998,Orlando, FL, USA. B. Fitzsimons, Thermal SprayMetal Coatings for CorrosionProtection, presented at UKEurocorr 94. K.P. Fisher, W.H. Thomason, T.Rosbrook, and J. Murali,Performance of Thermal SprayedAluminium Coatings in the SplashZone and for Riser Service, PaperNo. 499, Corrosion 94. E.R. Sampson, Thermal SprayCoatings for Corrosion Protection:An Overview, Materials