Flame-Sprayed Aluminum Coatings

Flame-Sprayed Aluminum Coatings
Used on Subsea Components
By: T. Rosbrook, W.H. Thomason, and J.D. Byrd

The performance of flame-sprayed aluminum (FSA) coatings on the Hutton Tension Leg Platform after three years of service in submerged, splash zone, and marine atmosphere areas in the North Sea is reviewed. An extensive assessment of the coating was performed when one tendon (consisting of 17 components) was removed after two years, and one production riser (13 joints) was removed after three years of service.

This article reviews the in-service performance of flame-sprayed aluminum (FSA) coatings on critical subsea components of the Hutton Tension Leg Platform in the North Sea. This was one of the first large-scale applications of this type of coating system for long-term (stand alone-not backed up by cathodic protection) corrosion protection of critical components in submerged service. Large FSA-coated components were removed from service and an extensive evaluation of the FSA coating conducted.

Conoco evaluated a range of protection systems for the production risers and tendons for the Hutton platform. Each riser was composed of 42 ft. (12.8 m) tubular joints and a 53 ft. (15.2 m) taper joint, and each tendon was composed of 31 ft. (9.4 m) tension-leg elements, a lower anchor connector with flex joint, and a cross-load bearing with flex joint. Protection for subsea components has been provided historically by coatings, cathodic protection, or a combination of both. The tension-leg elements and risers offered special coating problems because of the splines used for torquing the joints and sacrificial anode attachment problems because of difficulties in welding to the high-strength steel.

After extensive testing to identify electrochemical and mechanical properties of thermal- sprayed aluminum, a decision was made to adopt a thermal-sprayed aluminum coating for the production risers and the tendons. Both flame- and arc-spraying techniques were found acceptable, but the contractors chose to use the flame-spraying technique.

The Hutton Tension Leg Platform was placed on location during June 1984 in 485 ft. (148 m) of water at which time the tendons were installed. The installation of the production risers was a phased operation as wells were drilled. However, some wells had been predrilled using a semisubmersible drilling rig, and production risers for these wells were installed between June and August 1984 to enable production to commence on August 31, 1984.

Because of the revolutionary design of the platform, it was decided that (and agreed upon by the certifying authority) a complete tendon would be removed after two years of service to enable a detailed examination of the components to be undertaken. There are a total of 16 tendons with four located at each corner of the hull. Similarly, a production riser would be removed after three years to allow a similar examination, the platform being ultimately designed to accommodate 32 production risers. One tendon was removed during July 1986, and one production riser was removed during June 1987.

Coating Specification

The FSA coating to be applied to the various components was designed to provide a 20-year life in the arduous North Sea marine environment. Therefore, a tight specification was applied with stringent quality assurance and control. All components were coated with a temporary protective coating when fabricated. This protective coating had to be removed prior to the thermal metal spray coating. The application procedure was as follows:

Remove temporary protective coating.
Degrease.
Preblast using iron grit (G17/24 mix).
Final blast using aluminum oxide.
Apply two layers of aluminum at 100 microns per coat.
Apply polyvinyl butaryl wash primer and vinyl sealer on tension-leg elements and silicone sealer for production risers.

The components were subjected to two separate grit-blasting operations, the initial blast to remove scale and any corrosion product, the second to produce a white metal finish SIS 0050.SA3/BS 4232/1st Quality with a 75- to 115-micron surface profile. With the exception of the cross-load bearing and anchor-connector components, all the components were coated in England. The cross-load bearings (FSA) and anchor connectors (epoxy) were coated in the United States where they were manufactured.

This specification was issued to both coating facilities. The U.S. facility did not use the aluminum-oxide blasting media and elected to use GL-40 hardened steel grit.

Removal of Tendon in July, 1986

As stated previously, Conoco U.K. Ltd. agreed with the certifying authority that a complete tendon, comprising 17 components in total, would be removed after two years of service. The results of the ensuing examination would determine the future change-out requirements for the remaining tendons. As part of the project, sufficient tension-leg components had been purchased for 19 complete tendons, and thus three complete tendons were to be retained onshore as spares.

As a preliminary to changing out the tendon, sufficient components were withdrawn from the warehouse and subjected to an extensive examination of threads, coating, and bore. These components had been in storage since manufacture. The bore of each component had been wrapped with a bitumen-impregnated paper and wooden battens bound around the bore to prevent damage during transit and storage. The pin and box threads were protected by "greased" rubber thread protectors. Removal of the transit packaging showed that three of the elements had small blisters approximately 3 to 4 mm in diameter and 2 mm high, arranged along the bore of the elements. Obviously, the discovery of these blisters gave rise to some concern as to the condition of the in-service components. The examination of the blisters will be discussed later.

The removal of the components that comprise one tendon was done during July 1986. Each component was removed following the approved procedure and stored in the mooring compartment until the replacement tendon had been installed. As each element of the tendon was removed, it was subjected to high-pressure water jetting (4000 psi) to remove the majority of weed growth. This opportunity was taken to give each element a general visual examination.

The presence of blisters, similar to those found on the elements taken from storage, were found on all the components with the exception of the cross-load bearing. The absence of blisters on the base of the cross-load bearing may be an important factor regarding the cause of blistering of elements from in service and storage. Areas of mechanical damage were noted for further examination.

Removal of Production Riser

As previously stated, the production riser was removed as part of the ongoing requirement to satisfy the certifying authority that the various aspects of the riser were operating as designed and thereby not invalidating the Certificate of Fitness. A production riser was removed on June 6, 1987, having been in submerged service for three years. Removal of the riser showed very healthy marine growth on the upper components, with decreasing amounts on the elements nearer to the sea bed. The components were removed from the drill floor and laid out on the pipe deck to allow easier removal of marine growth and inspection.

The elements in service had been subjected to a variety of conditions, that is, totally immersed, splash zone, and salt spray environment. The upper components had been subjected to splashing because of platform wash down and salt spray driven by the wind. The coating on the production risers also had to contend with an additional parameter: heat generated by the flow of produced oil. External temperatures of the riser reached as high as 80oC (176oF), whereas the temperature of the seas was from 4 to 8oC (39 to 44oF). No blisters were found on the silicone-sealed FSA coatings on the risers.

Examination of Tension-Leg Components

Tension-Leg Elements in Storage

The three tension-leg elements removed from storage were found to have blisters on the shaft. There was no visual evidence of corrosion product beneath the blisters, nor was there any evidence of blister cracking. Careful removal of one of the blisters with a knife showed that the coating was detached from the substrate and a dark-brown deposit was found on the surface of the component. The deposit was not steel corrosion product, but it was tightly adhered to the steel substrate and could only be removed with the point of a sharp knife.

Tension-Leg Elements Removed from Service after Two Years

On completion of the change out, the various components were transported in their special carrying frames to the Aberdeen Warehouse. The components with the exception of the cross-load bearing and anchor connector were removed from the carrying frames and laid out on pipe stands that had a wooden protective surface. Removal of all marine growth was completed using high-pressure water jetting. This technique did not cause damage to the coating, nor did it remove the calcareous deposits.

Close examination of the FSA as applied to the cross-load bearing showed no signs of blistering on the lower portion that had been submerged. The upper shaft of the cross-load bearing that had been subjected to some immersion during installation and subsequent removal also did not exhibit any blisters.

Blisters were found on the remainder of the external surfaces of the tension-leg components. The blisters were uniform in size, approximately 4 mm in diameter and 2 mm high. The blisters were of three types: blistering of the vinyl sealer only, blistering of an outer layer of aluminum, and blistering of the aluminum coating down to the steel substrate. Since the vinyl blisters only occurred where the vinyl thickness was excessive and was cosmetic only, further blister discussion will only refer to blistering of the FSA. Examination of the blisters showed that they had broken and the space beneath the blisters filled with white aluminum salts. The blisters were tightly adherent to the surrounding coating. None was found to be loose; it was also noted that the blisters had not been "damaged" during the water jetting process.

The blisters were examined through a magnifying glass. The blisters cracked due to the pressure of corrosion product buildup beneath the cap. The blisters had cracked at either the base, circumferential cracking, or in a cross formation across the cap, star cracking.

Because occurrence sites were numerous, a random sample was examined on each component. Each blister, after recording the nature of the cracking, was removed using a sharp knife. The corrosion product was gently scraped away and the substrate examined. The cracking of the blisters was evenly split between circumferential and star-type cracks, and no particular pattern was established. The vast majority of the blisters, 95%, had not penetrated the total depth of the original coating (200 microns). The remaining 5% had penetrated to the underlying steel substrate.

Although the coating at these sites had been disturbed, there was no evidence of corrosion attack, either general surface or pitting, to the parent metal.

Adhesion tests were conducted on coating adjacent to the blisters, and no adhesion values of less than 1000 psi were measured. The original design of the coating system allowed for up to 6% of the total surface area of the submerged components to be devoid of coating without affecting the integrity of the corrosion-control system.1 During installation and removal of the components, some minor mechanical damage to the coating was inevitable. Where damage occurred during installation, no evidence of corrosion attack was found.

Possible Causes of Blistering

Aluminum oxide abrasive produces a superior surface profile that enables high adhesion for thermal-sprayed coatings. Aluminum oxide is usually manufactured from spent grinding wheels. This source, although inexpensive, does present problems of contamination. Therefore, unless the media has been cleaned thoroughly to remove all traces of grease and foreign bodies, contamination of the work surface is inevitable.

One reliable test for contamination calls for placing some media in a bottle containing demineralized water. Shake the mixture and allow it to settle. Any contamination will float to the surface.

In a subsequent coating job using aluminum oxide from the same source, dark-brown particles were found embedded in the blasted steel. The particles were similar in appearance to the spots found underneath the blisters. The problem of contaminated aluminum oxide grit also occurred in the United States to the extent that a high-purity (expensive) aluminum oxide was used for high-quality coating applications.

The blisters only occurred on components blasted with aluminum oxide and sealed with vinyl sealer. No blistering occurred on components sealed with silicone or components blasted with chilled iron grit and sealed with vinyl. Numerous laboratory and field tests with unsealed FSA coatings also did not produce blisters. Subsequent tests of the vinyl sealer for penetration into the porosity of FSA coatings showed it to penetrate less than urethane, epoxies, or silicone sealers. These data all suggest that the grit contamination provided a site for water to penetrate to the steel, and the aluminum salts formed generated a blister.

The better sealing silicone system prevented the water penetration. The silicone sealer stays quite soft unless cured at a high temperature; however, once inside the porosity of the FSA coating, it forms an excellent barrier to water penetration even without the high-temperature cure.

The blisters (95%) that occurred only in the outer layer of aluminum (two-pass coating application) are not as readily explained by the grit contamination. Perhaps small voids occurred between FSA layers, and these were not sealed by the vinyl sealer but were sealed by silicone sealer. Also, the tension-leg elements were probably hot when the vinyl was applied, so it dried quickly without good penetration. The large components coated in the United States may have cooled before vinyl application, and the vinyl formulation may have penetrated better. A final consideration is the wash primer used preceding the vinyl application may have played a role in the blistering.

Repair Procedure

Examination of the tension-leg components in storage had revealed that three were suffering blistering and a number of the other components had suffered mechanical damage to the packing and coating. In some instances, the coating had been scraped away, exposing the underlying steel. It was imperative that the coating be repaired where examination of the blisters had been undertaken and the mechanical damage had occurred.

A repair procedure was drafted based on the original specification, the major difference between the specifications being the use of manual preparation and application techniques. The recommissioning of the original automated coating line would not be economic or practical for dealing with small, localized patch repairs.

A local coating contractor who was involved with FSA coating applications was hired to perform the repairs. A number of 9 5/8 in. (24.5 cm) diameter test coupons from the original coating operations were made available to the contractor for procedure evaluation. The procedure called for the coupons to be blasted using an expendable abrasive followed by preparation using the best quality aluminum-oxide grit to give the required 75- to 115 microns surface profile. It was found that the final blast using aluminum oxide showed no visible improvement in the quality of the finish.

Examination of the steel surface after the final blast using a magnifying glass showed that the pores contained a dark-brown deposit - similar to the deposits found under the blisters taken from the elements in storage. It was decided, therefore, that both the expendable and aluminum-oxide abrasives be replaced with a mixture of G17/25 steel grit.

A number of coupons were coated with FSA, each one with different operating parameters for the hand-held gun. The coupons were given a pre-blast followed by a second blast using the G17/24 mixture of steel abrasive, then degreasing using a methyl ethyl ketone. Adhesion tests were implemented using the Elcometer standard adhesion tester. Once the operating parameter had been established for the gun, adhesion values in excess of 1000 psi were consistently achieved.

Unlike the original operation, the repair coating would have to overlap onto existing sound coating that had been sealed. Coupons were treated with sealer and then prepared by removing a strip of sealer 50 mm wide from the edge of a half-coated coupon. The bare steel was then grit blasted and the adjacent coating abraded using the established repair procedure. The FSA coating was then applied to the bare steel and overlapped onto the sound coating, the coating on the overlapped edge being subjected to an adhesion test. No problems were encountered in attaining the minimum pull-off value of 1000 psi.

The repairs were then carried out on the areas of mechanical damage using the previously mentioned techniques. A coupon was coated for every two repairs carried out and used as a quality-control measure.

Conclusions

The FSA coating is performing in a satisfactory manner. While the blistering is a cause for some concern, the rate of consumption of the aluminum is not excessive and the design life should be exceeded. FSA coatings have proved that they offer an excellent alternative for protecting subsea components, the major advantage being that the coating acts as both a tough barrier coating and a sacrificial anode.
The formation of the blisters appears to be a result of a combination of factors. The method of surface preparation was common to both the tension-leg elements and the production risers. Examination of the blisters on components in storage showed dark- brown deposits similar to the deposits found in the profile of the coupons after blasting with aluminum oxide. The tension-leg elements were sealed with a vinyl sealer applied over a zinc-chromate pigmented etch primer, while the production risers were sealed with an aluminum pigmented silicone sealer. Therefore, the blistering is believed to be a result of inadequate sealing of the aluminum by the vinyl sealer.

This short fall in sealing pore of the vinyl sealer allowed water to penetrate the aluminum coating, and thereby react with steel and aluminum at their interface or with any contamination encountered. The blisters on the storage elements were possibly caused by a combination of inadequate sealing (vinyl sealer) and moisture. The storage facility, although covered, is not heated; therefore, moisture could condense on tension-leg elements.
The silicone-type sealer is a good choice for sealing FSA coatings for subsea use, irrespective of temperature. Although it may not cross link without a high-temperature cure, it penetrates the porosity of the FSA coating well and provides a good barrier to water penetration.

The use of chilled iron grit, grade C17/25 overcomes the possibility of contamination encountered with the use of aluminum-oxide abrasives. Very clean aluminum-oxide abrasives are available but are expensive.

Acknowledgment

The authors wish to thank Conoco U.K. Ltd. and the Hutton Field Development Partners for permission to publish this article. Participating companies in Hutton are: Conoco (U.K.) Ltd. (operator), Britoil plc, Chevron U.K. Ltd., Amoco (U.K.) Exploration Co., Mobil North Sea Ltd., Amerada Hess Ltd., Texas Eastern North Sea Inc., and Enterprise Oil plc. The generous assistance given by colleagues in Aberdeen is also gratefully acknowledged.

Reference

1W.H. Thompson, Materials Performance 24, 3(1985); pp. 20-28.

         

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