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Study: Less Blasting May Save USN $10M

Thursday, August 9, 2012

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Why undertake the gargantuan task of removing a battleship’s entire underwater hull coating system when only a fraction of the coating is loose?

That’s always been the Navy way, but new research suggests that the longstanding requirement is “redundant and unnecessary.”

 NSRP’s Surface Preparation and Coatings Committee conducted a two-year project to compare spot-and-sweep blasting and total coatings removal on five drydocked U.S. Navy vessels.

 Photos: NSRP Surface Preparation and Coatings Committee

NSRP’s Surface Preparation and Coatings Committee conducted a two-year project to compare spot-and-sweep blasting and total coatings removal on five drydocked U.S. Navy vessels.

A targeted coatings removal method called spot-and-sweep blasting now commonly used in the commercial shipping sector is just as effective and could save the Navy $10 million a year over its current surface preparation protocol, concludes the National Shipbuilding Research Program’s Surface Preparation and Coatings Committee.

Removal Practices

Currently, the U.S. Navy system maintenance strategy requires the complete removal of underwater hull coating systems after an eight-year service life by abrasive blasting or ultra-high-pressure water-jetting (UHPWJ).

By contrast, commercial practice (for general cargo fleet, tankers, bulkers, and cruise liners) includes an option for so-called spot-and-sweep blasting. This involves repairing the system at 36- to 60-month intervals by removing only loose or delaminated coating; applying primer to exposed steel; additional anti-corrosive as needed; and applying full antifouling coats.

Spot- and sweep-blast of underwater hull from 1996 NSSRP study

“The concept is essentially that since only a fraction of the anti-corrosive coating breaks down periodically—say, 5-10% of total surface area—it is redundant and unnecessary to completely remove 90-95% of intact coating to bare metal as required by Navy contracts each time upon coating system renewal,” the NSRP committee reports.

2-Year Study

The committee’s two-year project involved drydock blasting demonstrations on five Navy vessels. Both spot-and-sweep blasting and full coatings removal were included. The spot-and-sweep demonstrations were performed by Florida-based Chariot Robotics LLC, which makes robotic coatings removal technology.

The high-speed semi-automated robots, in conjunction with ultra-high pressure water-jetting, have been performing spot-and-sweeping at yards throughout Europe, the Middle East, Japan, Singapore, and the Caribbean for several years, the committee said.

For this trial, the production runs of spot-and-sweep removal ranged from 1,120 to 2,545 square feet per hour—”up to 10 times faster than traditional abrasive blast removal to a SSPC-SP-10 condition, thereby saving time in dock and increasing the ship’s operational readiness,” the project’s final report concluded.

Spot- and sweep-blast of ballast tank from 1996 NSRP study

Adhesion of tightly adhering existing coating was “not adversely affected by the spot-and-sweep method, even when using pressures in excess of 20,000 psi,” the team found.

Moreover, all water and paint effluent used and removed in the process was vacuum-extracted directly into a tank on the drydock floor—”in stark contrast of having to remove large volumes of spent abrasive from a drydock floor,” the report said

1,000 Days, $10 Million

The committee strongly recommended that the Navy adopt the more limited practice as an approved alternative surface preparation method.

“In all cases, the spot-and-sweep was deemed a success by all observers and by the principal investigator of this project,” the report says.

The group said the Navy could save more than 1,000 days of surface preparation and more than $10 million in related costs in 2012 by using spot-and-sweep methods for underwater hull and freeboard maintenance surface preparation.

“This schedule reduction can improve the productivity of other trades and may reduce the overall time in dock,” said the report “The potential savings from the schedule reduction could be far greater than the savings associated with the surface preparation activity.”

The report said spot-and-sweep water-jetting could “make a significant impact on dry docking maintenance costs for aging surface combatants such as the FFG-7 and CG-47 class fleet.”

NSRP Presentation

The research will be presented in October at the annual Ship Production Symposium hosted by the Society of Naval Architects and Marine Engineers’ (SNAME) and the National Shipbuilding Research Program.

The symposium presents original technical papers addressing problems of shipyard techniques and production of merchant and naval ships. Abstracts for the 2012 Ship Production Symposium are now available online.


Tagged categories: Marine; Marine Coatings; Paint and coatings removal; Research; Shipyards; SSPC; Surface preparation; UHP waterjetting

Comment from Jeremi Day, (8/10/2012, 10:17 AM)

I just sit and roll my eyes. Although, I agree with the thought of, "why remove a sound coating", I have to wonder if this will be well accomplished every time. Truth be told, this will work IF, it is done correctly. Those of us who see this in the field, rather than in some "demonstration", knows better than to think this will be appropriately executed in a real - world situation on a consistent basis. Good Luck to them. The "human factor" is what will get them in the end. It's great to see them use some common sense, but this is something that has to be done correctly for it to be successful.

Comment from shane hirvi, (8/10/2012, 6:05 PM)

Spot repair and over-coating have a long history of success--surprised the navy took so long to figure it out.

Comment from Gordon Kuljian, (8/11/2012, 5:33 PM)

I appreciate the comments but there's no need to roll your eyes Jeremi Day. This technique has been in practice for over a decade in the commercial shipping world. I encourage you to read the final report, available on the NSRP website, We have accomplished the spot and sweep on cruise liners, cargo tankers, and have done so on a number of Military Sealift Command ships (each over 80,000 ft2 underwater hull). One of the demonstrations for the project was on a Navy Frigate - entire underwater hull and freeboard hull - almost 30,000 ft2. You will see that this is by no means a method on small test panels. As for human factors getting "them in the end", with proven equipment and experienced operators this does not happen. As the report states, only hulls with 90-95% intact coating are recommended for spot-and-sweeping. Also I encourage you to see the Chariot Robotics website to learn more at I am the principal investigator for this project and stand by its merits wholeheartedly.

Comment from jesse chasteen, (8/12/2012, 11:58 PM)

I like your thought Gordon, you must be bait junior to me as we were spot and sweeping 30 years ago as a way to save a customer money and still satisfy ABS certification..the other aspect would be the middle of the road and utilize SSPC-6 criteria which would still show a savings.

Comment from Jeremi Day, (8/14/2012, 6:05 AM)

(I probably shouldn't have tried to write that from my phone) .... You say, "With proven equipment and experienced operators this does not happen". This is an area that the industry has had problems with for quite a while. I completely agree with your assessment. I do not, however agree with The problem is, will you get quality craftsmen on every project? The answer is no. Ask any contractor and they should tell you that the industry is extremely starved for experienced craftsmen. This is the “human factor” I am speaking of. 90% of the people out there blasting couldn’t tell you the difference between an SSPC SP7 and an SP 11. I encourage you to study that. As I said in my original comment, this is something that has to be done correctly for it to be successful. I think you would certainly agree with that.

Comment from Per Gabrielsson, (8/14/2012, 6:36 AM)

The problem with spotblasting - especially when preparing solventfree epoxies with a high dft - is the feathering of the surrounding paint edges. As epoxy gets harder and more brittle by time, feathering may end up in blasting many times the required area. To spotblast damages during several dockings will result in a "potato field" surface, which then also will add to the frictional drag of the vessel. A full blasting, at least every 10 year, will prove to be a good decision, considering the ever rising bunker prices. Feathering of vinyl ester based "hard coatings" does not present the same problem as for epoxies.

Comment from john kern, (8/14/2012, 9:16 AM)

Gordan, Spot repair of intact coatings has been around for numerious years and the commercial world uses it on a continous basis. This due to economics has been a stable for commercial repair. They often get rid of their capital investments after 15 to 20 yrs of service. The Navy on the other hand maintains their vessel in the water for up to 8 to 12 years and maintains the vessels for up to 40 years. Life expendancy of the primers is normally 15 years average and therefore the primers for Naval vessels should last the docking cycle of the vessel. If you recall, in the 80's the Navy went to a spot repair and sweep blast of the underwater hulls. USSN BAINBRIDGE (a thorn in my side) being a prime example had this repair system. The sweep blast turned out to be a disaster due to the "work force" (Shop 71 at NNSY) being inexperienced performing the job. Too heavy grit, too high pressure and too long stand time with the nozzel were the determining factors. As time went by and more vessels received this repair it was noted by Naval Research labs (don't remember if it was Annapolis or Carterrock at the time) found out that during the sweep blast cycle the epoxy primers (either F-150 or a priority system) would micro crack and induce a high chance of premature failure. My question to you is; Has the Navy and your team looked into the micro-cracking of the "new" systems being utilized and what is the logevity affect of the added induced stresses place on the chemical bonding of the stands in the coating system after brush blasting? I do agree with Jeremi that work force ability will determine the final outcome of the "repair." There are programs out there to educate the blasters and painters and the requirements that the navy has placed on the contractors help to insure a better performing work force. Human nature being what it is, I still find a need to maintain a watchfull eye on the contractor's ability to perform the work and maintain an educated and experienced work force. Thanks for the ear...jk

Comment from jesse chasteen, (8/14/2012, 10:21 AM)

I will try to not make a typing mistake..Number one a contractor must possess QP-1/2 just to bid Navy work..Part of having those certifications is having a training program..Number 2 a contractor performing Navy work must submit a list of all personnel and the documentation that they have C-7 and C-12/14 current. This at least somewhat insures that you as a contractor are placing folks on the deck plates that know the difference between sweeping and needle gunning. I would start to worry only if the think they can streamline the inspection process and start cutting back on the nearly every other para with the Hold Point Check Point in a work spec..

Comment from Gordon Kuljian, (8/14/2012, 9:44 PM)

Everybody you are missing the point! If you would please read the subject latest report( , in addition to the 1996 NSRP hydroblasting productivity study, you would see that this is all about SPOT-AND-SWEEP BLASTING USING UHP WATERJETTING. All this talk of spot blasting using abrasives is decades-old news. We all know that spot blasting using abrasives has the potential to be a disaster, and is only successful with experienced craftsmen. Both the 1996 and 2012 reports show no anti-corrosive microcracking, improved tensile adhesion, etc. etc. The report shows that by optimizing the Ultra high pressure waterjetting system, in terms of design and rotational speed of the spin jet, stand-off distance, pressure and traversing speed, one elimminates any rough edges, swirl marks,and the resultant surface is clean/profiled intact epoxy while removing any compromized coating to bare metal. Only experienced operators with the correct equipment will be succesful with this technique. So let's all stay focused on the featured article, please - this conversation seems to have veered off. Cheers.

Comment from Gerald Burbank, (8/15/2012, 10:50 AM)

Spot preparation and overcoating never has the same service life as complete removal. However, when the maintenance cycle is considered and the Net Present Values of the cash flows are calculated, it is often cheaper to overcoat through several cycles rather than pay for complete removal. If the data is there, I have no doubt that the numbers would pan out. My only concern would be that the hulls are subject to immersion in salt water. This is a very aggressive environment. Quality control is crucial under the circumstances presented. Minor defects can create major problem in a short period of time, "undercutting" the authors assumptions. Only time will tell.

Comment from john kern, (8/15/2012, 8:04 PM)

Gordan, Thanks, I did read the article before commenting and do understand that we are talking HPWJ. Just wanted to borrow your ear for a second. I did miss comments on micro cracking and did not know a study had been completed with the HPWJ. Sounds like you may be on to something here. Maybbe the others will understand the efforts that the Navy is putting into the "quality" of the inspectors and inspection process. My next question is, Does the Navy intend to still use the definition of "spot repair" for this process as outlined in the 009-32. Will they compine all the spots or will they square off toatal square footage of repair areas within zones (aft-mid-fwd (port/stbd))Again thanks...see you soon jk

Comment from OLE JACOB MOE-HALVORSEN, (8/16/2012, 6:12 AM)

I’m agreeing with you PEG; one of the problems with spot-blasting (spot cleaning)- is the feathering of the surrounding paint edges. The question is, - are the edges feathered or have they been “cut off” by perpendicular cleaning Other problems are the availability of craftsmanship and QA (Quality Assurance) and I like to add, the risk, where the over coating of the existing system represent a problem, by building " a sandwich of layers” with different properties. Pretreated substrates that are clean and dry, paint products and skilled applicators in close cooperation with qualified QC are keywords regarding coating performance.

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