Now hear this, ship owners: Paint now, or pay later.
That’s the bottom line of new research that carefully quantifies—perhaps for the first time—the economic consequences of drag from ship hull fouling.
The study looked at the hull fouling penalty for the U.S. Navy’s conventionally powered, mid-sized surface combatant: the Arleigh Burke-class destroyer (DDG-51), the largest class of conventionally powered ships in the fleet. The study examined 320 actual individual inspection reports from Jan. 1, 2004, to Dec. 31, 2006.
Dr. Michael P. Schultz, Program Director of Ocean Engineering at the U.S. Naval Academy in Annapolis, MD, examined a range of costs, including expenditures for fuel, hull coatings, hull coating application and removal, and hull cleaning.
|The study tracked the Arleigh Burke-class destroyers.|
His conclusion: The main cost associated with fouling is the increased fuel consumption from increased frictional drag.
“The costs related to hull cleaning and painting are much lower than the fuel costs,” Schultz reports in “Economic Impact of Biofouling on a Naval Surface Ship,” published in the journal Biofouling.
Furthermore, Schultz said, a hull needn’t be fouled to drag. Even when the hull is free of fouling, frictional drag on some hull types can account for up to 90% of total drag, he reported.
Schultz estimates that the Navy’s present coating, cleaning and fouling level will be about $56 million per year for the entire DDG-51 class or $1 billion over 15 years. His study aims to provide guidance on the most cost-effective strategies to combat hull fouling.
For his study, Schultz noted that the Arleigh Burke class destroyers currently receive two coats (125µm dry film thickness each) of anticorrosive paint and three coats (125µm dry film thickness each) of copper ablative antifouling paint, Schultz reports.
About 760-910L is required for a single coat of paint. He assumed repainting every 7.5 years (although, he notes, actual repainting is based more on inspection results and/or performance criteria). Painting costs included only paint and labor for surface preparation and application.
A full cleaning cost between $26,200 and $34,200 in 2010, and an interim cleaning cost $15,000 to $21,500. The direct cost of fuel was assumed to be $104.16 per barrel, based on NAVSEA guidance from December 2008.
With those assumptions, Schultz calculated the cumulative costs for four hull roughness conditions over 15 years:
• An ideal hydraulically smooth paint (Case 1);
• A newly applied Navy-qualified ablative AF coating with no fouling (Case 2);
• A typical hull roughness, given the Navy’s present practices, including qualified ablative AF coatings and regular interim and full cleanings (Case 3); and
• A scenario featuring an upper bound for hull fouling (Case 4).
Schultz determined that the baseline (Case 1) cost for fuel is about $450,000 per ship, with no additional cost for hull paint roughness or fouling. Cumulative costs over 15 years, over and above the baseline cost, were entirely due to surface preparation and painting.
For Case 2, the 15-year cumulative cost (over the baseline cost) is about $3.33 million per ship. Although the vessel remains free of fouling, the typical paint roughness of as–applied AF coatings leads to an increase in fuel consumption of 1.4% per ship per year, he said.
Cases 3 and 4 “demonstrate the enormous effect of fouling on fuel consumption and subsequent operating costs,” Schultz writes.
With Case 3, the 15-year cumulative cost (over baseline) for operations under current Navy hull maintenance practices is approximately $22.7 million per ship—mainly from increased fuel consumption due to hull fouling. Even without hard fouling, he said, heavy slime can increase fuel consumption by 10.3% over Case 1.
For Case 4, the 15-year cumulative cost is about $43.8 million per ship, due to an average 20.4% increase in fuel consumption, or about $2.3 million per ship per year, Schultz found.
Estimates will vary, of course, based on several factors, including ship type, age and condition of the hull coating, labor and production rates, and region of operation. (The costs used in Schultz’s model were unweighted averages across several ports based on a NAVSEA contract with one vendor.)
Higher-price coatings had only a small effect on operational costs, Schultz also noted.
The hull fouling research is one of two projects Schultz is pursuing with funding from the Office of Naval Research, according to the Journal of Ship Hull Performance, published by The Hydrex Group, a supplier of ship protection equipment, products and services. The other project studies the fundamentals of flow over roughness. The goal is to rapidly test any hull coating and calculate what its drag would be when on a ship, bypassing the need for towing tank tests or live tests, the Journal said.
Meanwhile, count Schultz among those scientists who favor development of alternatives to copper-based antifouling coatings. “There are two sides to the story with copper,” he told the Journal.
“One is the environmental issues with copper release, and the other is that I don’t think people are satisfied with the performance of copper. One of the Achilles heels of copper antifouling paint has been especially the slimes and algae levels of fouling which require these sort of booster biocides to try to combat.
“Copper is not optimal anyway. Even if these things don’t kill off the ecosystem, it’s not good to be pouring large quantities of copper into the ocean.”