keeping up with the changes in abrasive requirements for use in shipyards is a bit like trying to ride a bicycle on an ice skating rink. You make progress slowly, and you are likely to fall flat on your face from time to time.
Some people often describe shipbuilding as if it were a single industry with simple problems common to all participants. In the automobile industry, a few large firms do essentially the same thing in several different countries. In shipbuilding, however, a huge number of facilities carry out activities ranging from construction of the largest bulk carriers to maintenance and repair of the smallest coastal fishing vessel or private pleasure craft. It is a grave mistake to try and characterise shipbuilding as a single industry.
Abrasive selection in shipyards is often driven by these differences in size and mission, and economics is often dictated by outside influences. The market might be divided along the following lines.
- Newbuilding vs. maintenance activities are perhaps the most basic difference.
- Size of the shipyard is important. Large facilities often have different and very special requirements not present at small yards.
- Local environmental requirements are becoming the single most important criteria in defining abrasive use, because costs associated with environmental control and compliance can far exceed the cost of abrasive.
No doubt there are other criteria, but this article presents a brief overview of the shipyard market for blast cleaning abrasives and suggests a simple means to estimate the true 70-481 cost of surface preparation.
The Cost Factor
Perhaps the most important criteria for abrasive selection is cost—either the real cost of the abrasive or the perceived cost of the value of the abrasive relative to its cost per tonne. A clear understanding of the true cost of abrasives and of surface preparation is critical to the success of a painting programme at any shipyard.
In all too many cases, however, the facility owner has no real understanding of the true cost of either the abrasive supply or the full cost of surface preparation. It is far too easy to assume that the yard should buy the cheapest material it can find when, in fact, there are other factors in the total cost besides cost per tonne.
In other cases, the facility manager may have more important or more pressing matters facing him than abrasive selection, and so he misses the opportunity to significantly increase his economic returns and to minimise undesirable environmental impacts that can result from using less expensive abrasive media.
The cost of surface preparation and the unit cost of abrasive are not the same thing.
70-484
A : Abrasive cost ($/tonne)
B : Consumption rate (kg/hr/nozzle)
C : Labour cost ($/hr)
D : Equipment cost ($/hr)
E : Environmental cost
X : Productivity (m2/hr)
The full cost of abrasive blast cleaning in a shipyard is the sum of the costs of material, labour, and equipment, as well as related environmental or management costs adjusted by the productivity of the tools used. The cost of the abrasive—one of the “tools” of abrasive blasting— is only a small part of the total cost, which is tied to the cost of producing the abrasive, delivering it to the user, and a number of other factors. However, the productivity gain in using a high quality product or material usually offsets the difference in price. It’s like the difference between buying a low cost axe or an expensive power saw. Which makes the most sense if you have a hundred trees to cut down?
Other Factors
What other factors influence the total cost of blasting in a shipyard? Since blasting is a labour-intensive activity, the labour costs for both blasters and their supervisors must be included. In many cases, this is the largest single expense.
Equipment costs—the costs of owning capital equipment as well as the costs of replacing expendable items such as nozzles—should be counted. The productivity of these items must be weighed, of course, because the cheapest abrasive available is of no value at all if it fails to adequately prepare a surface for coating.
Costs related to environmental controls and regulations, especially the costs of dust control and waste management, are a growing factor in the blasting business. Certainly they should be included. Waste management is becoming the most important part of the cost picture in some areas, and it will only continue to grow in the future.
Cost Formula
The formula in Figure 1 shows that total cost is the sum of all these factors, and the economics of abrasive selection depends on the level of each one.
The most important element of cost is productivity, which is reflected in the efficiency with which the various factors—material, labour, and equipment—are used.
| Total Cost ($/m2) | 12.52 | 18.58 |
| Factors | Garnet | Slag |
| Test Area (m2) | 4.6 | 4.5 |
| Time (min) | 11.8 | 18.5 |
| Consumption (kg/hr) | 400 | 790 |
| Efficiency (kg/m2) | 17.1 | 54.1 |
| Cleaning rate (m2/hr) | 24.1 | 15.1 |
| Cost ($/metric tonne) | 330 | 72 |
| Direct cost ($/m2) | 9.75 | 9.87 |
| Cleanup @ $50/tonne ($/m2) | 0.92 | 2.90 |
| Disposal @ $100/tonne ($/m2) | 1.85 | 5.81 |
(U.S. figures)
Small changes in productivity have large effects on the total cost.
Environmental costs are often assumed to be constant regardless of the abrasive chosen. This is simply not true. For example, an abrasive media that cleans a surface twice as efficiently as another abrasive not only allows the user to purchase only half as much abrasive to do the job but also requires the disposal of only half as much waste, which certainly impacts the environmental costs of the project. The formula should always include estimates for the cost of adequate dust control, material handling and disposal, and any other indirect expense that is likely to occur.
If the abrasive is to be recycled three or four times—as is common with some garnet media—then allowance for the small added costs of labor and equipment should also be made.
What Affects Productivity?
Since productivity of the assets and dust control are the key elements in determining total cost of blasting in a shipyard, it is important to understand what affects these factors.
The nature of the abrasive grain—its size, shape, and most important its mass—determine how efficiently the abrasive will work on the steel.
In addition, the type of steel surface, its location, and the complexity of its configuration must be considered. A deeply pitted, rusted surface will clean differently than a new steel plate covered with light mill scale. Likewise, a large, flat hull surface generally will clean much faster than the compartmentalized interior of a ballast tank.
Other factors to consider in determiner productivity are site ventilation and operator skill. Both can have a dramatic effect on productivity and, therefore, on total cost. In addition to the possible health risks, poor ventilation can cause even the most skilled operators to use excessive amounts of time (labor costs) and abrasive (material costs), because when visibility is low and they cannot see well, they tend to over-blast. Use of unskilled operators can result in the same problems. On the other hand, the quicker that dust and spent abrasive can be removed, the quicker the painting can be done, and the better the productivity will be. Dust control is a function of the
abrasive being used—some media are less prone to breakdown than others—and the conditions at the site. A yard employing 20 blasters is going to have a different dust problem than one using only one or two blasters, as shown in the photographs on page 25. Also, a poorly supervised or unskilled operator will tend to create excessive dust and waste material regardless of the abrasive media being used. An additional factor in dust control is the type of coating being removed. Some coatings are more brittle than others and, therefore, create more dust particles, as will removal of a three-coat system compared with removal of a single coat.
In Conclusion: An Example
Table 1 on page 25 illustrates the actual experience of one shipyard in evaluating a high-cost material—an almandine garnet— against a less expensive coal slag. In this case, the shipyard owner
did not attempt to recycle the abrasive, and disposal costs were considered minimal.
Note that productivity—the amount of material and time necessary to clean one square meter— was significantly reduced with the use of the high-cost media. That’s the message here: cost per tonne of abrasive should be only a small part of the shipyard’s decision.
As new technologies and environmental requirements are imposed on shipyards, compliance and cost control will increase in importance. However, significant savings can be realised simply by using the existing tools more efficiently and by taking care to accurately measure the costs involved.