Ships – Implements – Hull cleaning
Reexamination Certificate
2001-09-18
2003-07-22
Basinger, Sherman (Department: 3617)
Ships
Implements
Hull cleaning
C239S251000
Reexamination Certificate
active
06595152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method for the removal of surface coatings from various surfaces. In particular, this invention pertains to a method of using ultra-high pressure water to remove surface coatings, including paint, to expose the metal hulls of ships. A remotely controlled platform having a ferro-magnetic and motive means moves the ultra-high pressure nozzles about the surface to be treated. An alternate embodiment of the invention incorporates a recycling and waste disposal system whereby the water is recovered, the coating particulate removed and the water reused as an abrasive.
2. Description of the Prior Art
The marine environment is extremely demanding on coatings applied to hulls, or other bodies, which are submerged for long periods of time. The corrosive properties of salt water are well known, and in this hostile environment even the most durable coating must be replaced periodically.
The degree of surface roughness of submerged portions of ships has a great effect on both ship fuel efficiency and the speed which can be achieved at a given propeller revolution rate. Roughness can be caused by marine growth (“fouling”), degradation of hull coatings, and deterioration of unpainted surfaces such as propeller blades. For commercial, private or military ships, losses in ship performance can have a variety of consequences, both financial and in terms of meeting scheduled arrival dates.
Although the following examples are for a VLCC Very Large Crude Carrier; an oil tanker, with the following typical approximate specification: 272,000 tons deadweight; total engine horsepower (at 90 RPM propeller rate): 32,700 hp, examples could be given for any size or type of marine craft. A typical trip for a VLCC is from the U.S. Gulf Coast to the eastern end of the Mediterranean Sea, This round trip normally takes about 40 days. However, with an increased surface roughness causing a loss in peak speed of only 1 knot (nautical mile per hour), 2½ days would be added to the trip.
Considering the effect of surface roughness on efficiency, for a VLCC, each increase of 1 RPM in propeller rotation rate corresponds to an increase in ship speed of about 0.15 knot. Thus, a roughness caused loss of one knot would require an increase of about 6.7 RPM to maintain the same ship speed (i.e., to overcome the increased ship resistance). This increased propeller speed requires 20 tons (metric ton) per day of extra fuel.
Marine engineers estimate that an increase in the average roughness of a ship's hull of about 30 microns (peak-to-peak, RMS roughness) can cause a drop in peak achievable speed of about one percent. A new hull can have a surface roughness of about 160 micron and a deteriorating coating can be about 280 micron. This roughness increase could cause a four percent drop, which for a typical 16 knot VLCC peak speed is a loss of about 0.64 knots.
The foregoing clearly demonstrates the economic importance of maintaining the submerged surfaces of ships in as smooth a condition as is practical. Therefore providing a means to maintain surface smoothness of ships is a practical and economical objective for ship owners.
Fouling of ship bottoms not only reduces fuel efficiency, thus increasing operating costs, but also attacks the integrity of the coating which leads to corrosion and metal fatigue. Corrosion damage to hulls can lead to costly repairs, loss of operating time and, if unchecked, to the premature scrapping of the vessel. Environmental laws hamper fouling prevention by limiting the types of paint which may be used, especially those containing organotin/tributylin and cuprous oxides which are most effective in controlling calcerous fouling. Thus, because the most effective preventive measures against fouling are unavailable it has become necessary to replace coatings more frequently. The coatings which can be applied under current laws need a superior surface finish in order to extend the life of the coating on the surface. Astute ship owners realize a superior surface finish extends the life of the coatings and reduces drydock time and expenses in the future.
There are presently three existing cleaning methods which are used for cleaning ship's hulls:
1. Chemical paint strippers are currently used to remove small patches on a ship's hull, as is required for non-destructive testing and access cuts. This method is unsuitable for leaning the entire ship's hull and it creates large amounts of toxic waste for each area cleared;
2. Abrasive grit blasting is used for cleaning the entire hull. Current hull cleaning methods using abrasive grit generate tons of abrasive that must be detoxified if used on the organic paint, and millions of gallons of water that must be treated annually. Satisfactorily methods have not been developed to manage the organic waste during the application of the paint, removal of spent paints in dry-dock prior to repainting, and detoxification of the grit and other wastes generated during the present abrasive blasting method used;
3. High pressure water jet systems which use water, pressures of less than 10,000 psi, are usually applied by a hand held nozzle. The prior art water nozzles lacked sufficient pressure, typically less than 10,000 PSI, to completely remove paint from the surface of a hull Another major disadvantage or prior art rotary water jets is the slow rate at which multiple layers, or very hard coatings, can be removed.
To fully remove coatings from hulls it is necessary to construct platforms for operators from wooden bars or rods, or by suspending gondolas or cradles or the like from above, when blasting. Thus, the operators must work by hand on such platforms located at a considerable height. For this reason, the operation is dangerous and, in addition to this, the efficiency of removal is extremely low. To overcome this disadvantage, various steered vehicles have been proposed. However, such vehicles heretofore proposed have proved to be unsatisfactory, for example, in that when the vehicle is driven over a surface having a certain curvature instead of a flat plane of a structural member, it is not possible to obtain a sufficient adhesion force for supporting its weight and further that it tended to sometimes damage the operation surface due to the contact pressure exerted between the vehicle and the operation surface.
Some vehicles use permanent magnet means and/or electromagnet means mounted thereon, and are driven by magnetic belts provided on both sides of the vehicle. Changing the direction of travel of a vehicle of this type requires remarkable skill as the apparatus has the tendency of slipping at the contact area either of the right or left belts during the turn of the vehicle owing to the change in the contact pressure between the vehicle and the wall surface.
Some examples of the prior art are:
Hirosha et al U.S. Pat. No. 3,777,834 discloses a magnet vehicle supportedly adhered to and adapted to run over an inclined wall surface of a structural member made of a strong magnetic material such as iron and steel, characterized in that the vehicle can be altered of its direction of travel as desired on the surface.
Woods U.S. Pat. No. 3,922,991 discloses an apparatus for cleaning relatively large, flat, ferro-metallic surfaces of corrosion, scale, paint and undesirable marine growths, the apparatus having high pressure fluid blasting assembly, magnetic attachment device, driving motor, and a signal generating and receiving system for guiding the apparatus along the surface to be cleaned.
Cadutt U.S. Pat. No. 4,890,5.67 discloses a robotically operated device using an ultrasonic transducer for the cleaning of ships' hulls. The device may also be used for spraying paints or other chemicals on the sides of ships'hulls. The device includes a housing having an open face adapted to confront a ship's hull and apparatus disposed in the housing for impinging a flow of fluid through the open face onto the ship's hull. An ultrasonic transducer is
Basinger Sherman
McHale & Slavin P.A.
Ultrastrip Systems, Inc.
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