The Problem:
The Thames barrier was erected in 1989 to provide flood control to the upper reaches of the Thames River including the heavily populated areas of London. The gates could be raised from the riverbed in a vertical defense position in the event high surge tides threatened the London metropolitan area.
In October of 1997 a sand dredger, the Sand Kite, wrecked into one of the main gates of the Thames barrier. The ship was damaged and dumped its load of sand and aggregate, then sank onto the gate where it sat for several days atop its load. This caused paint failure and premature corrosion on the flat face of the gate. The failure of this gate could have had potentially disastrous effects on London, with flooding damage estimated at UK$21 Billion and extensive loss of life.
The UK’s Environment Agency had several requirements for any repairs that were undertaken on the damaged gate. The barrier could not be taken out of service and had to be able to be closed at any time with a 1-hour notice to the cleaning contractor. In addition, there could be no environmental pollution or potential release into the environment during the surface preparation procedures. The twice-daily 21-foot tides and heavily traveled river created logistics problems for any repair to the gate. Because of these constraints any surface preparation that required stationary staging was rejected.
The Solution:
The remotely controlled, vacuum attached JetTracTM system provided by UHP Projects, Inc. was used to clean and prepare the surface of the gate for re-coating. This system uses Ultra High Pressure (40,000 PSI) waterjets to strip coatings from the surface. A patented seal allows the remote JetTracTM crawler to attach itself to the gate using vacuum supplied by a remote vacuum skid. The paint and water is completely contained in a vacuum shroud and removed down a hose to a vacuum system located on a barge. The JetTracTM crawler is remotely controlled and can move in any direction in both the horizontal and vertical positions as well as overhead.
UHP waterjetting provided an environmentally safe surface preparation method as well as an excellent way to remove chlorides and other contaminants caused by the constant submersion in salt water. This surface preparation method also eliminated the handling of abrasives over the water and greatly reduced the waste disposal costs of the project.
The Procedure:
UHP Projects, Inc. supplied the following equipment which was loaded onto a barge and placed at the base of the gate; UHP pump, JetTracTM vacuum skid, JetTracTM crawler, filtration system, 185 CFM air compressor, 440V generator, two 5000 gallon tanks for fresh and effluent water storage, necessary painting equipment and materials. A tugboat was on 24 hour standby to move the barge in case of an emergency, and a rescue boat was always available during cleaning operations due to the swift tides. UHP Projects, Inc. staffed the project with a 4 man crew.
The project was started with the gate in the vertical (defense position) and the JetTracTM cleaning the top 25% of the gate surface down to the high tide waterline. A cable support system was designed and installed to allow a self tensioning winch cable from the vacuum skid to run to the top of the gate and back to the JetTracTM crawler. This allowed the crawler to be lifted and placed onto the gate’s surface from the barge deck, vacuum could then be applied to hold the crawler to the surface. This winch system also provided a safety backup to catch the crawler if vacuum was lost.
After the top section of the gate was cleaned and painted, the gate was placed in the overhead (maintenance) positions. UHP Project, Inc. designed a lift system that allowed the crawler to be lifted into place from the barge deck to the underside of the gate during any tide level. Vacuum and UHP water pressure was then applied and the crawler would be remotely controlled from the barge deck. The remainder of the gate was cleaned in this position.
Surface preparation in both gate positions was coordinated to be done on the outgoing tides down to the high tide waterline. Re-coating was accomplished on the next incoming tide. In the vertical position this allowed the painter access to the area from the barge. Painting done in the overhead position was accomplished using a small manlift placed on the barge. A 2-part high solids epoxy from Kemira was used as specified by the Environment Agency. All coatings were applied using 45:1 airless spray equipment.
The existing coating to be removed were a 20-50 mil hot applied 2-pack epoxy that had been applied at construction. The JetTracTM using ultra high pressure waterjetting removed the coating to a SSPC SP 5-WJ-2 condition with production rates of approximately 200 FT2 (18 M2) per hour. An independent paint inspector performed daily tests for chlorides, profile, holiday detection, and adhesion. Every effort was made to re-coat as soon after waterjetting as possible to minimize flashrusting
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