February 1, 2013
Four key tasks should be part of a dredge maintenance plan to help ensure long-term operation.
Proper equipment maintenance is the key to long-term success, since it reduces production uncertainty by increasing availability, reducing energy consumption, decreasing or eliminating environmental and safety penalties, and allowing major repair scheduling and procurement. Dredges deviate from the typical maintenance model, as they encompass additional challenges because many of the wear items are below the water’s surface and are not visible for frequent inspection.
Original equipment manufacturer (OEM) literature is often the single best source for creating a written maintenance program. Since all dredge manufacturers use other OEM equipment in their products, they must rely on their recommendations to support the dredge. The supplemental manuals provided for the individual components of the dredge should be read and understood. Due to this fact, adherence to a written maintenance plan can be the starting point for successful dredge operation and production. This plan should include sections that identify what systems need to be inspected, the frequency of equipment inspection/maintenance, and the tools required to perform the inspection/maintenance services.
Whether the machine is a pipeline dredge (hydraulic), a dragline (mechanical), or a clamshell dredge (mechanical), its maintenance requirements can be broken down into several key tasks: visual inspection and lubrication, routine sampling and analysis, performance-based testing, and scheduled replacement.
1. Visual inspection and lubrication
Of the four tasks, only visual inspection and lubrication is commonly practiced in the aggregate industry. While this component of dredge maintenance is critical for safety, environmental, and immediate operation concerns, it does not provide enough insight to management to reduce energy costs due to wearing components or to efficiently procure expensive parts on a timely basis.
Inspection and lubrication schedules, while commonly provided by the equipment manufacturer, can be created or modified by the dredge operator, plant operator, or management. These tasks are accomplished daily by the dredge operator or deckhand without taking apart any components and do not directly involve management; however, it is important that a written record of these tasks is provided to management.
The phrase “cleanliness is next to godliness” cannot be more applicable when dealing with the Mine Safety and Health Administration (MSHA) regarding a dredge. Often, MSHA inspectors do not want to be on the water in the first place, and the last thing they want is to encounter a machine in disrepair, coated with oil and grease, and laced with debris; the dredge should be cleaned at least at every shift change to promote an atmosphere of pride and respect.
Since a dredge is a floating machine, the trim and heel (stability front to back and side to side, respectively) of the vessel should be visually checked upon boarding, departure, and routinely throughout the day. Any noticed change in either draft or lean should be immediately inspected, corrected, and reported to management. Most flotation problems are a result of manholes that are not tightened properly or from corrosion, stress, and rubbing problems that cause a crack or hole in a tank. These issues can be quickly remedied with pumps, epoxy, wooden dowels, or concrete, and can be permanently fixed during regularly scheduled maintenance periods. It should be noted that dredge flotation tanks are often considered confined spaces, and company procedures for entering these tanks must be strictly followed.
On a daily or shift basis, swing, spud, and lift cables should be inspected for fraying, wear, and proper fleet. Lift cables should be replaced immediately, as a failure on a ladder or bucket lift system can be financially, as well as humanly, catastrophic. Frayed swing or spud cables usually only result in downtime and should be replaced at the first scheduled maintenance time.
Sheaves, bushings, and bearings — especially those that are submerged — should be lubricated every shift to prevent wear. Lubrication and inspections usually require less than 30 minutes per dredge shift; the installation of automatic greasing systems can greatly decrease lubrication time, as well as ensure that every grease point is properly lubricated.
Pressurized hydraulic oil lines and water hoses should be visually checked for leaks and frays anytime the equipment is above the surface and operating. Most modern dredges provide operator alarms for leaking hydraulic systems. A leaky hydraulic system will, without fail, end in an injury or an environmental fine. All equipment oil levels should be checked daily or at the OEM’s recommended intervals.
All electrical equipment should be visually inspected for rubbing or loose external connections.
Structurally loaded components should be visually checked for cracks, and abrasively attacked surfaces should be checked for wear. Clamshell bucket shells and lips, dragline booms and buckets, cutter heads and drives, dredge ladders, and spuds are good examples of these components. Additionally, all equipment guards should be inspected for proper bolting and fit.
Lastly, dredge manufactures should provide sufficient alarms to warn operators that a hazardous condition or problem exists. These situations should be corrected promptly; the manufacturer or the maintenance personnel should eliminate nuisance or faulty alarms if they occur.
2. Routine sampling and analysis
Routine sampling and analysis provides insight into the health of the dredge and its likelihood to need major repairs in the near future. These major tasks often require more skilled maintenance personnel and additional scheduled service time. The OEM often provides maintenance and testing schedules, but common sense can be used to create a schedule if none is provided.
Diesel engine oil should be sampled and analyzed in a lab at least every 500 hours and hydraulic oil at least every 1,000 hours. The presence of water in the oil during the normal inspection should trigger more frequent testing or immediate corrective action. The analytical results from the testing lab should indicate any required testing schedule change or upcoming corrective equipment repair.
Baseline temperatures of all equipment, including electrical switchgear, motors, and connections, should be taken upon commissioning with an infrared heat gun. Monthly comparative readings should be observed; a high temperature is often an indication of a loose connection or a failing component.
On more critical systems, baseline and semi-annual vibration reading comparisons should be used as an indication of the bearing health and balance of the item. On hydraulic dredges, the dredge pump impeller to front door gap should be checked and adjusted at least once a month.
An ultrasonic thickness tester, or UT tester, should be employed to take readings of the dredge pipeline thickness at the same location to gauge the wear; on mechanical dredges, the same test should be done on the bucket to predict the wear rate. Additionally, once a year, the dredge hull thickness should be checked in several repetitive locations for early indications of coating failure or corrosion. Monthly, bolts holding rotating or vibrating equipment should be checked with a torque wrench to correct the condition and help locate failing components.
3. Performance-based testing
Like routine sampling, performance-based testing generates information on the condition of dredge systems, but also provides important information on the efficiency and energy consumption of the wearing components. This testing is primarily designed to reduce energy costs associated with worn equipment, and many of the tests listed can be automatically performed by the dredge’s control system or PLC.
On hydraulic dredges, the dredge pump should be periodically tested to compare it to a performance curve baseline established when the pump was new. If greater pump speed is needed to achieve the test discharge pressure, a worn impeller is usually the culprit. Lower discharge pressure with higher energy consumption is associated with a worn front liner or shell, or a large gap between the impeller and front liner.
Similar tests can be performed on service water pumps, jet pumps, fire pumps, sand pumps, and silt pumps. Worn pump components consume more energy and should be replaced during the next scheduled maintenance period.
Hydraulic pumps should be routinely brought to their maximum pressure and flow conditions while the power consumption and temperature is recorded. Internally leaking pump valves cause reduction in speed and an increase in temperature. Internally leaking motors and cylinders characteristically operate at lower speeds and with reduced force while still consuming full power. Applying a known load at a set speed and measuring the fuel consumption, turbo boost, and manifold temperature can compute a diesel engine’s efficiency.
4. Scheduled replacement
Often, the scheduled replacement of a component is the easiest way to prevent downtime, if the component is one that does not exhibit wear or failure characteristics prior to failure. On diesel engines, batteries, oil, filters, and belts should be replaced at scheduled intervals even when testing and inspections show it is not necessary. Scheduled engine overhauls and replacements can reduce unplanned failures, as well as allow newer more fuel-efficient engines to be used.
Rotating bearing seals in cutter drives, bearing tubes, dredge pumps, etc. should be replaced annually to prevent unplanned failures. Gears, bearings, friction plates, and brakes in winches, motors, and drives have finite lives and should be replaced prior to failure; OEMs can provide information on the effective life of such components at differing loads. Brushes on DC electric motors should be replaced in batches, as opposed to all at once, to eliminate commutator-seating issues. Ladder and bucket lift cables should be replaced or flipped annually.
Hydraulic and water hoses breakdown with UV exposure and should be replaced before cracking or color changes are noticed. Dredge hull coatings are typically designed for a five- and 10-year service life. The dredge should be pulled from the water prior to these intervals, sand blasted, patched if necessary, and re-coated.
The four maintenance tasks described above form the basis of an equipment sustainability program. Each of these should be part of a dredge maintenance plan that is strictly followed by the user and sanctioned by the manufacturer.
William J. Wetta II, PE, is CEO at DSC Dredge , LLC. He can be reached at firstname.lastname@example.org.