New Generation Of Self-Unloading Cargo Ships
When Canada Steamship Lines (CSL) went shopping for a new generation of self-unloading bulk carriers, the company had a few basic requirements, namely: to reduce up front and maintenance costs; and to improve flexibility in cargo handling and environmental protection.
According to Rob Jones, president of CSL International, Seabulk Systems of Richmond, B.C., the company satisfied all of those requirements with the discovery of an innovative new design. In fact, those working closest to the project claim that this design will be the template for all self-unloading bulk carriers of the future. Seabulk, which has designed several self-unloaders in the past, has continually battled the problems of uneven belt loading and the resultant spillage and product loss, jamming of product in the hoppers, time loss due to a mismatch of ship and shore capabilities, as well as environmental issues such as dust and noise.
Various attempts have been made to solve these problems by automating existing hopper-andhold designs. However, according to Sidney Sridhar, president of Seabulk Systems, these solutions resulted in only marginal improvements. Seabulk's radical design approach reportedly solves many of the problems plaguing the industry with completely redesigned holds and delivery system. Hopper and gate changes Seabulk's Controlled Feeder Gate (CFG) is designed to eliminate the delivery problems inherent in the standard hopper/gate configuration. The 80-ft. (24.4-m) hoppers are open bottomed and capped with a reciprocating panel. Each panel has eight openings, the size of which is regulated by a closure plate. Hydraulic cylinders adjust the closure plates, and hence the opening size, to control the product flow rate. Another cylinder moves the entire panel back and forth along the hopper bottom. The hydraulic system is remotely operated by a computer designed by Siemens Electric, Industrial Systems Division, of Montreal, Canada.
By continually moving the opening along the bottom of the hopper, cargo is constantly shifted, eliminating the compacting, bridging and tunneling problems associated with stationary opening.
Operators will no longer have to pound the sides of the hoppers with a sledgehammer to dislodge a compacted load. Even "stickier cargo such as wet bauxite" won't present a problem, said Mr. Jones. Likewise, hit-or-miss methods of controlling product flow are a thing of the past. Computer analysis of flow rate based on plate opening size and product characteristics, combined with the onshore facility's receiving rate, results in a smooth, continuous offload. Belt overloading and numerous temporary shutdowns of the onboard operation so the onshore facility can "catch-up," are bypassed. Offloading progresses at a steady, unaltered pace, programmed to adjust the vessel's transfer speed to the receiving facility's capabilities. The volume of material on the belt is controlled by the gate. Offloading rates are regulated by flow from the gate with the belts moving at a constant speed. There are two variables involved in controlling flow rate. The first is the opening size and/or number of openings. For example, to operate at 50 percent capacity, a choice can be made to open all gates halfway, or to fully open every other gate. The second variable is the reciprocating panel speed.
The hydraulic cylinders utilized in this process are electrically controlled through a PLC (Programmable Logic Controller) designed and supplied by Siemens. Remote I/O stations are located throughout the ship. Speed control of the conveyor belt is possible, but not generally used to control the speed of the offl oad. Rather, when the vessel is working at a port with a limited receiving capacity where the maximum flow rate may be achieved with only minimal gate openings, the operating team may choose to slow the conveyors as an alternative method of controlling the flow rate. Slower belt speeds save some wear and tear and will reduce overall maintenance costs. Computer oversight protects against flooding the belts. Variable frequency drives and invertors control the belt speed. Computers control most of the offloading phase.
During the process, a PLC looks at the power drawn from the motor and gearbox and determines the volume of material on the belt. The predetermined offloading capacity of the delivery port is noted by the computer system which then sets the gate openings and the panel speed. Algorithms were developed to determine the flow rates of standard cargo. Because lump size and humidity are variables which may influence product flow, the computer begins each offload at 50 percent of the projected capacity, takes readings from the PLC and adjusts the gates automatically to achieve optimum speeds. A built-in memory achieves the optimum offload goal in less time on subsequent visits to the same port loading the same material.
With a top-notch computer system in place, the decision was made to utilize the system to perform other tasks as well. Siemens, which specializes in alarm and monitoring systems, brought all of those functions into the computer program. Sensors installed at the bearings relay temperature information to a video display unit on the bridge for monitoring and early warning of potential problems.
The system also keeps a record of alarms, motor failures and bearing temperatures to help isolate problem areas and streamline overall maintenance tasks. Another item on CSL's list of desired improvements was a clean tank top. The company wanted no barriers on the floor of the tunnel. Previously, conveyor idlers welded to the floor made washdown difficult because of build-up around the multitude of support posts. In Seabulk's design, the conveyors are suspended overhead by chains, resulting in an obstacle-free deck.
Installation of watertight hold doors, as required by IMO, proved to be another challenge met with ingenuity. The challenge laid in how to maintain an uninterrupted flow of material for the length of the hold, yet enable sealing of each of the eight compartments when necessary. Working with the other vendors, Seabulk developed a system whereby in an emergency, the belts drop to the floor, and guillotine type doors seal each hold.
When the doors open, the belts return to their working position. The receiving conveyor belt transfers the product to the incline conveyor which cuts through Holds No. 7 and 8. Although this results in some slight storage space loss, at 70,800 dwt, it was decided the loss was minimal and more than compensated for by the reduced cost and maintenance of an internal belt. The transfer system is able to achieve a steady 2,000 tons/hour offload speed.
Bottlenecking at the transfer point is always a worry during offloading. To facilitate smooth operation, infrared cameras located in the tunnel and at all transfer points, including the end of the boom, allow operators to monitor belt loading conditions from the pilothouse. By eliminating shutdowns due to receiving hopper overfills, jammed products, or belt overloads, the system is designed to easily match the receiving capabilities of most ports. Mr. Sridhar estimates the improved delivery system will save valuable time in port, and, according to Mr. Jones, the system will "enable the crew to focus on system maintenance instead of operation." Delivery system Another new design feature, a 250-ft. (76-m) telescoping boom at midship, capable of 270 degree rotation, is designed to allow for greater flexibility in delivering cargo to onshore hoppers. A hydraulic actuator slews the boom. Siemens produced a design in which the actuator remains stationary while the boom rotates, propeller-style. The luffing, slewing and shuttling action of the boom are all hydraulically controlled. According to Mr. Sridhar, "The biggest challenge of this job is the hydraulics. For hydraulics to work we need the electrical, because it all requires a 20-milliamp control signal, and that's what Siemens controls." The two companies worked closely on the control philosophy. Seabulk set forth the system requirements and Siemens developed the hardware and software utilizing PLC technology.
Environmental improvements As more urban ports deny docking permission to bulk carriers due to dust and noise pollution, CSL wanted a system that would eliminate these restrictions. To ensure these requirements, Seabulk replaced the standard open-lattice boom with a completely enclosed boom system to contain airborne pollutants. Even the belts within the body of the ship are covered to reduce dust in the internal environment. Additional dust extraction equipment is integrated into the system throughout the ship, improving air quality for crew members.
Vibration damping on all the drives helps with noise suppression.
The boom chute has a lining of sand between the inner and outer walls for additional noise abatement. An indirect benefit of careful regulation of product flow is that by eliminating overflows, pollution from spilled product is no longer a problem. With these safeguards in place, these vessels will reportedly be able to unload in any port, even congested urban loca- tions, without dust or noise. Selling the idea Mr. Sridhar presented his design to CSL International in Boston. To prove it would work, Seabulk built a $850,000, full-size prototype consisting of one complete cargo hold/hopper and conveyor.
They tested gypsum in the prototype and were able to demonstrate the 2,000 tons per hour flow rate.
CSL has purchased exclusive rights to the design for a four-year period. It currently has three ships underway at the Jiangnan Shipyard in Shanghai, China. Two of these vessels are for CSL International, and the third is for Egon Oldendorf.
The Seabulk engineer onsite during construction, which is scheduled to begin in October, will be Brian Carefoot. Seabulk subcontracted the fabrication of the boom and the feeder gates back to Jiangnan.
Other vendors include Siemens, supplying the electrical control system; Flenders, supplying the gearbox and reducers; Continental Conveyors of Toronto, Ontario, in its first marine project, supplying idlers and pulleys; Fox Fluid Bar of Toronto supplying the hydraulic actuator; and the Walz & Krenser firm in Rochester, N.Y., designing and supplying watertight doors. The unnamed vessel's first customer is New England Power. It will carry coal to New York in the spring of 1998.
