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Shallow-draught trailer TONG TAN for China
On 26 October 2002, the 3,500m³ trailing suction hopper TONG TAN was launched at IHC Sliedrecht. After playing the national anthems of China and the Netherlands, and the symbolic severing of the ship's ties with mother earth by two CHEC directors and two from IHC, Mrs Bianca Balkenende, the Dutch prime minister's wife, named the vessel. The owner, CHEC/Tianjin Dredging Company (TDC), is one of IHC's most faithful customers. As successor to the Hai Ho Conservancy Board, established in the mid 1890's, the company is China's oldest dredging company and already bought dredgers from IHC's eventual founder members in 1901. When, in December 2000, the building contract was signed, it was the 140th such order from the Chinese account. Hard on its heels came the 141st contract, for a 10,000m³ trailer. CHEC accounts for 10% of IHC Holland's turnover.
TONG TAN was delivered in the spring of 2003 and sailed home under its own steam. In these days of ever larger trailing suction hopper dredgers, 90.30m long (over all) TONG TAN stands out for its compact, beamy (19.10m) proportions, which gives a shallow draught (6.50m), relatively big hopper and load capacity (5,300 tons), generous stability for trailing and excellent manoeuvring. Propellers in nozzles, free-hanging twin rudders (max. rudder angle 370 on either side) and a bow thruster further enhance the vessel's docility. She is designed for versatility, for maintenance as well as capital dredging, with limited draught for negotiating China's many siltation-prone waters. Since waterborne freight traffic in China is increasing rapidly, so is the demand for dredging capacity, especially for smaller ports. With single 900mm suction pipe the deft dredger's maximum dredging depth of 28m is more than enough for creating and maintaining navigable channels for today's largest commercial vessels. With its single suction pipe, TONG TAN is a novelty for China, as further evidence of Tianjin Dredging's receptive mind for sound ideas. Likewise, they were the first Chinese dredging company to embrace the self-emptying system with bow connection as well as the first to buy an environmental cutter dredger. Single-pipe trailing greatly optimises the loading process and especially reduces overflow losses when dredging finer fractions.
The power of dredging Below deck, TONG TAN boasts two identical main (propulsion) engines for a service speed of 13.5 knots (loaded): 2,400kW to starboard, also for boosting the dredge pump when pumping ashore, and 2,600kW to port, for propulsion as well as for driving the jet water pump. Each engine moves a controllable pitch propeller as well as a water-cooled, self-regulating, brushless AC generator of 650kW continuous output at 1,500rpm. Both are suitable for running parallel with the auxiliary generators for take-over duties. A dedicated 1,320kW diesel engine drives, through a two-speed reduction gearbox, the high-efficiency, single walled centrifugal dredge pump with double shrouded four-blade impeller. (All engine outputs mentioned are at 1,000rpm). The ship's power plant further includes a 340kW diesel for the auxiliary generator and a similar engine for the emergency generator.
When the load is discharged through the self-emptying-system over the bow, either by rainbowing or by delivery through a floating pipeline, the starboard main engine can be deployed in tandem with the dredging diesel, through a twin input/single output gearbox. When discharging through an 800mm pipeline over a distance of 1,500m, TONG TAN will unload in less than an hour. Alternatively, the spoil can be dumped through two rows of bottom doors. When trailing at her maximum dredging depth, TONG TAN will typically make some 7 knots.
The suction pipe's flexibility ensures that the draghead will stay flat on the bottom at all times, exerting maximum vacuum. Two types of draghead are supplied: a California type and a universal draghead of the improved IHC type. For softening up compacted soils, the jet pump on the California draghead, works very well in the terribly compacted fine sands of which China's estuaries are awash. TONG TAN's semi-integrated Dredging Control System is based on independent programmable logic controllers (PLCs), communicating over a high-redundancy glassfibre network. The PC-based supervisory system has two operating stations with slave PC, membrane type keyboard and display. The operating stations are connected to two identical server PCs (gateways) for back-up purposes. The system gives some 20 pre-programmed process pages and 40 calibration, service and diagnostic pages.
Floating village Although TONG TAN may not seem particularly big in today's jumbo trailer environment, she would, with her 7,000kW installed power and complement of 26, be a rather big dredger, say, 20 years ago. Such a vessel is in fact a floating village, with all community functions on board to stay away from the shore for extended periods. All accommodation is forward in the superstructure and forecastle. The accommodation and switchboard room are fully air-conditioned.
On deck, the usual supply of windlasses, hydraulic 50kN capstan aft and winches, two of these to handle the 2,475kg high holding-power type stockless bower anchors, each with 495m stud link chain cable. For handling heavy weights such as dragheads and other parts of the dredging installation, a fixed mounting, revolving electro-hydraulic deck crane is fitted, with 12 tons hoisting capacity at 23m outreach. Winches, gantries, crane and bottom doors are hydraulically powered. The hydraulic system consists of a stainless steel hydraulic oil tank, three main pumps and three auxiliary pumps, all of the constant-displacement internal-gear type. All hydraulic cylinders are electro-hydraulically operated through PLCs.
On the bridge is the usual panoply of navigation and communication instruments for a dredger of this level of sophistication. It includes GPS and Differential Survey GPS, radio tidal measurement, ground-speed log and Inmarsat-C these days. The dredger master's console, with all controls and monitors, is situated aft on the bridge behind a large glass bay, allowing an excellent view of hopper and suction tube.
Since the launch, TONG TAN has been dredging quite a few smaller ports, where her manoeuvrability, shallow draught and great powers of dislodging obstinate soils have worked wonders to boost navigability and, in its wake, the prosperity-creating flow of goods. (Source: P&D 160, 2003, edited with kind permission)
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