Looking toward Mecca ? Japan?s largest ever built dredger

Holland may be the obvious country for dredger newbuildings but there are a number of other shipbuilding nations where this expertise is available .

Japan which is about to deliver its largest ever dredger to date. The trailer suction hopper dredger, named Mecca, is nearing completion at the Kobe shipyard of Mitsubishi Heavy Industries (MHI) for the account of the Suez Canal Authority (SCA) of Egypt and will have a hopper capacity of 10,000 cubic metres.

Dredging equipment

The newbuilding is slated for delivery in June this year and will be capable of dredging silt, mud, sand and gravel by means of two drag arms on both sides of the vessel. Dredged materials will be discharged through 20 conical valves installed on the bottom of the hopper hold and the vessel has a shore and bow discharging system. Amsterdam-based Vosta LMG is the primary contractor for the ship’s dredging system and is providing engineering assistance to MHI. In addition to delivering key dredging components such as the dredge and jet pumps, two 1,100 mm inner diameter trailing suction pipes with dragheads and hoisting equipment, an advanced dredge control and monitoring system, the hydraulic installation and the bow coupling will also be supplied.

The trailer suction pipes will be equipped with two identical double-walled inboard Vosta dredge pumps containing wear parts made of high wear resistant chromium white iron manufactured by GIW Industries of the USA. These dredge pumps are installed in the pump room in the fore-part of the ship and driven by frequency controlled variable speed electric drives from Siemens with a maximum power of 3,300 kW each. The shaft lines include a reduction gearbox with flexible couplings and strong journal and thrust plain bearings, both with white-metal sliding surfaces and, for shore discharging, both dredge pumps can be used in series.

For diluting the soil, two single walled jet pumps will be installed, which were also manufactured in close co-operation between Vosta and GIW. These jet pumps are also installed in the pump room and are driven by electric drives with a maximum power of 1,100 kW. The design of the dredge and jet pumps has been verified by hydraulic tests in the laboratories of GIW.

The two trailing suction pipes are designed for dredging depths up to 35 metres and can operate either the two ?California? dragheads or the two ?Universal? ones with wide visor. The former type work on hydraulic-mechanical principles and consists of two self-adjusting visors that increase the erosion effect as a result of the extended length of bottom contact. The Universal draghead consists of one heavy pivoting visor with cutting teeth which loosen the soil in conjunction with the effect of jet water nozzles. All dragheads are Vosta?s latest design and enable efficient dredging in different kinds of soil. Draghead and intermediate gantries are designed with A-frame and swiveling sheaves, while the trunnion gantries are of the parallelogram type. Pole-changeable motors drive the hoisting winches and special facilities are provided to allow emergency hoisting of the trailing suction pipes in the event of failure of the main system.

Sophisticated systems

The vessel is claimed to have the latest and most advanced control system, called ?dredge control & monitoring system? (DCM), which was developed by Hamburg-based STN Atlas Marine Electronics in association with its partner, Vosta LMG. Three control desks are provided, one central desk and two wing desks for individual control of the suction pipe on each side during trailing suction dredging. Designed for control of the entire dredging process, the DCM will facilitate monitoring of the suction pipe position, as well as draught and load, automatic cutter control, reporting and logging, trend curves and diagnostics for all machinery equipment. All relevant data will be available for instant display to the dredge master via dedicated keyboards and a series of control levers.

The system is tailor-made according to the specific properties of the dredge and the special requirements of the owner. Main features are the user-friendly human interfaces, the easy operation, the high functionality and the integrated automatic procedures for routine tasks.

Key components of the system include the PC-based Automatic Cutter Control (ACC) facility for acquisition and processing of dredging processes with data being relayed from dredge drives via serial interfaces. Profile data, feed values and alarm limits are entered via a control keyboard which also allows selection of four modes. Similar DCM systems have been installed onboard Jan de Nul?s trailer suction hopper dredgers, Vasco da Gama and Alexander von Humboldt as part of earlier joint SAM Electronics-Vosta LMG projects carried out under a collaborative agreement signed in 2002.

The dredger is also equipped with state-of-the-art navigation and communication systems which include a complete DGPS (Differential Global Positioning System) installation. The machinery control system allows operation from the bridge with unattended engine room.

General arrangements

Mecca has an overall length of 127.50 metres, a breadth of 26 metres, a depth of 10.7 metres and a deadweight, on dredging freeboard, of 17,100 tonnes. The ship is of twin-screw type with aft deckhouse and pump room in the fore ship, conical bottom valves for load dumping and a shore discharge installation. The deckhouse is designed to accommodate up to 74 persons representing two crews and the air-conditioned living quarters are arranged on three decks.

The machinery arrangement covers all operating conditions of the dredger with high flexibility. Fitted with two MAN B&W 7L48/60 main propulsion diesel engines with a power output of 8,400 kW at 500 rpm, each of these four stoke medium speed diesels drive a 4.2 m diameter Kamone controllable pitch propeller through a reduction gearbox and a clutch coupling.

Each main engine also powers two Japanese Nishishiba generators arranged parallel with a distribution gearbox and a rated output of 3,800kW and 2,300kW respectively. In this way, a high flexibility of power distribution is achieved and the four generators feed the main switchboard with a voltage of 690V. For anchoring and harbour duty, two identical auxiliary gensets are installed.

Because of the need to be highly manoeuvrable for accurate station keeping when dredging, the vessel is fitted with two Kawasaki KT-72B1F bow thrusters in addition to two rudders.

The dredger is built for long lifetime, high availability and safe operation for the special working conditions in the Suez Canal. Stand-by units are installed for most of the auxiliary equipment and the owner can easily maintain a large proportion of the equipment locally.

The newbuilding is classed by BV and has the capability of operating over 15 miles from shore. After undergoing sea trials in Japan, Mecca will sail for Egypt to undergo dredging trials before delivery to SCA scheduled for June after which it will be used for maintenance of the sea-channel entries to the Suez Canal. n