Operational characteristics of self unloaders, mini-bulkers, forest product ships, log carriers, retractable tweendeck vessels, vessels with Munck cranes and combination carriers
Self-unloaders
SELF-UNLOADING bulk carriers which discharge cargo by means of ship-mounted conveyor belts have been known on the Great Lakes for many years, but only moved into the international trades in the 1980s.
In European waters one of the pioneeering shipowners of this trade has been A/S Kristian Jebsens Rederi of Bergen with ships such as the 10,000 tonnes dead- weight Telnes, built in 1982 and operated by Jebsens' UK subsidiary. (Fig. 18.1)
After an initial period during which the vessel successfully carried a wide range of traditional cargoes between North Sea ports she has been employed mainly in the coal trade, delivering coal to power stations on the Thames estuary from Amsterdam, Rotterdam, Zeebrugge and the North East Coast of England. In this trade she will often carry three car- goes a week with loading, loaded passage, discharge and ballast passage each taking about 12 hours. The Telnes averages 100 voyages, or 200 port visits, a year.
The trade is one in which lack of experience on the part of the ships' personnel can lead to considerable delays, since failure of the ship's discharging equip- ment results in a complete cessation of work with little opportunity for discharge by alternative means.
Self-unloaders have a capital cost which is 20-35 per cent higher than conventional bulkers and provide less space for cargo than do conventional ships of the same size. These disadvantages can be more than balanced by their reduced port time for discharging, so they are most profitable when employed in shuttle services with very frequent port calls.
Ship's particulars: The Telnes is a four-hold self- unloading bulk carrier with bridge, accommodation and machinery spaces located aft. She was built by Kleven Mek Verksted A/S of Ulsteinvik, Norway, in 1982, classed by DnV with the notation 1A1 ICE 1A, EO, and is registered in London. Her principal dimensions are length overall 118 metres, breadth moulded 20.5 metres, depth moulded 11 metres and loaded summer draft 8.466 metres with a correspond- ing deadweight of 10,110 metric tonnes. Her gross tonnage is 6,792 rt and net tonnage 4,249 rt, whilst her trial speed was 14.0 knots.
The four holds have a 'W shaped cross-section, with the gates for discharge of the cargo being situated at the two base points of the 'W. Interconnected upper and lower wing tanks for ballast occupy the spaces created by the self-trimming shape of the holds, and double bottom ballast tanks are also fitted.
When discharging, the cargo is gravity-fed from the vessel's holds through a number of hydrau- lically-operated hopper gates on to conveyor belts which run forward beneath the holds. It is raised from hold level to boom height by a vertical belt, from where it joins the boom conveyor belt to be dis- charged. The boom can unload cargo at any point up
to 30 m from the ship's side and up to 15m above the main deck.
Further details of the vessel and her equipment are contained in Appendix 17.1, whilst the cargo discharging equipment is described in more detail later in these notes.
Loading: Preplanning of the loading is done without the aid of a loading calculator: none is pro- vided since the calculations are elementary. For a normal coal cargo the ship will be filled before she is loaded to her marks, so no decisions as to the distribu-tion of the cargo are required. All holds must be filled. If a denser cargo is to be carried the cargo distribution is planned to ensure a satisfactory trim, and calculations will be routine ones using stability formulae and trimming tables.
The loading sequence, hold by hold, is normally 3, 1, 4, 2, with a final trimming pour of about 400 tonnes in No. 4, aft. This sequence ensures that a stern trim can be maintained throughout loading, thus assisting deballasting. Loading is by shore- based shiploader or grabs.
Before loading the forward hold the boom, which has a length of 40 m and a maximum outreach of 30 m, must be swung out to the offshore side at right angles. Since this would cause a substantial list, ballast must be discharged from an offshore tank to keep the ship upright, this process taking about 20 minutes. When swung out the boom end must be marked with a flashing orange safety light. The purpose of the trimming pour at the completion of loading is to bring the ship upright after the boom has been swung back to its stowage position amidships. Subject to this requirement as much as possible of the remaining space will be filled. The stowed boom is located over Nos. 1-3 holds, but is clear of No.4.
Deballasting takes place concurrently with loading and is arranged by the duty officer—
normally the chief mate—from the control room where controls for the ballast valves and pumps are placed.
Steel debris in the cargo loaded, originating from rail wagons or previous ships to carry the cargo, is likely to cause enormous damage to the ship's discharging gear. The system can accept items no larger than a football, but sharp edges are likely to cause damage. Such items of debris cannot always be seen in a bulk cargo but where they are noticed every effort must be made to prevent them from being loaded.
Loaded voyage: sometime before commencement of discharge the chief mate will inspect the discharging gear to ensure that all appears in good order. If planning a discharge in an unfamiliar berth the chief mate needs to know whether there are any weight restrictions. There may, for example, be maximum permitted loadings for a quay where stockpiling is carried out. Discharge is likely to be to hopper feeding another conveyor belt or to stockpile or barge.
BULK CARRIER PRACTICE 203
204 THE NAUTICAL INSTITUTE
Discharging: A cargo hold tunnel runs along the ship's centreline with, on each side, a main cargo conveyor belt positioned below the gates in the hold bottoms. A selected pair of gates, one port and one starboard, is partly opened to release a flow of cargo on to the main belts which carry the cargo to the fore end of the ship. Here transfer belts bring the cargo amid- ships to the loop belts situated in the 'cathedral', so- called because of its height. The cargo is compressed between the inner and outer loop belts which raise it vertically to the start of the boom conveyor belt. From the end of the boom conveyor belt, enclosed within the boom, the cargo is discharged ashore.
Communications between ship and shore are usually by means of VHP handsets provided by the shore installation. Alternatively portable telephones may be used. Shore installations accepting cargo direct through a hopper on to a conveyor belt may have a siren to indicate when the belt has stopped.
All discharging operations including positioning of the boom, commencement and stopping of cargo, and rate of discharge are controlled from the ship's cargo control room. Three closed-circuit television cameras are strategically sited in positions in the tunnel, the cathedral and at the boom end to allow the operator in the cargo control room to monitor the flow of cargo. A wandering lead can be used by an officer on deck or ashore to position the boom for discharge into barges.
Operation of the gates at the bottom of the holds to govern the flow of cargo on to the belts requires skill and experience, particularly when cargoes are not free flowing. If there is a mechanical failure of the belts, or if cargo is allowed to flow out too fast, it will jam and overflow the belts. When this happens tonnes of cargo must be shovelled out of confined spaces in the tunnel by hand to free the belts and allow a resumption of discharge. Cargo which overflows in this way will generally be shovelled back on to the belts and discharged.
Control of the flow of cargo is achieved by the amount the gates are opened. For dry coal or for grain the gates are only cracked open. The only indication the operator has of how much cargo is dropping on to the belts is given by the load on the belt motors. If the cargo is sticky, for example moist coal, it may be that initially no cargo drops at all. The flow of cargo can be assisted by operation of the vibrators which are situ- ated next to each gate. These vibrators, also operated from the cargo control room, are in the form of eccen- tric weights which when activated shake the frame- work to which the hold plating near the gates is attached.
If cargo starts to overflow a belt at a time when several pairs of gates are open, the operator has no sure way of knowing which pair of gates is releasing too much cargo. A quick-close button permits him to close all gates instantaneously, after which he can gradually reopen the gates in turn. When discharge is interrupted, the belts should if possible be emptied before they are stopped, since it can be very difficult to start the belts when they are loaded. Maximum ton- nage likely to be on the belts is 50 tonnes when discharging at maximum rate.
During discharge the ship must be kept even keel as far as possible, with a maximum acceptable trim of 2
metres by the stern, since a greater trim puts an exces- sive load on the belt motors. If it is necessary to restart the belts when they are fully loaded it may be neces- sary to trim the vessel by the head to assist the forward momentum of the cargo on the belts.
List must also be kept to a minimum, with a maximum of no more than 1 degree. A list causes the belts to 'wander towards the list', causing scuffing of the belt edges, which reduces its life. This is expensive in replacement material and in repair time. List and trim are monitored in the control room by means of sensitive inclinometers consisting of weighted 1 metre lengths of line attached to the transverse and fore-aft bulkheads.
Holds may be discharged or part discharged in any convenient sequence calculated to preserve the desired trim. One favoured sequence is: (i) empty No.2 hold;
(ii) alternate between Nos. 1 and 4 holds until they are empty, keeping trim below 2 m; (hi) complete by discharging No.3 hold. Discharging rates of 1,800 tonnes/hour can be achieved if coal is very dry, and a rate of 1,000 tonnes/hour can comfortably be main- tained throughout discharge with most coal cargoes.
On deck and ashore the discharging process is not a dusty one, because the discharging belt is inside the boom. The holds can be kept covered throughout dis- charge, although the lids must be raised to allow air to enter and they are normally opened for convenient monitoring of progress. When the ship is on a regular coal run, holds are not cleaned between cargoes and the ship sails with some cargo residues between the frames on the transverse bulkheads. The ship's constant, which includes cargo residues and sediment in the ballast tanks, is about 240 metric tonnes.
Discharge can be controlled by a computer program which can be instructed to maintain a stipulated rate of discharge, although with some cargoes manual oper- ation is preferred as it offers more control. Ship's staff find that the program is unable to recognise instances in which the cargo is sticking and will open the gates wider and wider until a large volume of cargo is suddenly dumped on the belts. This can be prevented by setting a maximum permitted opening percentage for the gates. Ballasting is carried out from the control room whilst the cargo is discharged, with care to ensure that list and trim are kept at a minimum.
Two sailors remain on duty throughout discharge.
In addition to normal mooring and gangway watch they are responsible for making an inspection in the cargo tunnel at hourly intervals. Any belt roller which is seen to be not turning is freed by tapping or, if that fails, is noted for changing at the first opportunity. A normal 650 mm roller takes about 10 minutes to change. The sailors look out for any spillages of cargo, which must not be allowed to build up under the belts, and they shovel any overflows of cargo back onto the belts or if this is not immediately possible clear of the working area.
Whilst discharging is taking place the tunnel becomes dusty. The tunnel is ventilated by fans and to ensure that it remains well ventilated it is only possible to engage the cargo breaker and commence discharge when the fans have been running for 15 minutes. On completion of discharge the boom is swung inboard and the ship is brought upright with ballast, one tank
BULK CARRIER PRACTICE 205
206 THE NAUTICAL INSTITUTE
having been left slack for this purpose.
Belt system: Power to the seven belts which in combination form the discharging system is provided in a variety of ways. The two main cargo belts and the two transfer belts are driven by 440v AC electric motors through Dodge reducers fitted onto the driving rollers. The inner loop belt main drive wheel is driven by two electric motors through two fluid clutches into two Hansen reducers, one each side of the loop casing.
The outer loop belt is driven by the inner loop belt by friction. The main boom belt, like the inner loop belt, is driven by an electric motor through a fluid clutch into a Hansen reducer.
The tension in each of the belts is adjustable, and belt tension is an important factor in efficient discharge. Too much tension in the cargo belts will reduce the concavity of their cross-section, and their ability to contain cargo, thus squeezing cargo out of the system. Too little tension will permit the belts to wander. Tension in the main cargo belts is adjusted by adding or removing tensioning weights, whilst screw tensioners are used to adjust tension in the transfer belts and the main boom belt. The tension of the loop belts is adjusted with hydraulic rams, with typical pressures for coal being 1.400 psi for the inner loop belt and 1,250 psi for the outer loop belt. The belts are automatically stopped if pressure is lost. Suitable ten- sions for the discharge of a particular cargo have to be found by trial and error, widi finer grained cargoes requiring lower belt tensions.
Bins are situated between the transfer belts and the loop belts and between the loop belts and the main boom belt. The bins act as hoppers which feed cargo on to the next belt in the system. Pressure pads fitted in the bins will detect any build-up of cargo and will automatically close the cargo gates to stop discharge.
In the bins at belt ends, defector plates are installed to guide the cargo down on to the centre of the next belt.
These have to be adjusted for a change of cargo and may seize up if not operated from time to time. A trip line runs the length of the tunnel and can be used to stop the belts instantly in the event of emergency.
Ballast voyage: Washing of the tunnel is carried out every voyage or at the very least every second voyage. The washing is done with sea water and the objectives are to ensure that the cargo gates are clean and free of any material which might clog them, to wash away any cargo spillage from the tunnel, and to remove the dust. Washing below the gates is done with the gates closed and is difficult, as the space is restric- ted. The water and all the residues are removed from the bilges by eductors and discharged into the sea when regulations permit. Washing the tunnel normally takes about two hours. Washing the tunnel with sea water encourages corrosion, which may become a problem as this type of ship becomes older, so frequent fresh water rinsing must be carried out.
The cargo holds are not washed if the next cargo is to be similar to the previous one. If holds have to be washed prior to carriage of a different cargo they are washed with seawater from handheld hoses with the gates open, and water draining into the tunnel for dis- charge by the tunnel eductors. If necessary the sea- water wash will be followed by a fresh water rinse.
i If a very fine grained cargo, such as nefelin syenitt,
is to be carried, the hold gates must be lined with poly- thene sheeting or tarred paper before commencement of loading to prevent cargo from leaking through the closed gates during the loaded passage. This lining will drop through the gates when discharge commences and be discharged with the cargo.
Maintenance: All the deck machinery, including all the discharging gear, is thoroughly greased on a continuous cycle once every six weeks. The full greas- ing programme takes about four days, with half a day spent on deck fittings including hatch covers and three and a half days spent on the cargo gear. Every large belt roller is provided with a grease nipple at each bearing. The outer nipple is difficult to reach in the restricted space outboard of the belt. Each gate has four greasing points which are not easily accessible.
The ship has the capability to make small repairs to the belts using special clips, but the belts are vulcan- ised and cannot be permanently repaired by ship's staff.
The cargo belts are connected to the motors by drive belts. If the system is overloaded or if the drive belt is worn, the drive belt will part. It is routine to look behind the guard frequently and to renew the drive belt when it is seen to be worn or damaged.
The main and transfer belts are fitted with skirting at the sides to prevent cargo from falling off the belts, and this skirting must be renewed as it becomes worn.
The belts are also fitted with scraper plates to remove residues from the return portion and they, too, require renewal when they are worn out. The main rollers at the ends of each belt are very heavy and difficult to change, chain blocks being required for this process.
The cargo gates are electro-hydraulically con- trolled. Repairs to the hydraulic pipework, a category of work which becomes larger as a ship grows older, are done by available ship's staff drawn from deck and engine departments.
Cargoes carried: The Telnes has successfully carried coal, gypsum, limestone, magnetite, ilmenite, petcoke, chalk, barley, oats, wheat, pyrites, olivin, coke, quartz, salt, stone, iron ore pellets, iron ore fines, cement clinker and nefelin syenitt. Alumina, however, was found to be too fine and presented serious problems with the belt tensions in use at that time.
At the other extreme, any cargo which was exces- sively sticky would be impossible to discharge with ship's gear and would have to be discharged by grab.
Such cargoes would not be carried. Cargo which is spout-loaded will generally discharge more easily than cargo loaded by grab, since the latter process tends to compact the cargo.
Mini-bulkers
Bulk carriers of up to about 10,000 metric tonnes deadweight are often called mini-bulkers. Whilst some vessels of this size are engaged in worldwide trading and many are equipped so to do, their main trades are the European short-sea trades. In European waters several thousand mini-bulkers are employed primarily in trades with a draft limitation of about 8 metres or where the requirement is for comparatively small ton- nages of cargo. Mini-bulkers are more likely to make long passages upriver to small inland berths, and can
BULK CARRIER PRACTICE 207