Bulk Cement (Powdered Cement) in bulk is fluid enough to be pumped and, as a result, there exist cement-tankers which have tanks rather than holds and which rely on pneumatic pumps to discharge their cargoes.
Some such ships supplement or substitute this facility by using shipboard mechanical equipment, such as Siwertell handling systems to discharge, whereby cement is lifted by Archimedean screw out of cargo compartments and carried overside by conveyor-belt or bucket-line. Such ships are usually too specialised for tramping and will be employed on and built for long-term contracts, serving the cement industry.
Bulk Cement can be quite readily transported in conventional bulk carriers, though, cargo-handling in these cases being effected by one or more of a variety of methods. The simplest, perhaps, is to use grabs fitted either to shoreside apparatus or to the ships’ own gear for both loading and discharging, the problem with such a system being the amount of dust created.
Consequently, some bulkers are loaded by pneumatic means, small holes (cement holes) being cut into the closed hatch-cover of cargo compartments, and pipelines connected to each; one or more through which to load cement, and one or more others via which to extract dust and air from the compartment. Upon completion of loading, the pipelines are disconnected and the small holes (cement holes) made good to the Ship Master’s and/or Classification Society’s approval as to seaworthiness, a typical charterparty clause covering such a loading method being:
“In order to facilitate the loading operation, Charterers to have the option to burn up to three holes of approximately 100 centimetres diameter in the hatch of each hold which is to be loaded, such burning to be performed under supervision of Master and class surveyor. The holds are to be rewelded to class surveyor’s satisfaction prior to departing from last load port. It is agreed that all such work shall be carried out in Charterers’ time and at their expense.”
After Bulk Cement Shipping, to avoid dust pollution at the cargo receiving end, discharge may be effected by shore-based pneumatic or mechanical means or, in the case of vessels involved on only short distances and thus discharging before the cargo has had a chance to compact, by vacuvator.
Where these facilities do not exist, however, a further alternative is to discharge into a Cement Mothership anchored off-shore, where atmospheric pollution will be less obnoxious to the local environment. Such Cement Mothership may be fitted with mechanical equipment to help them to unload bulk carriers secured alongside, storing product on board and thereafter discharging cement over side into smaller coastal craft, or proceeding into port to discharge via a sealed unloading system into silo, lorries or railway wagons.
In case of need, cargo can be stored onboard for some time, and certain, sophisticated Cement Motherships incorporate even a bagging plant, by which method bulk cement can be converted prior to discharge by conveyor belt for onward distribution.
Bulk Cement is thus a perfectly feasible cargo for conventional bulk carriers, although it is important that before loading cargo compartments are scrupulously clean and odour-free.
The slightest amount of sugar residue from a previous cargo, for example, can ruin Bulk Cement by destroying its binding properties, whilst ammonia fumes create quick-setting tendencies in the cargo.
Bulk Cement loaded direct from factory kilns, incidentally, may sometimes be as warm as 75°C.
The angle of repose of Bulk Cement can vary widely, and mechanical trimming equipment may be employed to obtain a reasonable cargo level, a ship occasionally being required to remain alongside the loading berth for twelve hours or so, to enable trapped air to exit from the cargo and for a stow to settle.
Ship’s cargo compartments should be thoroughly swept and cleaned following the discharge of bulk cement, although this operation may be hindered if dampness or water ingress into the hold has caused portions of the cargo to adhere to hold sides, necessitating the laborious task of chipping away solidified pieces.
Bulk Cement Stowage Factor:
- Bulk Cement Stowage Factor 22/27
The carriage of cement at sea has grown steadily in volume until today it provides a major source of merchant ship employment. There are various types of cement requiring a variety of ingredients based on supplies of chalk or limestone, clay, ores, gypsum and flyash, (once a waste product from coal-burning power stations). But from a seaborne trade perspective, the product can be divided into bagged and bulk, the latter being capable of sub-division into finished cement and clinker. Whether for bagged or for bulk, nearly all carriage terms are negotiated on adapted versions of the GENCON Charterparty Form. Many nations possess cement factories, and thus the business takes the form of a complex trade of criss-crossing shipments of raw materials to these factories; partly refined cement (clinker) transported between cement works perhaps hundreds of miles apart; and the final, vital product for construction projects being delivered around the world in bags or in bulk. This final product is a fine grey powder which contracts as much as 12% from an aerated condition following handling, to a non-aerated state once it has settled, and which can be very dusty. Obviously it should be kept scrupulously dry so as to avoid solidifying, and conventional, non-sealed cargo-handling work cannot be conducted in rainy conditions.