MacGREGOR’s hopper and blow pump bulk handling systems for offshore supply vessels (OSV) can achieve capacity increases of up to 75 per cent compared with a conventional bulk tank set-up. The two main tasks of a bulk handling system – storage and discharge – are performed by two separate units in a hopper and blow pump installation. Cargo is received and stored in a hopper, whereas discharge is performed by a blow pump using compressed air. Technical solutions based on this concept have been employed on cement carriers for the past 20 years.
The main weakness of the bulk tank type concept is its basic principle – the use of pressure vessels as bulk tanks. The two most widely used bulk tank shapes share a circular footprint which is not an optimum shape for efficient use of the valuable space on an offshore support vessel. This has limited the options available to the ship designer for maximising the space available below deck for other liquid cargoes and various equipment.
The hopper and blow pump combination offers a very competitive alternative. Since the hopper is not subjected to pressure it can be rectangular and easily integrated into the ship’s hull. The existing longitudinal and transversal bulkheads form the four sides, and the main deck forms the top of the hopper. To complete the hopper only the bottom needs to be added, approximately 2.2 to 2.6m above the tanktop. Cargo capacity of this hopper is considerably larger than that of conventional bulk tanks occupying the same space - an increase in capacity of around 50 to 75 per cent may be realised. For segregating different cargoes, the hopper may be further divided into two or more compartments by adding transversal and/or longitudinal partitions.
The initial system cost expressed in dollars per m3 capacity according to MacGREGOR estimates is calculated to be in the cost ratio 3:5 in favour of the hopper system. Some of the underlying reasons for this are:
- much higher capacity achieved by the hopper system compared with the bulk tank system – could be in the range of 50 to 75 per cent
- reduced need for refrigerant air dryers since cargo is not stored in huge tanks that are pressurized during discharge
- no special internal coating required for the hopper (black steel) unlike the conventional system (epoxy primer).
The installation costs work out to be in the cost ratio 4:5 in favour of the hopper system, which is attributed to:
- simplified piping work thanks to the two-way valves
- no internal coating required for the hopper
- elimination of dependence on the timely delivery of conventional pressure vessel bulk tanks
- elimination of the storage and installation required for huge bulk tanks.
According to MacGREGOR, investigations into the operation and maintenance of both systems resulted in a cost ratio of 3:4 in favour of the hopper system, which is attributed to the following factors:
- higher degree of automation operational reliability, requiring minimum attention from the operator
- two-way valves eliminate the branching pipes, where most of the wear and frequent clogging occurs
- the inspection covers on two-way valves facilitate easy maintenance
- between each fill and discharge cycle of the blow pump the whole discharge line, including the long flexible hose, is blown clean; this almost eliminates the risk of clogging in the hose, resulting in higher operational reliability.