Conversion of a bulk carrier into a selfunloading cement ship
Introduction
The majority of the selfunloading ships utilized for international cement trade are converted bulkcarriers rather than newly build ships. The main reason for this is the fact that international cement trade is a volatile market, unlike the domestic distribution markets which are quite stable. Therefore, ships on the international market require a much shorter payback period. This can be achieved by conversion of a 8 ?12 year old bulkcarrier into a selfunloading ship. The total costs after conversion are usually half the cost of a newly built selfunloader.
A further difference between selfunloading ships used for international trade and ships used for domestic distribution is their versatility.
Ships used for domestic distribution are usually dedicated for one or a few fixed routes only. Their equipment is therefore purpose designed. Selfunloading ships for the international cement trade have to be suitable to meet changing routes, loading facilities and unloading terminals. Their equipment has to be able to meet these changing circumstances. Conversions of bulkcarriers into selfunloading cement ships is therefore speciality work.
One of these ships is the m.v. Thai Ho . This article describes, the conversion of this ship in detail to show the requirements and scope of work behind these ship conversions.
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Requirements
There were 3 main requirements for the conversion of the m.v. Thai Ho:
- The ship structure should be kept intact as much as possible to save conversion costs and time.
- The cargo capacity, taking into account weight and volume, should be affected as little as possible.
- The equipment should be able to load and unload the ship at a high capacity for a wide range of loading and unloading terminals.
As a basis to comply with the above requirements the Carlsen unloading system was selected, which combines a fluidized hold bottom with vacuum extraction to unload the cement from the holds. This system can be easily combined with a range of loading and unloading systems.
The system is extremely suitable for ship conversions.
- The inclined fluidized hold bottom can be prefabricated, including all piping, and put on top of the existing double bottom of the ship. All remaining equipment can be put on deck. The structure of the ship is hardly affected and because of the high amount of prefabrication work the actual conversion time that the ship has to spend at the shipyard is very short.
- Ships are flexible. In high seas the structure of the ship flexes considerably. The pneumatic system is insensitive to these ship movements. It does not require special alignments, heavy support structures or other special features. Installation is very simple.
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The Carlsen vacuum system has additional advantages:
- High reliability (no moving parts in the cement flow other than a butterfly valve).
- Low energy costs, as the fluidized bottom strongly reduces the required energy for the pneumatic conveying.
- Low maintenance costs.
- Compliance with all shipping regulations including the new SOLAS requirements.
The hold volume and weight of the conversion were carefully considerated. The extra weight of the conversion consisted of the new inclined fluidized hold bottoms, the machinery for the loading and unloading system and a deckhouse for this as well as some new deck sections. The reduced weight of the conversion consisted of removing hatchcovers, hatchcoamings and 2 of the 3 deck cranes (one was retained as a hose handling crane).
The final result was a ship weight almost equal to that before the conversion.
The hold volume of a ship is reduced by a conversion, as a result of the inclined fluidizing bottom and the removed space between the hatchcoamings. Bulkcarriers however are usually designed with a hold volume for light cargos such as grain (SG=0.8). Cement is a heavy cargo (SG=1.3) requiring much less space.
By using a diamond shaped fluidized bottom a minimum of hold volume is lost whilst a high inclination angle of the bottom is achieved. The result is an excellent cement flow to the suction pipe, giving a high overall unloading capacity, and excellent cleaning capabilities.
By designing the loading system in such a way that the holds can be filled almost level to top, an effective hold volume of ample size for the cement cargo was achieved.
Loading and unloading system
The m.v. Thai Ho is equipped with high capacity loading and unloading equipment.
The loading system is suitable for gravity loading by a mechanical shiploader as well as pneumatic loading with the cement being blown to the ship through a pipeline.
The loading capacity is 1200 tph maximum.
The loading system consists of a cement receiver/cyclone and an airslide distribution system. The cement receiver/cyclone is situated midships on the top of the cement handling machinery room. With pneumatic loading the cement is blown directly into the reveiver/cyclone. The conveying air is vented into the holds and from there to the large dust collecting system in the cement handling machine room.
From the receiver/cyclone the cement then flows into the aft or forward distribution airslides. These airslides guide the cement to the port and starboard loading inlets of the 4 holds. Flow control and direction valves take care of a controlled and automatic distribution. Level detectors in the hold prevent overloading.
For mechanical loading the ship is equipped with loading airslides on the port and starboard side. These airslides are located low and have their loading points close to the shipsides. This makes it possible to load this large vessel with relatively small mechanical loading systems. The loading airslides convey the cement to the centre line of the ship. Here the cement flows into a high capacity vertical screw that conveys the cement into the receiver/cyclone. From there the cement is distributed similarly to the pneumatic loading.
The m.v. Thai Ho has a fully pneumatic unloading system. Two pressure tanks are alternately filled with cement by vacuum and then blown empty to the shores silos by means of pressured air. The fluidization system in the holds ensures an even and controlled flow of cement to the centre of the hold. Here the fixed suction pipe is located which leads to one of the pressure tanks. At the suction inlet the fluidized cement is mixed with additional air and vacuumed to the pressure tank. When pressure tank 1 is full, the system switches over and starts to fill pressure tank 2 by vacuum. Meanwhile, the cement in pressure tank 1 is mixed again with pressured air and the mixture is blown through a pipeline system to the shore based silos. A flexible, heavy duty rubber hose connects the ships piping system to the shore piping system.
The m.v. Thai Ho is equipped with four compressors providing 7 bar pressure. This makes unloading at 600 tph possible using a single pipeline of only 14 inch. Because of the possibility of using different numbers of compressors and conveying pressures the m.v. Thai Ho is suitable for unloading a wide range of pipeline lengths and diameters.
The m.v. Thai Ho is designed for the possibility that a mechanical outloading system will be added in the future. Instead of emptying the pressure tanks by pressured air, the tanks are then emptied by gravity into a screw conveyor system. This screw conveyor system conveys the cement into an outloading boom equipped with an airslide and outloading bellows. This boom can be swung above a conveying belt and the bellows can be connected to the loading point of this belt. It is also possible to load bultrucks directly. Maximum capacity would be 600 tph.
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