Bulk Handling Systems (BHS) are standard systems on Offshore Support Vessels (OSV), Offshore Rigs, Cement Carriers and other related maritime applications in performing their storage and supply function. The main task of the Bulk Handling System
is to receive cargo store and discharge it. In conventional systems, the storage and discharge of cargo is performed by pressure vessels (Bulk Tanks). With advances in High volume BHS concepts, the Sender Tank System (hopper & pressure vessels).
This configuration performs by way of two separate units – cargo is received and stored in the non-pressurized Hopper, which is integrated into the vessel’s hull whereas the discharge is performed by a pressure vessel/tank using compressed air. As the hopper is a non-pressurized it is not required to be pressurized for discharge, which means it can be shaped to fit the overall hull design and utilize the available space more effectively.
With this bulk handling system solution the dry bulk capacity is increased by 50 – 75% within the same space occupied by conventional spherical pressure bulk tanks.
This blogpost discusses the advances in high volume bulk handling system and merits from technical and operational perspectives and towards meeting tomorrow’s even more stringent requirements to protect the environment.
Present Bulk Handling Systems (BHS)
A 3D illustration of the dominating Bulk Handling System (BHS) concept that is found on almost all existing, and for that matter even those under construction, is presented below.
The basic principle of this concept is using pressure vessels as bulk storage tanks and for discharging bulk cargo (Cement, Barite, Bentonite) by means of compressed air. The main specifications of a BHS based on this concept primarily consist of:
- bulk tanks designed for max operating pressure 5.6 barg; V: 30 – 80 m3; Ø: 4 – 5 m; H: 5 – 6 m
- bottom part of each tank is covered with aeration panels to fluidize the cargo when discharging to rig or terminal
- each tank is equipped with a level indicator, a pressure indicator and a safety valve
- the tanks are provided with relevant class certificates
- valves for selecting a tank to be filled or emptied: these are usually butterfly
- remotely-controlled valves are equipped with either pneumatic or electric actuators
- quick couplings for connecting the onboard piping system to the shore or rig piping through flexible hoses
- ejectors, for vacuum cleaning bulk tanks before switching cargo type
Main Weaknesses & Limitations of this concept
- The main weakness is its basic principle – use of pressure vessels as bulk. All shapes have one thing in common – circular foot print which is not an optimum shape for efficient use of valuable space on an OSV. This has limited the options available to the ship designer for maximizing the space available below deck for other liquid cargos and installation of various equipments.
- Complicated piping layout and inconvenient location of pipes and valves again due to the basic principle of the concept which is further worsened by use of butterfly valves. The piping layout demands considerable amount of attention both during design, fabrication and installation phases. Even operators of the system point out the clogging problems they regularly face in
the areas where pipes branch out and the difficulties encountered in maintenance due the inconvenient location between main deck and top of the tank.
Lion Bulk Handling, together with their well-known brand name’s Carlsen, Ruyter Offshore, Kintec, Bulk Academy and Kovako are recognized for its reliable and innovative solutions, especially in the field of pneumatic conveying of cement. Check also our energy efficient bulk handling system for Cement Carriers, named E3-Cargo System here. This system is supplied by our brand Carlsen, which is in operation since 1966. Over the past 50 years, Carlsen pneumatic conveying systems have been installed onboard more than 40 cement carriers sized between 1,000 dwt and 40,000 dwt (figure 6) and at multiple marine cement terminals. With conveying rates of more than 500 t/h of cement through a single line have been achieved.
Sender Tank System
The fundamental principle of this type of bulk handling system is that the two main tasks of a BHS storage & discharge are performed by two separate units – cargo is received and stored in the atmospheric Hopper and the discharge is performed by a pressure vessel/tank by using compressed air. Technical solutions based on this concept have been successfully employed on cement carriers for the last 20 years.
Since the hopper is not performing the discharge operation it is not subjected to pressure and therefore can be rectangular in shape and easily be integrated into the ship’s hull. The existing longitudinal and transversal bulk heads form the 4 walls and the main deck the roof of the hopper.
Only the floor needs to be added approximately 2.2 m above the tank top to complete the hopper. It is obvious that the cargo capacity of this hopper would be considerably larger than that of conventional bulk tanks occupying same space. Approximately 50 to 75% increase in capacity may be realized. For segregating different cargos the hopper may be further divided into two or more compartments by adding transversal and/or longitudinal partitions.
The floor of the hopper inclines in both the longitudinal and transversal directions towards the outlet of each compartment. The complete area of the floor is covered with aeration fabric. At the lowest point of the floor there is an opening for discharging cement in to a pressure vessel/tank located directly under the opening.
Cement is loaded into the hoppers pneumatically by means of compressed air provided by compressors on shore. To maximize the net cargo volume the filling line to each compartment is split into two or more branches before entering the hopper thus providing several drop points in each compartment. Dusty air is evacuated and cleaned by two onboard dust collectors before releasing it to atmosphere. The dust that is separated is returned to the appropriate hopper pneumatically by means of compressed air provided by compressors onboard the OSV.
At loading the operator will select the compartment(s) to be filled on the control panel located in the Wheel House and once the appropriate loading and venting valves are indicated in correct position will allow the loading process to start. The transport air is evacuated, from the compartment(s) being filled, through venting pipes by the onboard dust handling system which separates the dust and conveys it back to a compartment.
As soon as the cement level reaches a preset value, a high level guard in each hopper will activate an audio-visual warning prompting the operator to take appropriate action.
At unloading the operator will select the compartment(s) to be emptied on the control panel located in the Wheel House and once the appropriate discharge valves are indicated in correct position will allow the unloading process to start. The BHS control system will then automatically activate the aeration system in the selected compartment(s) and start the Sender Tank System. The air supplied by the blowers passes through the fabric of the aeration panels and fluidize the cement which will start flowing by gravity towards the outlet at the lowest point of the hopper floor. The cement is discharged into the pressure vessel/tank in batches of approximately 3 m3 (net capacity of the pressure vessel) which conveys it to the rig or terminal pneumatically. This is a cyclic operation. The compressed air for pneumatic conveying is supplied by the onboard transport air compressors. Only the pressure vessels/tanks are pressurized; the storage hoppers are never pressurized.
The implementation of the Sender Tank System extends benefits to various stake holders that can be summarized as follows:
To the Shipowner, the initial system cost expressed in dollars per cubic meter capacity according to our estimates works out to be in the cost ratio of 3:5 in favor of the Sender Tank System. Some of the underlying reasons for this are:
1. Much higher capacity that is achieved by the hopper system compared to the bulk tank system – could be in the range of 50% to 75%.
2. There is lesser need for refrigerant air dryers since cargo is not stored in huge tanks that are pressurized during discharge.
3. No special internal coating (black steel) required for the hopper as compared to the conventional system (epoxy primer).
To the Shipbuilder, the installation cost works out to be in the cost ratio of 4:5 in
favor of the hopper system. This is attributed to:
1. No internal coating required for the hopper
2. Eliminates the dependence on the timely delivery of the conventional
pressure vessel bulk tanks
3. Eliminates the storage and installation for huge bulk tanks
To the Ship operator, our investigations into the operation and maintenance for
both the systems arrives at a cost ratio of 3:4 in favor of the Sender Tank System. This is attributed to the following factors:
1. Higher degree of automation operational reliability requiring minimum attention from the operator.
2. Between each cycle of fill & discharge of the blow pump the whole discharge line including the long flexible hose are blown clean. This would almost eliminate the risk of clogging in the hose resulting in higher operational reliability.
Lastly, to the Ship Designer, the Sender Tank Systems frees up precious space:
1. At the tank top previously taken up by the foundations of the conventional pressure vessel tanks.
This allows for more efficient space utilization by the Ship Designer
As a closing note, we remind that owing to the Sender Tank System being INTEGRATED with the structural design of the vessel, it should not be treated as loose equipment to be INSTALLED into the vessel. Therefore early interaction between the ship designers and the system designer is vital for the successful implementation.
Do you have any questions about the Sender Tank System, contact us here.