In this article, we learn about the Marine Loading Arm Types
Introduction
Marine Loading Arm (MLA) used to transfer liquids/gas from liquid carrier ships to the jetty, and then the liquid will transfer via pipelines from the jetty to the storage tank area or from the tank to the liquid carrier ship.
During loading or unloading of liquid/gas from a ship, the loading arm follows the movement of the ship due to change in seawater level, Ocean current, drift and sway movement of the ship.
Marine Loading Arm Types
Basically, the Marine Loading Arms are classified into three types.
- Fully Balanced Marine Arm
- Double Counterweight Marine Arm
- Rotary Counterweight Marine Arm
Fully Balanced Marine Arm (FBMA)
The Fully Balanced Marine Arm is fully balanced in all position. This type of loading arm had a simple structure design and using two counterweights.
- Main counterweight and
- Sub-counterweight
These counterweights ensure the loading arm has full balanced in all positions. The main counterweight attached to the inboard arm and the sub-counterweight attached to the outboard arm.
FBMA can be operated with few personnel and have an easy-to-maintain structure. As a result, FBMA is the most commonly used type of marine loading arms for smaller range ships.
The commonly available size of the FBMA loading arm range from 3” to 12”. FBMA loading arms are widely used for loading and unloading of barge and small tankers range from 100 DWT to 10,000DWT.
Rotary Counterweight Marine Arm (RCMA)
The Rotary Counterweight Marine Arm (RCMA) also a fully balanced loading arm in all position. RCMA mainly used for loading and unloading medium to large tankers.
Today, RCMA widely used in most terminals. RCMA uses a single counterweight to balance both inboard and outboard arms simultaneously (Refer figure).
The counterweight, assembled on the beam interdependent of the lower pulley. The pantograph sheave and pantograph cable to transfer movement from the inboard arm to the outboard arm or vice-verse. This type of loading arm required more space for counterweight movement when compared with a Fully Balanced Marine Arm and DCMA.
When compare with DCMA(same operating range), RCMA type marine loading arms are lesser weight.
Double Counterweighted Marine Arm (DCMA)
The DCMA loading arm uses two independent counterweights, as well as a pantograph to ensure the arm full balanced in all position. Separate counterweights are used to balance the inboard and outboard section of the arm. The outboard counterweight assembled on a beam, fixed to an idler sheave which rotates around the same horizontal axis of the trunnion elbow (style 50 swivel joint). The rotations are adjusted to balance the outboard arm through a pantograph cable to a second sheave which is fixed to the outboard arm at the Apex elbow (style 40 swivel joint). The inboard counterweight fixed to the inboard counterweight extension beam and adjusted to assure the correct balancing of the loading arm assembly.
The DCMA required a smaller area for counterweight movement (rear side of loading arm) then compare with RCMA. This type of marine loading arm widely used for loading and unloading very large tankers range from 3000 DWT to 500,000 DWT.
The available size of DCMA range from 6” to 24”.
Author Note:
Some manufacture provides rigid pantograph type design and others are cable pantograph type design. The basic principle of pantograph working remains common for both rigid and cable type design.
The pantograph used to transfer the movement of both the inboard arm and outboard arm to the counterweight. The larger arm movement transfer to the smaller movement of the counterweight.
Conclusion
While loading and unloading operation of liquid/gas from the ship to shore or vice versa, the marine loading arm adjusts with the ship movements due to high tide, low tide, transfer movement of ships, ocean currents, etc.,
The selection of a marine loading arm depends upon the various factors, some are listed below
- Based on Terminal throughput, if terminal throughput is very high then large size loading arm will be selected
- Rate of loading and unloading from the ship.
- The range of ship handle by the jetty
- Product handle by marine loading arm
DCMA type marine loading arms widely used due to their robust construction and longer service life.
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Happy learning !!!