Marine Loading Arm (Fig. 1) is used to load or unload the ship or vessel carrying the Petroleum products, chemicals etc. Loading arms are made up of several sections of pipe, connected by swivel joints. The section on the shore side of the ‘apex’ of the loading arm is known as the inboard arm and the section on the tanker side of the ‘apex’ is known as the outboard section.
Loading arms must be specified/designed to allow for movement of the tanker during loading/unloading, due to tides/waves/wind and due to the tanker’s cargo increasing/decreasing. Each loading arm is designed with a certain operating ‘envelope’. If a connected loading arm is forced outside this envelope, it should be disconnected immediately (manually or, for some systems, automatically). The main factors that affect the required operating envelope are tanker DWT, tidal range, maximum wave height, elevation of jetty structure, and the size/number of loading arms at the berth.
In general, loading arms are hydraulically powered. A hydraulic system, which often serves all the loading arms at a berth, is used to move the arms into position at the tanker’s rail, and to retract the arms back into the stowed position after use. It is common for loading arms to be one (or even two) sizes smaller than the associated pipelines.
When loading a small coastal tanker (up to say 5,000 DWT), it is normal to use only one loading arm (subject to loading arm diameter), whereas when unloading a large vessel (eg a crude oil VLCC in excess of 160,000 DWT), it is common to use up to four loading arms in parallel, in order to achieve the required flowrate.
There are a number of systems for connecting loading arms to tankers. The simplest connection is made using a flanged coupling. It is common practice for loading arms to be fitted with powered universal couplers. Loading arms handling LPG (or any other pressurised/refrigerated material) should have a powered emergency release coupling (PERC), connected to the ESD system.
If the occupancy of a berth is very low by design, it may make sense economically to use hoses rather than loading arms. But there are factors that may outweigh the cost benefit of hoses relative to loading arms, as follows:
- In order to handle hoses with a diameter of 4” and above, a crane is required
- In order to prevent snagging, hoses often need support that has to be adjusted during loading/unloading, for which a crane is required the environmental performance (in terms of emissions to air and to water) of a berth with hoses is generally not as good as that of a berth with loading arms
There are a number of standards relating to loading arm design and hose manufacture. Loading arms can be divided into two categories; those designed to handle liquefied gas (pressurised, refrigerated or semi-refrigerated) and those designed to handle Refinery products which are stored at ambient conditions.
Loading Arms for Refinery Products:
For a berth handling Refinery products which are stored at ambient conditions, it is common to have two pairs of loading arms to handle all non-refrigerated/pressurised products. Each pair of arms has a manifold such that one pair is dedicated to ‘white oils’ and one pair is dedicated to ‘black oils’. Product contamination is avoided by stripping and draining as part of the loading process, as follows:
- After pumping to the tanker is complete, the MOV upstream of the manifold is closed. These MOVs are generally the double seal design, to ensure product segregation.
- Liquid remaining in the manifold and the inboard section of the loading arm is then pumped out by the stripping pump. The stripping pump discharges either into the outboard section of the selected loading arm (i.e. onto the tanker) via a small ‘piggy back’ line, or back into the supply pipeline upstream of the MOV, or to the on-shore slops system.
- Any unpumpable material in the manifold is drained into an on-shore slops drum at the berth, leaving the manifold and loading arms empty.
For hydrocarbons stored at ambient conditions, the stripping pump should discharge into the outboard section of the loading arm because generally speaking the material has already been metered and therefore belongs to the customer.
Loading Arms for LPG at Ambient Temperature:
For a berth handling LPG or any other material stored under pressure, it is common to have a vapour return system to take the vapours displaced from a tanker during loading. The flow of returned vapour should be metered. The vapour return can be a separate loading arm, but it is common for an LPG loading arm to be designed with a ‘piggy-back’ vapour return line i.e. two loading arms in one.
The returned vapour can be routed to the storage vessel from which product is exported. This enables high loading rates to be achieved, but can be impractical/uneconomic over large distances, particularly if fans/blowers are required to boost the pressure in the vapour return pipeline. Returning vapours to the storage vessel gives rise to the risk of a storage vessel being sent off-spec by contaminated returned vapours.
An alternative is to route the returned vapour to a local flare/vent system, with a pressure controller in the vapour return line to back-pressure the tanker.
When pumping LPG onto a tanker is complete, stripping of the loading arm(s) is usually achieved as follows:
- The loading arm/manifold is de-pressured into to a dedicated closed sump, from which the drainings vaporise (with heating as required) into the vapour return system.
- The loading arm/manifold is purged with nitrogen to remove hydrocarbon vapours. Nitrogen purging is often carried out before connecting the loading arm, in order to remove air.
Loading Arms for Refrigerated LPG:
Many of the considerations applicable to loading arms for products stored under pressure also apply to the design of loading arms for refrigerated (or semi-refrigerated) LPG, and for other products stored at below ambient temperature.
An additional design consideration for loading arms handling (semi) refrigerated products is the provision of a liquid return line to permit chilldown of the supply system prior to loading.
Marine Loading Arms are Designed as per OCIMF (Oil Companies International Marine Forum) guidelines.
Main Components (Fig. 2) of Marine Loading Arm:
- Riser Pipe
- Inboard Arm
- Outboard Arm
- Counter balance
- Connection Flange / QCDC
- Apex Swivel Joints
- Emergency Release Coupling
- Hydraulic System
How Marine Loading Arm Works (Fig. 3):
- Marine loading arm is designed based on its Operating envelop
- Marine Loading Arms is operated by using the hydraulic system.
- Heart of the marine loading arm is swivel Joint. Which plays a important role in operating the Marine loading arms.
Swivel Joints (Fig. 4):
- Enabling the loading arm once connected, to follow the movements of the vessel within the working envelope.
- Allow the rotation between two items of a product line whilst ensuring no product leakage, even under external pressure.
- ln a standard marine arm there are six swivel joints with the direction of rotation in three planes giving the arm six degrees of freedom. This allows the arm to be maneuvered to and from the vessel and, once connected, allows the arm to follow all the motions of the vessel.
- Each swivel joint is made up of a male and female part joined by two or three bail bearing raceways allowing the free rotation of the joint. A compression type packing seal is used to seal the two parts of the swivel joint.
- Swivel Joints are available in various materials including carbon steel, LTCS, Hastelloy, Titanium etc.
End Connection (Fig. 5) of Marine Loading Arm:
- The End connection of the MLA is a standard ASME B16.5 flange.
- It is available in various size and rating.
- It is also called as Quick connect / disconnect coupling (QCDC)
- End connections are of two types:
- Manually operated.
- Hydraulically Operated.
Emergency Release Coupling (Fig. 6):
Emergency Safety Systems are used to ensure best possible safety in fluid loading/unloading operations with Marine Loading Arms.
The system allows full automatic and safe disconnection of the loading arm from the ship without product spillage, when the arm exceeds the operating envelope’s limit line.
Hydraulic/Control System in Marine Loading Arm (Fig. 6):