The mast riser is normally used during loading for tank vapour pressure control. Its exit location, being at least 6 meters above the deck, allows for the free flow of the vapours displaced from the cargo tanks by the incoming liquid crude oil at the rate of loading of the cargo.
The rate of displacement of VOC (Volatile Organic Compounds) vapours from the cargo tank system will be the same as the loading rate but the concentration of VOC vapours in the displaced stream will be greater dependent upon the extent and rate of evolution of VOC vapours from the incoming cargo that would add to the volume of gas/vapour mixture already existent in the cargo tank prior to loading.
A mast riser is a vertical pipe that connects the ship’s manifold to a point above the ship’s deck level. It is used on tankers to prevent spillage or overfilling of cargo during loading or discharge operations. The purpose of a mast riser is to provide an additional layer of protection against spills by allowing the crew to see and monitor the cargo flow while it is being transferred.
As per the International Safety Guide for Oil Tankers and Terminals (ISGOTT), the use of mast risers is recommended for all tankers during cargo operations. The ISGOTT specifies that mast risers should be made of a suitable material, such as stainless steel or non-sparking material, and should be designed and installed in such a way as to ensure that they do not interfere with cargo operations or create a hazard to personnel.
The ISGOTT also provides guidelines on the installation, maintenance, and inspection of mast risers. For example, it recommends that mast risers should be checked regularly for damage or wear, and that any defects should be repaired immediately. Additionally, the ISGOTT recommends that all personnel involved in cargo operations should be trained on the proper use of mast risers and other safety equipment to ensure the safe and efficient transfer of cargo.