Tanker Loading Systems
The loading and unloading of tankers with petrochemical fluid ranging from crude oil to liquefied natural gas is a daily occurrence throughout the world. However, it is an operation fraught with the dangers of spillage, fire, explosion as well as the fury of the sea. The design of a ship transfer facility is usually a compromise of loading times (demurrage) and construction cost. The overall combination of high flow rates, relatively short distances coupled with intermittent operation lead to high velocities. IMO and US Coast Guard regulations require transfer flow to be stopped rapidly in the event of any emergency, which usually means valve closures of less than 30s and the generation of high and rapidly applied surge pressures. Experience has shown that the potential for damage to pipework and hoses, spillage and failure of equipment can be easily realised.
TYPICAL HYDRAULIC PROBLEMS
The problems encountered in a typical ship transfer system will include:
- Excessive surge pressures caused by Ship ESD valve closure causing damage to loading hoses or arms, loading buoy and feed pipework.
- High surge pressures caused by closure of breakaway couplings following ship disconnection causing damage to loading hoses or arms, loading buoy and feed pipework.
- Low pressures and possible vapour pocket formation following unplanned pump trip.
- High pressures during pump start up, particularly of concern if there are any vapour pockets formed during the connection which will lead to high pressures.
- Check valve slam at the pumps or along the system during rapid flow changes.
- Excessive pipe loads and pipe movement caused by high pressures and rapid pressure changes.
- Unstable operation due to control valve misuse or mis-setting.
- Low pressure caused by closure of valves upstream of the system.
SERVICES AND SOLUTIONS
Hydraulic Analysis Limited, through the use of a detailed mathematical model can offer advice, assistance and solutions based upon experienced gained through offering consultancy services on more than one hundred ship loading and offloading systems throughout the world.
Hydraulic Analysis Limited can typically assist with:
- Review and optimisation of the system's steady state design.
- Review of system controllability and control of the system during normal and abnormal conditions.
- Review and definition of optimum start up and shutdown sequences
- Surge analysis of the system under normal and abnormal operating conditions including a ship ESD.
- Sizing and installation details of surge alleviation equipment such as surge relief valves, accumulators, spring relief valves to control surge pressures caused by a ship's ESD.
- Definition of acceptable closure times for system ESD valves and breakaway coupling closures.
- Definition of set pressure and position of high pressure trips to shutdown pumps and close ESD valves to reduce surge pressures.
- Review of leakage and lost volumes in the case of a system failure.
As with all types of projects Hydraulic Analysis Limited would be pleased to offer a free appraisal of any scheme given some basic details of the system's layout and operation.