Preparing the cargo tanks is one of the crucial operation step on Gas Carriers after Drydock.
I would like to share one of my preparation operation details.
After drydock, the vessel’s tanks had wet air. Therefore, to get the vessel ready to load a cargo of Propane, the following steps were performed ;
Drying the tanks with dry air from IG plant.
Purging with IG to get oxygen percent below 5%.
Gassing in 1 Cargo Tank (Tank 2 in this case), alongside.
Cooling down 1 Cargo Tank & receiving heel alongside.
Gassing in the remaining Cargo Tanks outside the port.
Cooling down, ready to load.
DRYING
Cargo tanks were dried by dry air from IG plant by BITO, dry air being heavier than moist air. Pipelines, pump shafts, level gauge shaft etc were dried out thoroughly. The dryers were in operation 24 hours.
The vessel was on anchor at Singapore OPL, awaiting orders. The tanks were maintained under a positive pressure of approx 40 mBar.
The charterers proposed to employ the vessel for a Propane/Butane voyage. The cargo tanks were tested for Ammonia content to ensure avoidance of contamination. Tanks were found to have as much as 50 ppm Ammonia around the bottom. It was decided to open up the tanks again & the bottom cleaned off any oil deposit.
It was decided to dry the tanks while purging with dry IG.
INERTING
The tanks lined up in cascade, BITO, for inerting. Dry IG was introduced at the cargo tank three bottom & cascaded through the other tanks & vented off the forward vent mast. The tanks were inerted till O2 content was found to be 4%. The tanks were pressed up with IG to 200 mBar to avoid any inadvertent leak of air inwards. The hold spaces were also purged similarly to O2 content below 4% & pressed up to 120 mBar.
GASSING IN
The vessel was lined up BITO, in cascade in sequence 2-3-1 as cargo nomination required Cargo tank 2 in Propane & Cargo Tanks 1& 3 in Butane. The cargo system pipeline was segregated in 2 systems, System 1 being Cargo tanks 1& 3 and System 2 being Cargo tank 2.
The following connections & disconnections were made;
Vapor & liquid line elbows connecting the two systems together, below the hose handling crane platform, were removed & the lines were blanked.
Vapor line elbow on the port side of tank dome 2, which was connecting vapor line of Cargo tank 2 to vapor line system 1, was realigned to connect it to system 2.
Condensate line elbow on the port side of tank dome 2, which was connecting condensate line of Cargo tank 2 to condensate line system 1, was realigned to connect it to system 2.
Vapor line elbow on the port side of pump dome 1, which was connecting vapor line of Cargo tank 1 to vapor line system 2, was realigned to connect it to system 1.
Condensate line elbow on the port side of pump dome 1, which was connecting condensate line of Cargo tank 1 to condensate line system 2, was realigned to connect it to system 1.
Booster manifold was connected to vapor manifold of system 2 by flexible hose.
Vapor line & liquid line were joined by elbows on tank domes 1 & 2.
IG top line on tank dome 2 & liquid line on tank dome 3 was connected to IG line.
IG top line on tank dome 1 was connected to vent mast.
Vapor line & condensate line valves were aligned to isolate the 2 systems inside the compressor room.
A flexible hose was connected to the condensate line on tank dome 2 & was led to the manifold for receiving the heel directly into the condensate line.
A small diameter hose was connected to the sample point on the condensate line & was connected to the inlet side of the booster heater.
Per the charterers’ instructions, the vessel proceeded to Kharg Island Terminal to load 300 MT of Propane heel in Cargo tank 2.
It was chosen to load Propane heel via a flexible hose directly into the condensate line on tank dome 2. The small diameter hose was used to introduce a controlled quantity of liquid Propane into the booster heater.
The vapor generated was routed from the booster manifold, via hose connection to the vapor manifold of System 2, to the vapor line on tank dome 2.
This was routed into the liquid line of tank dome 2 via the elbow connecting the lines. This sent the hot vapor from the booster heater into the bottom of tank dome 2.
The IG was allowed to evacuate out of Cargo tank 1 via the IG top. This was connected in cascade to the bottom of Cargo tank 3.
The IG evacuated out of Cargo tank 3 via the vapor line & was sent ashore through flexible hose connected to the vapor manifold of System 1. The inlet valves on tank dome 1 were kept closed throughout the vessel’s stay in port for heel to ensure nothing needed to be vented to the atmosphere via the connection of IG top of Cargo tank 1 to the forward vent mast.
COOLING DOWN
Once the top of cargo tank 2 registered 98% propane, the top spray was started & vapor from the booster heater was stopped. The excess pressure from the tank cascaded into Cargo tank 3 for continued gassing up.
Once the bottom temperature was reduced adequately, the liquid was slowly introduced into the bottom of Cargo tank 2 by crack opening the liquid-condensate crossover valve. The rate of liquid introduction was slowly increased, basis the temperatures in the cargo tank.
Cargo compressors port & center were started to control the rise in pressure of tank 2. Both compressors required continuous attendance to run correctly due to the improper functioning of various expansion valves.
By the time loading 300 MT Propane was completed, Cargo tank 3 was completely gassed in.
After the vessel’s shift to the anchorage, the compressors were stopped & the excess pressure from Cargo tanks 2 & 3 was used to gas in cargo tank 1. The residual IG was vented to the atmosphere.
Once Cargo tank 1 was completely gassed in, all 3 compressors were started. Suction was taken from all 3 tanks & condensate return was sent to top spray of Cargo tanks 1 & 3.
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