It’s mid-afternoon at Petronet Dahej LNG terminal in Gujarat, western India. At the port’s newly commissioned South Jetty, the 158,000mᶾ LNGC Aseem is discharging.
Looking out from the vessel’s expansive, highly automated bridge is Rakesh Jhang, who has been ship’s master since 2009, when Shipping Corporation of India (SCI) took delivery of this membrane type Mark III design LNGC from Samsung Heavy Industries (SHI).
Aseem is SCI’s third LNG carrier after Disha and Raahi, each of 136,000mᶾ. Aseem delivers LNG from Ras Laffan in Qatar to Dahej under a long-term contract between RasGas and Petronet LNG Ltd (PLL).
“The challenges of berthing the vessel and unloading are manifold at Dahej compared to Ras Laffan’s protected calm terminal,” says Captain Jhang.
There is heavy traffic in the Narmada channel from the mouth of Gulf of Khambat and leading to the LNG terminal, a distance of 115km.
“Full-scale operations at other terminals create the traffic but there is also a 5.5 knots current near the terminal [and] waves reaching 2m and above during the monsoon, with wind at 10-12 knots.” says Captain Jhang.
Sudden high wind can wreak havoc. “With high membrane tanks exposed to winds, the vessel can suddenly drift away while coming alongside, damaging the fenders and jetty structure,” says Captain Jhang.
When the monsoon winds hit the coast of western India, heavy rain may hinder visibility too.
Dahej relies on its experienced, trained pilots and tug operators during storms. Their close communication keeps the risks under check.
“Bringing the ship alongside and completion of mooring and cargo transfer, within the given time is always challenging during such conditions,” Captain Jhang says.
Dahej’s tidal variations of 4.5m-11m and the lengthy process of berthing and starting cargo discharge makes timing crucial when getting the vessel alongside. If Aseem misses the window between low and high tide, it faces a six-hour wait for the next one.
“Being bigger, the LNG vessel takes time for berthing compared to ships calling at the nearby terminals. We try to enter the channel six hours prior to the pilot boarding time,” says Captain Jhang.
“The pilot boards the ship three hours in advance of the slack period. Since the waves are higher during the monsoon, accompanied by strong currents, we provide leeway to the pilot for safe boarding to avoid time loss,” he says.
Although 24 hours is the allotted time for berthing, mooring, cargo discharge and sailing, SCI prefers to finish in 18-20 hours, maintaining contact between the vessel master and pilot.
The pilot boards and briefs the master about the mooring lines required for berthing, based on the wind and weather, and briefs him about the officers in charge at the forward and aft mooring stations.
The pilot also informs the master about the tides, wind speed and currents.
“Expected weather changes are also intimated to the master in advance, to maintain the appropriate manoeuvring speed of the ship while coming alongside as sudden weather changes are not unnatural at Dahej,” says the master pilot on board Aseem, Captain Rajesh Sanil.
“Vessels are usually berthed at a 1.5 knots current speed.”
Captain Jhang briefs the pilot about Aseem’s propulsion systems and instruments and they discuss limitations of the equipment. This enables the pilot to make the berthing plan and the master to optimise operation of the equipment.
Captain Jhang says: “We cannot afford to have the slightest engine breakdown during berthing and unberthing, [as this would] result in missing the slack period. Hence, as per the pilot’s instruction, we run our four Wärtsilä generators at a total power of 36,000kw.”
Aseem uses an electric propulsion system. Two generators deliver 12,000kw of power and the other two 6,000kw apiece.
To optimise unloading time, the vessel’s liquid header and crossover lines are cooled to -90°C one hour before the pilot boards the carrier. While transferring the cargo, tank pressure is maintained at 10-12 kPa using LNG vapours from shore.
Pressure gauges display tank pressure, which is monitored to ensure that it does not rise above 22 kPa before shore arms connect to the ship’s manifold for cargo discharge, before which there are safety checks to perform.
The tests include leak tests in the ship manifold, first using nitrogen then a soap-based solution. Purging follows.
The manifold’s oxygen content must be less than 2 per cent to avoid fire hazards during transfer.
During the tests, the manifold’s pressure is maintained at 650 kPa.
After purging comes custody-transfer monitoring. Before any cargo transfer, the vital warm emergency shutdown test (ESDS) is carried out manually, on shore and ship side, from various locations.
Cooling of the shore unloading arms is undertaken from the ship side through LNG to bring the temperature to about -80ºC.
Simultaneously, a fire pump located in the vessel’s engineroom creates a 100 m3/hour water curtain on the ship’s manifold. In the case of a leak this avoids contact with LNG, which is at -160°C and could cause brittle fractures in the mild steel of the ship’s manifold side.
Next comes the cold valve operations test. This is followed by cargo discharge at 700 m3/hour.
The crew gradually raises this to 2,800 m3/hour, then to the rated 12,000 m3/hour, using eight pumps, each of capacity 1,850 m3/hour, installed in the ship’s tanks, which are fitted with safety alarms.
Pumping remains steady until near completion, when pump loads are gradually reduced to prevent cavitation at low cargo levels.
As a safety precaution during cargo transfer, the crew continue to monitor the tensions on the mooring lines. Cargo transfer stops and the arms are disconnected if the wind goes above 35 knots and swell height above 2m.
“The tests and transfer are monitored through our state-of-the-art integrated automation system in the cargo controlroom,” says chief officer Sunil Yadav.
“The touch-screen-based system can deliver extensive information on all functionalities, including warning signals. However, to doubly ensure there are no safety lapses, we monitor the entire operation.”
As the cargo transfer approaches completion, the pumping rate is reduced. The shore line is then disconnected, drained and purged to check that the methane level is less than 1 per cent, and CTM analysis follows.
Captain Jhang concludes: “The key to Aseem’s successful operation is SCI’s elaborate operating practices for LNG ships and superior technical management.”
These practices will be vital, allowing Dahej to handle more cargoes as expansion makes it busier than ever.