Excelerate Energy has transferred LNG ship-to-ship in its first operation involving a floating storage and regasification unit (FSRU) moored to a subsea buoy. Four transfers in, vice-president operations Charles Ruehl tells Karen Thomas about the challenges
Subsea unloading buoys are not new. The oil industry developed the technology for receiving terminals where land is at a premium, or the port is too shallow, or where building a deepwater port is too expensive or too tricky.
Moored to the sea bed, the STL system incorporates a subsea buoy that is connected into the hull of the vessel. A subsea riser then connects the buoy to a pipeline manifold on the seabed to allow for the delivery of natural gas.
When the vessel disconnects, the buoy floats 30m-50m below the surface until it needs to reconnect. A standard-size mating cone connects the turret to the vessel. This means that the buoy is both a mooring device and the conduit to deliver gas to shore.
US-based Excelerate Energy spotted an opportunity to use buoy-based systems to support floating LNG operations when it launched the world’s first floating storage and regasification units (FSRUs). It deployed STL buoy systems to work with first-generation FSRUs, delivering natural gas to shore using subsea pipes linked to the turret.
And so, in 2005, Excelerate launched Gulf Gateway Deepwater Port, 116 miles (187km) off Louisiana. It went on to launch a dual-buoy system at Northeast Gateway off Boston, Massachusetts in 2008 and Hadera Deepwater LNG terminal, the subsea unloading buoy-based system that in November 2012 opened Israel to LNG imports. Gulf Gateway is now decommissioned.
Israel Electric Corp (IEC) charters Excelerate’s 138,000m³ FSRU Excellence to deliver up to 3.7 mta of LNG. The terminal was built to meet increased demand for energy in the region while the country’s gas reserves are being developed
Israel is a gas-hungry market. There is a disadvantage to using STL-based FSRUs. When the FSRU runs empty, it must disconnect from the turret and sail to an LNG-reload terminal or to a sheltered harbour to top up its cargo ship-to-ship (STS).
This takes the vessel off-site and out of operation, in the case of Excellence, sailing 12-14 hours to Cyprus, to carry out STS at Limassol.
The world’s first LNG ship-to-ship transfer involving an FSRU moored to an STL buoy took place last autumn.
Altogether, the operation took six to eight months to plan. Excelerate held meetings with the port authority, the harbour master, the tug-boat operator, the STS fender provider and the operator of the LNG carrier.
“Because this had not been done before, we had to work through various risk assessments, looking at the sea conditions and to ensure that the mooring system and the STL design made it capable of handling two vessels simultaneously while carrying out the transfer,” says Excelerate vice-president operations Charles Ruehl.
“We carried out several studies, looking at haz-ops and manoeuvring to demonstrate this – to the industry, to the third-party shipowner, to the customer and to Israel Natural Gas Lines, the owner of the STL buoy and pipeline. We needed to reassure everyone that the design and the systems could support this transfer.
“Previously, we have never had a commercial reason to carry out this kind of transfer. There was a commercial reason for this project, however; it ensures that the FSRU can continue to send gas out to the network for industrial use and power generation, while refilling. In other words, it ensures a continuous supply of gas from the vessel, with no interruption of service, while reducing overall costs for the operation.”
The east Mediterranean is known for its calm summer waters. Excelerate planned the transfer with a 72 hour operating window.
Calm seas are a must. The FSRU weathervanes around the STL buoy when connected to minimise the wind and wave forces on the vessel. And so, when an LNG carrier arrives alongside, the FSRU moves away from it. This presents a challenge for the tug crew.
As luck would have it, high seas delayed the debut transfer by a day. Excelerate waited and the weather turned. The first tug was fixed to the stern of the FSRU to hold it in position as the LNG carrier approached. Three additional tugs then brought the LNG carrier alongside.
With the vessels secured, using the LNG carrier’s standard mooring lines, the transfer proved fairly straightforward. Excelerate connected the hose saddles from the FSRU to the LNG carrier, before starting the cool-down and transferring the cargo, which came from Trinidad.
“A lot of people have the misconception that LNG is a volatile, dangerous cargo,” Mr Ruehl says. “As a cargo, in fact, it’s not. This is about getting people comfortable with the processes involved in carrying out an STS. Having carried out more than 1,200 STS operations, Excelerate is very comfortable with that.”
Excelerate took several hours to connect gear and cool down the emergency release couplings, LNG cargo-transfer hoses and hose-support saddles, before starting the cargo transfer. That done, it completed the custody transfer and warmed the gear. Having disconnected all the equipment, the two ships prepared for unmooring.
The mooring master or pilots came alongside again for manoeuvring, which can take place during daylight or at night. The weather remained calm throughout.
Mr Ruehl explains: “All LNG STS operations are subject to strict met-ocean or weather parameters in place primarily to protect the cargo tanks from sloshing damage. On the conventional carrier delivering LNG, cargo at certain levels within the tank can damage the inner liner due to cargo sloshing around inside.
“This is especially critical with open water STS operations, considering the location’s environment… The met-ocean parameters in place for operations to commence are under 2m for height of significant swell and less than 8 seconds for time period of swell. Some suppliers, or even locations, may impose stricter restrictions.”
Four transfers in, it is standard procedure at Hadera to place the location mooring master or pilots on board both ships and to use tugs to assist with manoeuvering and mooring. Excelerate and its partners have also agreed to restrict the tug manoeuvres to daylight hours.
Excelerate has compared the timings of the first buoy-based transfer with the time it takes the Israel-based FSRU to sail to reload at Limassol in Cyprus. Refilling the FSRU in situ takes just under 48 hours, compared with 98 hours if the vessel needs to depart – less than half the time. Pumping the cargo takes around 24 hours.
At the time of writing, Excelerate was about to carry out its fifth LNG transfer in situ, delivering and loading cargoes onto an FSRU moored to an STL buoy. The first four transfers, a total of more than 481,626m³, transferred an average 120,407m³ load.
So pleased is Excelerate with the new transfer method that it has not sent Excellence to Limassol for reloading since last September’s trial operation. It has the option to return to Cyprus if there are storms off Israel. If all goes to plan Excelerate hopes, with practice, to cut in situ transfer times to just 39 hours.
“This transfer technique offers cost savings to our customer in Israel,” Mr Ruehl concludes. “It’s cost-effective, because it means not having to disconnect the vessel from the buoy, nor then sailing to another location, hiring tugs and service vessels at that location to bring fenders alongside, reloading then sailing back to connect to the STL buoy.
“It’s cost effective – it’s simply a more efficient operation. Israel’s gas demand changes on a daily basis. For the offtake partner, it improves both efficiency and security of supply.”