Mobil launches shipboard LNG regasification

Mobil Technology Co. and Mobil Shipping & Transportation Co. have jointly introduced a new technology for converting liquefied natural gas carriers to perform the added duty of offshore regasification. The system, called Mobil Shipboard Regasification Terminal (SRT), uses a conventional LNG carrier.

A 125,000 cu m carrier can deliver as much as 300 MMscfd at pressures as high as 70 bar, says Mobil. It also can deliver to conventional LNG terminals without any further modification.

In contrast to other offshore terminal designs, SRT involves no offshore transfer of cryogenic fluids. SRT is especially suited for deliveries to emerging markets and environmentally sensitive areas, says Mobil.

Mobil has also developed a large-scale floating liquefaction plant design (OGJ, Mar. 9, 1998, p. 67).

The design

In Mobil's SRT design, two LNG carriers would be retrofitted to hold the entire regasification system (see schematic). The necessary modifications include:

Seawater inlet and outlet.
LNG booster pumps.
Additional LNG piping.
Intermediate fluid LNG vaporizer.
Natural gas piping.
Bow discharge coupling.
Electrical distribution.

When operating, both retrofitted ships would contain full LNG tanks. Upon arrival at the discharge site, one ship would be moored to a piled single-point mooring (SPM) system (see diagram).

The SPM would be equipped with a boom and rotating head, thus allowing the ship to weathervane. A tug would be required to pull the vessel at the stern in order to maintain constant tension on the mooring hawser.

LNG then would be pumped to delivery pressure, regasified onboard, and discharged at ambient temperature via a high-pressure flexible hose. The gas would flow through a high-pressure swivel on the SPM, from which it would be transported ashore via a subsea pipeline.

After the entire cargo has been regasified, the first ship would disconnect from the SPM and return to the supply port. The second carrier would then connect in its place.

This dual-carrier system provides nearly continuous delivery of gas, says Mobil.

The number of days on station depends on the delivery rate required. A cargo discharged at a 300 MMscfd rate would be delivered in less than 9 days.

"For a small added cost, two SPMs can be employed to provide even greater availability by permitting the second carrier to begin regasifying before the empty carrier disconnects."

Advantages

An SRT can be set up in 24 months-half the time required to build conventional onshore LNG terminals. Capital costs are much lower.

"SRT allows customers to purchase small quantities of LNG," said Mobil. "Should demand grow to levels that make a full-scale terminal more economically attractive, SRT can supply LNG to a buyer while its conventional terminal is being constructed, providing more flexibility in the development of downstream infrastructure."

It also is useful for supplying additional peak or baseload gas volumes after the conventional terminal is built.

In addition, says Mobil, "SRT can provide new buyers with a means to demonstrate LNG offtake capability and sound cash flows while constructing downstream facilities."