Addressing the challenges of offshore LNG transfer

LNG has developed into a worldwide commodity. LNG can be transported anywhere in the world, which is often based on the highest bid for the resource. This serves as both a pro and a con. Liquefaction provides extreme mobility, but the necessary transfer from tanker to terminal seems to be the downside of LNG. In the US, a number of onshore transfer stations have been applied for, only to become entangled in massive regulatory issues.

One way to alleviate complications with the logistics of onshore transfer is to take this process offshore, although the offshore environment can present its own unique set of challenges such as waves and wind. Engineers have been challenged for decades by the prospect of performing high-velocity transfers of LNG from a vessel to a shuttle carrier in high-wind, high-wave offshore environments without creating a spill. FMC Technologies has designed its Chiksan Articulated Tandem Offshore Loader (ATOL) as a safe, efficient solution to this problem.

The design of the ATOL system is the result of more than 50 years of R&D in the LNG industry. FMC’s first onshore LNG system was in Algeria in 1956, and the company has continued to make strides in LNG transfer, both on land and offshore. The ATOL system is the culmination of decades of experience and serves as the vital link to enabling safe, efficient offshore transfers of LNG.

To prevent clashing between the 20-in. cryogenic lines during connection and transfer, the system latches them together by means of articulated ball joints, assuring a permanent spacing between them. (Images courtesy of FMC Technologies)

The technology
The two most important aspects of offshore LNG transfer are separation distance and flow rate. When two vessels come within transfer distance of each other, wave height and wind conditions can easily force them to collide, which potentially endangers the lives of the crewmembers and could cause catastrophic damage to the transfer vessels. To determine the safest distance between vessels, FMC interviewed and surveyed a range of ship captains. It was determined the separation distance had to be between 213 and 263 ft (65 to 80 m).

Traditionally, side-by-side LNG transfers have been preferred due to the need for a high rate of flow, but this method is often limited by rough sea conditions. Offshore, it is natural for sea conditions to average 8.2 ft (2.5 m) Hs. Such conditions would make it impossible to perform a side-by-side transfer without the two vessels colliding. The tandem method of transfer allows for a greater distance between the vessels, allowing transfers to take place when sea conditions are as treacherous as 18 ft (5.5 m) Hs.

The ATOL system was designed to achieve a safer separation distance during transfers while ensuring a flow rate of 565,035 cf. The separation distance is accomplished by transferring through a tandem method using two liquid lines and one vapor line. Each line is 20 in. in diameter and is constructed of stainless steel.

With sea conditions such as these, one of the most critical and difficult aspects of developing a safe and efficient method of LNG transfer is connection. Movement of the vessels is not restricted to up and down; the vessels are also constantly rocking side to side. Connection and disconnection must happen quickly in case of an emergency, and they must be done automatically to ensure crewmember safety.

To ensure a perfect connection under these conditions, the ATOL system uses a patented targeting cable system, which uses a hydraulic winch attached to the shuttle carrier to send the main cable out to pick up the extremity of the system and then brings it down to the carrier to connect. Connecting the central line takes 30 seconds and is the only human interface in the entire process.

In addition, the ATOL system is designed to provide a smooth connection of the coupling. When connecting to the shuttle carrier manifold, flanges can collide, with the possibility of damaging the gaskets or seals causing leakages during LNG transfer. To prevent this from happening, FMC developed a coupler seal protection metal ring that is spring-energized and plays the role of shock absorber in case of sudden relative motions during the connection phase. Additional precaution is provided by permanent spacing between the lines. To prevent clashing between the 20-in. cryogenic lines during connection and transfer, the system latches them together by means of articulated ball joints, assuring a permanent spacing between them.

Making improvements
Since installing the first offshore LNG transfer system in the 1970s, which performed shipments for 23 years without fail, FMC has made continuous improvements to its design. One major improvement has been the inclusion of a fixed crane. Early versions of the ATOL system used an extremely heavy vertical crane. Even though this version was functional, its large footprint was problematic for the space constrictions on offshore platforms. The ATOL system incorporates a lighter, more compact fixed crane into its design and now has one of the smallest footprints in the industry.

The ATOL’s initial design included a fixed pipe attached to the carrier and a vertically articulated system. This design was modified to use two legs in place of the fixed pipe; each leg is attached at a different spot on the crane to accommodate larger movements and expanded separation distances between vessels. The new system offers more flexibility for offshore transfers.

Development challenges
One of the challenges in developing the new ATOL system was the need to accommodate the existing fleet of LNG carriers. The current fleet of carriers includes a vast array of designs, including different sizes, connection-types, and manifolds. After discussions with shipyards, FMC concluded each ATOL system would require a modification to the shuttle carrier, which means the system requires a dedicated fleet.

The ATOL system incorporates a lighter, more compact fixed crane into its design and now has one of the smallest footprints in the industry.

Another challenge in designing the ATOL was the need for a quick disconnect of the entire system in the event of an emergency. To address this, the system was equipped with an emergency release that allows the flow of LNG to be remotely stopped immediately, with both valves closing within 10 seconds. This safe and quick disconnect helps to alleviate environmental concerns of offshore transfers of LNG.

The future of ATOL
As the technology behind LNG transfer becomes more advanced, operators are now looking for systems designed specifically to address the individual circumstances of the locations in which they work. As an example, FMC is currently developing specific applications to meet the operating challenges of the Timor Sea off the coast of Australia. The conditions in the Timor Sea boast a combination of waves and wind that are perpendicular to one another, making this location one of the most challenging in the world for safely connecting LNG transfer vessels.

Adapting to new and ever-changing sea conditions will be the primary focus for the future of the ATOL, which will make its debut in 2015 as large-scale LNG projects come onstream.