There is a smart solution for LNG offloading using conventional gas carriers that unlocks the market for FLNGs in harsh environmental conditions. Side-by-side offloading with articulated rigid arms is currently the reference solution for FLNGs (floating LNG). This cost-effective solution uses mature technologies and procedures that are similar to exposed jetties. Despite its restrictive weather threshold inherent to the close proximity of the FLNG and the LNG carrier moored alongside, typically 2.5-m significant wave height, the side-by-side mooring provides sufficient offloading uptime for FLNG projects sanctioned to date.

But for future FLNGs – large-capacity FLNGs requiring increased offloading frequencies or those located in areas with persistently harsh environments--side-by-side offloading may no longer be a viable solution.

The industry proposes several tandem LNG offloading systems. They provide great safety separation distance between the vessels and high weather thresholds, but they require the use of a dedicated LNG carrier fleet with bow loading systems or long and large-diameter floating LNG flexible lines to reach the midship manifold of conventional LNG carriers.

Parallelize your offloading operations

In late 2014, TechnipFMC, together with HiLoad LNG AS, began developing the HiLoad LNG PLS (parallel loading system), an offloading solution that combines the best of both side-by-side and tandem solutions. The HiLoad LNG PLS:

• Allows the loading of any LNG carrier for full trading flexibility and minimum shipping costs;
• Accommodates high sea state threshold, typically above 4-m significant wave height;
• Improves offloading uptime and regularity;
• Minimizes fluid transfer line length for high flow and reduced-pressure drops; and
• Uses proven or qualified technologies.

The solution expands on the proven HiLoad® technology. The HiLoad vessel is a dynamically positioned semisubmersible L-shaped vessel able to dock onto and provide DP (dynamic positioning) station-keeping to any tanker vessel. Such a vessel has already been operated for oil offloading operations in Brazil. For LNG applications, the HiLoad LNG Loading Terminal is docked amidship and is used to interface the FLNG offloading lines and the LNG carrier midship manifold using standard articulated rigid arms, as in a conventional loading berth.

The overall motivation behind developing the HiLoad LNG PLS was to move the LNG carrier midship manifold significantly closer to the FLNG compared to the tandem arrangement to shorten the LNG transfer lines. The LNG carrier is kept parallel to the starboard side of the FLNG with her midship manifold in line with--and about 100 m away from--the stern of the FLNG. The DP system keeps the position of the LNG carrier midship, the heading control being enabled through the LNG carrier’s own propulsion in “dead slow ahead” and autopilot to avoid manual control of the LNG carrier’s rudder.

For established wind, waves and current conditions, the “optimum heading” of the LNG carrier is set so the environmental force acts partly on her port side to ensure that the ship naturally drifts away from the FLNG in case of emergency situations such as blackout scenarios. In addition, flexibility is given to the LNG carrier to weathervane to accommodate changing wind, waves or current conditions without changing the FLNG heading.

For high flow and low pressure drops performance, the HiLoad LNG PLS can accommodate any type of offshore LNG transfer system proposed by the industry, aerial, floating cryogenic hoses or articulated rigid arms. In combination with TechnipFMC’s ATOL articulated rigid arm LNG transfer system, the HiLoad LNG PLS can achieve the highest LNG carrier loading rates--typically 14,000 m3/h--with minimal pressure losses (about 2 bars), comparable to a side-by-side system.

The feasibility of parallel loading operations has been confirmed through time-domain analysis of established and changing environmental conditions and using MARIN’s Full Mission Bridge Simulator, which account for human factors similar to a full-scale operation.

The HiLoad LNG PLS, in combination with the ATOL, is ready for future FLNG projects--large-capacity facilities or those operating in harsh environments--opening the FLNG market to new geographic areas.

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