Jun-2021

Stable vessel design for FLNG

Advances in floating liquefied natural gas production technology are becoming an important factor in maintaining sustained growth of the natural gas industry.

LARS ODESKAUG and SAEID MOKHATAB
Front Energy AS

Article Summary

As global demand for natural gas increases, the development of offshore floating liquefied natural gas (FLNG) production technology is becoming an important factor in maintaining sustained growth. Although offshore LNG production has been the focus of research and development for decades, it is only in the recent years that a few FLNG projects have progressed to detailed design, construction and eventual operation. In fact, some special challenges exist in the design of an FLNG facility in the harsh offshore environment that require special solutions. The key aspect in developing a successful FLNG project is the proper design of the hull to provide: a seaworthy and stable platform for production and product offloading as well as safe accommodation of the crew in a remote, potentially hostile environment; and enough deck area to accommodate the topsides process/utility units, required product storage and offloading systems, and support facilities.

This article presents the Cefront hull design, which provides a more stable and economical platform for the offshore gas pre-treatment and liquefaction processes than conventional hulls. It is a further development of the axisymmetric hull and is more fabrication ‘friendly’ and thereby less costly than earlier designs. The Cefront FLNG vessel has a more efficient topsides layout than the axisymmetric units, and at the same time it has significantly less pitch and roll motions than a conventional ship-shaped hull which eliminates the need for expensive turret and swivel solutions.

Floating liquefied natural gas (FLNG) production technology is a technically innovative solution and potentially a commercially viable means of exploiting remote offshore gas reserves. It may also provide an economically preferable option to flaring associated gas at oil fields. FLNG technology may offer lower production cost, reduced time to first production, and fewer environmental impacts than land based alternatives. In addition, a potential advantage of a floating facility is that it can be moved relatively easily to an alternative offshore location as the original gas resources decline or economics or politics change. This allows the operator to save money on future gas field developments or earn revenue by charging third parties to process their gas through the FLNG facility. While principally aimed at remote offshore gas reserves, FLNG production technology can also be considered for the development of nearshore gas fields with limited infrastructure or as a combined liquefaction and storage solution for onshore gas.

Initial FLNG developments were focused on building large scale facilities that can move and process large quantities of LNG, typically 5 million t/y and up, which require huge capital investment. However, the current trend is to mitigate project risks by developing small to mid-scale FLNG projects, limiting production capacities to 0.5-3 million t/y. In large scale FLNG projects, the liquefaction facilities are mounted on a barge-like structure or a ship-shaped vessel (depending on the location) with the LNG stored in the hull underneath. In small to midscale FLNG projects, the liquefaction facility is built on a converted LNG carrier or on a purpose-built vessel that is sized more as a conventional LNG carrier.

When it comes to the design and construction of an FLNG facility, every element of a land based LNG facility needs to fit into a limited and compact deck space, whilst maintaining safety and flexibility of production. Cargo containment and product offloading systems also need to withstand the effects of the wind and waves at sea. Some of these technical challenges have already been addressed, while others such as hull design and offloading technologies are still being developed and enhanced.

Cefront hull concept
The Cefront hull design is based on decades of experience with various types of offshore vessels. The focus has been to design a hull that does not require a turret and is as stable as the axisymmetric hull regardless of wind, waves and current direction, and at the same time is suitable for Asian yards’ fabrication facilities. The result is the Cefront hull shown in Figure 1.

The hull is spread moored with three clusters of mooring lines, one in the bow and two aft. It has extremely favourable motion characteristics which are achieved by the geometric relationship between length and breadth in combination with a bilge keel. Hull sizes with storage capacities ranging from 150 000 m³ to 300 000 m³ (LNG and condensate) have undergone extensive testing in the ocean test basin at Marintek, Trondheim, Norway, verifying its exceptional stability in sea states up to significant wave heights of 17m. As can be seen from the example — response amplitude operators (RAOs) in Figure 2 and roll and pitch amplitudes in Figure 3 — the Cefront hull is stable compared with a very large crude carrier (VLCC) based hull. In summary, it has a number of advantages compared with a traditional ship-shaped hull, making it ideal for FLNG applications:
- Low pitch and roll motions, reducing sloshing and providing a stable platform for the gas pretreatment and liquefaction facilities  
- No need for turret and swivel
- High payload capacity
- LNG storage capacity of 225 000 m³, and additionally 45 000 m³ condensate or LPG
- Insignificant hull girder loads and deflections regardless of loading
- Modules supported by strong points in main deck
- Small deflections give simpler topside interface — sliding supports not needed
- Well known structural arrangement based on “standard” scantlings/ dimensions
- Stiffened plate structure as for standard tankers
- Stiffener and stringer/girder spacing as for standard tankers
- Not fatigue critical and hence high tensile steel may be used throughout
- Easily scalable

Hull design and arrangement
The hull has double sides and bottom, and offers flexibility with respect to tank arrangement. Typical dimensions for a 3 million t/y FLNG unit are waterline length of 130 m and breadth of 100 m. The hull is flared above the waterline, and deck dimensions are typically 155 m length and 125 m breadth.

The hull is arranged with seven LNG tanks with a typical total capacity of 225000 m³ and four condensate/LPG tanks with a total capacity of 45 000 m³. Ballast tanks are arranged in double side and in double bottom to the International Convention for the Prevention of Pollution from Ships (MARPOL) damage point. The double bottom inside this is void. Number of ballast tanks is 14. The ballast and condensate tanks extend up to the main deck while the LNG tanks extend up to the process deck.

The geometry of the holds in which the LNG containment system will be installed is shown in Figure 4. The hold space will have a controlled atmosphere in order to reduce/eliminate the risk of fire and explosion, and also to avoid condensation and a humid atmosphere.