Floating LNG (FLNG) is finally starting to become accepted as a technically feasible and competitive solution to onshore LNG production. Currently, FLNG is split between two development schemes: neashore (or atshore) and offshore. Both types of projects have reached a positive final investment decision (FID). Shell’s Prelude and Petronas’ Kanowit and Rotan projects are located offshore and will produce directly from subsea wells, while Exmar’s Caribbean FLNG and Golar’s Hilli projects were developed and designed to produce LNG from partly treated pipeline gas nearshore.
The design criteria for nearshore and offshore FLNG projects are similar, but nearshore developments offer various advantages, including simplified marine systems. Typically, the vessel will be jetty moored, resulting in simpler offloading requirements and no need for a turret or dynamic positioning system. However, the topside still needs to be designed for a limited footprint, and safety is important as there are limited options to escape in case of an emergency situation. Some additional challenges must, however, be expected for an offshore FLNG vessel with respect to topside design:
- Logistics – refrigerant import may be even more of a challenge in the open waters than close to shore.
- Motion – many offshore FLNG project sites may be subject to harsh conditions. As such, greater care must be taken to ensure, for instance, correct distribution in the heat exchangers to guarantee performance.
- More changes in feed gas and frequent shutdowns – while pipeline quality feed gas may provide a relatively stable feed gas source and limited variations in composition and pressure, the conditions change more frequently offshore as wells come on and offline. More shutdowns must also be expected.
- Possibly more complex pretreatment – while quality pipeline gas typically has a maximum of 2.5 – 3.0 mol% CO2, virtually zero H2S and limited N2, offshore FLNG may also require handling of slugs, high water cut and condensate production, as well as a higher concentration of sour components.
The topside equipment is one of the most cost-intensive investments, and the liquefaction system is one of the major contributors to this cost. Selecting a technology that requires minimum deck space, whilst at the same time being inherently safe, is, therefore, a crucial factor in trying to reduce the overall CAPEX.
Offshore logistics may be challenging.
Choosing the right technology for offshore FLNG
There are currently two main liquefaction technologies proposed for FLNG: mixed refrigerant (MR) cycles and nitrogen cycles. In general, MR cycles have the advantage of having high theoretical efficiency, while nitrogen cycles have the advantage of not being flammable and having high availability. When producing LNG nearshore from pipeline gas – where fuel gas comes at a price – high liquefaction efficiency may influence project economy more than when producing LNG from your own gas field where slightly higher fuel gas consumption only results in a few months shorter field lifetime.
However, experience from the offshore oil and gas sector shows that factors such as availability and flexibility for feed gas composition changes, as well as increased start-ups and shutdowns, are just as important for the overall project economics. Applying this experience to offshore LNG leads to the consideration of using nitrogen as a refrigerant. The nitrogen cycle is non-hazardous, single-phase, and simple to operate, resulting in optimised availability and higher project net present value. For any FLNG project, availability and safety must be the most important factors, with efficiency a secondary concern.
Sourcing and delivery of refrigerants is also a major consideration when selecting an FLNG liquefaction technology.....
Written by Inga Bettina Waldmann, KANFA Aragon, Norway. Edited by Callum O'Reilly
Read the article online at: https://www.lngindustry.com/special-reports/08062016/safety-at-sea-2569/