Floating liquefied natural gas (FLNG) vessels are processing facilities that float above offshore gas fields. They treat and process natural gas using marine versions of the same technologies found on a land-based LNG plant -- only much more compact – floating LNGs are approximately 1/4 the size for the same LNG output. They offer operators the ability to process gas at or very close to the source field. The value of FLNGs is that they can tap into smaller and more remote fields. When a resource is exhausted, they can be unmoored and reconfigured for a new feed gas composition range and/or consumer non methane component specification and can be moved to another location to continue operations.
Offshore operation underscores the importance of maintaining the reliability of equipment on these vessels for extended periods. Operational shutdown of a key piece of equipment, such as a gas turbine (GT), can cost millions of dollars per day in lost production. Limited personnel and access to spare parts could mean further delays. Continue reading to learn more about the benefits and challenges of FNLGs as well as recommendations to ensure dependable GT operation.
The benefits of operating and processing gas offshore, at source, using a nonpermanent structure are clear.
- There is no costly subsea piping or compression stations required to move the gas to a shore-based facility for processing.
- No need to reclaim or develop a suitable coastal site for processing, storage, and transmission infrastructure, including harbor facilities. This helps to alleviate environmental concerns and complexities with local community groups. It can also accelerate the permitting processes and reduce time to first production.
- FLNGs facilities are more compact than land-based plants.
Space is the biggest challenge facing FLNG engineers. LNG production requires a large amount of specialized equipment, including pretreatment systems, gas turbines, compressors, expanders, head exchangers, etc. Storage space for the LNG in its super cooled stage and natural gas liquids (NGLs) is also necessary. On top of that, all the systems and facilities needed for the ship and crew must be considered.
In contrast to land-based sites where designs may be considered as field proven, layout optimizations, design experiences, and engineering best practices with regards to implementing FLNG production are still relatively new and are evolving. To date, only a handful of FLNG production vessels have been commissioned - Petronas’ FLNG Satu and Dua, Golar’s Hilli Episeyo, Exmar’s Tango FLNG, and Shell’s Prelude. Projects currently under construction include Golar’s Gimi and ENl’s Coral South.
The importance of maintaining equipment
The refrigerant compressors hold the key to maximizing production. In most cases, refrigerant compressors are mechanically driven (rotated) by gas turbines. That said, the reliability of the GT becomes equally critical to production. Aeroderivative GTs are preferred as refrigerant compressor drivers over frame engines because they are smaller and lighter and have components that are quick and easy to interchange, making maintenance easier. They are also designed to offer high reliability, and can be quickly ramped up and down, allowing for any forced interruptions caused by adverse weather conditions to not have a prolonged effect on
Gas turbine air intake system
One key piece of GT equipment is the GT combustion air intake system. GTs take in huge amounts of air as part of their combustion process. Air that is left untreated contains several destructive contaminants, which can cause serious damage, erosion, corrosion, and fouling of the precision engineered GT internals. The harsh weather conditions found in offshore environments are particularly brutal on any piece of equipment, let alone one that needs to run continuously.
Gas turbine filtration requirements
Issues relating to the ingestion of ambient air particulate, salts and hydrocarbons account
for 60 - 80% of overall gas turbine losses. Controlling these contaminants with the right air intake filtration is a huge step in assuring reliable plant operations and maximizing LNG output for extended intervals.
Salt is particularly damaging to the GTs on FLNGs because there is such a large quantity of it churned up from the sea. While the filterhouse and internals are typically manufactured from 304 or 316 grade stainless steel, sodium from sea salt (NaCl), if allowed to get downstream of the filters, will combine with sulfur in the fuel to create sodium sulfate (Na2SO4). This chemical reacts with the base metal of the turbine blades in the high temperatures of the hot gas path, causing rapid corrosion and component failures. This is a common effect known as hot corrosion or sulfidation. Chlorine in the salt also acts as a pitting corrosion initiator in colder parts of the turbine, potentially leading to catastrophic damage.
Because of its hygroscopic nature, salt can be difficult to control. It readily absorbs water and can easily move from solid to liquid form with changes in ambient relative humidity.
Filtration recommendations to defend against salt contamination:
- Allow for input concentration, aerosol size distribution, and aerosol physical state – whether droplet or particle.
- Consider the impacts of increased pressure drop introduced by adding multiple filtration stages with salt protection.
- To prevent salt from traveling downstream to the GT on an FLNG production vessel, using hydrophobic filtration is essential.
Sand and dust
Sand and dust can cause numerous issues for an installation in terms of both damage to machinery and degradation of turbine performance. Large dust particles greater than 2μm in size can cause erosion and affect turbine efficiency. If the erosion causes parts in the front end of the equipment to fail, contaminants may travel through and cause severe machine damage.
Finer dust can stick to parts of the machine and change the operating aerodynamics. This, in turn, reduces turbine efficiency, requires online and eventually offline water washing, reduces availability, and increases operational costs. Moisture in the inlet air stream can combine with dust to form mud which can block a filter.
Filtration best practices for sand and dust:
- Consider using efficient particulate arrestance (EPA) rated filters as they are designed to stop the very finest of particulate passing downstream to the GT.
- Filter media selection is a critical for reliable operations. Glass fibre media is the preferred choice for high efficiency hydrophobic filtration. It is thick compared to other technologies, and this greater pore volume makes it naturally less prone to sudden blockages. This means that very high efficiency filtration can be achieved without the risk of alarm/shutdown triggers from sudden pressure spikes, and it provides the capability of much longer and more predictable running between maintenance intervals.
Turbine air inlet filtration system design considerations
There are several factors that should be considered in the design of an FLNG turbine air inlet system:
- Size and weight of the system.
- To maximize GT availability and uptime, the system needs to enable online changing of prefilters.
- The increased air velocity and compact nature of filter systems on FLNGs means the aerodynamics and air turbulence created as the air flows through the filtration and acoustic systems need to be carefully modelled and optimized to create smooth, laminar air flows and to meet the GT air inlet distortion limits as defined by the GT OEM.
- Operators also need to select a filtration solution that can accommodate multiple filter options without mechanical changes – one that easily adapts to new filtration requirements depending on the location of the vessel and seasonal variations in concentration and size distribution of the airborne challenge.
To ensure profitability, the reliability of the systems used to liquefy gas onboard an FLNG vessel is critical and, although GT filtration systems may seem like a smaller part of the overall puzzle, they are vital to ensuring ongoing smooth operations. GT air intake systems need to be designed for the real-world environment in which they will be used.
They must also be able to effectively and efficiently handle a diverse range of seasonally varying contaminants such as salt, dust, oily hydrocarbons, and moisture. For FLNG vessels, it is essential that these systems are physically compact and flexible enough to allow operators to easily change filter types depending on location. Designing GT air intake solutions for the offshore environment requires a thorough understanding of the very specific challenges such systems will face, but when undertaken correctly offers operators a rapid return on investment.
This post was contributed by Pete McGuigan, global LNG market manager, Parker Gas Turbine Filtration Division, Parker Hannifin Ltd, UK.
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