Striking a Balance
Published by Jessica Casey,
Editor
LNG Industry,
There is a major push in the LNG industry to minimise bespoke facility designs in favour of standardised offerings in order to achieve cost and schedule improvements, particularly for smaller plants where speed to market is paramount. While it is nearly impossible to apply entire facility designs to different areas of the world with different feed gas conditions, various pre-engineered components within those designs can be pieced together to meet the needs of a specific project. By utilising past and current project experience, along with the development of philosophies consistent across projects, standardisation can be realised, in some cases, to a great extent.
But what are projects sacrificing when using ‘off-the-shelf’ designs? Do the short-term gains turn into long-term success? There is not a common answer to these questions; determining the proper path will be project-specific, and can be both technically and commercially complex.
What is a ‘standard’ LNG plant?
Unless a true parallel facility is being built – same location, same feed gas, same capacity, etc. – there will not be a common solution from the pipeline to the tank. Even small variations in site/ambient conditions and feed gas pressure or gas composition can amount to a poor fit and sub-optimal plant performance if applying the same design to different scenarios. But, when a large library of designs for typical unit operations are available from an experienced licensor and contractor, suitable combinations can be found that keep costs down and efficiency up, all while accelerating the timeline to first LNG.
Many developers are now viewing LNG solutions as products, particularly in the small scale market, but this ideology is also becoming popular for mid scale LNG. The difficulty in providing such an LNG plant is that it is made up of many products from multiple suppliers that must be engineered to fit together and operate seamlessly. In order to achieve a cohesive final product that can be duplicated and offered to multiple customers in multiple locations, a basis must be set that balances flexibility for the variations discussed with reasonable cost and efficiency. The schedule savings of a standardised LNG unit can be undermined if only a small portion of gas conditions can be treated and liquefied. Conversely, if the design tries to accommodate too wide of a range, efficiency and operability can be reduced and the cost will inflate due to oversized equipment or highly complex flowsheets.
Liquefaction and refrigeration
The refrigeration train is the heart of the LNG plant, but it is actually the simplest unit to standardise and apply across a wide range of conditions. Because it supplies indirect heat transfer, the same duty can effectively be applied to any feed gas in the main cryogenic heat exchanger. When a single mixed refrigerant technology, such as PRICO®, is utilised, the ability to customise the refrigerant composition across the entire cooling curve further enables this flexibility to handle a wide range of conditions while maintaining high efficiency.
The key components in a liquefaction train – the refrigerant compressor/driver, and main cryogenic heat exchanger (MCHE) – can and should be custom-designed for each project while all the interconnecting components – piping, valves, vessels, coolers, etc. – can be mostly copied from project to project. By designing the refrigerant compressor specifically for the project, the best efficiency point will be provided; applying a common centrifugal compressor design for various temperatures and flowrates could render the unit unable to provide proper pressure, or even be permanently operating in recycle. Also, decoupling the compressor from the fixed equipment design allows for optionality in driver, both in size and either as a gas turbine or motor. In a truly bespoke design scenario, all the equipment would be sized for a specific operating scenario. In a standardised design, using this philosophy for the fixed equipment will minimise the cost of over-sizing or the efficiency debit from under-sizing.
The MCHE, commonly a cold box containing brazed aluminium cores for small and mid scale plants, should likewise be tailored to the specific project. This is obvious if heavies removal is integrated within the liquefaction train; however, even just a difference in feed gas path design pressure can impact the core size and fin selections, and thus the efficiency of the refrigeration process along with it. The mitigating factor in terms of schedule for semi-bespoke design of these two long-lead items is that they are the first pieces to be specified by the licensor and can be purchased even before the full release of project work. Detailed engineering is then integrated around them.
This is an abridged version of an article that was originally published in the December 2022 issue of LNG Industry. The full version can be read here.
Read the article online at: https://www.lngindustry.com/special-reports/29122022/striking-a-balance/
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