Smooth sailing

There are a lot of choices to be made when planning an LNG facility, and there are many additional decisions when developing a floating LNG (FLNG) facility. First off, for an FLNG facility, the weight of these decisions literally affects the cost of the entire vessel. Once the LNG production target has been determined from commercial considerations, the work begins. Determining the optimal number of trains and train size is very important to the overall equipment weight and layout. The compressor drivers and machinery arrangement decisions are often made in parallel to train size and number of trains.

This article will compare two mixed refrigerant (MR) process options frequently considered for FLNG applications, along with train sizes and machinery arrangements. The impact of these options on the overall equipment count relative to equipment weight and train availability will be discussed.

In all options discussed in this paper, the liquefaction cryogenic heat exchangers that will be considered are coil wound heat exchangers (CWHE). CWHEs provide a significantly smaller footprint and are inherently robust and safe to operate with dual containment, which allows for continued operation even in the unlikely event of a tube leak until a scheduled turnaround. They also provide the ability to economically scale up in capacity without requiring parallel configurations vs the alternative, brazed aluminium heat exchangers (BAHX). Air Products has done extensive marinisation testing and development over many years to meet both the mechanical and process design requirements to account for the effects of motion on CWHEs caused by sea states and continues to improve the equipment design. That has been covered in other papers and will not be discussed here, but is also an important factor for FLNG applications.^1,2,3

The very nature of an FLNG facility means that all equipment is modularised. The number and weight of the modules required for the facility are directly influenced by the count and weight of the bare equipment. Liquefaction processes using MR are the most efficient. Since MR processes use the latent and sensible heat of the refrigerant to remove enthalpy from the natural gas, the relative flow rate of these refrigerants compared to gas expansion processes is much less. Therefore, the refrigeration equipment including the cryogenic heat exchanger(s) and refrigerant compressors are much smaller than those in a gas expansion refrigeration process for the same LNG production.

Two MR processes will be highlighted in this article. The AP-SMR^TM LNG Pro-cess has been proposed for many floating opportunities. The AP-DMR^TM LNG Process is currently in operation off the coast of Mozambique on the Coral Sul FLNG vessel. Both processes are proven and take advantage of the compact, robust, and safe features of CWHEs.

A single MR process

The AP-SMR liquefaction process (Figure 1) is a simple process that combines all the required refrigeration into a single refrigeration compressor and main cryogenic heat exchanger (MCHE), which has the benefit of minimising the total equipment count. However, this low equipment count comes at a cost. All refrigeration duty for the train is accomplished in a single CWHE. As a result, the maximum train size is limited to approximately 1.4 – 1.7 million tpy by the practical limits of refrigerant compression, gas turbine or motor drives for the refrigerant compressor, or construction and shipping of the MCHE. Train sizes greater than approximately 1.7 million tpy are expected to require parallel equipment of one of those three items, which is feasible, but limits the benefits of a compact AP-SMR LNG train. At this train capacity, the refrigerant compressor aerodynamic efficiency is best matched with the high output shaft speeds of aeroderivative gas turbines. If, however, the compressor is driven by a motor, then a gear box or super synchronous motor is needed to increase the rotational speed of the compressor to optimise aerodynamic efficiency, adding to the equipment count and required plot space of the train.

In single MR processes, the refrigerant must contain refrigerant components needed for all three duties: precooling, liquefaction, and subcooling. Although the AP-SMR process has three separators to optimise the refrigerant in each section of the MCHE, the individual duties of the process are still constrained because the entire temperature profile cannot be optimised for a specific cooling curve. This reduces the liquefaction efficiency and increases the power consumption compared to precooled MR processes.

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