Virtual commissioning of a gas 
handling system

Gas reliquefaction units were designed by means of stationary and dynamic process simulation models

RAINER SCHEURING, Cologne University of Applied Sciences
TORSTEN FELIX, Flemming Automationstechnik
MICHAEL BRODKORB, Honeywell Process Solutions

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Article Summary

Gas cargo such as liquid natural gas, ethane, ethylene and propylene is transported in liquefied form under cooling temperatures and pressures. Typically, pressures are low, which leads to very low temperatures. Although the cargo tanks are well insulated, gas continuously evaporates from the liquid cargo. This gas is called boil-off gas and has to be reliquefied during maritime transport in the reliquefaction system.

The reliquefaction system by TGE Marine Gas Engineering for five new vessels of Navigator Gas shipping company was designed with UniSim Design. First, a stationary simulation model was 
used for the basic design. Later, a dynamic simulation model was developed for the design of the automation and control system.

There are many differences between automation systems that are installed on a vessel and automation systems of a production plant. For instance, it is much more difficult to provide support and maintenance on a vessel than on land. As a consequence, sea based automation systems have to be designed, verified and tested very carefully. The requirements in this field are to some extent similar to the requirements in the automobile and aviation industries.

In order to develop automation systems that are safe, reliable and error free, automobile and aviation industries rely on hardware in the loop simulation. With this technology, the automation system can be verified within a simulation based test environment.

TGE has decided to use hardware in the loop simulation in the design of the automation system of the new cargo handling system, too. This allows not only the development of an error-free automation system but also a substantial reduction in commissioning time. 
Reliquefaction system

About 50% of the existing fleet of liquefied ethylene (LEG) carriers uses TGE designed cascade reliquefaction units to cool down cargo and maintain tank pressure. Liquefied ethylene is usually transported using semi-pressurised, fully refrigerated vessels that are equipped with IMO type C tanks (see Figure 1) and cascade refrigeration systems.

The basic technology of the cascade refrigerant systems have remained unchanged for years. The cargo tanks are cooled by an open cooling system compressing the BOG in a two-stage process and condensing it against a refrigerant (see Figure 2). Two crank, oil-free piston compressors usually serve as cargo compressors. The refrigerant (propylene or R404A in place of R22) is compressed by oil injected screw compressors and condensed against seawater.

To date, the biggest ethylene carriers in operation are 22 000 m³ vessels built by Jiangnan Shipyard for Navigator Gas. A new generation of these vessels with 21 000 m³ capacity is under construction and the first of five vessels are to be delivered in 2014.

The vessels are designed to carry 21 different LPG, chemical gas and chemical cargoes. For 15 of them (including ethylene, propylene, ammonia, VCM and butadiene), refrigeration is provided. The vessels are capable of transporting up to three different grades, two of which may be cooled at the same time. Operating conditions and modes vary widely depending on the specific cargo.

A challenge in the design of the reliquefaction units was to improve performance and operability in order to meet increased requirements for loading rates and cooling down times. In addition, the same reliability and robustness of the units, which have operated successfully for 14 years, had to be maintained. As an example, cooling down capacity for ethylene in one-grade operation is 50% higher for the 
new systems. In order to meet 
all requirements, compressors 
of higher capacity and new 
modes of operation have been introduced.

Dynamic plant simulation and design of automation system
Dynamic Process Simulators like Honeywell UniSim Design, Invensys Dynsym, or AspenTech Aspen Hysys Dynamics are based on so-called first principle process modelling engines that allow realistic modelling of the transient behaviour of processes typically found in the oil and gas and chemical industries. In order to create a process model, the user selects readily available components and thermodynamic packages to define physical properties and phase 
equilibria of the system and then creates a flowsheet by adding and linking generic unit operation models (such as pipes, vessels, pumps and distillation columns) and control equipment (valves, PIDs, and so on). The resulting model can be initialised to a specific initial condition and run through different predefined scenarios as part of a dynamic simulation study.

Dynamic simulation studies are a standard tool in the process industries for analysing and optimising transient process behaviour. Application examples for operability or safety studies include dynamic flare load estimation in 
refineries1 or onshore gas fields,2 and compressor studies.3

In order to design a control concept for the reliquefaction system, a dynamic simulation 
model was developed (see Figure 3).

This model served as the basis for the design of a new control concept that makes it possible to operate the reliquefaction system with all cargoes and all operation modes within a single control structure. It is TGE’s first fully automated cascade refrigerant cycle for LEG carriers. In addition, the reliquefaction system is kept in a stable and stationary operation point independent of any load on the system. In order to ensure that the reliquefaction system operates in a satisfactory manner in all operating conditions, special attention was paid not only to efficiency but also to robustness of the solution.

Hardware-in-the-loop simulation
Hardware-in-the-loop (HIL) simulation is a technique that is used in the product development cycle in which real components interact with simulated components.4 Early 
applications date back to the 1970s.5 Today, HIL simulation has become an integral component in the development process of electronic control units especially in the automotive and aviation industries.4,6 In the basic structure of a HIL simulator, the control unit is a real component and the plant is simulated.

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