Simulating a Conceptual FLNG in Waves

Some 475 kilometers off the western coast of Australia, Prelude FLNG, the world's first floating liquefied natural gas platform, is about to revolutionize the way natural gas is produced. As the largest offshore facility ever constructed, Prelude FLNG boasts a length of 488 meters, a width of 74 meters and weighs around 600,000 tons.

Still in its early days, the FLNG technology will allow the freshly extracted natural gas to be processed and stored aboard, before being loaded onto LNG tankers, thereby permitting the exploitation of offshore resources that had been too costly or difficult to develop otherwise. 

In a scientific study undertaken by the Australian Maritime College (AMC) – a specialist institute at the University of Tasmania in Launceston, Australia that focuses on seafaring and maritime engineering – numerical simulation was used to investigate how various wave scenarios will affect the motions and operations of such a facility. The computations were performed using STAR-CCM+.

The Prelude FLNG project, initiated by a consortium in which the energy group Royal Dutch Shell is the majority shareholder, is the first of its kind. In principle, the FLNG processing units are similar to the FPSO facilities used for oil extraction, although Prelude FLNG will work on a much bigger scale. 

The natural gas produced at the field will be cooled to -162°C, at which temperature it turns into a liquid and its volume is reduced by a factor of 600. The liquefied gas can then easily be stored in tanks and loaded onto LNG tankers for onward transportation.

To reach the full potential of this technology, it must be ensured that in extremely adverse weather conditions, such as storms and heavy seas the ship's structure is able to withstand the enormous strains that arise and it is possible to maintain operations with as little disruption as possible, including the docking and loading of the LNG tankers.

In order to gain a detailed knowledge of the conditions to be expected and ensure undisrupted operation, the AMC has analyzed, in a scientific project involving numerical simulation and experimental validation, how such gigantic FLNG facilities behave at sea.

The project

The three-year research project started in March 2014. The initial phase, which has now been completed, consisted of investigating the influence of different wave frequencies on the motion response of a conceptual FLNG unit.

In the second phase, which is still in progress, the primary focus is on operational aspects of the facility, specifically on the interactions between an FLNG facility and the much smaller LNG tankers and supply ships during approach and mooring. These include the emergence of frequencies causing pitching and rolling movements, and undesired resonance waves.

The project is conducted by Yuting Jin, who currently is a Ph.D. candidate at AMC. His intent is to provide specific information to help with the development of the following target areas:
?    Planning: determine design configurations suitable for critical conditions;
?    Operation: establish efficient procedures for safe operations;
?    Crew training: enable precise and practical crew training.

CFD simulations at AMC Search

The AMC specializes in shipping and maritime engineering. The institute has an extensive range of testing equipment, including a 100 meter long towing tank, a circulating water tank, a cavitation tunnel and a 12x35 meter model test basin. Also, it has access to a computing capacity of over 1,500 cores.

AMC Search, the commercial arm of the institute, has been making the acquired knowledge and the developed techniques from research and experimental testing available to the maritime industry in Australia, New Zealand and across the world for over 30 years. 

Dr Max Haase is responsible for implementing CFD simulations into commercial projects. He states that in recent years, CFD has played an increasingly important role, due to more sophisticated requirements in performance evaluation and design optimization which cannot be achieved by model testing in a timely and cost-effective way. At AMC Search, STAR-CCM+ is a popular choice for CFD studies due to its versatile simulation capabilities, its user-friendliness, and its computational speed.

Towing tank versus simulation 

Towing tanks have been an indispensable tool for ship design, optimization and performance assessment for over 150 years. Over time, procedures used have proved their value and achieved a high degree of accuracy. However, model testing is typically not available until a late development stage, when design and construction are well underway. In addition, the construction and alteration of the prescribed scale models can be both time-consuming and expensive. 

Overall, the flexibility and ability to innovate as required in today’s development cycles is clearly limited by the use of towing tanks only. Furthermore, they are limited to scale models that are significantly smaller when compared to the full-scale device, potentially restricting the ability to investigate innovative designs.

As a result, a growing number of engineers are turning towards numerical simulation in order to assess complex systems at a much earlier stage of the design process. Simulation software, such as STAR-CCM+, has been proven to be as accurate as towing tank tests, and given realistic assumptions, allow ships and offshore platforms to be simulated at full scale, thereby eliminating some important uncertainties introduced by the scaling process. Scale model testing remains relevant in terms of not only demonstrating software robustness, but also the validity of assumptions relied upon in carrying out various design investigations. 

Analysis

The dimensions of the computational domain for the full-scale calculations were 3,000 x 800 meters. For these calculations, meshes from four to 12 million cells were used depending on the wave frequency being investigated. A total of 40 calculations were performed. The calculations required around 700 hours using between 48 and 64 cores. Water depths of between 80 and 800 meters were simulated in order to assess the shallow water effects that may occur during towing tank tests and lead to inaccuracies.

The following STAR-CCM+ features were used:
?    Overset mesh: The overset mesh capability permitted easy positioning of the LNG tanker in the vicinity of the FLNG unit, for example to analyze the effects of approach and mooring (for example, resonance waves).
?    Motion model: The dynamic fluid-body interaction (DFBI) model was used in order to account for the coupling between waves and ship movement.
?    Wave model: The non-linear Stokes 5th order wave model was chosen for its accurate representation of wave propagation in open water. The wave height, set to 4 meters, was determined using BMT Global Wave Statistics for the sea area of interest. Particular attention was paid to wave damping in order to avoid unwanted wave reflection.
?    VOF model: The Volume of Fluid (VOF) multiphase model was used in order to correctly capture the interface between water and air, and accurately depict the interaction between the hull and the free surface.

The simulations revealed that:
 * The wake from the FLNG overlays the ocean waves and forms a relatively calm area;
 * With high frequency ocean waves, steep waves (with deep troughs and sharp crests) are formed around the FLNG.

Future investigations will look at how the berthing of LNG tankers and supply ships will affect this configuration, in particular how to avoid resonance waves between the different hulls, how to control the pitching movement of the ships involved, and whether regulations need to be adopted to make the operation safe.

The comparison between simulation and model test results at a model to full-scale ratio of 1:100 shows an excellent agreement over the entire frequency range. It highlights the impact of a limited water depth especially on the pitching movement of the FLNG for waves of low frequencies.

Conclusion

This study highlighted how the use of CFD simulations can help engineers make decisions concerning not only hull design and layout configurations, but also ship operations.

At AMC Search, these results will be used to develop recommendations and operating guidelines for three target areas: planning, operations and crew training. For Haase, this project is valuable for another reason: "Increasing attention is being paid to CFD technology in the hitherto rather conservative maritime field. Nevertheless, compared with Europe where there are a number of model test basins, organizations and service providers with comparable interests, CFD has not yet unfolded its considerable potential for the maritime industry in Australia. We believe that with this project we have demonstrated the capability and scope of CFD simulations and have achieved an important milestone in the establishment of this method for maritime applications."

Acknowledgements

The project has been initiated and supervised by A/Prof Shuhong Chai, Prof Neil Bose, Dr Jonathan Duffy and Dr Chris Chin at the Australian Maritime College. All research services are also available for consultancy project through AMC Search.