sensors supplement


Measuring wave impacts under extreme conditions


to gain insights into the complex physics inside cargo containment systems (ccs) partially filled with liquefied natural gas (Lng), the dutch research institute mArin has developed a new facility named the Atmosphere. one hundred pressure sensors from Kistler inside the Atm deliver accurate feedback on sloshing wave impacts – a major contribution to improving ccs designs not only for Lng but also, potentially, for hydrogen in the near future.


are both used to transport this fuel from its natural origins to its destination of use, but shipping is the more economical method – especially over longer distances. Liquid gas is carried at temperatures of around -162°C by special-purpose ships known as LNG tankers, sometimes recognisable because of their partly visible spherical tanks. More than 600 of these vessels are already in service throughout the world, and the market is expected to continue growing in the coming years. In terms of design, there is still plenty of scope for optimising the tank geometry of these ships, but this requires groundbreaking research into the fundamental physical mechanisms behind the processes involved. One phenomenon that is not yet fully understood is sloshing – the motion of a liquid (LNG in this case) inside a container due to external forces (such as the ship's movements): and this is where MARIN plays a key role.


N


atural gas is a cornerstone of cleaner and more sustainable energy provision at global level. Pipelines and shipping


AccurAte pressure


meAsurement in demAnding environments


601C piezoelectric pressure sensors from Kistler have been designed with special features for challenging test and measurement applications:


Short rise time, high sensitivity and natural frequency


Optimised membrane to reduce thermal shock


Very wide operating temperature range (-196°C to +350°C)


Pressure range up to 250 bar (3,626 psi)


Variants with charge (PE) or voltage (IEPE) output available


These extremely compact sensors – with a diameter of only 5 mm – can be mounted with different housings, adapters and cables to best fit the application scenario.


The Maritime Research Institute Accurately capturing wave impacts: signal


processing and data acquisition for 100 pressure transducers from Kistler are handled by 25 LabAmp charge amplifiers with automatic


synchronisation at 100 kHz. Source: Kistler group


Netherlands (MARIN) in Wageningen is one of the world's leading marine research centres. Since 1932, MARIN has built up a vast stock of expertise based on a combination of numerical simulations and experiments performed in large-scale facilities comprising various basins for ship model tests, together with on-board measurement and testing. With the backing of a public-private partnership including Dutch universities, industry partners and federal funding, a new globally unique facility was commissioned in 2016. The Atmosphere (ATM) is a giant research facility capable of creating different atmospheric conditions by varying the pressure (0.02-10 bar), temperature


(15-200°C), type of gas (He, N2, SF6 and water vapour), relative humidity (0-100 per cent) and other parameters that


influence the sloshing dynamics in a real LNG tanker. The ATM consists of an autoclave with an outer diameter of 2.5m and a length of 15m – people can actually enter it, but events inside are usually observed through one of the 17 inspection windows.


precise resuLts under vArying environmentAL conditions Rodrigo Ezeta, a researcher at MARIN who holds a PhD in Fluid Mechanics, explains the approach as follows: “The Atmosphere offers full environmental control of temperature and pressure: this is relevant for LNG carriers, which normally operate close to the boundary between liquid and vapour. This gives us a better understanding of the sloshing phenomena that occur in a real container inside a ship.” Inside the autoclave lies a flume where waves can be created artificially. As soon as these waves hit the impact wall, pressure sensors from Kistler record the impact loads with a very high degree of accuracy. 100 of these sensors are installed in a T-shaped array. Ezeta explains: “During the commissioning phase of the facility, we found that this is the best layout to obtain precise impact loads. The upper part of the T-shaped array allows detailed capture of the loads generated by the wave crest, while the lower part of the T registers the effects of the gas pocket that can be ‘trapped’ in between the wave and the wall.” According to Ezeta, research has already shown that these gas pockets – in addition to the loads generated by the wave crests – can have a non- negligible effect, so they could also exert a detrimental influence on the container.


synchronised signALs from A hundred sensors Due to their high natural frequency and short rise time, the pressure sensors from Kistler can capture the highly dynamic sloshing events inside the flume. They also feature an optimised membrane to minimise the thermal shock that occurs when sensor exposure changes suddenly from


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August 2021 Instrumentation Monthly


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