Digital solutions for sloshing management for LNG vessels

Apr 29, 2023

Ascenz Marorka | Jun 05, 2023

Introduction

GTT cargo containment system (CCS) is directly fitted on the inner hull of the ship. LNG is directly in contact with the insulation system. When, a tank is not 100% filled, the liquid inside it can move. The liquid impacts on insulation system resulting from the liquid motion is called Sloshing.

Sloshing description

Sloshing can be described globally and locally. The local behavior depends on local factors like escape of gas, compression of gas, condensation, shock waves, fluid/structure interaction, free surface instabilities… These factors are hard to predict in a deterministic manner. This is the reason why, sloshing is considered as a stochastic phenomenon that is modelled in a probabilistic manner.

The local effects induce a high variability of loads.

The global behavior depends on environmental factors (mainly sea-states), the ship design (ship Geometry, ship seakeeping, cargo tank geometry, tank position…) and operational factors (ship loading characteristics, tanks filling level, ship/wave incidence, ship speed…)

Sloshing effects

The direct mechanical effect of sloshing is the impacts on the cargo containment system. This leads to local loads against the primary membrane which cause banging noises and increases the probability of high pressure impacts. Another mechanical effect of sloshing is the drag forces it induces on the pump tower.

In addition, severe sloshing has also thermodynamic effects as it increase boil off gas and the pressure rise.

Sloshing management using digital tools

Crews manage sloshing based on heuristic indicators (e.g. banging noises) and habits or beliefs with uneven results.

Thanks to GTT unique expertise in Sloshing, Ascenz Marorka provides innovative digital solutions to manage sloshing in an effective manner with tangible benefits.

Sloshing impacts induce vibrations of the tank structure which feature a recognizable "signature". By adding sensors around the tanks, SloshieldTM constantly monitors the vibrations of each tank's structure to insulate these signatures from the rest of the signal. This allows the crew to identify the sloshing activity and its evolution in time. In fact, by detecting the early signs preceding the establishment of a severe sloshing situation, the crew can mitigate the effects by changing the speed or the course of the ship for example.

Unloading or loading cargo can sometimes be jeopardised by a sudden external risk like harsh weather conditions or an issue at the receiving/loading party. The potential sloshing negative effect is high when the tanks are in partial filling conditions. The Emergency Departure tool helps the crew reach a safe filling levels situation by suggesting tank-to-tank transfer sequences.

The sequence can optimize either minimize the time spent in an unsafe configuration or optimize the autonomy of the vessel.

GTT's Sloshing Virtual Sensor characterizes the liquid motion activity from the weather forecast. This was made possible thanks to hybrid models trained on GTT unique sloshing database.

The two main characteristics estimated are the sloshing frequency and the sloshing pressure. The combination of both helps derive the sloshing intensity.

Conclusion

LNG carriers’ membrane tanks are designed to support the sloshing load in all conditions. However, proper management of the sloshing using digital tools help optimize the maintenance cycles, the boil of gas and reduce the risk in emergency situations.

About Ascenz Marorka

With more than 1,300 vessels equipped, Ascenz Marorka, a GTT brand, is a leading provider of digital solutions for Smart Ships in the maritime industry, offering the one of the most comprehensive, innovative and reliable digital platform for ship owners and charterers around the world. Ascenz Marorka portfolio comprises solutions for LNG Cargo Management, Electronic Fuel Monitoring, Vessel Performance Management, Emissions Monitoring, Operational and Regulatory Reporting, Weather Routing… More details on the company and the products at: https://ascenzmarorka.com

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