Over the last several years, Bureau Veritas has been researching and responding to the challenge of building secure containment systems for LNG carriers, floating LNG vessels (FLNGs) and floating storage and regasification units (FSRUs).

More recently, the French classification society has been focused on LNG bunker tank designs for the increasingly wide range of LNG-fueled ships and LNG bunker vessels—everything from the largest containerships ever ordered to small tugs and specialised ships such as the ice-breaking dual-fuel electric hybrid expedition class cruiseship penned by Ponant at Vard in Norway.

All of these ships have one unique thing in common: a special focus on the containment systems.

There are three main containment options: Type-B, Type-C and membrane tanks.

The groundbreaking 23,000-teu dual-fuel containerships being built for CMA-CGM and the Commandant Charcot for Ponant both feature GTT membrane tanks. Membrane tanks, when used in LNG carriers, are typically required to be operated at less than 10% capacity or at more than 70% of tank height. When tanks are either mostly empty or mostly full, the impact of sloshing is mitigated.

WHAT IS SLOSHING?

Sloshing of LNG is a hydrodynamic phenomenon that can lead to high-magnitude impacts on walls with potential consequences on the containment system response. Sloshing is primarily an issue when LNG tanks are void of internal structure and also occurs in partially filled conditions.

In ultra-large containerships, with their large beams, a partially filled tank spanning the breadth of the ship is potentially subject to heavy sloshing impact in beam seas. One possible option is to select a two-row tank configuration as developed for offshore FLNG units. Additionally, with breadth being constant, tanks that are relatively higher are subject to lower sloshing impacts. However, two-row or multiple tank configurations may entail higher construction costs by requiring additional gas-handling equipment.

With proper assessment, calculation and—if necessary—adjustments to the design of the tank, the risks of sloshing can be effectively addressed. Bureau Veritas has a methodology to assess loads and determine the appropriate design responses needed to build form-fitting fortified containment systems.

At the same time, LNG-fueled ships need to be able to operate when tanks are at various fill levels—including partially filled conditions. As such, sloshing is a critical safety issue that needs to be addressed.

Seakeeping analysis

Initially, the entire range of the ship’s operational loading conditions is ordered in different groups reflecting different operational conditions, such as variations in draft. For example, at a given draft, the worst-loading sloshing condition is that associated with the greatest metacentric height (GM) and the lowest natural roll period.

3-STEP SLOSHING ASSESSMENT & CALCULATION PROCESS
  • Seakeeping analysis calculates the motions of the ship and, more specifically, tank motions
  • Sloshing model tests (carried out by the designer) and computational fluid dynamics (CFD) calculations by Bureau Veritas (both using calculated tank motions) are conducted in order to determine sloshing loads
  • Sloshing loads applied to entire containment system

Coupling effects between liquid motions inside the LNG tank(s) and the ship’s motions need to be taken into account as well. For a tank spanning the full beam of the ship, coupling must be considered as the tank’s natural periods (for all filling levels) are out of the range of the ship’s roll periods and, as such, may require a strengthened cargo containment system.

Sloshing analysis

In addition to sloshing model tests to be submitted by the designer, Bureau Veritas carries out its own computational fluid dynamics (CFD) calculations for sloshing model test verification and to calculate the loads for the inner hull and pump-mast strength assessments. These CFD calculations are complementary to model tests. CFD calculations, by recording all data at each time step, in all cells, provide a total representation of the sloshing impacts on all the tank walls. By applying a ‘Dynamic Probes’ post-processing tool, different designs can easily be compared to each other, as seen in the comparison below.

‘Dynamic Probes’ is a specific post-processing tool applied to CFD computations that enables users to count and store all the sloshing impacts occurring at any time and anywhere on the tank walls. It creates sloshing impact cartographies, as seen in this graphic. A model experiment is only able to measure a sloshing impact where a pressure probe has been located and therefore only gives a partial cartography of sloshing impacts.

Sloshing loads applied

The final step is applying sloshing loads to the entire containment system—including the inner hull (i.e., of the ship) and the pump mast inside the tank—to ensure the design passes the Bureau Veritas strength assessment.