Rollover Risks of LNG

The phenomenon of rollover is a long-standing risk associated with liquid natural gas (LNG). The consequences can be severe.

Although more associated with shore-based storage, LNG rollover is a potentially dangerous issue for sea based storage units, both as a cargo or as a marine fuel.

To date, this thermodynamic effect has generally not been evident on LNG vessels. But as trading and chartering patterns shift, it is important we understand not only what rollover is, but also how to detect it and what can be done to mitigate the risks.

What is Rollover?

Fundamentally, a rollover can only take place if there are two separate layers – this is known as stratification. For stratification to occur, two elements must be in place:

1. There must be a retained volume of LNG (or heel) within the tank; and
2. There must be a difference in the densities of the LNG being loaded and the retained LNG in the tank.

Stratification is more likely to occur if the density of the loaded LNG is greater than the density of the retained heel. The filling line on an LNG vessel is usually designed with the outflow at the bottom of the tank. Therefore, heavier LNG will remain at the bottom and the lighter heel will sit on top.

This has the effect of creating two separate and distinct layers within the tank space.

Over a period of time heat will be absorbed into the both layers. The upper layer will evaporate (or “boil off”) in a similar manner to what would be expected under normal circumstances. As the surface evaporates there will be an increase in the surface density due to cooling. The surface liquid will then sink and a convection cell will be created within this layer. Subsequently, the amount of or boil-off gas generated from the surface will reduce noticeably – perhaps as much as 10%.

The lower layer will also experience a heat gain and therefore become warmer. But as it is subject to a static pressure exerted by the upper layer, it can’t physically evaporate or boil. It essentially becomes superheated, existing in a liquid state when at a temperature above its natural boiling point. The resulting lack of evaporation removes the cooling effect that is normally evident at the surface and so the cargo will continue to warm up. As this lower layer warms the density decreases.

As the densities of the two layers equalise, the static pressure that had been exerted by the top layer is overcome. The lower layer rises through the top layer towards the surface. The LNG loses its superheat and boils rapidly, producing a large volume of vapour.

Preventing Rollover

So what can a vessel do to prevent this from happening? Society of International Gas Tanker and Terminal Operators (SIGTTO) has produced guidance for the prevention of rollovers in LNG ships. They suggest adopting the following procedure when loading higher density LNG cargoes with a large volume of lower density heel whilst alongside a terminal:

1. Consolidate heel into one tank
2. Partially load a second tank to a level such that there is room to transfer into the tank the entire heel.
3. Close manifold liquid values but keep vapour manifold open
4. Transfer heel into partially filled tank as quickly as vapour pressure allows
5. Do not load any further LNG into the tank containing mix
6. Complete loading other tanks in normal way

It is important to note that this operation should only be carried out in close cooperation with shore facility. Large volumes of vapour could be generated and it must be carefully managed.

There are occasions where either the density of the LNG being delivered is not known or has changed from the original analysis. Such cases can exist where LNG has weathered (or aged) or if transhipped and the original certificate of quality is not up-to-date.

So what can a vessel do if it is in the unfortunate situation of loading a heavier LNG under a large volume of retained heel and the risk of a rollover is assessed to be high?

SIGTTO advises that the safest and surest method to prevent a rollover in this situation is to discharge all LNG as soon as possible in into a shore receiving tank with appropriate mixing arrangements. This might be an effective solution but it can have huge commercial and operational implications.

Spotting the Signs

Prevention is always the best tool, but the ability to detect that stratification has occurred and a risk of rollover exits is the next best thing. Understanding a developing situation and proper preparation allows the risk to be managed before it becomes too great a problem.

Two very simple signs that may indicate that stratification has occurred and a risk of rollover exists are:

1. A reduction in the normal boil-off from a tank
2. Increased temperature readings in the lower levels of a tank while the upper readings are largely constant

Suspect Rollover?

If a vessel has identified the potential for a rollover to happen then it is important they put steps in place mitigate the risk.

These steps may include:

• Increasing gas flow to the engine from the affected tank – the increased consumption should result in reduced tank pressure and create a buffer in case of rapid boil-off
• Using a spray pump in an attempt to “mix” the layers internally within the tank

Changing Times

Historically, rollovers on seagoing vessels have been very rare events. But as the nature of the LNG market changes, we may see vessels being asked to retain larger volumes of heel or load multiple grades from multiple ports or load “weathered” LNG via transhipment operations or when bunkering.

All these scenarios could result in heavier LNG being loaded under a lighter heel which ultimately could lead to a rollover.

Further details can be sought from North’s dedicated LNG team,  whose experience include Master Mariners, commercial operators, LNG project management, technical superintendency and technical operations.

Author: Rod Maclennan
Executive (Loss Prevention)