Impact of boil-off gas in LNG transfer

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Evert van Bokhorst, TNO Technical Science, Delft, The Netherlands

Integrity and efficiency in LNG transfer

Caused by the global increase of demand for natural gas the interest in offshore liquefaction, transport and re-gasification grows rapidly. The installations that are presently developed are in essence copies of the onshore systems that are already many years in operation. However, the fact that the installation is installed on a ship imposes limitations and leads to special system requirements. An important issue is that offshore offloading systems, in contrast to ship-to-shore offloading, have to function under harsh sea conditions. Systems should be flexible to deal with the ships moving relative to each other and extreme cases may require instantaneous disconnection. Besides, the offloading system has to transfer LNG at the highest possible rate, which requires that the equipment has to withstand high flow velocities and pressures.

At present ship-to-ship transfer systems are being developed that apply loading arms or flexible pipes and hoses. Such systems have already been successfully operated in a number of cases. A question that remains appropriate continuously is whether the integrity of such a system can be guaranteed under all circumstances and in all configurations. This is of particular relevance in corrugated hose/pipe systems, where due to the inner profile local pressure and velocity fluctuations occur near the hose/pipe wall, a potential cause for bubbles to form. These vapor bubbles can affect the flow behavior and impose operational complications. The research presented enables development of  tools for the prediction of safe operation conditions and optimization of the LNG transfer and transport systems.

Even more so than in the established onshore LNG industry, the physical behavior of LNG at the storage and transport conditions plays an important role in the design of offshore (floating) LNG installations and transfer systems. LNG is stored and transported at near to boiling conditions at -162 deg. C and low pressure (around atmospheric) and pressure and temperature fluctuations can cause vapor (gas bubbles) to develop.

The research described enables development of tools for the prediction of safe operation conditions and optimization of the LNG storage and transport components. The results will contribute to efficiency and safety in LNG transfer and processing by:

  • accurately predicting the pressure loss across the transfer system;
  • increasing the operating envelope for safe and undisturbed operation;
  • restricting the duration of the LNG transfer operation;
  • extending the lifetime of LNG multi-composite hoses and other transfer components;
  • enabling accurate in-line custody transfer flow measurements;

Pressure loss should be minimized

Next to fatigue and damage tolerance testing flow tests on multi-composite hoses have been carried out, both at ambient (water) and cryogenic (LNG) conditions. These tests are part of the qualification process according to the code EN1474-2,which presents guidelines for qualification, but does not specify details of testing or allowable levels with respect to mechanical vibrations, pressure pulsations or pressure loss.

The limitation with respect to pressure loss is necessary to prevent that the internal pressure downstream of the hose is above the vapor pressure so that no cavitation can occur. Next to the flow rate the pressure loss across the hose also depends on:

  • internal pressure (resulting in elongation of the hose)
  • overall length
  • (un)loading configuration e.g. corresponding shape of the hose
  • hose roughness, pitch and profile
  • impact of boil-off gas

Impact of boil gas on LNG transfer

In addition to the tests at ambient conditions LNG flow tests have been performed on the same 8-inch hose at a  LNG Test Facility with similar flow conditions with respect to pressure and flow rates.

It is concluded that possibly local boil off gas formation in the hose may result in large deviations of the pressure loss as a function of the hose inlet pressure.

While flowing, the temperature of LNG near boiling conditions may increase as a result of energy input via the pumps, and as result of heat influx via the wall of cryogenic piping and hoses. The required transfer rates in LNG Offshore transfer are high due to economical reasons, resulting in considerable flow velocities in transfer hoses/piping up to 10 m/s, whilst usual flow velocities in processing liquids are typically between 1 and 4 m/s only.

A high flow velocity in the hose results in high turbulence levels and possible vortex shedding downstream of the inner wire/corrugation in the transfer hose/pipe. This effect, in combination with the liquid near boiling conditions, may result in gas formation in the corrugated hose/pipe, which has a considerable impact on the pressure loss across the hose. It could also result in cavitation at the downstream end of the hose, where the average pressure is low and close to vapor pressure. Implosion of gas bubbles may result in an increased internal wear and damage of the cryogenic hose, which should be prevented under all circumstances.

The impact of boil-off gas is also important in relation to the propagation of fast transients in LNG transfer due to start-up, (emergency) shutdown, flow control and leakage. Like in onshore LNG terminals for loading and offloading of ships also LNG Offshore transfer systems should be provided with emergency shutdown valves (ESD) and emergency release systems (ERS). Fast closure of ESD valves in combination with upstream and downstream piping to pumps and storage tanks may result in high pressure surges affecting piping, seals and flexible cryogenic hoses. The combination of a high transfer rate, which means a high flow velocity and a fast closing ESD may result in high amplitude shock waves.

Further research required

Fundamental questions, which need to be answered are:

  • When do vapor bubbles occur in a liquid near boiling conditions and at which location in a pipe flow and in a corrugated hose?
  • What is the impact of the internal corrugation and the hose roughness on vapor bubble occurrence?
  • What are the physical properties of the vapor bubbles, like size, distribution, and deformability, depending on local flow conditions?
  • What is the impact of vapor bubbles and gas content locally in the corrugated hose on flow resistance?
  • What are the local flow conditions that stimulate vapor bubble implosion?

A combined numerical/experimental investigation will be carried out on the effect of boiling in pipe flow with both smooth and rough walls. The project, which is still open for new participants, builds on existing expertise in observing gas bubbles in wall turbulence (as applied in investigating drag reduction with gas bubbles for ships) and in investigating multiphase flow in pipes.

Evert van Bokhorst will be presenting Impact of boil-off gas in LNG transfer at the Cryogenics stream at Gastech 2012 Exhibition’s Centres of Technical Excellence (CoTEs). To learn more, register here for this free seminar on 10 October 2012 at the ExCeL centre in London.