.
Our site needs your help!
Site categories

Strategies and Best Practices of Ballast Voyage for Liquefied Gas Carriers

Join Our Telegram (Seaman Community)

Navigate ballast voyage for LNG carriers adeptly with this comprehensive guide, covering heel management strategies, considerations for gas-burning propulsion and reliquefaction plants, and additional insights for smooth sailing. Enhance your ballast voyage operations with expert advice and best practices.

For cargo/deck or related work, the status of the hazard or risk increases when the LNGC is fully laden, so significant maintenance, especially of an invasive nature, should be avoided unless it is critical to the safety of the ship.

Overview of Ballast Voyage

Therefore, although following this guideline may increase the workload on ballast passage, it is a better time to manage non-essential work.

On the ballast passage, the main objectives are:

  • cold conditioning of the cargo tanks, resulting in the best possible cold state on arrival at the load port;
  • maximisation of the use of the LNG BOG from the retained heel, as a supplementary fuel source will help to conserve bunkers;
  • maintenance of cargo tank pressures within design recommendations;
  • completion of any potentially disruptive planned maintenance (PM);
  • with operator approval, completion of any cosmetic maintenance that is in or adjacent to a gas dangerous zone;
  • completion of any E/R maintenance on the LNGC’s power plant that has the potential to cause blackout, loss of way through the water or a delay to the schedule;
  • completion of any alarm or trip tests that could disrupt the schedule.

Heel management

On the ballast voyage, the cold state of the LNGC’s cargo tank will be maintained according to the charterer’s requirements.

Maintaining the cold state allows the LNGC to start Terminal Operations for LNG or LPG Carrier after Arriving in Portloading at the terminal on arrival, reduces time in port and keeps turn-around time to a minimum. It will also determine the loading procedure required at the terminal.

On longer ballast voyages, the lighter fractions of the liquid can evaporate and, the CH4 content will be minimal, with only the heavy fractions (for example, propane, butane, ethane) remaining. Consequently, this liquid mass will have a relatively high saturation temperature and density, which may prevent pumping because of the high ampere loads induced in the pump motor. As a coolant, it also becomes ineffective, carrying a risk of droplet entrainment into the LNGC’s vapour system. On extended ballast voyages, heel ageing is a phenomenon that the operator must consider.

Heel management will depend, primarily, on the type of propulsion and whether the LNGC is to arrive ready to load or not.

Heel management of LNGCs with a gas-burning propulsion system

Heel management of an LNGC with a gas burning propulsion system will be determined by the charterer’s voyage orders. The following items may be considered:

  • intended loading date;
  • expected condition of the cargo tanks on arrival in the loading port;
  • ballast voyage fuel mode.

If a gas only fuel mode is ordered by the charterer, the heel quantity will be determined by the Overview of Alternative Propulsion Systems for the LNG Vesselpropulsion system consumption for the required voyage duration. In addition, sufficient heel will be retained for cargo tank cooldown prior to arrival.

Normal practice will be to take additional heel as a “buffer” in case of delayed loading or adverse weather conditions during the voyage.

The charterer may request that the cargo tanks, on arrival at the loading port, contain sufficient liquid at the bottom of each tank to perform the opening CTMS. This will be at or above minimum accuracy level of the CTMS.

Where LNG is used for pre-cooling of Low Duty Compressor(s) on the Liquefied Natural Gas CarriersLD compressors, such as on DFDE/TFDE ships, additional heel will be required to keep the spray pump running. In this case, the minimum restarting level of the spray pump is also taken into consideration.

Heel for cooldown is estimated through experience of the operator. Cooldown is usually done in a sequence of spraying so the amount of heel will be above the quantity stated in cooldown tables.

Most charterers will also have a limit on the heel quantity remaining at the end of the voyage.

An example of heel management:

  • conventional membrane LNGC of 170 000 m3 cargo tank capacity with DFDE propulsion;
  • duration of voyage is 15 days + 1-day buffer;
  • ship should arrive cold and ready to load;
  • all tanks should contain liquid at or above minimum accuracy level of CTMS;
  • gas only mode to be used;
  • the daily consumption of the LNGC at given speed is 210 m3/day (15 days + 1-day buffer = 3 360 m3);
  • coolant required for ready to load on arrival is 600 m3.

The total heel required from the above calculation is 3 960 m3. As minimum CTMS accuracy level is to be maintained in the loading port, heel will be retained in all four cargo tanks.

  • Cargo Tank No. 1 – 19 m3.
  • Cargo Tank No. 2 – 37 m3.
  • Cargo Tank No. 3 – 37 m3.
  • Cargo Tank No. 4 – 34 m3.
Heel distribution will be as follows:
Cargo Tank No. 1Cargo Tank No. 2Cargo Tank No. 3Cargo Tank No. 4
Volume at minimum accuracy level50 m3100 m3100 m3/
Volume at minimum spray pump restart///450 m3
Voyage BOG304 m3592 m3592 m3544 m3
Coolant600 m3///
Total minimum954 m3692 m3692 m3994 m3
BalanceVoyage requirement – minimum heel from above = 628 m3 (this is divided in tanks where heel is retained for cargo tank cooling and LD pre-cooling)
Balance314 m3//314 m3
Heel retained at end of discharge1 268 m3692 m3692 m31 308 m3
Note: the above example can be useful guidance in calculation. Hovewer, knowledge on heel management is gained mostly through experience of the operator with their particular ships.

Heel management of LNG carriers with a reliquefaction plant

Heel management on LNGCs with a reliquefaction plant is less demanding than on those with a gas burning propulsion system. This is because the reliquefaction plant is continuously running to condense evaporated BOG, giving the operator options for heel distribution and cargo tank cooldown.

Every operator will have a different approach to a ballast voyage where a reliquefaction plant is fitted.

Read also: Liquefied Natural Gas Reliquefaction Plant

There should be sufficient BOG available to run the reliquefaction plant. In addition, even though BOG is not consumed, there is still an efficiency aspect of the ballast passage to consider. The reliquefaction plant should not run on loads higher than required for cargo tank management as this would mean an increased fuel consumption for power generation. An optimum operational practice for such an LNGC is a ballast voyage with heel retained in all cargo tanks, so that the cargo tank bottom is kept cold and less cooldown is required, while sufficient BOG for plant operation is provided.

An advantage of the system is the availability of condensate return, which can be utilised for continuous maintenance of the cargo tank temperature without the need for a spray pump. A disadvantage is the high power demand of some reliquefaction plants, with a subsequent higher fuel consumption.

Whenever the spray pump is in use generated BOG is increased, increasing the need for reliquefaction. However, it is good practice to avoid constant changes in the load and the use of condensate return will provide this.

Further considerations on ballast voyage

Whichever principle of heel distribution is used the limitations of cargo tanks must be considered. Moss type cargo tanks can be loaded to any partial condition, but membrane tanks must be kept outside of their sloshing limits.

Sloshing limits will be clearly stated in the ship’s certificates and documentation and must be strictly complied with.

Author
Author photo - Olga Nesvetailova
Freelancer
Literature
  1. The international group of liquefied natural gas importers (GIIGNL). LNG custody transfer handbook / 6th Edition: 2020-2021.
  2. ©Witherby Publishing Group Ltd. LNG Shipping Knowledge / 3rd Edition: 2008-2020.
  3. CBS Publishers & Distributors Pvt Ltd. Design of LPG and LNG Jetties with Navigation and Risk Analysis / 4th Edition.
  4. NATURAL GAS PROCESSING & ITS ENERGY TRANSITION ROLE: LNG, CNG, LPG & NGL Paperback – Large Print, November 14, 2023.
  5. OCIMF, ICS, SIGTTO & CDI. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases / 1st Edition, 2013.
  6. The Society of International Gas Tanker and Terminal Operators (SIGTTO). Ship/Shore Interface / 1st Edition, 2018.

Footnotes
Sea-Man

Did you find mistake? Highlight and press CTRL+Enter

Май, 17, 2024 67 0
Add a comment


Notes
Text copied