Like many fuels, liquid natural gas (LNG) is potentially hazardous and to help reduce the possibility of an incident during berthing or when a vessel is alongside a jetty, integrated mooring systems are becoming standard specification. These incorporate advanced software focused on remote operation and second by second recording of events during berthing and while the vessel is moored.

The growth in LNG terminals and shipping traffic to service these has been the subject of considerable public scrutiny. Although the industry has a high safety record, LNG is viewed as hazardous because it is inherently volatile. LNG carriers are specially designed to minimise any risk of spillage, having double hull construction and many other safety features. The highest operational risks are typically during approach, berthing, mooring, loading or unloading. Considering that future LNG carriers will have capacities above 215000cubic metres, it is vital for operators to understand the external effects imposed on the vessel from the sea and weather and once berthed, the interaction between the vessel and jetty needs to be monitored.

A leader in this field is Australian company, Trelleborg Harbour Marine, an operating unit of Trelleborg Engineered Systems, who has supplied equipment to approximately 70percent of LNG terminals over the past three years.

“Mooring and monitoring systems for LNG marine terminals have undergone exciting and significant development,” says Tom Toth, the company’s Technical Director. “With the advantages offered in terms of safety and efficiency, their use will undoubtedly be wide spread. The safety record of LNG carriers is excellent. However, it is important when planning and upgrading LNG facilities, that they are equipped to ensure that a 100percent safety expectation remains a reality.”

The mooring equipment and monitoring systems now being specified on state-of-the-art LNG jetties typically include the following:

o Quick release hooks.

o Remote release systems.

o Vessel docking aid systems.

o Mooring line load Monitoring.

o Environmental and oceanographic monitoring.

Recommended by both SIGTTO and OCIMF, the foundation of any hazardous jetty mooring system is the Quick Release Hook (QRH). These provide the point of restraint between vessel and jetty, allowing release of the vessel without the need to manhandle the large mooring lines while in an emergency they enable release, even under tension. Located along the jetty at each dolphin (mooring point), the QRH are generally in a combination of double, triple or quadruple hook unit configurations.

“Hook capacity and configuration are determined from a detailed mooring study,” comments Tom. “This should carefully consider the range of vessels likely to visit the facility over the life of the project. QRH specifications are becoming more rigorous to give higher levels of operator safety, improved reliability, better functionality and minimised downtime. Hook capacity is also increasing to accommodate larger vessel designs and 150TonnesSWL is now quite typical.”

Most LNG and condensate facilities include a QRH remote release system. This enables the operator to release each hook from a remote, safe location, using either a console or optional ‘virtual’ panel. This minimises the risk of injury to personnel who are not required in the ‘risk zone’. It also increases productivity, as fewer personnel are required to release a vessel.

“A growing trend is to physically locate the remote release control console some distance away from the jetty. This is a safety requirement that allows remote release well away from the jetty if normal release procedures cannot be undertaken due to unsafe conditions,” continues Tom.

The recommended solution to ensure maximum effectiveness of the remote release is to have the following release options:

o Local manual release override at the hook totally independent of the remote release.

o Local push button release at a safe distance from the hook unit.

o A jetty located release panel for normal operations.

o An emergency release panel in a building separated from the jetty.

“Importantly, release panels should allow hooks to be released and their status monitored individually. Simultaneous release is still often specified though it is not recommended by many major LNG operators due to the inherent danger of released lines fouling propellers of carriers or standby tugs. Amazingly the use of ‘break bolts’ or an automated release operation is also commonly specified. This should be avoided as it is very dangerous, potentially resulting in a sudden, uncontrolled release.”

The Berthing or Docking Aid System (DAS) is used by the Pilot and Vessel Master to assist in manoeuvering the vessel towards the jetty during the last 200meters/ 218 yards of its approach. A typical DAS consists of two laser sensors, a controller, central PC with software and remote display devices such as a display board, for each berth. The docking system measures vessel distance, angle and speed of approach using lasers mounted on the jetty. The data is displayed and logged in the marine monitoring PC in the control room and also made available to the Pilot on a jetty mounted large display board and/or over telemetry to a handheld monitor.

As vessel size increases, berthing dynamics will also change. A DAS provides the data necessary to enable personnel to clearly follow agreed procedures, for example keeping vessel docking speed within acceptable limits. A good quality laser Docking Aid System will provide second by second data logging and have the capacity to produce scalable distance, speed, time and vessel angle graphical relationships which can be interpreted to ‘paint a picture’ of the complete docking event.

“No two dockings are identical and the majority of dockings will be event free,” says Tom. “However, there are occasions, either during manoeuvering the vessel to the dock or upon impact with fendering that a snapshot of an event can prove to be very valuable. Learning from each docking manoeuver through analysis of results can provide important feedback to berthing operators, assisting all parties in reducing the time needed to berth the vessel and improve safety procedures.”

The critical structural elements that limit vessel surge (longitudinal drift) and sway (drift out) are the mooring pattern and balanced distribution of mooring line tensions over all the mooring points. This is a fundamental requirement of effective mooring.

Once the vessel is alongside the jetty and mooring ropes secured to hooks, it is vital that their tensions are monitored. This is performed by load cells installed into each mooring hook, which output load tension data and transmit this to the jetty control room. Information displays are also available at each hook location and may be linked to an optional high load alarm siren and strobe. This alerts operators at the jetty and vessel crew immediately a line becomes over-loaded. Corrective action can then be taken in timely response to changing conditions. The local, real-time display is also extremely useful in removing the guesswork during pre-tension of mooring lines to ensure loads are correctly balanced and the vessel remains safely moored from arrival.

Vessels have a multitude of forces imposed on them from many sources. These include wind and current, speed and direction of waves, surge effects from passing vessels, under keel blockage effect, tidal changes and vessel draft changes due to loading or discharge of product.

These external events are monitored through environmental and oceanographic sensors. These will usually include a weather station and jetty mounted sensors that monitor current, wave height, wave profile and tide data. In addition, current and wave sensors may be installed on the seabed or as a wave rider buoy arrangement along the approach channel or within a turning basin. Information can then be transmitted back to the central jetty system over radio telemetry or undersea cable.

In the latest fully integrated systems information from the environmental sensors is combined with that from load cells and docking lasers. Data is relayed over a digital network to a central PC-based system, usually located in the jetty control room. The system raises alarms if data exceeds a user set range. It can also be networked to other locations over LAN or telemetry if required. A portable monitor for use by the pilot and mooring supervisor provides seamless communication between jetty and vessel bridge.

Alarm data generated by docking and mooring monitoring can be interfaced to the facility’s Distributed Control System (DCS) using standard industry protocols. Mooring tension information can then be transferred to the vessel over the fibre optic Ship-Shore Link commonly used at LNG facilities. This meets SIGTTO recommendations requiring the transfer of mooring tension data to the vessel.

“Trelleborg Harbour Marine has examined the efficiency of many systems insitu and the consensus of opinion from most Operators is that the integrated system concept undoubtedly contributes to safety, while providing substantial operational benefits,” says Tom Toth.

Natural gas makes up half the world’s hydrocarbon resources, but it’s not always found where it’s needed. As the developed world begins to exhaust its own domestic natural gas supplies and emerging nations require low cost energy to grow, there is a need to move natural gas from source to place of consumption. In its gaseous state this can only be done by pipeline, but the cost this infrastructure is high.

The alternative is liquefied natural gas (LNG). When natural gas is cooled to temperatures below minus 162°C/ 260°F it condenses into LNG. As a liquid, natural gas occupies only 1/600th the volume of its gaseous state, so it is stored effectively in a limited space and can be transported by container ship.

Substantial investment is being made into LNG facilities. In 1990 there were just nine LNG production sites with a total of 13 trains, the term used for a liquification and purification facility within an LNG plant, producing 56milliontonnes of LNG. By 2006 this had grown to 66 trains producing 130milliontonnes. This though is just the beginning of a boom, with forecasts showing that the number of liquefication facilities will grow to increase capacity five-fold by 2030.

Inline with this growth the number of LNG vessels is also expanding. Around 150 LNG tankers were in operation worldwide in 2003. By the end of 2005 the world fleet of LNG carriers was up by 27percent to 190. On top of this, a further 133 vessels are currently on order or under construction.

Not only are there more ships but also, to improve economies of transport, their size is increasing. Currently the most common size of larger tanker in service is 138000m3 but of the vessels on order, there are only a few of this size or below.

The average size of those commissioned is 175000m3, with the largest ones, a massive 270000m3.o

For more information, visit www.trelleborg.com