In response to balancing the issues of environmental concern, access to suitable land sites and the economic reality of getting gas to market within an acceptable timeframe, an increasing number of LNG terminals are being developed or studied for location offshore. Projects include gravity base structures, conversion of existing offshore platform facilities and the use of Floating Storage and Regassification Units (FSRU vessels). The approach and safe mooring of the gas carrier to the offshore terminal must consider not only all factors that affect a typical near shore jetty facility, but also increased sea state, access to and operation of the mooring equipment and relative vessel motions in the case of FSRU’s.
The concept of a fully integrated Ship-To-Ship (STS) mooring system is a key component of offshore LNG terminal design. Integrated mooring systems are custom engineered, long lead purchases requiring independent testing and an ISO9001 accredited approach to design and manufacture. For the construction of the marine facilities to proceed smoothly, the earlier the specifications are developed, the better. There are however vast differences in features and the quality of standard jetty-based mooring equipment and careful selection is essential to ensure the safe and efficient mooring of the LNG carrier offshore. Development of the integrated system is ideally carried out by a specialist, full service supplier providing performance and capability input across the range of equipment to the design engineers starting at pre-FEED and beyond.
This article briefly addresses the design of mooring equipment and monitoring systems for offshore LNG facilities and how they are integrated to provide centralised data to the operators. The equipment is based on design philosophies used successfully in near-shore jetties and typically includes:
• Vessel Docking and Piloting Aid Systems
• Environmental and Met Ocean Monitoring
• Quick Release Hooks and Capstans
• Remote Release Systems
• Mooring Line Load Monitoring
Docking Aid and Piloting Systems
At near shore jetties, a laser Docking Aid System such as SmartDock® DAS measures vessel distance, angle and speed of approach using lasers mounted on the jetty. The data is used by the Pilot and vessel Master to assist in manoeuvring the carrier vessel safely alongside the terminal during the last 200m of approach and is displayed and logged at the central monitoring PC and also made available to the Pilot on a jetty mounted large display board or over telemetry to a handheld monitor.
The same equipment can be used offshore, however it is often complemented by measurement of carrier position via GPS. This is particularly important for application on floating terminals where the dynamic movement of both the FSRU and the carrier may reduce the practical range of the laser system to less than 100m. Here, a DGPS or RTK-GPS based piloting system can provide display on a carry-on laptop during approach and may interface with the laser DAS once the carrier is within a preset distance of the FSRU to provide increased accuracy and/or redundancy of approach data. Because the laptop communicates with the terminal over telemetry, all approach data can be logged centrally. In addition, environmental and sea state data may also be sent from the terminal to the pilot laptop providing a further layer of information to effect a safe berthing operation.
Environmental and Met Ocean Monitoring
Wind, wave and current forces acting upon the vessel have a significant effect on vessel handling, particularly at slow speed and manoeuvring under tug assistance.
Offshore LNG marine facilities typically require additional environmental and oceanographic sensors in comparison to a near shore jetty, to provide valuable data during approach, docking and whilst alongside and enable both terminal and carrier personnel to make informed decisions on operations.
The data is collected from a variety of sensors such as:
• A weather station on the LNG terminal (typically including visibility linked to a fog horn).
• Current monitoring, both at the terminal/FSRU and in the approach route.
• Measurement of wave height, profile and direction data using a buoy or seabed mounted device.
• Monitoring of salinity, water temperature, and other sensors specific to each project.
The sensors communicate with the central PC over cable or telemetry and data can be relayed to the laptop carried by the Pilot. Quick Release Hooks and Capstans The Quick Release Hook (QRH) is the foundation of any integrated moor ing system and is recommended by both SIGTTO and OCIMF.
Hook units are mounted on the LNG facility and provide the point of restraint between carrier and fendering. QRH’s are typically of triple or quadruple configuration on a common support base. The units are designed to withstand static and dynamic forces exerted by the carrier from wind and current load, whilst alongside. As with jetty mooring equipment, we are seeing an increase in hook capacity to accommodate the projected increase in vessel size to 200,000m3 plus, where 150 tonnes SWL capacity is now being specified.