Vessel Mooring and Monitoring Systems at LNG Marine Terminals
Simon Wilson and Tom Toth, Harbour & Marine Engineering (HME), Australia
The safe, reliable and efficient mooring of LNG carriers is most important. This article describes the integrated, high-technology mooring equipment and monitoring systems that are now the state-of-the-art in new LNG terminals.
Quick Release Hooks, Capstans, Load Monitoring and Remote Release at Cove Point LNG, Maryland USA
A key component of the LNG terminal is a reliable mooring system. There are vast differences in product features and quality, which must be carefully selected to ensure safety and efficiency of the LNG carrier mooring. The SIGTTO Publication "LNG Operations in Port Areas" notes that monitoring and control systems for operations on jetties are primarily aimed at enhancing safety of operations and personnel. Indeed, with the level of public scrutiny currently aimed at LNG marine facilities, particularly receiving terminals in the United States, specifying the right mix of equipment on the jetty becomes critical not just to operating safety, but to demonstrating the owner's commitment to safety.
Mooring equipment has in the past been low on the priority list for design review; however, with the increasing attention to documenting operations and the more recent availability of robust monitoring systems for this unique marine environment, this is changing. Fully integrated mooring systems are custom engineered, long lead purchases requiring independent testing and an ISO 9001 accredited approach to design and manufacture. For the construction of the marine facilities to proceed smoothly, the earlier the specifications are developed, the better.
This article discusses the mooring equipment and monitoring systems now becoming standard for new LNG jetty and gravity base structure specifications and how they are integrated to provide centralised data to the operators. Typically, these include:
- Quick Release Hooks and Capstans
- Remote Release Systems
- Mooring Line Load Monitoring
- Vessel Docking Aid Systems
- Environmental and Met Ocean Monitoring
The discussion draws upon recent LNG projects awarded to Harbour & Marine Engineering including Damietta & Idku, Egypt; LNG3, Ras Laffan; Isle of Grain, UK; Darwin, Australia; Atlantic LNG Trinidad and Southern LNG, USA.
Quick Release Hooks and Capstans
The Quick Release Hook (QRH) is the foundation of any hazardous jetty mooring system and is recommended by both SIGTTO and OCIMF. The basic design has been improved over the last 40 years and they are used at all major oil and gas marine facilities worldwide.
The QRH facilitates the release of the vessel without the need to reduce tension in the mooring lines. QRH's are designed for either manual or remote release from zero load up to the Safe Working Load (SWL) capacity of each hook.
100T capaciy Triple Quick Release Hook unit incorporating Capstan and Remote Release
Hook units are located along the jetty at each dolphin and provide the point of restraint between vessel and jetty. For LNG jetties, QRH's are usually in double, triple or quadruple configuration on a common support base. The base also incorporates a capstan used for hauling in each mooring rope to the hook using a lead line passed from the vessel. The capstan motor should ideally be positioned within the support base to improve protection from mechanical and environmental damage.
The units are designed to withstand static and dynamic forces exerted by the vessel, often from wind or current load, whilst alongside. Hook configuration and capacity are determined by a mooring analysis usually performed by the Owner's consultant. This considers the range of vessels likely to visit the facility over the life of the project and recent specifications have noted the projected increase in vessel size to 200,000 cubic meters plus. Hook SWL capacities have typically been 125 Tonnes for vessels up to 140,000 cubic meters; however we are now seeing 150 Tonnes SWL capacity being specified.
The key considerations for selecting a quality quick release hook package are as follows:
- Manufacturer can support all aspects of the structural design and provides traceability for materials operating under ISO 9001:2000 accreditation
- Pre-shipment testing of each hook including proof load test to 125% or 150% of SWL and Release Test at SWL
- Importantly, hooks fitted with load cells should undergo a release test with load cells fitted in order to shock-test the load cells
- Maximum protection to capstan by fully enclosing within the support structure
- It has been demonstrated that it is most desirable to have a single vendor for design and manufacture of the complete package of Hooks, Remote Release and Load Cells
Remote Release
Integral to the hook is the release mechanism. The purpose of the release mechanism is to release the mooring lines, which may be under tension, in a safe and controlled manner. Most manufacturers offer manual release at the hook using a removable lever as a minimum feature.
At many LNG and condensate facilities, a remote release system is installed. This provides the operator with the means to release each hook by electrical signal from a remote, safe location, via either a console or computer screen, as well as by push button at the hook unit itself.
Remote release systems for quick release hooks are installed for two primary reasons:
(1) Safety
The risk of injury to personnel is greatly increased during the release of a vessel from its moorings. Mooring crews are at risk in close proximity to mooring lines that may be under tension.
In addition the terminal may have little control over the condition of the mooring lines, which are the property of the visiting vessel. If these are in poor condition they can be prone to failure. Failure of a mooring line, with an operator close by to the QRH, can result in serious injury or death. The recoil from the parted line results in a section of the line possibly coming in contact with the operator.
During the release of a vessel, as fewer and fewer lines are restraining its movement, additional load is placed on the remaining lines, increasing the probability of a failure. This is especially the case in bad weather where the tugs are having difficulty restraining the vessel's movements.
Remote release systems minimize the need for the operator to be in the 'risk zone'.
(2) Productivity
The number of personnel required to release a vessel can be minimized on facilities with remote release systems fitted. A senior employee, such as the Marine Manager, would normally operate the system from the console and although not mandatory, if hooks are within line of sight, up to two other personnel may be positioned, one forward and one aft, to assist in the event of tangled ropes or local power failure (back up UPS systems can be incorporated).
Operating modes are often overlooked: whilst the actual purpose of the remote release functionality is simple, the circumstances that require the hook release are sometimes confusing. If a crisis situation were to occur where the vessel was to be released from it's mooring quickly then a remote release system would provide a quick and safe method for a controlled release.
Release Mechanisms and Control Systems
Release mechanisms generally include either Electric or Hydraulic operation. Hydraulic based Remote Release systems require a separate power unit, hydraulic lines and an oil reservoir to allow operation and are typically high maintenance systems. There has been a strong swing toward electric based systems, which offer the same level of functionally in terms of release control and safety, with substantially lower maintenance requirements.
Control Philosophy
A number of recent LNG terminal project specifications demonstrate some of the misconceptions in the industry relating to Remote Release of LNG vessels. The association of automated remote release with cargo transfer Emergency Shut Down (ESD) is a prime example.
A cargo transfer or plant emergency situation does not necessarily constitute a reason to disconnect the vessel from the berth. The authors encourage industry discussion and wish to highlight the requirement for guidelines on this subject.
The traditional plant / transfer instrumentation methodology may not align with the control system logic used for remote release. A growing trend is to physically locate the remote release control console some distance away from the jetty. Whilst this may be convenient for construction of common control rooms, cargo transfer operators and marine operators are likely to be separate teams with different operational requirements.
As the line is released from the shore with shore staff safely out of the immediate vicinity of the hooks and mooring lines, vessel crew can be endangered if not forewarned of impending line release. This is a compelling reason to perform a risk assessment for the ship-shore interaction during docking, whilst moored and on de-berthing.
The control and monitoring systems should be designed, manufactured and integrated with the mooring equipment by the one dedicated mooring system manufacturer to ensure the total mooring package provides the best possible solution for safety and functionality.
Docking Aid System
Jetty-mounted large display board provides vessel speed and distance to the Pilot
A Docking or Berthing Aid System (DAS) is a tool used by the Pilot and vessel Master to assist in manoeuvring the vessel towards the jetty during the last 300 m of approach. 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.
Although experience is an essential ingredient for effective mooring management, it cannot be completely relied upon at all times and may be limited, particularly at greenfield marine facilities. In addition, as vessel size increases, berthing dynamics will also change and a DAS provides the data necessary to enable personnel to clearly follow agreed procedures, for example keeping vessel docking speed below the agreed site limit.
A typical DAS consists of two laser sensors, a controller, central PC with software and remote display devices such as the display board, for each berth. As with all the jetty monitoring systems, multiple berths can be interfaced to a single control point for integrated operator display, logging and reporting.
In the event of a docking incident, a history of the final approach to the jetty and impact with fenders can be replayed on the docking software. Data is logged at one-second intervals and a series of graphs and reports printed for analysis and training purposes.
Mooring Line Load Monitoring
Once the vessel is alongside the jetty and mooring ropes secured, line tensions and their distribution must be monitored effectively as this is the critical structural element which limits vessel surge (longitudinal drift) and sway (drift out). Measuring line tension is by way of load cells installed in the pivot block at the rear of each hook.
There can be a multitude of forces imposed on a vessel from many sources, including 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.
Harbour & Marine Engineering's SmartHook® Load Monitoring provides local display of hook loads
The efficiency of the mooring system is largely dependent upon layout, length of each mooring element, stiffness and construction of mooring lines. The management of the vessel's mooring system represents a great responsibility to both terminal operator and Master and requires considerable understanding of the factors discussed above, as well as effective information feedback in order to allow time to make the right decisions.
Therefore in addition to display of load data in the jetty control room, it is recommended that mooring loads are available at each hook location, linked to a high load alarm siren and strobe, to alert jetty operators and vessel crew immediately a line becomes over-loaded. Corrective action can then be taken in timely response to changing conditions. This 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.
Environmental and Met Ocean Monitoring
Wind, wave and current forces acting upon the vessel can have a significant effect on vessel handling, particularly at slow speed and manoeuvring under tug assistance.
Wave Profiling / Tide laser sensor at Pyeong Taek LNG receiving terminal, Korea
LNG marine facilities require environmental and oceanographic sensors to provide valuable data during docking and mooring and allow both terminal and vessel personnel to make informed decisions on vessel operations.
The data is collected from a variety of sensors that may typically include the following: -
- A weather station often located on the roof of the Jetty Control Building
- Current monitoring, mounted off the jetty
- Wave height, profile and tide data, off the jetty (see Figure 5)
- Current and wave measurement 'offshore' either in a turning basin, approach channel and/or offshore mooring point
The sensors communicate with the central PC over cable or telemetry, and data can be relayed to the portable monitor carried by the Pilot.
An Integrated Approach To Central Monitoring
The data from load cells, docking lasers and environmental sensors is relayed to a central PC system usually located in the Jetty Control Room. Signals from these sensors are now over digital communications transmitted over twisted pair conductors, fibre optic or radio telemetry and provide a significant reduction in cabling requirements, coupled with an increase in reliability, compared with just a few years ago. For example, Harbour & Marine's SmartHook® Load Monitoring system utilises a digital link, which can also accommodate remote release control.
A dedicated PC-based integrated system requires minimum operator specialist skills within a user-friendly interface to provide display and recording of data during the vessel mooring and cargo transfer operations. The system raises alarms, which must be acknowledged by the mooring operator, if data from the load monitoring, docking or environmental sensors exceeds a user set range. The data can be networked to other locations over LAN or telemetry if required and often specifications call for display of docking data, line load and environmental data on a single portable monitor for use by the Pilot and Mooring Supervisor.
Centralized monitoring screens provide a single user interface for all docking (left), mooring (right) and environmental data
Increasingly, alarm data generated by the mooring monitoring is being interfaced with the plant Distributed Control System (DCS) over industry standard protocols such as modbus. These can then be transferred to the vessel over the Ship-Shore Link. SIGTTO recommendations, for example, call for load monitoring data to be available at the vessel whilst alongside, however this could be extended to provide drift warning, release warning and environmental data.
Conclusion
Mooring and monitoring systems for LNG marine terminals have undergone exciting and significant development during the past 5 years. Not so long ago, each of the above systems may well have been designed and supplied by a separate manufacturer, each having its own PC control. The industry has certainly seen a positive move towards fully integrated mechanical and instrumentation systems, with the one manufacturer responsible for the total package, including ongoing service; however, there remain outdated specifications based on 'old' information. From the authors' experience, mooring systems need to be considered during project FEED and at each main design review to ensure specifications incorporate the latest technology and operational feedback and provide the LNG facility with the tools to effect safe and efficient vessel mooring.
Biographies
Simon Wilson, Sales & Marketing Manager in Harbour & Marine Engineering (HME), is responsible for international business development. He is a Naval Architect with experience in marine and offshore oil & gas in the North Sea, USA and Asia.
Tom Toth, Technical Director in HME, has a degree in civil engineering and over 25 years experience in the marine field. He is responsible for the company's technical management and product development.
HME was established in 1971 in Melbourne and is a leading designer and supplier of mooring and monitoring systems to the oil and gas marine industry worldwide. The company has supplied equipment to approximately 75% of LNG terminals (excl. Japan) over the past 3 years and specialises in the design of integrated jetty mooring systems with both a strong mechanical and instrumentation resource in-house.
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