Monitoring stresses and strains.
Strainstall examines load, stress and strain monitoring solutions for the LNG industry.
The relatively rapid rise in the intercontinental transportation of LNG has coincided with a corresponding development of ship and shore-side infrastructure to facilitate this transportation. As the industry develops, the safety of transporting this cargo is paramount.
An essential element in the safety of such facilities is ensuring the structural integrity of both ship and shore-side equipment. Load and stress monitoring equipment can support operators in ensuring safety is maintained, as cargo is transferred or transported worldwide.
This article will examine the equipment available for the LNG industry, as well as benefits and applications in both existing and future facilities.
Hull stress monitoring systems (HSMS) provide real-time monitoring and display of the movements and stresses in a ship’s hull structure, both during a voyage and during loading and unloading operations.
The value of such monitoring is recognised by the international class authorities, and it is a DNV GL class requirement for such ships to be fitted with an effective and operational HSM system.
Examples of such systems are installed on two sister ships, the Arctic Discoverer and Arctic Voyager, both operated by K-Line. These vessels operate in extreme temperatures, ranging from the cold of north Norway to the heat of South America.
The 270 m long ships carry over 147 000 m3 of LNG and can travel at speeds of up to 20 knots (much faster than most oil tankers), using spherical tanks to hold the LNG. Sensors are used to monitor the stresses in the hull itself, and in the tank support structure. These installations also incorporate
the real-time monitoring of environmental conditions, such as wind speed and direction, and wave height, direction and period. Due to the speed of the vessels, climatic conditions have a large impact on the stresses within the hull structure. The HSMS provides live information for the provision of feedback to the Master, on which he can base decision on improvements to course and speed, to optimise the vessel performance, while minimising the stresses experienced. This is especially important when partial loads are carried, as the sloshing of the LNG within the tanks can have a significant impact on the stress levels experienced.
The HSMS is a flexible monitoring system that can be configured to interface with a wide variety of sensors and existing ship equipment via the industry standard NMEA0183 interface. Additional electronic interfaces can also be added to connect with additional sensors.
The standard software interfaces with the following sensors:
- Long base strain transducers for deck mounting
- Strain sensors for structural component monitoring
- Bow pressure sensor (to measure bow slamming)
- Fore and aft draft
- Ship motion sensor (heave pitch, roll, surge and sway)
- Wind speed and direction
- Air temperature and barometric pressure
- Wave height, period and direction (using ship mounted radar)
- Position and heading via a ship’s GPS
- Propeller shaft speed and torque
Additional parameters can be added via in-house custom-designed software. The standard software provides a number of processing, display and recording features, including real-time data, mimic displays, trends, statistics, cycle counting, spectral data, system integration, alarms and DNV GL fatigue analysis.
FLNG and ship-to-ship transfer
One of the more recent developments in the industry has been the design of Floating LNG (FLNG) and Floating Storage and Regasification (FSRU) ships. A FLNG vessel would be permanently moored at the production field to extract the natural gas from the earth, and process it for transport using an LNG carrier. The transfer from the FLNG to the carrier would be performed using a ‘side-to-side’ mooring operation.
During such transfers the relative motions of the two ships are constantly monitored by the mooring master to ensure both vessels remain in a safe condition whilst the cargo is transferred.
Once the LNG tanker has reached its destination, an FSRU can be used to return the LNG to its natural gas state for entry into the domestic gas supply system.
In addition to HSMS, Strainstall provides equipment and monitoring systems to provide safe mooring when in the side-by-side configuration and to provide the mooring master with ’live’ information on the loads being applied to the moorings by the vessel motions during the cargo transfer process.
This equipment includes:
- Quick release hooks used to moor the two vessels together in the side-by-side mooring configuration. Mooring lines from the LNG carrier are passed over to the permanently moored LNG FPSO and anchored on board by use of the quick release mooring hooks.
- Mooring monitoring systems. These enable loads in the mooring lines to be measured and recorded via load pins in the quick release hooks. If required, the emergency release of each individual line from various ship locations, either at the hook or in the operations room, if the climatic conditions change during the transfer operation.
- Load monitoring for the FLNG mooring chain-stopper loads.
- FLNG anchor chain monitoring system to ensure the integrity of the FLNG turret moorings.
- Winch load monitoring.
- Dynamic separation distance monitoring (via high accuracy GPS).
The company manufactured and supplied the quick release hooks for Shell Prelude, the world’s first FLNG FPSO, currently under construction in South Korea.
LNG terminals have similar ship berthing facilities to oil terminals, utilising the same equipment used on the FLNG for offloading operations but with additional equipment to handle the modified docking procedure including:
- Vessel approach speed and direction (using jetty mounted lasers or ship mounted GPS).
- Environmental monitoring (e.g. wind speed and direction, air temperature and humidity, barometric pressure, sea current, visibility and rainfall, wave height and period, and tide height).
All systems are configured for each individual installation using standard or customised software modules. The computer systems may be single displays or complex server-based multiple display screens, to meet client requirements.
Remote displays using radio communication and/or ship-to-shore hard-wired (fibre optic) communication can also be required.
The company has supplied equipment to approximately 300 oil terminals across the world, as well as an increasing number of LNG terminals. These include the Dragon Terminal (UK), Teesport Gas Terminal (UK), Reganosa (Spain), Valencia (Spain), Zeebrugge (Belgium), Tesoro (USA), Sakaide (Japan), Yokkaichi (Japan) and Shandong Qingdao (China).
LNG port case study
The latest system delivered by Strainstall was for the Qingdao LNG terminal, located in Shandong Province, China, for Sinopec Corp.
The equipment supplied is typical of a modern LNG terminal to aid the berthing process, displaying all data in real time, and includes the following:
- Six triple 125 t SWL (Safe Working Load) quick release hooks, with integral capstans and remote release.
- Four double 125 t SWL quick release hooks with integral capstans and remote release.
- Load monitoring on each hook, with the ability to perform in-hook calibration checks without removing the sensor or dismantling the hook. Load displays at both the hook and in the jetty control room.
- Remote release control panel in the control room, enabling individual, group or emergency release of the mooring lines.
- Docking approach distance and speed monitoring using jetty-mounted lasers.
- Large digit display showing vessel approach information (bow and stern distance from the jetty, approach speed and angle of approach) as the vessel undertakes the berthing operation.
- Wind speed and direction, air temperature, barometric pressure, humidity and rainfall monitoring.
- Visibility sensor located on the jetty.
- Sea sensor monitoring the current and direction, wave height and period, tide level and wave energy spectra.
- Dual-computers for processing the data, with information displayed in Mandarin Chinese for local operator understanding.
- Rugged laptop to display the approach information to aid the pilot when berthing. The data is displayed in real-time and is transmitted via an ultra-high frequency (UHF) data link incorporated in the unit.
- Hazardous area certified pager display, indicating parameters in alarm conditions, etc.
The equipment is certified for use in hazardous areas. Following successful installation of the hooks and berthing system, the governing body signed off that the LNG terminal was safe to operate, and the first vessel docked on 14 November 2014.
A wide range of equipment and monitoring instrumentation is available for use in the LNG industry to optimise both ship operations at sea and the berthing process in port to ensure the safety of both personnel and equipment.
The adoption of these various systems provides the LNG industry with the information to maintain and record ‘loads and stress’ on equipment, protecting assets and maintaining the integrity of LNG as a viable fuel of today and the future.