DEVELOPMENT OF LASER BASED TECHNOLOGY FOR CUSTODY TRANSFER SYSTEM

摘要

The Custody Transfer Measurement System (CTMS) remains today the most important measurement on board an LNG Carrier. The key element of the CTMS, is its primary level measurement sensor. Since the Mid 1970's; the Capacitance based level sensor has been the most widely used measurement principle in the industry. Twenty years later, the Radar based measurement technology had come of age in the LNG CTMS application. Today the Laser based sensor technology represents the next logical progression and most advanced level measurement technology in the CTMS application on board today's modern LNG carriers. Having carefully studied the various factors which can affect a Laser based sensor within the LNG cargo tank; Invensys Foxboro has developed this measurement technology with the use of a unique Digital Signal Processing technique for an accurate measurement. (Product Patent Pending). The Laser unit is installed above the deck and is isolated from the process by a sight glass and it targets the liquid level surface with a pulsed low power Laser beam. This beam is protected from the in-tank adverse measurement conditions by an ordinary stilling well. It is important to note that the stilling well used on a laser based system is not used a wave guide and that its construction is simple and remains relatively inexpensive. Unlike the Laser based system; a Radar requires a stilling well which is critical and more complex. With Radar sensors this well is used as a wave guide which resembles a gun barrel; and this is a critical requirement by the Radar sensor.rnThe new laser based level sensor is based on the Time of Flight (TOF) measurement principle. An electrical pulse generator is utilized to drive a semiconductor laser diode to send out infrared light pulses. These are collimated and emitted by the transmitter lens down to the LNG Liquid surface. The echo signal which is reflected by the target (LNG Liquid) then hits a photodiode via the receiver lens,. This then generates an electrical signal at the receiver. The time interval (TOF) between the transmitted and received pulses is accurately counted by means of a quartz-stabilized clock frequency. This time is then converted by the laser unit to distance. The level measurement system remains unaffected by changes in the LNG composition or by the changes in physical properties of the still pipe. When performing level measurements at the bottom of the tank, echoes or reflections near the bottom do not affect the accuracy of the system. The level measurement as well as the measured tank pressure and temperatures, are all transmitted to the computer which then uses the stored volume tables to process and prepare all of the various data outputs required for display and reporting in a typical CTMS application. This real-time information can also be historized/archived or transferred to other systems on board the Carrier. The first commercial application and certification of this new laser based technology, resulted in late 2005 following a 2 year R&D effort on board the Gaz de France owned vessel "LeTellier" .