Marine Radar Technology
The Rosemount TankRadar® gauges have been developed over a period of more than 25 years. A number of factors affect the performance of a radar level. Our engineers have developed Rosemount TankRadar® gauges that give our customers excellent performance under the most diverse conditions.
There were many reasons why radar technology for level gauging was first adopted on tanker ships, despite being a new, unproven and expensive method at that time. Firstly, the tank environment is quite harsh, both in terms of the various chemicals (even in crude oil) and in mechanical terms, from the waves in high seas as well as the cleaning guns. In that environment the new technology soon proved its superior ability to measure where other systems failed. The float gauges (fairly common in the past) were literally wiped out when new regulations during the late seventies required cleaning guns to be used during discharging. The primitive pressure gauges used at that time were not reliable enough when inert gas made it impossible to open the tanks and look in as a back-up method. Radar technology soon proved to be fairly reliable, partly due to the absence of moving parts, as well as to its location well above the cargo, with the sensitive parts completely outside the harsh tank environment. The gauges for the Rosemount SUM-21 system, the only level radar system in the years 1975-1985, for instance, had an experienced mean time between failure of 65 years, and later models improved that figure. With the antenna being the only part exposed to the atmosphere inside the tank, it was possible to have it made of acid-resistant steel and PTFE only, in order to allow it to withstand a wide range of cargoes.
The first ever radar level gauging system installation was carried out on this ship
- M/S Havdrott - with Rosemount SUM21 radar level gauging system.
The antenna is the most critical part
for use in marine systems. The tanks are not simply empty space: pipes, ladders, steel beams and other obstacles can cause reflections that disturb the measurement. The surface of a cargo of oil has a rather weak reflective power for radar waves: about 3% of the power is reflected, as compared with a metal surface, and consequently it is far from easy to make the strength of the disturbing echoes much weaker than the echo desired from the oil surface. To achieve that, the antenna is an important tool for making the radar beam narrow enough to pass between typical tank constructions, thereby avoiding false echoes. With the antenna diameter 39 cm SUM-21 and its follower TRM, the antenna beam could get the right beam-width for avoiding echoes from pipes in beams even in deep ships of 25-30 m. For smaller ships, e.g. 10-12 m deep, an antenna with a smaller diameter can sometimes be used with a wider antenna beam. Regardless of the type of antenna, the width of the antenna beam is closely related to its diameter as counted in wavelengths. For level gauges in industrial tanks, higher frequencies can sometimes be used, giving a narrower beam. In marine tanks, however, where the antenna is occasionally washed over by the cargo when at high seas, the higher frequencies would be attenuated too high, due to the layers of wet cargo on the antenna. Large-diameter antennas are typically parabolic antennas, but for diameters under 20 cm various kinds of conical horn antennas are used. In the highly condensing tank environment, antennas that include a horizontal surface are known to be subject to problems when condensation creates large droplets on surfaces such as these. Interesting enough, the antenna beam-width cannot be too narrow either, as the reflection from the surface will move slightly over the surface, depending on trim and list conditions (reflection is always perpendicular to the surface). The optimum antenna diameter is therefore fairly large, and depends to a certain extent on the size of the ship.
The second generation TankRadar® Marine - the TRM.
An increasing number of tanker
ships carry liquid gas. There is little electrical difference between liquid gas and a liquid (mainly a question of a certain interval of temperature, pressure and chemical composition) but from time to time the liquid gas will boil, temporarily making the echo much weaker. In order to guarantee accurate measurements under all such conditions, including typical installation methods, liquid gas is measured through a pipe guiding the radar waves, and because liquid gas is very clean it does not produce any deposits, which otherwise could easily ruin the accuracy of the measurement. In some liquid gas tanks (those under pressure), the typical property of waves, i.e. that they depend very little on atmospheric conditions, will not apply fully, and it will be necessary to correct the value by several hundredths of a percent or even up to one percent, depending on pressure etc. that can be measured.
An LNG (liquified natural gas) carrier and the TankRadar® for LNG applications.
All marine systems to date
use some kind of FMCW technique, meaning that the microwave frequency is swept over the frequency band required for the distance resolution desired. The radar echo from the tank is quite complex, and normally contains a number of disturbances in addition to the real echo from the surface. The echoes arrive over slightly different times (15 m corresponds to 0.1 microseconds) but the FMCW method provides a good way to rescale these short times to a frequency spectrum ideal for analysis with modern signal processing techniques. Due to internal tank echoes, waves on the surface of the liquid etc., a lot of information is required from the tank for selection of the correct echo, and this is provided by the tank spectrum. Conditions in the tank change over time, as the tank and ship are not quite rigid, and as layers of cargo will change the reflections from different parts of the tank; this means measured signals from the empty tanks are generally obsolete next time the tank is empty. Different manufacturers have used various methods for hardware solutions over time. Many units use a well-known piece of cable as a reference for achieving a good level of accuracy in the rescaling from time to frequency, but other methods are possible and are used on high precision refinery gauges. The cable solution in the early models, however, made it possible to use circuits with low power consumption. Today low power counter solutions offer a better solution for controlling the frequency.
The third generation TankRadar® Marine - G3.
The full marine tank gauging system consists of one gauge per tank (10-40 tanks on one ship) connected by a cable to a common feeding unit. The feeding usually has to be “intrinsically safe”, meaning that currents and voltages going out onto the deck must be so low that they are unable to create any igniting sparks, whatever failure might occur. That means that only low current drain is possible. A cargo control room typically contains a display unit with signal processing and data distribution features. All cargo handling is done from inside, and typically from the bridge. Often other sensors are integrated in the level gauges, such as temperature and inert gas sensors.
At Saab Marine Electronics, the second generation TankRadar Marine (TRM) was launched in 1985, followed by the third generation G3 in 1995, both of them with a number of continuous improvements and cost reductions. The G3 system included liquid gas tanks, and the system was fully integrated with some cargo handling capabilities.
In 2002 G3 was followed by the STaR system - a system with "intelligent" transmitters enabling much higher accuracy and the possibility to include a cost-effective redundancy where two or three electrically independent radar level transmitters could be sharing the antenna and encapsulation. Typically one of the level transmitters is used as a high-level alarm instead of another system.
The TankRadar® STaR with its 3-in-1 functionality: independent radar based level gauging, high-level and overfill alarms.
The main advantages for using radar for tank gauging are:
- Radar waves never get stuck.
- Radar waves are not affected by the atmosphere above the product in the tank.
- The only part located within the tank is the antenna.
- No moving parts - High reliability.
- High accuracy.
- On the Rosemount TankRadar® systems, the Electronic Box can easily be changed with closed tank conditions.
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