Manchester, UK –?? AJ Consulting of Inverness, UK – in partnership with the University of the Highlands and Islands – have developed a technique for the calibration of tension meters to improve mooring safety on semi-submersible oil rigs.
The method relies upon the fact that tight moorings allow less movement than slack ones. Following initial computer simulations, which proved the technical viability of the method, a scale model rig (see Figure 1 at windmill.co.uk/loadtest.html) was constructed for testing in the inner Moray Firth near Inverness.
The data logging equipment present on the “mini rig” included a motion sensor, load cells connected to each anchor line and a RTK receiver. The bespoke load cell circuit was designed to transmit data from the eight load cells at a rate of 8 Hz.
The company therefore required a serial data logging application that was capable of receiving and processing that volume of data in addition to the data received from the RTK and motion sensor.
After several unsuccessful trials with other serial data logging packages, they came across the Windmill website (windmillsoft.com) and purchased Windmill 7 with the COMIML Serial Driver. They were able to use this software to parse the output strings of all data logging equipment on the rig.
This was a great benefit as the RTK and motion sensor were transmitting ASCII characters, while the load cell circuit was outputting binary strings.
Another requirement was that the logged data was accurately timestamped.
Other RS232 loggers looked at used the Microsoft Windows serial driver. This is incapable of logging data at the rate required, and is affected by other Windows processes running which can introduce varying delays to data acquisition.
Windmill Software write all their own drivers, so the other benefit of Windmill was that it allowed them to log at the required rate and to precisely time stamp the data.
The data gathering phase of the project is now complete and the company have significant volumes of data to analyze and interpret in order to prove the theory in practice on a small scale. Early results look encouraging and the next stage is to test the technique on a full size, offshore moored installation.
The Windmill COMIML software – with logging, charting, display and control programs – is currently reduced to ??50 from Windmill Software Limited at http://www.windmillsoft.com/daqshop/rs232-modbus.html
Using Thermocouples in a Data Acquisition System
Thermocouples are widely used because they:
– are cheap
– can be glued to a variety of substrates
– will withstand harsh environments
– cover a wide temperature range
– have a fast response time owing to their small size
Thermocouples comprise two dissimilar metals joined together, making a continuous circuit. If one junction has a different temperature to the other, an electromotive force (voltage) is set up. This voltage varies with the temperature difference between the junctions. If the temperature at one junction (the cold junction) is known, the temperature at the other junction can be calculated.
Types of Thermocouple
There are several types of thermocouple, labelled with letters according to their constituent metals. A K-type thermocouple, for example, is made up of chrome and Alumel. The metals give the thermocouples differing properties, such as temperature range and accuracy.
Three Potential Pitfalls in a Thermocouple Monitoring System
1. Attaching the Thermocouples to Metal Surfaces
If the thermocouples are directly attached to a metal surface, particularly one carrying its own voltage such as a heating element, you need to isolate the signals. This will prevent high voltages in the monitored item damaging the data acquisition equipment.
It will also make the measurements “float”, letting you record the small thermocouple voltage in the presence of high voltages.
The voltage produced by a thermocouple does not change linearly with temperature – presenting a problem for some data acquisition systems. Windmill will obtain the correct temperature for you automatically in, say, ??C or ??F, with built-in linearisation for B, E, J, K, N, R, S and T type thermocouples.
3. Using the Wrong Type of Thermocouple Lead
You need to connect the thermocouple to the data acquisition unit using the correct type of extension or compensating lead. This is made of either the same material as the thermocouple metals, or material with similar characteristics.
For more information on measuring temperatures with thermocouples, go to www.windmill.co.uk/temperature.html
To purchase a thermocouple monitoring system from Windmill, with over 50% off, go to www.windmillsoft.com/daqshop/thermocouple-measurement.html
Windmill Software Ltd,
PO Box 58, North District Office,
Manchester, M8 8QR, UK
Story courtesy of Windmill Software.