Suppose: you check your draghead depth of STPM by temporarily attaching a pressure sensor on the heel of the draghead, and comparing the depths with calculated depths of the STPM-system. And you find an error....
Here is how to analyse the error.
The best approach is to lower the dredgepipe and stop on regular depth intervals. While lowering, do not hoist intermediate or draghead for any minor adjustements, for example to make the draghead stop at an exact depth. This will mask error type (3).
On every stop, note draghead depth, as calculated by STPM, and depth calculated from the pressure sensor.
When you reached maximum depth, repeat the excercise in reverse; hoisting the dredgepipe, stopping at several depth intervals, noting the values. This may prove important in case (3), below, to check the repeatability of the error.
This whole procdure is rather crude, don’t expect accuracies in a range of 5-10 cm, especially if the ship is heaving, pitching, rolling, in open sea conditions, at the time of this test.
Best is to note all the values in an excel-sheet (found here), and arranging the found values in a graph. This allows a diagnosis.
There are three distinct patterns, to be recognised in this graph:
1. The error is fixed, within 10-20 cm variancy.
A fixed error of 10-20cm is inside the accuracy envelope of an STPM-system. (see article on accuracy). A larger -fixed- error can be caused by a lot of resaons, excluding reasons under (2) and (3).
An fixed error, varies between 0 en 20 cm value. This is all within the measurement accuracy.
Some reasons for this type of error may be:
a. Tidal height not set to "0", at start of calibration; error equals set tidal height.
b. Incorrect value from suction inlet draught transmitter. This may –typically- be caused by marine growth, clogging this sensor. Correct measurement can be checkd with a measure tape overboard.
c. Incorrect setting of density seawater. If e.g. freshwater is set (d = 1.000) instead of seawater (d = 1.025), this may cause an error of 25 cm on a draught of 10 meter. (Error varies with draught).
d. Incorrect setting for offset draught transmitter versus centerline of suction inlet.
2. The error varies with depth.
If the error becomes larger with depth; this is typically caused by wrong input of lengths of parts of the dredgepipe. You may check all the dimensions of the pipe in the dredgecomputer.
On a newbuilt vessel, it pays to check all dimensions of pipes on deck. You may find that some pipe lengths are up to 20 cm out of specs. All this adds up to noticeable errors, which will definitely show up when the dredgepipe is first lowered to maximum depths, this is: to larger vertical angles.
3. The error is not repeatable, hoisting versus lowering; and arranges itself in an crude hysteresis loop.
Vertical angle transducers have mechanical components that are prone to wear, or can be hampered in movement by some contamination in the sensor; … enough to block or hamper the free movement of the pendulum, and to make it drag. This is translated in a lagging angle measurement, which shows up in the graph.
A variable, non-repeated error, arranged in a crude hysteresis loop; clearly a sign of (a) worn vertical transducer(s).
Which angle transducer is to blame then ?
If hoisting / lowering the lowerpipe reflects only a fixed error (or no error), the sensor on the upperpipe is to blame, and vice versa.
Some brands of angle transducers are very susceptible to faulty behavior, and quickly develop errors, often within months.
Further errors on STPM systems may be caused by water ingress into cabling and connectors on the dredgepipe.
Often, this creates large errors in transmitted signals, where the dredgepipe on the STPM-display oscillates in very short periods of time.
Marc Van de Velde