Are Azimuth Thrusters Always the Best DP Solution?

When I tested my first few vessels dependent on a small number of azimuth thrusters, I found a worse dynamic fault response than I was used too.?But the offshore industry likes azimuth thrusters and I recently read industry guidance recommending them over fixed direction thrusters.?I wondered if I was wrong, and I thought this mismatch might be worth looking at.

Azimuth thrusters have a number of advantages over conventional thrusters.?They take up less room than a shaft line and are easy to place in large, relatively flat, hull sections.?Their ability to direct the thrust where it is most needed sometimes allows replacement of multiple fixed thrusters and they are more efficient than tunnel thrusters.?They are ideal for large semi-submersibles, where multiple thrusters can be placed far apart from each other, to prevent losses from thruster interaction (thruster wash across another thruster reduces thrust).?They can be stuck out of the bottom of the pontoons and the wash angled slightly down to avoid hull interaction (thruster wash across the hull reduces thrust).?There may be mission critical areas or transducers that their wash must not affect, but with many thrusters, large vessels, and more than two redundant groups, this is not a significant problem.

Their problems become more significant in smaller vessels with very few azimuth thrusters.?It is common for designers to replace two main props, two rudders, and two stern tunnels with two main azimuth thrusters.?It takes up less room and is cheaper to purchase and run in a diesel electric plant.?But if there are only two main azimuth thrusters then the failure of one becomes very significant to dynamic positioning (DP) and the rotation time needed to compensate for the fault delays system response.?Let’s illustrate with some examples.

The pictures above show three common thruster configurations operating in a beam environment.?Reddish shows the port redundancy group, green shows the other, and all three vessels are biased to reduce hunting (a problem that we will ignore for now).?If the main azimuth thruster opposing the beam environment fails then the remaining azimuth in line with the environmental force needs to stop thrusting, turn 180 degrees, and thrust hard to recover position.?This can take from 10 to 20 seconds and it will need to hunt a bit to maintain surge control.?Even if biased, the remaining stern tunnel thruster, in the center vessel, can respond much more quickly.?The right, single tunnel thruster vessel has a problem, as loss of the stern tunnel’s redundancy group means the remaining main prop needs something to push against to maintain stern sway and yaw control, so the configuration has to bias that group’s main prop against the forward azimuth to provide wash for the rudder to allow a limited environmental envelope.?Bias has some problems that I won’t discuss but the third configuration obviously has limited beam, and almost no stern, redundant capability and should be avoided.?There are alternate single stern tunnel configurations and it is possible to make most work, but it is hard, usually not understood or maintained, and better avoided.?The use of bias in the first two configurations is an exaggeration for the purpose of illustrating the problem created by having to wait a long time for the azimuth to turn.?There are failure modes where large turns are needed and turning and ramp time can be critical to position keeping, but the bias provided a simple, clear, first example of the problem.

The pictures above show the same situation but without bias and skips the problematic single stern tunnel configuration.?In both vessels, the environmental load is the same, but the tunnels are running at 25% thrust each, and the azimuths are running at 15% thrust and slightly offset to avoid interaction.?The interesting failure mode is no longer loss of thruster, where the responses are similar, but failure towards full thrust.?Unless the faulty thruster is immediately detected and stopped, both the stern tunnel and azimuth thruster will need to reverse thrust to compensate for the excess power and that takes an azimuth thruster longer to perform.?As mentioned, this can be avoided with a quick-acting thruster watchdog detecting the fault and shutting down the malfunctioning thruster but most vessels lack these and even those rare vessels with automatic shutdowns often take a few seconds to detect the fault.?I’ve found three seconds to be typical for most operator or automatic intervention.?Some people may think that failure to full is only found in controlled pitch and not found in fixed pitch, electric, variable speed thrusters.?They are mistaken - the electric failure mode is rarer but worse because it can be faster.?This is worse with a common power plant, as it can deny power to the opposing thruster if the malfunction ignores power limits.?Independent, automatic, rapid, reliable shutdowns become vital, but are rare.

The above drawing shows an additional complication that reduces the healthy and fault response of azimuth thrusters.?Smaller ships and semi-submersibles need to worry more about thruster interaction and work & transducer zones that cannot have thruster wash.?These forbidden zones limit the directions that the remaining healthy azimuth can thrust, which reduces healthy DP capability and dynamic failure response.?For example, if forward wash is forbidden then capability will be limited on the stern, and after a fault, the remaining thruster may be slowed by having to go the long way round to correct position.?Even healthy stern sway and yaw thrust will be limited.?The fixed thrusters have defined wash areas that the work areas (but not the transducer in the picture) are placed out of, so its work and capability can remain unaffected.?Large forbidden zones can make azimuth thrusters a poor choice.

The above drawing shows another concern in ships.?Thrust losses vary with direction due to hull interaction.?It’s fine to assume equal thrust in all directions in passive conditions where the error will get built into the DP model, but the resulting errors are unacceptable in dynamic fault handling.?When the wash is away from the ship, there are no problems, but when the wash is over the hull or into the skeg then the resulting losses need to be scaled into the model.?The same is true for the main props but that is usually captured during scaling.?Main azimuth thrust reduction with direction sometimes isn’t.?A DP control system without this scaled is less capable of reacting safely to sudden changes (e.g. faults).

It can be seen from the examples that most of my concerns are with azimuths in small DP vessels.?I can’t quantify the effect of azimuth delay from testing and fixing various vessels, but slow azimuth response sometimes had a noticeable negative effect on dynamic fault recovery and capability.?Fortunately, Voith has compared different thrusters in the same ship and documented the response of azimuth delay on dynamic position keeping capability.?The picture below is from their 2017 MTS DP Conference presentation (https://dynamic-positioning.com/proceedings/dp2017/Performance%20-%20Pivano%20-%20presentation.pdf).

The left plot appears to show a sizable loss of 30-40% of capability, until you look at the scale and realize the center is not zero.?When you realize that the center point is 10m/s, azimuth delay causes an average loss of 15-20% compared to the fast acting Voith thrusters’ wind envelope.??With a 1.75kt collinear current and very tight position and heading control limits (1m, 2.5 degrees), the effective overall loss with larger limits (3/3 or 5/5) might be 5-10% and the proportion of wind and current loading are unknown.?This can still be significant compared to the 20% dynamic margin used in passive DP capability plots but it is getting close to the size of calibration errors.?The right plot shows a frightening difference between the standard, passive, DP capability plots and actual dynamic capability, but some of the difference is caused by the tight limits.?Passive plots are a known problem and need replaced with proven dynamic ones.

Finally, hidden thruster azimuth feedback calibration faults are more of a concern when you have a smaller number of main azimuths, and unlike with a rudder swing, there may be no wash to diagnose the problem with.?An azimuth thruster that fails to hidden azimuth offset or rotation is often minor when a vessel has twelve main azimuth thrusters, but can be critical when it has two.?Hidden azimuth angle control faults have generally been the most insidious azimuth faults that I have found during testing.?Hidden faults in thrust speed or pitch control can be detected by comparison with thruster load, but azimuth angle usually lacks an independent feedback and has common failure modes.?This makes regularly checking the angle of orientation of the azimuth thrusters especially important.?Azimuths that provide independent local indication (not through the common feedback gearbox) can be regularly compared with feedback or cameras installed to allow the DPO direct access (preferred solution).?Without independent feedback or direct physical indication of angle, azimuths need regularly tested (e.g. during setup or before critical operations).

The industry loves azimuth thrusters for their obvious and immediate benefits, but there are limitations that need to be considered and managed.?Small numbers of azimuths are less suited to tight position keeping, more vulnerable to faults, and more impacted by forbidden zones, but the drawbacks of azimuth delay were smaller than I expected.?All thrusters have trade-offs and there are some solutions, but azimuths are not always the right tool for the job.