August, 2025 DP Questions
Introduction: This month’s question article has ten questions. Each question covers an important dynamic positioning (DP) issue, but the original question has sometimes been edited for length or focus, name removed, and ship and system identifiers disguised. There are sometimes supplementary questions and issues in a question. While the answers are mostly what I gave at the time, extra time, sober second thought, and readability means that I got to add extra information and links. The ten topics are:
Question 1: We have been operating for years with incomplete DP capability plots so we had them redone. What we got back showed all thrusters running at 100% even though there wasn’t enough power to supply them like that. It’s a traditional main engines driving shaft generators and main props with shaft generators driving tunnel thrust design. It has a single switchable stern thruster.
Answer 1: Yes, the new plot is no good as a DP reference. It is essentially a piece of sales literature. It doesn't reflect actual redundant DP capability, but a magical capability that could be used in vessel advertising. They haven't included real limits and losses, and that increases the imaginary capability of the vessel. They didn't consider the real worst case capability, and it’s bad because the vessel has main props and a single stern thruster.
I’ve written quite a few articles on redundant DP capability, but let’s stick to IMCA M140 Rev 2. They require a 25% dynamic margin to allow recovery after a fault. This is reserved thrust, so the power available for capability is 75%. But they recommend average 15% hull losses. That might be a bit low for some tunnels (20-40% due to a combination of losses for length, grill, and angle), and is high when the mains props are thrusting forward, but let’s keep it simple and average 15%. Capability is now based on 60% of thrust. But we haven’t yet considered drivetrain and electrical losses. IMCA recommends 10%, which I think is a bit light, but we will follow that. Are the shaft DGs big enough to supply the tunnel thrusters at 100% despite 10% losses. Yes, they are, so we don’t need to worry about that and can subtract 110% of the used thruster power from the main engines. The drive trains (main gearbox, etc.) have their own 10% losses. Using the simple IMCA suggestions, the tunnels do not contribute more than 60% thrust to capability calculations (25% margin + 15% losses) and the main props are 90% power minus 110% of tunnel power, which leaves about half the main prop power before taking away the 25% dynamic margin and 15% loss when thrusting astern. Translating to apply the 60 to 75% thrust reduction to the already reduced power requires knowing the power/thrust curves. That does not include other losses, such as for current inflow or main prop directionality. It will vary with thrust allocation, as most of the power is available to the main props when the environment is on the bow, while the design is weaker with an environment on the stern.
A rough analysis using standard industry values shows a big difference from the assumption of 100% power and thrust. Your common sense and other information already told you it was impossible, and the rough math just backs it up.
Unfortunately, the designers did not use common sense and the vessel only has a single stern tunnel. The simple worst case failure leaves one main prop and one bow tunnel to control the vessel. The bow tunnel can control the bow sway and yaw, and the main prop can control vessel surge, but the main prop cannot control stern surge and sway without considerable environmental force to push against, so it can use its rudder. This is worse if it is a switching tunnel thruster that is capable of disrupting both redundancy groups.
Related articles: SSST, New M140, Footprints, Rough capability estimates, Old M140
Question 2: I would like to ask if it is a DP system requirement to perform a Heading dropout and it to behave similar to Position dropout? I mean, I do a DR test in my system and it behaves well, like 6m in 10 min. But the heading dropout, it goes 90 degrees within 1 min.
Answer 2: It depends on the system. At one point they tried to make the heading model as robust, so all the systems they sold with just two gyros didn't look bad, but it was generally unsuccessful, and eventually, all the DP rules required three independent position references, which should make it moot, so long as they really are independent (no common power, network, master gyro command system, or unprotected common load). Best of all is heading references based on different principles.
Heading model was an unsuccessful, short-term fix for aggressive, questionable, sales practices, but they didn’t work well. If you think about how quickly heading forces can change with just a small change in direction, then you can see why a heading model based on assumption of constant forces didn’t work out in practice. Position model has the advantage of active heading and wind references, and can still be thrown off by changing forces.
Question 3: Does a DP vessel have to be in CAM all the time?
Answer 3: No, but this is a mixture of contract and common sense. They are supposed to be DP CAM when within the 500m zone, unless agreed with the customer for a particular operation. Outside the 500m zone is usually TAM (reduced redundancy), unless specified by the customer. Assuming they are outside the 500m zone, drift off, and not in a position to damage anything (e.g. not using tautwires that they could drag into subsea equipment), then a vessel could close their bus ties, even go down to a single DG, perform maintenance, or perform drills during their non-critical, non-redundant operation. All such operations should have a proper risk assessment. As the old Hippocratic oath says, “First, do no harm.” Of course, a customer could require CAM standby.
One of the first DP new builds that I analyzed and tested blacked out a week after entering service. What went wrong? I worked so hard to find and clear problems, had a strong background, and dug deeply through the systems, but was new to the field. What had I missed? They were in standby outside the 500m zone, closed the bus ties and went down to one DG to save fuel, and then lost the running DG, because Murphy is right. You can lead a horse to water, but… It was a non-critical failure during a non-critical operation, and they eventually restarted my heart. After that lesson, I invested more time talking to crews about the contents hidden inside their DP Ops manuals.
Question 4: Is this also not a topic in DP? We have vetting, we have consultants running around ships conducting tests and sometimes pressing buttons where this should be the task of the crew. I have even seen ships with a "test" crew on board, because it was convenient and the systems could be tested more quickly. But is it systems, or humans? Last DP conference I attended there were still people focusing on systems and not humans. I get it, blaming systems is not personal. But in the end, most accidents can be prevented by humans. But then the industry needs to take a long and very hard look in the mirror. Because it is not the seafarers I blame, it is the bureaucratic system onshore which is to blame. Too much on the plate of the seafarers and they get less time than before to do what they need to do. Then they start to do less, they start to let things slide as they don't have time to do it. Then maintenance suffers and with lack of maintenance and knowledge of systems come accidents. Testing systems is fine, but are we not overtesting? Should the focus not be on the human and how they interact with the system? If the system is maintained properly it will work in 90% of cases.
Answer 4: You are very right. The human systems are the most crucial ones and the ones where we are failing the most, but we concentrate on technical systems, because we aren't crossing into other people's jobs when we do so. Some managers even insist that all problems are technical problems and require technical solutions. For years, I have had secret trials tests that I do to evaluate crew readiness and then try to stick in some light training and suggestions to fill gaps. It should be a major portion of safe DP operation, but managers don't like interfering with each other and are mesmerized by magic paperwork. Marine operations are so financially squeezed that the paperwork is all that some operators have time for. It is a dangerous abstraction that distracts people from the real vessel, real operations, and real crew. But it is simple and makes some people money, so it increases. Rather than using more paperwork to solve paperwork problems, we need to get back to basics.
In other industries, safe and reliable operation includes the operators. The heart of DP operation isn’t the DPC or other equipment, but the people who use it, and make it work. The base DP rules are blind to this. IMO MSC 645 doesn’t think about them, and IMO MSC 1580 wants the crew to be good on paper. IMCA and NI have some paperwork requirements. Our whole problem is that everything is good on paper, but the purposes behind the paper have been bypassed and violated. We are in the Wild West when it comes to the crew, and the guardians are part of the problem. Crews who don’t know how to operate their equipment or how things work should be a serious mark against a vessel, but it’s not in the acceptance criteria, and partially not their fault.
We hire the cheapest possible people (look at what has happened to the wages in the last 20 years) and get every bit of extra work out of them until they are dead on their feet, and then complain that they aren’t as good as the people with years of training and experience, who won’t work for those wages or put up with those conditions. Then those problems become a profit center and an excuse for kingdom building. I’m one of the bad guys and will threaten to fail a vessel if they can’t use their IJS, and will pause trials for training when the crew is a bigger danger than the equipment. When I was the technical authority for a DP consulting company, our trials had a crew section. It was small, but it’s only common sense.
If we don’t like what is happening, it needs to become visible and there need to be negative consequences. All of this is being driven by vessel clients who demand ships at the lowest possible price. We need a practical operation quality floor, not a paperwork one. We need to put Dynamic People back in DP. This is a skilled trade, not something that you throw bodies at. We need to address the real causes if we want this fixed, rather than another paperwork solution to be bypassed. In the meantime, good crews, good owners, and good clients at the upper end of the market need to seek each other out, as the rest are too dangerous, and undercut the good guys, unless those are recognized and rewarded for doing work right.
Question 5a: I'm working onboard a heavy lift boat, what are the most important considerations during the Lifting operation which affect the station keeping?
Answer 5a: Interaction between the heavy lift and DP control system is of particular attention and requires heavy lift modes, well defined station keeping limits, and well defined environmental (movement) limits.
Question 5b: Thanks, but how does the crane create forces that cause loss of position?
Answer 5b: It's not so much the crane, as the pendulum movement of the heavy load applying force on the ship and moving it. If you swing a yoyo back and forth, you can feel a little force, but nothing important. If you swing a heavy case, back and forth with your arm, not only are you moving it, but it is moving you, so you need to have good footing, and apply noticeable force to stay still. A ship doesn't have good footing, so it moves when the heavy lift applies force. The DP system doesn't measure the force, but sees the resulting movement and tries to correct for it. The pendulum moves the ship, the DP system moves the ship as a result, and the two interact. The DP system isn't good at dealing with invisible interactive loads, and the results can oscillate out, like wave excitation or someone pushing a swing. Heavy lift software tries to reduce the interaction.
This isn't just a position keeping problem. Remember how it takes a lot more force to swing a heavy case back and forth? With the pendulum movement exciting the DP system and the DP system exciting the pendulum, the heavy load swings further and further, but the crane is built to exert vertical force, has limited horizontal force tolerance, and can collapse under the dynamic load. It’s a bit like a man straining himself or breaking his arm by trying to handle too much moving weight too fast. From your own experience, you know that moving loads are harder to handle than passive ones.
More technically, there is something called DP stiffness, which is essentially how much force is applied per meter deviation from the set point. The pendulum with the heavy load has more stiffness (more force per meter movement) than the DP system does, so DP force needs to be carefully managed by specialized software that ensures stability. The load has almost no stiffness when connected but not tensioned for lift (even then if you move enough, there will be lots of tension), so heavy loads change from low stiffness to high stiffness as the lift begins. The heavier the load and the shorter the pendulum, the more the stiffness. You know this from playing with a yoyo or other pendulum – swinging at full length exerts little force but your fingers can really feel it when it has the same ark but is inches from your fingertips – that is stiffness. The DP equivalent of stiffness is the proportional control gain used to rev up the thrusters to oppose movement. DP stiffness can be changed and I’ve seen examples from 7kN/m to 46kN/m, but they are normally 30kN/m or less. The Converteam paper has more detail.
Use the DP trend view to catch oscillation. Plot your sway thruster force, residual (error) force, and speed, and look for oscillation, especially increasing oscillation. Trend plots are a powerful system operation and health tool that many DPOs don’t use very much.
All of this is just considering the basics, any heavy lift vessel should have dedicated heavy lift software, and they take different approaches, depending on the manufacturer, so read the heavy lift operators manual. The software gives more tools than I have discussed, but these vary by vendor and by software version.
Question 6: What happened to the auxiliary system articles? In April, you said your next one was going to be on combustion air.
Answer 6: I know, I know. DPEs are important too, and I will be getting around to it, but it might be a month or two. I kept getting interrupted by other concerns.
Question 7: Why do we use the terms open and closed bus tie when split and common bus show immediately what that problem is? The first terms are sort of neutral and using them concedes ground. The second set of terms gets to the heart of the problem. It’s like the difference between pro life and pro choice. Also, it is easier to understand because valves are opposite – closed provides isolation. Even closed doors isolate.
Answer 7: We tend to use ‘open/closed’ and ‘split/common’ interchangeably. ‘Open/closed’ describes the physical state of the bus ties, and ‘split/common’ describes the state of the electrical system. I agree that the latter one is more important, but it is also slightly abstract. While the first one feels more concrete, because it is regularly seen on the control system mimics. ‘Split’ and ‘open’ probably have the same neutral value, while ‘common’ and ‘closed’ may depend on the viewer. People who want to save money by taking more risk will see ‘closed’ as positive, and ‘common’ as a partial slander, and people who want less risk will see the opposite. I don’t think changing the use of terms will change the minds of informed people. It’s a good point about the valves and doors though, and maybe I should start favoring ‘split’ and ‘common’ more to avoid that confusion.
Question 8a: Can you help me? I’ve been given the opportunity to speak.
Answer 8a: Use it. Find a subject that you want to speak about, and cover that. Even if you do badly, it is a learning experience, but covering something that you care about makes that less likely, and is more likely to be appreciated.
Question 8b: I’m undergoing training, but lack a laptop and it handicaps me. Can you help?
Answer 8b: I understand. If you work on a ship, that is your library. Read everything there and learn from it. Think through it. Question it. It isn't always right. Many people have more resources, but do not use them. Use what you have as well as you can. Ignore the words and learn the principles behind them, and you will be far ahead of those with libraries who never read their books or never looked beyond the words. Every advantage is a disadvantage, and every disadvantage an advantage. You are forced to concentrate on what is most important. You do not have the resources to waste. Being forced to focus on the most important things can be very useful. What you learn from that will put you in a good future stead. I also had limited resources. It forced me to pay attention to the ones that I did have and to think deeply through them. The trick to life is making the best of limited resources. We all have limited resources. Anyone who says otherwise is selling something.
If you do not have a laptop, you do have a phone and can download free documents to master, such as MTS guidelines and IMCA DPE. I do not have a phone, but I see others use them that way. Resources.
Question 9a: I have a concern about the ship's outline display in the posplot and its possible effect on position keeping. My vessel is 115m long, but it is only displayed as 100m in the posplot. If the displayed ship's outline is not important, I will just continue without any action. However, if the displayed ship's outline shows it is calculating forces wrong, I need to report it and take appropriate action. In my experience, ship surge movement shows no problem. However, in the sway direction, the carrot is faster than the ship's movement. Therefore, I suspected the ship's projection data was not properly saved in the DP system.
Answer 9a: I think that the vessel display in the position plot is more diagrammatic. It is meant to show the general type and size, and most especially its heading, but is not used for measurements. I'd be much more concerned if the wind information was similarly wrong.
I'm concerned that you can see the length of the vessel on your DP display. While many DPOs don't like seeing the stubby mini-vessel when you zoom in, that should be your normal display. You should zoom in until the alarm limits take up most of the screen. Then turn on the trace, so you can watch the vessel movement, learn to correlate it with other information, and use it to detect developing problems. That invaluable understanding is lost if you are always zoomed out.
The control carrot should gently lead the vessel. The vessel lagging behind the carrot is usually an indication of a control problem. That is worth investigating. Does it start running to catch up? Does it run by? Or is it just slow to start and/or slow to follow? How do the thrusters react when you watch this?
Question 9b: My ship's work type is a cable layer. Honestly, I don't setup the position alarm, because the ship is almost always moving during the cable work. But trace and trend symbols are always set up. In sway movement, the ship starts and follows more slowly than carrot. And thrusters are using full load within the run-in setup boundary. In case of surge movement, when stopping at the set point, sometimes runs past the set point. At that time, the thrusters are full to the opposite direction. Furthermore, the ship is run by again from the set point. And this movement repeats. I am using the run-in function to reduce the thruster's output to control the speed in this case. I am still not sure whether this problem is caused by the DPO's control or DP system's control.
Answer 9b: Because you are laying cable, there are forces that are invisible to the DP control system, and the ship's motion interacts with the cable force. This is why changes in speed are desired to be gradually ramped, so the unknown forces from load interaction don't throw off DP control. From the DP desk viewpoint, you are trying to maintain constant or at least slowly ramped force in the cable lay feedback, so the DP control system stays happy. It can adapt to unknown changes slowly, so they need managed.
Your DP control system would have been tuned without a cable load, and probably has similar and consistent surge and sway responses in those circumstances. When you have been running cable for a while in a straight line and at constant load, the surge control has built most of the consistent force into the DP model, but the sway force is more variable and harder to put in the terms of error current, because it’s more like a spring. So, your sway control will be sloppier than the sway force, because it is proportional to how far off from "zero" that the vessel is. The interaction with the unknown force degrades position control, so the movements can't be at high DP speed. The vessel always has more surge power, but still needs to adapt to the spring effect of the load, and needs to try maintain a constant tension. Delay and overshoot is a response to the changing unknown forces, and is managed by limiting the speed of change to stay within DP control adaptation limits.
Question 9c: I am always mindful of the tension caused by external forces from cable work or other related equipment. Perhaps, I have been increasing speed a little too quickly. I will share this issue with my officers, so that we can increase and reduce speed at a more gradual ramp. Honestly, I had never heard about 'Spring effect' and 'Error Current'.
Answer 9c: The "current" displayed on the DP systems isn't based on real measurement, but is a combination of all the unknowns. To distinguish from the unmeasured water current, which can be importantly different, some professionals call it Kongsberg current, DP current, or error current. The further the error current is from the water current, the greater the danger to DP control.
The spring effect is the directional tension changes. It’s more of a shallow water effect. In deeper water, it will be a load effect.
Question 10: What happened with that SPRINT-Nav problem you reported?
Answer 10: I had a reliable report of a problem with a SprintNav installation that I shared. It was interesting and concerning, because the independent position reference was dependent on the DGPS when tested, so someone asked about it. The selling point of SprintNav is that it is an easy to use, beacon-less, independent, hydro-acoustic inertial position reference (HAIPR?) for shallow water. Sort of like an upside down DGPS as far as usefulness, but different principle. It looks like a potential, redundant sensor that would be really useful for a number of industrial missions that right now can only use DGPSs, so that was disappointing to hear. An experienced man noted that this had happened with some HPR-INSs when they were new, and that had proven to be commissioning problems. I heard that this was third party rental equipment, so Sonardyne wasn’t directly in the loop but wanted to help, because this was a test of their system for an important industry player. According to reliable report, it ended up being a setup problem that was resolved, and the system was tested with some box moves. It will be tested, logged, and used with other position reference sensors for a few months, as part of an independent evaluation by one or more big industry players. This is progress towards wider adoption, and there is an unrelated new paper on SprintNav in the October 2025 MTS DP Conference. (I don’t work for any of the related companies)
Conclusion: Hopefully, you found something interesting, and learned or remembered something useful. Learning is a process, not a goal. The modern world tells you to believe in yourself, as if it was doubt, rather than ego, that holds us back. I believe in curiosity - questioning what you think you know, exploring what you don't know, discovering what you should know, relearning what you have forgotten, and seeing/doing what you can make better. I suspect that curiously examining things and playing with them is more effective than rote learning. Interest is vital. I hope you found something interesting or useful in one of the questions and answers.
DP Incidents:
The Aug-24 DP incidents article is next week, so please share any DP incidents, lessons learned, or service bulletins by private message.
Older Multi-Question Articles:
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3wCap information about references Best por dp 3 ....
Online Course Presenter (Recoa.eu)
4wThought provoking as always, Paul. Thanks for sharing.