Rule of 25 - Limitations

Introduction: Last week, I introduced a simple redundant power rule of thumb for fixed pitch propeller (FPP), variable speed (VSD) thrusters that allowed a DPO or engineer to look at thruster power feedback and identify when they were no longer redundant. I simplified a lot of things to keep the article short and to emphasize the main idea. Now that you have the main idea, it’s time to add some minor complications. If you haven’t read the last article, or have doubts about it, then go read it here.

Power Feedback: First things first, we need to compare apples with apples. The thruster feedback is the drive motor load, not the motor load plus auxiliaries. The power/thrust curve is true of the ideal propeller drive train, but at very low levels, auxiliaries, losses, and friction start to dominate. Some ships provide lump load feedback (motor + auxiliaries + losses) and some vessels measure motor load. If you are running at 35% of full rpm and have an auxiliary load of 5%, then the auxiliary load is bigger than the power used for thrust (0.35x0.35x0.35=4%). Know whether you are looking at combined (raised base floor) or motor power, as the base load is significant at low speeds. If you have a base floor of 5% of thruster power, then the rule of 25 can no longer be true. The power feedback goes from 5 to 100%, instead of from 0-100%. 25% becomes conservative because the motor load only represents 95% of the maximum feedback. Zero motor power is 5% feedback and 100% motor power is 100% feedback, so 25% is 5% for base load and 20 out of 95 for motor load. That is 21% of motor capacity (20/95) and more conservative than a direct motor measurement, but that might be a good thing, as we will see. I’m going to assume motor power measurement for the rest of the articles, and ignore motor and drive train losses to keep things simple, but you can see how to adjust things (whether 3% or 5% base), if you need to. (Yes, frictional losses will increase with speed but we are keeping it simple)

Dynamic Margin: The rule of 25 is based on the standard 20% dynamic margin. Sometimes 20% isn’t enough. Experienced DPOs know what I’m talking about, and it has been measured and documented in a study that made a major DP company “unhappy” by exposing performance gaps where vessel spent an unexpectedly larger percent of the time not on position. The environment can vary by more than 20%. Take this story from Bill: “I'm an engineer but was a DPO in the early days of DP in the North Sea, the thrusters would sit at 25 per cent, then a big wave would come along and push the vessel off station and they would go to 80 per cent for 5 seconds and then settle down again.” That’s back when engines hadn’t heard of pollution or teeny tiny load steps, but you get the idea. If you are an experienced DPO, you can probably think of some examples of your own, and provide stories in the comments. The excess dynamic load might be current, it might be waves, it might be gusts, or all three, but the environment is not tame and will not respect the 20% boundary. There is a reason experienced engineers and captains don’t wait for the PMS, but add DGs “prematurely” to stay safe. 20% is a slightly conservative base benchmark, but more is sometimes required. Sometimes the waves are coming from two or three different directions, the wind is swirling, and so is the current.

Dynamic Excitation: Sometimes even simple environments can overexcite a DP system with a form of resonance. You’ve seen videos of how a small sound at resonant frequency can shake a glass so much that it shatters, or of machinery or structure vibration at resonant frequency. The same things can happen to ships rolling and position control. The difference between the active centers of gravity and buoyancy create a period that can be excited by waves and cause excessive rolling. The DP system can be worse, because it actively adds power into the system to control position (like pushing someone on the swings who is already using their legs to build up power). If the gusts or waves hit at the DP system’s control frequency (DP + thruster response times), then the DP system adds power in synch with the environment. A wave period near a DP control frequency or a roll frequency can cause building hunting, unless the DPO changes the heading or gain. Wind gusts can be relatively simple to catch but is harder to catch when there are waves from different directions, and only one of them at a critical period. In that case, it doesn’t matter how much redundant power and thrust is available, because that isn’t the problem. DPOs need to learn the ship specific DP & roll frequencies over time, and the roll frequency will change with vessel loading. Dynamic capability can be very different from static assumptions, but only the people operating the vessel can estimate by how much at that particular time. The needed dynamic margin varies by vessel and condition, and resonant movement needs removed if the rule of 25 is to be useful. Know your ship, know your conditions, minimize vulnerability, and increase your dynamic margin to match the environment you face (more in a future article).

Power Rate: You can have all the power in the world, but if the vessel doesn’t have the ability to apply it quickly enough to be useful, then the 25% power limit is too much. This can go both ways. Tight position control limits require greater responsiveness and accuracy, but wider limits require less response and control. At wide enough limits (deep sea drilling), even the normal static DP plots are true, if properly done. They usually aren’t. I wouldn’t recommend going below the 20% dynamic margin, but I have been applying greater margins to vessels that need to maintain standard position limits, but have slow azimuth or slow ramping. If you have done DP trials and paid attention, then you have probably compared the thruster response with the decreasing distance from the acceptance limits. It can sometimes be painful to watch. I have some unofficial rules of thumb that I will share in a future article.

Power Available: I’m old fashioned. 25% of available power is the power currently on the switchboard. Engines may not start and generators may not connect, especially during an electrical fault or when the load is changing quickly. Some people don’t know this and get caught out by trusting their PMSs. As an electrical engineer, it never occurred to me that people didn’t know this, until I started talking to them. Synchronizers have limits, and when they are exceeded, the DG won’t connect because the risk of connecting out of phase is too high. DGs might start and connect, but if you know that you are going to need them, then put them on the board. Better to spend a little fuel, than to not have enough power, lose position, and gain a few days of unpaid investigation, or worse, a serious incident. All the old timers will tell you, “Redundancy costs fuel.” Operators want to save money, but they also need to manage risk. Rare but expensive incidents have outsized costs that need considered. There is a reason the experienced captains and engineers put more engines on.

Conclusion: So we have looked at a few limitations. The rule of 25 is based on the right kind of feedback and doesn’t cover dynamic control faults. Those need detected and avoided by the crew. The rule of 25 is based on the 20% dynamic margin, but that may not be enough in some environments and slow system response may require a greater margin to stay within the position limits. Tighter position limits require faster and better control and a greater margin. Finally, make sure that you have power on the board when you need it, because the system may not be able to connect offline DGs, and you might not want them to try to. Next week, the Rule of 25 and thruster losses.

Apologies for the delayed article. I was trying to hammer some work out. I probably should have used a hammer, rather than my head.