60 Centimetres of Steel: The High-Stakes Game of Digital DUKC Navigation

Dynamic Under Keel Clearance (DUKC) replaces legacy, static depth rules with real-time mathematical modelling—integrating high-density bathymetry, met-ocean telemetry, and vessel squat hydrodynamics—to maximise cargo payload while maintaining an unblemished safety record against groundings. While commercial charterers champion DUKC for its economic gains and marine insurers value its data-driven risk reduction, the system narrows physical safety buffers to a Net UKC as low as 0.5–0.6 meters. This leaves clearance calculations exceptionally sensitive to speed variations, swell period forecasts, and sensor integrity delivered to Portable Pilot Units (PPUs). 

Barry discusses Under Keel Clearance, the effects of too little of it, and the fine balance between a complex DUKC approach and the basic but ultimately proven static approach.

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Transcript

SPEAKER_00 0:00

A very warm welcome everyone to the Maritime Education Podcast. My name's Barry Sadler. To those of you who are our regular listeners, welcome back. It's great to have you on board once again. Those of you who are new to the podcast, a very warm welcome to you, and I hope you find something here that's both educational and entertaining. Those of you that are here out of interest who perhaps don't even work at sea or not part of the maritime community, you are more than welcome to listen to and enjoy some of the facts, figures, and debates surrounding maritime activities in the modern world. Today I'd like to talk to you a little bit about dynamic underkill clearance systems and how they are evolving on a worldwide basis, what effects they're having on maritime shipping, in particular the effects that they're having on pilotage, and how they can and perhaps cannot be used in the future to enhance maritime safety. Most underkeel clearance calculations are done on a static basis. Certainly in the port of Southampton we work on static underkeel clearances. What do we mean by that? Well, what we mean is that we have a fixed figure that we use when calculating the amount of water that we would prefer to have between the hull of the ship and the seabed. In other words, making sure that when we move ships of any size in any area of the port of Southampton, there is sufficient underkill clearance, taking into account the size of the ship, the speed that the ship is going at, any turns that the ship may make, any adverse weather that the ship may encounter causing swells, and of course the intricacies of the manoeuvre of the vessel taking into account adverse dynamic effects that may exist when there are very low underkill clearances. Now, the principle of maintaining a safe underkill clearance is based on lots of different factors. The main factor, of course, that it's based on is the amount of the ship that is beneath the surface of the water, known as the ship's draft. Now, ship's drafts in the port of Southampton vary wildly. Our smaller vessels that occasionally go up the river itchin, or our smaller gas ships, can have drafts as less as three to three and a half meters, whilst our ULCC container vessels, if they come in deep, can have drafts of up to 16 meters. So we have this huge variance of draft in the port of Southampton. Now, draft has more of an effect than just the distance between the keel and the seabed. In other words, we're not just calculating an under keel clearance based upon the fact that we don't want the vessel to ground, it's based upon displacement as well as the physical amount of ship that's under the water. So what do I mean by that? Well, obviously, the bigger the ship, the heavier the ship is, the heavier the ship is, the more water it will displace. In other words, if we've got a very large container vessel and it's coming in at 15 meters draft, it could be displacing somewhere in the region of about 250,000 tonnes. That means that as it moves around the port, as it's maneuvered alongside its berth, that vessel has got to basically brush aside 250,000 tons of water in order to move around. Brushing aside of this water is relatively easy when there's lots of distance between the keel of the ship, the underside of the ship, and the seabed. But when that distance is reduced, particularly when it's reduced to the minimal amount, the ability for that water to move around becomes constrained, and therefore the hydrodynamics of handling the ship do become perhaps a little unusual, sometimes, dare I say, a little unpredictable if we're running on very very short distances between the keel and the seabed. Static underkeel clearance would take into account what we call squat, which is basically a displacement of water from around the ship, which can reduce the amount of depth between the hull and the seabed by essentially displacing the water from around the ship and the water not being replaced quick enough as the ship moves. Now, squat is a factor of speed, and the faster you're going, the faster that water will have to flow to replace the hole that you've left. So if you're moving an ultra-large container ship displacing 250,000 tons at 15 knots, then basically the water around the ship is going to be moving rapidly to fill the hole that that 250,000 tonne ship has left as it moves. If that water cannot move freely, in other words, if it can't flow freely under the hull, where there may be a reduced underkill clearance, then that causes squat, and that's a very very dangerous situation to be in. Now, squat directly related to speed, as I say, so we can we can eliminate quite a lot of squat by slowing the vessel down, which means that as a vessel is approaching its berth, uh it's going very very slowly, uh, and therefore squat becomes less of an issue. But that movement of water and the change in hydrodynamics is an issue that exists regardless of whether the speed of the ship is high or not. In other words, you might be trying to push the ship sideways with tugs in an area with very very low underkill clearance, and that might take a lot of power from those tugs because, again, you're trying to move the water through a very very restricted space. Static underkill clearances will also take into account heel, that is, when the vessel heals over, the side of the vessel that we heal towards will effectively go deeper in the water, and that reduces the distance between the keel and the seabed as well. Weather, very very notoriously difficult to predict, and let's look at the summer that we've had here in the UK, where we've had heat waves now for weeks and weeks on end. The weather is getting even more difficult to predict. So being able to predict large swell waves coming in from the approaches to a port, very, very difficult to do these days. We've had numerous times where the weather systems that have been forecast to come over the UK have changed course, been less intense than forecast, more intense than forecast, lasted longer. So weather at the moment is very, very much a variable that we are still desperately trying to get on top of. So for instance, a ship approaching the outer pilot station to come into the port of Southampton and using the deep water channel, if it's an ultra-large container ship, the minimum static underkill clearance that we will use is 2.2 meters. Now we're very very rarely at that minimum. That would mean that we're either at the start of our tidal window or at the end of our tidal window, but that is a minimum static value that we will use. That static value has historically been very very successful. We have not had any groundings due to the reduction in underkill clearance to, for instance, that 2.2 meters, and we've had no adverse handling effects of these ultra-large container ships as we bring them up the deep water route with 2.2 meters underkeel. So that's just an example of a static underkill clearance. Now, why is underkill clearance so important? The obvious reason that it's so important is we don't want the vessel to ground, so the vessel must always operate in sufficient water so that it never touches the seabed. A vessel that touches the seabed is said to have grounded, and even a very very brief grounding on a relatively soft bottom on an ultra-large container ship can cause damage to the hull. We can touch the propeller or the rudder on the seabed, that's going to cause massive amounts of damage and has the potential to incapacitate the vessel. So we need to ensure that we've got sufficient underkill clearance UKC for that not to happen. We also need a half-decent UKC to be able to operate the ship at the speeds at which are safe for the particular area in which we're transiting. Berthing and unbirthing the vessel at ULCC, the speed's going to be well under 2 knots, and therefore we're not so concerned about that underkill clearance because we're moving so slowly, we're not creating adverse dynamic effects. However, if we let that underkill clearance go down to bare minimum, then we could see the hydrant dynamics of the vessel change and we need more power to achieve the same result, or the ship may become a little bit unpredictable. On passage, though, underkill clearance is equally as important. This is because when we are doing certain turns, particularly in the port of Southampton, we like to have a ship at a certain speed because we know the ship will be able to handle the turn and will be easier to manage through those turns. Now, the two big important turns that we talk about in the port of Southampton are a big important turn at what we call the Nab Deep Water Route, which is a deep water channel which is about 10 miles off Portsmouth, allows us to enter the Solent safely. And of course, our notorious West Bramble and Cowshot turns. So inward bound, that's a 140 degree turn to starboard, followed by a 90 degree turn to port, a chicane, if you like, around the notorious Bramble bank. All of these turns require the ship to be at a speed where it can be managed in the turn. If the ship is going too fast, then we can't turn the ship enough to make the ship fast enough to make the corner. If the ship is going too slow, then the ship will labour around the bend, will become overly affected by tide and wind, and there may be insufficient flow of water across the rudder to make the rudder efficient enough to be able to turn the vessel as required. So our underkill clearance is crucial when the ship is moving at speed. Now, a ULCC will typically do the Nabdeep water turn at about 12 knots and the West Bramble and Cowshot turns at about 10 knots. This varies sometimes a little bit below that, sometimes a little bit above that, but they're around the rough figures that we would use for being able to efficiently turn these huge ships across these bends, and therefore underkill clearance is imperative to ensure that even at speed we have the ability to turn the ship as required on passage. Adverse hydrodynamic effects of a reduced underkill clearance, even at very slow speeds, are well documented. The fact that ships are sometimes hard to stop when they approach a berth that is at the end of a waterway and is carrying all that water with it with a reduced underkill clearance. The carriage of the water as you move tends to keep the ship going. In other words, because you're displacing so much water, and in the case of one of these ULCCs, as I say, displacing around a hundred and sorry, two hundred and fifty thousand tons, because you're basically moving that mass of water into a cul de sac, it has the effect of the ship stopping, but the mass of water still moving, and a reduced underkill clearance will mean that instead of the ship stopping and the water flowing nicely underneath the ship to its destination, the ship will try to stop. The water around the ship's got nowhere to flow, so it will literally move the ship on. So one adverse effect of a very very reduced underkill clearance is this effect that vessels are difficult to stop at the ends of waterways. We have that problem in the port of Southampton when we go up to our container berths, berths one to four, these are basically at the end of the deep water part of the port, and therefore the water has nowhere to go. So we sometimes find that if we're taking a very big ship to those berths with a deep draft and minimum underkill clearance, we feel that push ahead from that body of water moving. Even at very slow speeds, we have some very tight turns to make and some swings to make in order to position the vessel at the correct heading and distance off aberth to be able to put the ship alongside, and a very reduced underkill clearance will make the handling of the ship, how the ship behaves, difficult to predict. We may need a lot more power on the tugs or the engine. It may be that the ship is not responding to helm orders and engine the way that I described on the turns. This can also happen as we're approaching berths. So reduced underkill clearance can make ships a lot more difficult to handle when we're approaching the berth and a lot more unpredictable. So sufficient underkill clearance to try and minimize these somewhat unpredictable effects are extremely important. So I've talked about static underkill clearances, I've talked about why underkill clearance is so important. I guess I should define underkill clearance. The end of the day, underkill clearance is the distance between the hull of the ship, so the deepest part the ship is in the water, and the seabed itself. And it's this measurement that is critical, as I've described, to keeping the ship safe and being able to manoeuvre it efficiently. Now we talk about dynamic underkeel clearance systems. A dynamic underkeel clearance system will not use a static number to ensure that the ship is safe, it will use a variable number based upon the predictions that can be made as to how that ship will behave in certain waterways at certain speeds in certain weather and tide conditions. In other words, by initially surveying and measuring the effects on a ship as it dynamically moves through the water, we can hopefully predict exactly how much underkill clearance we need in order to minimize the amount that we require in order to have the effects desired, sorry, the desired effects that I've just described. In other words, to be able to manoeuvre, turn and stop the vessel efficiently. Dynamic underkill clearance systems take into account exactly the same factors that a static system accounts for. Just that a static system is a fixed number that accounts for these effects. The actual resultant dynamic underkill clearance after effects of speed, healing, wind, weather, and tide are known precisely on a dynamic system, but are perhaps a little bit unknown, but known to be safe on a static system. So a dynamic system has lots of perceived benefits over that static system. One is that of precision, in that static underkill clearance rules rely on, if you like, a gross estimate of a worst-case scenario underkill clearance. In other words, we're working on a minimum static number, which may be too much, but if it's too much, who cares? We'd rather too much underkill clearance than too little. Whereas a dynamic underkill clearance system is a lot more precise. It takes, if you like, a bottom-up approach, calculating the precise real-time safety margin based on exactly what the water and the ship are doing at any given moment. This can be seen to eliminate the risk of grounding, but it may reduce any safety buffer that you have. In other words, whilst it's not going to reduce the underkill clearance any more than perhaps the static system would, what it does do is it eliminates the risk of grounding due to an unexpected event. So because it's taken into account the weather and the tide, etc., these are no longer unmeasurable and unexpected events. Now you could say that a static underkill clearance deals with that as well, it just deals with it in a in a gross, very, very perhaps um uh overdoing it kind of way. But dynamic underkill clearance systems reduce some of the guesswork by saying this is exactly what's going to happen, so this is exactly where we need to be with our underkill clearance. This sometimes means that we can make the ship a little bit deeper in the water, we can increase the draft, which of course can maximize the cargo capacity of the ship. And most dynamic underkill clearance was set up to do exactly that to move a ship off the berth and take it into deep water at its absolute maximum draft, so it has absolutely maximized every ounce of cargo space that it has on board the ship. Dynamic underkill clearance can expand what we call the the tidal windows. In other words, tidal windows based upon a static underkill clearance may be seen as being a little bit overly cautious, whereas a dynamic underkill clearance can use the precision associated with the ability to measure these effects to expand the tidal windows and allow ports to operate to more commercial advantage. The other thing that it can do, of course, is reduce the amount of dredging and hence the amount of environmental impact when trying to maintain a channel depth to allow these huge ships to come and. Go. Now that sounds all very, very modern. It sounds all very, very smart and precise. And yes, we've got the kit that can measure exactly what these effects are. But those of us that have been at sea for a long time know that the effects on a ship that happen one day may not be the effects that happen on the ship next time it calls at the port of Southampton. The ship may behave differently. Ships are not like trains and cars and things like that that follow tracks and roadways. Ships will very, very rarely follow exactly the same track going in and out of a port. They may be 20-30 meters left or right of the last time they came into port. The precision of where we take the ship is not that straightforward. Not straightforward like a train, for instance, which can only ever follow the prescribed line of the track. A ship can wander off its intended route even by just a few meters either side, which is going to have this, if you like, unpredictable effect on the prediction of the underkill clearance and the prediction on what the ship is actually going to do from a dynamic sense. So sometimes to the learned mariner, this can this can produce quite a lot of scepticism, and um most pilots, uh ship handlers are trained to rely on on instinct, historical precedent, physical cues, things like that, and also rely on the static underkill clearance calculations that they've been doing for years and years. These static underkill clearance uh calculations have worked. Certainly worked for me after 25, nearly 26 years of piloting. I've not used anything other than a static underkill clearance, and I've never had an issue with the handling of the ship. So there is this skepticism from those of us that have been doing it the old way for a long time. Dynamic underkill clearance models are a complex mathematical model, and we can sometimes be a little bit resistant to trusting this algorithm that they have, taking into account these fine measurements, knowing that these fine measurements may not be the same from one ship to another or even one visit to another. So at the end of the day, if the software dictates a historically safe transit is mathematically unsafe or vice versa, then we may start to doubt the kit itself. Dynamic underkill clearance systems may result in what is effectively a much smaller static number being derived. So, in other words, when the dynamic underkill clearance system does its calculations, it may turn around and say that the equivalent static number is smaller than the static number that's been used by experienced pilots and masters for a long time. Dynamic underkill clearance systems depend a lot on data accuracy, and I think this is where I have my most um scepticism about the whole dynamic underkill clearance thing. It relies massively on data. It relies massively on accurate data for charts and depths, and it relies massively, hugely on very very accurate differential GPS systems that are capable of being corrected to accuracies of below five centimeters accuracy. So this is very very new ground here. Even a GPS that is corrected by a basic form of correction, such as satellite-based augmentation systems that use satellites to constantly correct the signals coming from GPS and other global navigation systems, these systems are only accurate to below about 50 meters accuracy, in other words, within 50 meters of where the ship is. Sometimes down to 30, sometimes even down to 10 if we've got a very, very good S-Bax system working. But to really get the accuracy that we need for these dynamic underkill clearance systems to work, we need real-time kinetic connection corrections. Sorry. That means corrections from a base station relatively near to where the ship is actually operating, so that the corrections that the base station receives are the same corrections that the ship would need. Satellite-based augmentation systems are exactly that, they're based up in the uh up in the heavens with the other position fixing satellites, and although they are an excellent way of eliminating errors with position fixing systems, they are not accurate enough to allow a dynamic underkill clearance system to work effectively. Even if that were the case and we did have such an accurate GPS measurement, we still once again cannot guarantee that the ship will be on exactly that position at the exact time that the dynamic underkill clearance system says it will be. In other words, I'm coming in on my big ULCC, I've got a uh a yacht that's uh sailing a little bit close to the wind, getting a bit too close to me. I decide to move across the channel by a hundred meters in order to keep out of its way, then it doesn't matter how accurate my GPS unit is, the ship may still be out of position. And anybody that's driven anything in the solent will know that it's a very, very common occurrence for us to have to adjust our position in the channel slightly to account for all sorts of external influences such as leisure activities, survey activities, uh yacht races, all these kind of things. Dimish diminished long range predictability is another problem with dynamic underkill clearance because, as I explained before, weather and swell conditions are very very volatile, they affect dynamic underkill clearance systems massively. If you attempt to predict a sailing window more than let's say 24 hours in advance of what's actually happening with the weather systems, then your algorithms become less and less accurate. This is what I was mentioning earlier on about historical weather data becoming less accurate, models becoming less accurate because they are based on observations and the weather is massively changing. Who'd have thought that we would have had nearly one month of temperatures above 30 degrees Celsius in any part of the United Kingdom? A one-month heat wave. If you'd have said to somebody 20 years ago, we're gonna have a one-month heat wave where temperatures are gonna be above 30 in the UK and that's gonna happen in ten years' time. I'm sure there'd be a lot of skeptics out there that say, no, I think you're being a bit pessimistic. Yet here we are. So our ability to predict the weather beyond 24 hours is severely diminished at the moment, and a dynamic underkill clearance system requires accurate weather forecasts in order to operate. High implementation complexity. What do I mean by that? Well, upgrading to a dynamic system is one fairly expensive, and it requires buying and maintaining these physical sensors that measure the effects on the ships coming and going to and from the port. Now, these sensors have to align with heavily digitized inputs such as uh the chart, the tidal height, the actual wind speed and direction. And this can cause a clash when we try to bring in complex modern digital technology and feed it with basically what is somewhat older, more established and refined data such as chart data from the area in which we're operating. Don't get me wrong about dynamic underkill clearances. Currently, there are no recorded worldwide cases where a certified dynamic underkill clearance system has caused a vessel to ground or suffer hull damage. That's the claim from the dynamic underkill systems, chaps. But don't forget, as I said earlier on, we're not just talking about grounding, we're talking about the physical handling of the ship, the ability to push that ship through the water to achieve your objective of berthing or unberthing the vessel, swinging the vessel, whatever it is. You might not ground, but you might require more power, you might have a few close calls where you are unable to maneuver the ship as required because a dynamic underkill clearance system has calculated so precisely how much water you need that it has reduced the same effective and equivalent static number down. At the end of the day, there are arguments for and against a dynamic underkill clearance system, but at the end of the day, I always go back to what I was taught in secondary school about computers, and that is the garbage in, garbage out risk. In other words, if everything isn't aligning to the conditions that the dynamic underkill clearance system wants, in other words, if we've got garbage going in, garbage going in could be a dodgy position fix, it could be a dodgy weather forecast, it could be a dodgy sounding off the chart, it could be a dodgy ship, it could be a dodgy encounter with another vessel that causes you to move off track. Once that input into that complex system is garbage, then you're gonna get garbage out. Speed adherence failure. Speed will always be one of the massive governing factors as to how much resultant underkill clearance you've got the faster you go, then effectively the less underkill clearance you're going to have. And if you can't adhere to the speed, then you've got a problem. A great example could be a delay when you board the ship. It's happened numerous times to me. I've boarded a ship coming towards the end of its tidal window, needed to get in, and there's a ship sailing late, or another ship has perhaps impeded me, and I've had to slow down. My static underkill clearance kind of takes that into account because I've got that little bit extra up my sleeve. The dynamic system will not. Now the decision will be the pilots as to whether to keep going, having been delayed for whatever reason, whether to turn around and get back into deep water, but ultimately this is pressure that we don't particularly want when handling a ULCC. At the end of the day, we are at a very very infantile stage of dynamic underkill clearance systems here in the port of Southampton. They've been operating successfully in many ports around the world based upon many different objectives. Here in the Port of Southampton, we have so many variables that apply. The length of the passage into the port of Southampton is 26 miles, and in that 26 miles, so much can happen that can cause the ship to no longer follow the exact route, timings, or speed the DUK system requires. And therefore, because we're such an early stage here in Southampton, I think we'll have to wait and see on the results of our dynamic underkill clearance trials, which we've started, and see whether or not these accurate underkill clearance systems are everything that they claim to be. I hope you've learned a little bit about uh these dynamic systems and some of the thinking behind them, particularly some of the thinkings behind manoeuvring a ship with a static underkill clearance, which is what I do for a living at the moment, and uh the success and safety that's generated by doing things that way compared to the relatively unknown route of the dynamic underkill clearance system. Thanks for listening, everyone. We look forward to welcoming you back to the Maritime Education Podcast. Please do hit subscribe if you haven't done so already, and I look forward to welcoming you back soon. Thank you very much, guys, and have a great day.