Making DP Plots Great Again?

Introduction:? There were major problems with the standard passive DP capability plots.? We will start with the background, before we see if the new revision of IMCA M140 resolves them.?

Capability:? Old articles expressed my dissatisfaction with the standard passive dynamic positioning (DP) capability plots being used in the industry.? Operators used to be told that these reflected the real redundant capability of the vessels, and to plan safe operation with that information.? It was repeatedly proven that DP plots generally didn’t reflect the real redundant capability of vessels, and eventually the last revision of IMCA M140 put in a note saying not to use them that way and to use footprint plots instead.? That wasn’t much of a solution and, rather than making lots of footprint plots to cover all different conditions, everyone continued using DP plots (intellectual inertia or slow news distribution), but tempered with ASOG thrust level warnings.? Usually the ASOG thrust levels were too high and didn’t reflect redundant operation, so I wrote a series of articles about redundant thrust levels, and about using independent power feedback as a check against false thrust (or power) feedback (Rule of 25 1, 2, 3, 4, and use of thrust/power tables).

Comparison:? The old revision noted that DP plots could still be used for vessel comparison, but that wasn’t really true, as a slow azimuth gave less dynamic capability than a Voith-Schneider of the same thrust, due to its limited ability to react, but looked the same in the passive plots and were cheaper.? This encouraged bad designs.? Real operation is dynamic, so I recommended adopting dynamic DP plots going forward, and use of proper redundant thrust levels as an interim measure and check during operation.

Sales:? DP plots are important sales documents, and that distorts apparent capability limits and vessel comparison.? Outside of DP, engineers don’t just build things and hope they will work.? They test them repeatedly.? DP plots need to not just reflect the design of the vessel, but the current state of the vessel, if they are to be used for planning safe DP operation limits or comparing vessels.? That requires an engineering system approach to testing and verification.

Result:? IMCA noticed the criticism early, did their own independent review, put together a group of experts, consulted industry, and updated IMCA M140 in Jan/25.? Let’s look at what they did.

Capability Resolved?? Improved, but not resolved.? The updated rewrite covers almost everything that needs considered, but does not require that they be included in analysis.? Static analysis is allowed without limit, but with good advice.? Dynamic analysis is introduced and defined, but the loop is not closed with actual engineering model and response validation testing, and regular retests.? Both consider keeping position after benign failures, like simple loss of equipment, but only the dynamic analysis considers the active failure response and if the resulting deviation was acceptable.? Neither analyzes position keeping capability with aggressive failures, like a thruster that runs toward full thrust but is tripped by protections, even though that is an expected failure mode that the vessel must stay within limits during and after.? The safety margins recommended used in static analysis are increased compared to the previous revision.? For example, dynamic margin is increased from 20% to 25%, which might be OK for some semis but not for most ships (see the rule of 25 series), 10% electrical losses included (I use 15%, or more if known), non-thruster load considered, hull losses increased to 15% when applicable, and skeg losses of 40% are included.? However the executive summary appears to indicate that these don’t have to be used, but what is and is not used needs to be clear.? Dynamic analysis can be based on assumptions, rather than real data, does not include some identified sources of error, and does not have recommended safety margins to make up for these faults.? For example, position references have error ellipses and hunt, and both get worse with increased vessel motion.? This analysis will not solve the problems, but it might be all that the industry is currently willing to stand.? We are told that the plots should be updated, when they no longer reflect the vessel equipment, procedures, or operations, but there a probably more criteria than that.? I’ve seen a vessel redundancy wiped out by software updates or hidden limits in equipment.? The more criteria used in the analysis, the more vessel factors that may change.? It’s why I like detailed annual testing and wish we weren’t just eyeballing it.? Finally, shouldn't updates be triggered by real operational capability not matching DP plot estimates? Or do we just add each example to the list?

Capability Use:? There will be no scientific or rigorous engineering basis to plan safe DP operation with yet.? Like the previous revision, this update is sometimes of two minds, DP plots are absolutely fundamental to planning safe DP operation and must be used in ASOG creation, or are unreliable and not to be used for such purposes (usually not in the same paragraph).? The useful purpose of the DP capability plot cannot be denied, but the cost to fulfil it is not demanded.? Usually, the middle position of testing DP plots against a series of footprint plots and tempering it with addition ASOG [thrust and power level] protections is recommended.? That is probably the best position and the one to follow.? I’m a little cautious of footprints, as the current trendy substitute, as, while footprint size normally increases linearly as the edge of capability is approached, this isn’t always the case.? Generating and analyzing many footprint plots for comparison, and proper ASOG redundant thrust and power levels are still vital to keeping DP plots honest.? They can be expected to occasionally detect loss of redundancy inside most static and some dynamic DP plots.? Users and clients beware: the problems are unresolved and the advice is to distrust, verify, and have backup redundancy protections in the ASOGs.? I forgot to add that IMCA recommends operators use their own judgement and experience to identify weaknesses in specific DP plots.? That’s good advice.? Know that it is required and take time to develop that judgement.? Like the DP control system, the DP plot is a DPO aid, but not a complete solution.

Comparison Resolved?? Generally, no.? The update still allows using static capability plots for vessel comparison, but requires that the factors included and excluded from the analysis be clearly defined.? This doesn’t do anything for experts who already know those factors, and had the uncomfortable habit of detecting lies or exaggerations in DP plots, and cornering providers with them.? It also doesn’t help people who don’t understand each factor, and when and where they are important.? It does educate the sober second thought of the people in the middle.? I miss some of the now missing old rules of thumb that were useful to people, such as typical power to thrust ratios (is the official thruster curve reasonable or exaggerated?) and the old requirement to do wind envelope plots at different currents for easy extrapolation (0m/s, 0.5m/s, 1m/s, & 1.5m/s).? Like to like comparisons are further complicated by using wave or current envelop plots instead.? These, heave, motion, and other limiting factors can be vital to individual projects, but allow obfuscation when comparing apples to oranges.? The coverage of issues is good, but the possible application can cause further confusion.? Finally, there is the hidden vessel specific knowledge that only the experienced DPO of the vessel knows about plot validity.? That isn’t available for outsider comparison.

Sales Resolved?? Not even addressed.? This is the major distorting factor in the process and the cause of much of the problems.? Listing important analysis factors, without providing constraints on how they are applied, just gives the amateurs more ways to cheat.? I watch for things like high thrust/kW, lack of reasonable hull or thruster losses, thruster interaction, forbidden zones, or power limits, strange wave or current limits, and reduced dynamic limit as initial indicators of trouble.? I’ve had good people apologize to me when caught cheating on their DP plots, and say that they needed the contracts.? I’ve had providers not understand what was wrong, or try to cheat in different ways when caught.? Getting the plots right is vital to the operators, but getting the job is vital to the providers, and sometimes that means “finding the right” answer (“Why, yes, your design can sustain the contractually required 27.6kts of wind on the beam in 1.5kts of current, and not what that other nasty provider said.”).? Sales are the main goal for designers and shipyards, and the cheapest path to approved official capability is often considered best.? Owners also like to avoid unnecessary expense and like paperwork that makes them look good to clients, but operation means they also need to know the real limits.? The crew certainly need to know the real limits for safe operation.? Incorrect values have caused many DP incidents.? Vessel clients are split between those who only care about price and those who care deeply about safe operation, with most clients scattered between the two extremes.? Owners and operators who aim at the high risk, high reward end of the market will need to specify how DP plots will be done and validated to meet capability requirements, if they need something that is closer to true.? Vessels designed for the low end of the market should probably not be compared with ones from the high end, as the low end vessels will claim more capability with worse kit for a lower price.? At the very least, such vessel plots almost need re-engineered and tested to be compared and validated, but that ignores problems in other areas, such as standard operating procedures (official & unofficial).? Don’t get cheap if you need things done right.? It can be an expensive way to save money.?

Conclusion:? IMCA M140 Rev.2 does not resolve the outstanding problems, but it does explain some of the factors more clearly, educates people in what is involved, provides good illustrations, and explains mitigation.? The industry does not currently seem to want reliable DP capability plots, but just wants something “good enough”.? “Good enough” varies from viewer to viewer, and IMCA cannot move too far ahead of the market, so M140 Rev.2 explains the options, looks at more of them, does it more clearly, and provides clearer warnings.? It provides options and stop gap measures for those who want more.? While I might want to constrain some of those options to ensure safer operation and better design, IMCA knows the industry better than I do and consulted widely.? DNV made a step change in the quality of capability plots that they would accept years ago, but I fear that it did not bring them much new business, despite a free static DP capability app that managed some of the concerns.? In a perfect world, providing such good value and quality would be rewarded with more business.? When it comes to DP plots, let the buyer and operator be aware, and let them beware.

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P.S.? I had considered adding a suggested specification for correct DP plot use and a check sheet as an aid to DP capability plot reality checking, but the more I considered it, the more I demanded to close the gaps.? Some of these gaps haven’t been properly explored, like defining the maximum environment that will allow satisfactory dynamic over-thrust failure positioning keeping response, so I’m less sure my partial answers are helpful.? The only way we can get definitive answers are by doing full dynamic engineering analysis and tests for each vessel, and at some point, the costs will outweigh the benefits on almost all vessels.? Perhaps “good enough” is all we can get for low cost.? People, who want more, will need to pay more to be surer.? But, I suspect no vessel has a full representation of its position keeping weaknesses.? Perhaps, IMCA concluded the same things.? I still wish we had a better defined floor.? Perhaps vessel clients need to set their own minimum standards, with the understanding that they won’t catch everything.? Only some high risk, critical projects can probably afford full capability analysis.? I wonder how the consequence analysis joint industry task group will handle these problems.