DP Capability Plots Are Dangerous & Need Reformed

Note:?A magazine asked me to rework an old article for publication, so if you have read the original article, in September 2021, then don’t bother reading this one.?If you haven’t, then this version is much shorter, more reader friendly, and covers an important gap in safe DP operation.?How can you stay within the redundant DP capability of the vessel when you don’t have anything to tell you what it really is??The standard passive analysis for DP plots doesn’t define safe operating envelope or allow fair comparison of vessels.

I used to wonder why so many vessels operated outside their redundant dynamic position keeping (DP) capability.?You could see it in the incident reports and accident investigations, usually labeled as equipment failures.?But the loss of a single thruster/engine, or of a whole redundant group, is survivable if a vessel is operating within its redundant position keeping environmental capacity.?A DP vessel must always operate within these limits, if it wants to be redundant, so I often assumed operator error.?Now, I know that assessment was unfair.?The DP capability plots that define these limits are usually wrong.?It’s the things you know that aren’t so, that get you.

The DP capability plots were historically used to define the limits of safe operation and to compare vessels.?If the plots are usually wrong, they can’t do either.?Providers and equipment suppliers exaggerate capability to meet specifications and make sales, calibrate the thrusters to provide less thrust than shown in the analysis, overlook power and configuration limitations, overlook hull interaction thrust losses, and assume a static environment with a small buffer for dynamic variation that is inappropriate for many vessel designs, missions, and environments.?Then there are the operator problems of purposely operating outside, or not noticing when they are operating outside the redundant capacity of the vessel, and failing to maintain equipment’s capacity to produce power or thrust at 100% or to adapt the redundancy guidelines for this loss.?DP plots are often brought into question by disagreement with the station keeping analysis, footprints, and practical testing, but few check them, and too many people trust them.??

There is a hint in the name, DP vessels do not perform static positioning, it is a dynamic process and the 25% dynamic margin is usually an inadequate margin to ensure position keeping after a fault, or in an active environment.?After a fault, the vessel is moving, due to the change of thrust and previous control and environmental variations, and the remaining thrusters are realigning and changing thrust to regain control.?Slow ramp or turning speed and forbidden zones mean thrust correction may be too late to avoid position loss.?Most vessels tested needed a larger dynamic thrust margin.?Lots of other people have noticed this problem.

Marine Cybernetics presented a paper at the 2013 MTS Dynamic Positioning Conference demonstrating that standard static DP plots did not guarantee position keeping in dynamic situations.?They demonstrated that static DP plots with small dynamic margins were insufficient to ensure position keeping in many vessels.?Their 2017 presentation demonstrated the effect of slow corrective response reducing real capability by comparing the changes in the dynamic redundant envelope with three different thrusters in the same vessel.?To their engineers, it was obvious that the dynamic limits need to be tested and defined dynamically.?Static DP plots didn’t cover the real vessel response and needed replaced by dynamic plots.

Problems with DP plots had been demonstrated before and are still not resolved.?It is an uncomfortable problem at the heart of our DP safety assumptions that needs to be addressed.?Our use of generally static, benign environment FMEA & annual testing, and of static DP plots makes us overestimate the safe, redundant, dynamic positioning capability.?There are quite a few papers on the need for dynamic analysis but little industry change.?IMCA & MTS have added hints of warnings to their documents but still recommend the old static DP plots.?DNV updated their rules and rolled out a new standard (ST-0111) and web application to address the issue, but ABS and Lloyds have not properly addressed the problem.

Detailed engineering analysis and tests are needed but most DP surveyors can’t do them.?System response will drift with equipment capacity & calibration, and will need similarly verified during annual trials.?It requires a control engineering approach to testing the system, performing system modeling, and producing vessel specific, data verified, and accurate dynamic DP plots.?For some vessels, the envelopes will be much smaller, but finding that is important to safe and reliable DP operation.?Other vessels, with better equipment and designs, will have their hidden comparative advantage revealed.?This is probably the best industry solution but needs to be adopted.

These solutions have not yet been widely applied, so stop gap measures are needed to improve the chances of operating within the real redundant DP capability of the vessel.?Maybe an increased dynamic safety margin, improved worse case fault testing, and increased DPO training in detecting and resolving more unusual control conditions can make the cheaper, static DP plots safe enough for now.?DPOs can use heuristic, adaptable WSOG rules of thumb to estimate redundancy, if they reflect actual vessel equipment capacity, use higher dynamic margins, and verify the DP system assumptions.?In the end, people providing and hiring vessels will need dynamic DP plots, so they can better manage risk.

Modern IMCA guidelines are a little schizophrenic about the issue.?M140 Rv1 2017 still recommends static capability plots but warns that they are for “comparison purposes only.”?IMCA 103 Rv4 2019 claims DP plots are fundamental to vessel operation.?IMCA 109 Rv3 2019 references DP plots as important.?IMCA 183 Rev3 2018 emphasizes the value of DP plots and recommends their use in safe operations.?Almost all recommend comparison against DP footprint plots and one document notes that “adding dynamic force interaction to the calculation will provide a more accurate result”.?Static DP plots are not a fair comparison between vessels or an indication of actual redundant DP capability, and the normal operation shown in footprint plots does not represent the system fault response that defines the real redundant DP envelope.?IMCA knows that there is a problem with DP plots and needs to resolve it.?Both current risk mitigation and long term solutions (dynamic plots & testing) are needed.

MTS guidelines are similarly inconsistent.?They recommend that DP plots be used to define the safe redundant operating capacity of a vessel but notes that they are static plots that do not consider dynamic forces or possible excursions.?If they don’t, then they can’t really define the redundant safe DP operating envelopes.?MTS operation guidelines recognize DP capability plots as theoretical and recommends validating them with footprint plots.?These documents do not reflect the evidence proving serious deficiencies in most static DP capability plots.?MTS is aware of the problem - one of their leaders presented a paper in 2006 showing a mismatch between the real and the calculated redundant capability of a vessel.?MTS should consider issuing a TechOp covering the subject and incorporating its advice in the future revisions of their documents.

DNV moved to resolve the risk of static DP plots.?It rolled out a new DP capability standard (ST-0111), changed their DP class rules to require its use, and provided an associated web application.?The basic, free service still uses static analysis but tries to reduce the gaming of the system, while the paid services offer greater quantification, including dynamic DP plots.?Non-DNV vessels and grandfathered vessels can still benefit from these services, so designers, DPOs, vessel owners, and potential clients can use them to evaluate their own or other vessels.?While the DNV application looks promising, the basic version only looks at the vessel response at a single current, so the aging IMCA M140 Rv1 still has that advantage.?ABS has only updated its DP rules enough to reflect unfocused IMCA concerns and does not appear to be aware of the problem.?Lloyds Register DP rules are unchanged.?Further movement is required to resolve these threats to safe operation.

Conclusion:?When it comes to DP plots both the buyer and the operator need to beware.?Standard, IMCA M140, static, DP plots cannot be used to meaningfully compare vessel capability and do not define the real redundant DP envelope.?They can be deeply wrong.?DP footprints and simplistic testing can’t resolve this but can detect some problems.?The operator can use well designed WSOG rules of thumb to improve the chances of vessel operation being redundant.?Voluntary use of the DNV capability services can fill some gaps, especially as it can be used to quickly check capability, but an overall industry improvement is required.?Vessel clients with safety critical applications should greatly prefer dynamic DP plots over static DP plots and include it in their requests for bid.?DP testing and DP plots need to become dynamic to reflect real operation & faults, and to determine true limits.?This requires coordination between the major industry players.?False position keeping expectations are still a major unaddressed safety issue in the DP industry.?The associated risks need to become better known, so they can be resolved.