Are all DP systems flawed?
Out of all the environmental forces acting upon a vessel, only wind speed and direction are actually measured and compensated within a DP system. The remaining environmental forces such as?current and?waves are all thrown into something which is called "DP current".
"DP current" is in other words a mix of basically a whole lot of uncertainties thrown into a pot. "If you can't measure it, you can't manage it" is a famous quote and when combining that with highly practical engineering, it doesn't really leave you with a great taste in your mouth.
So the DP current is calculated by a mathematical software model. It is shown on the DP screen, and it is equal to the total thruster force minus the actual measured wind force. A major challenge is that this algorithm assumes that the current speed and direction as well as wave direction change slowly and therefore the DP system then compensate primarily for slowly time-varying environmental forces.
As seen below from our 2D wave spectra onboard one of our clients vessels in the North Sea wave direction & current direction do change rapidly, all the time.
To be fair it kind of makes sense as there hasn't been a source from the vessel itself that can provide reliant accurate measurements in real-time of the ocean surface. However, that's not the case anymore.
Obtaining such measurements and data is now an ease feat and by no means an expensive feat.
Why continue to use estimates as input source when one can actually use actual measurement as input?
This will surely reduce and/or eliminate any measurement errors, and model errors within the DP system no?
The Miros Wave System could input real-time accurate wave and current parameters to any DP integrator system. Imagine having the possibility to input the full directional spectrum into your DP system, but why stop there? Go big or go home!??
Imagine the fun if one could also input 120 seconds highly accurate wave and vessel movement prediction into the DP systems.
How would that improve DP operations?
Could it improve DP capability plots?
Would this make positioning more efficient as it would improve the?predictive?nature of the model, which predicts and meets the thrust requirement rather than reacting to modelled environmental forces?
Would that mean that one could reduce excessive movement and power of the thrusters and?therefore reduce fuel consumption? Wouldn't that also reduce wear and tear of the thruster and machinery systems, extending the lifetime of the components?
Would love to hear some thoughts and feedback from you guys who actually know something about Dynamic Positioning. Leave a comment below.
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S/DPO Chief Mate COC.
1 年Absolutely agree! We have systems quite accurately measuring windwaves and swell, why not to have them integrated to DP systems and applied to particular vessel's shape
Captain - AHTS / Comandante - CCB, SDPO
1 年Should be ok, but as many people knows, sometimes, this measurements fight against themselves and it hard to find a better heading for operation…. Then, “other” sometimes, experienced seafares’ eyes help a lot…. But, come in “The Future”! Let us know you!
Offshore DP Consultant
1 年It 8s not stricly true to say the dp 'uses' the dp current. It appears on the screen as an estimate of a current that might produce the force and turning moment as you rightly say left over aftef deducting the wind force estimate. So although we measure the wind, the force is calculated from a wind model so even that is not 100%. In fact the dp is applying thrust in reaction to off position and off heading. The dp then estimates those force from controlling pitch / speed etc thesd will not be correct due to inflow, hull interaction etc so the over-all force and moment ard an estimatd at best so the calculated current will alo be wrong. If we coukd measurd current and wave we would still have to have to apply it to a hull model to estmate the force , so having done that znd added to the wind, i think we may find ig does not equal the estimated force so we end up with a 'lost thrust ' that we wojld not know what do do with. Where it may help is being able to input better curreng and wave data into an online capability plot?
Senior DPO
1 年This is a hard ask, in the first instance, you would need a minimum of three such 'sensors' with two different principals. The DP Current isn't perfect but does work provided the operator is fully engaged.
Engineering Management Professional | Experienced, Practical, Registered Professional Engineer | Dynamic Positioning Subject Matter Expert (DP SME)
1 年I agree that this is an issue that does need solved. The right kind of measurement is better than an algorithmic band-aid. The assumptions of the band-aid are sometimes wrong and have caused DP incidents, as shown in the annual IMCA reports. At this point, it would probably be useful for the DP control system providers to define what kind of measurement would be useful to them, so the available measurement can be compared with the kind that will make them take action. Failure modes of the system are vital to practical DP implementation, as well as the proper format and use of the data. Whatever DP company solves the wave & current measurement gaps will gain a position control advantage over their competitors. https://www.dhirubhai.net/pulse/why-dont-dp-vessels-measure-use-current-paul-kerr