Position and economy?

21 Dec 2013
DP capability plots can be useful to determine actual responses to conditions

DP capability plots can be useful to determine actual responses to conditions

“You can’t assume much about what’s really happening to fuel consumption when a vessel is in Dynamic Position mode,” David Barton of Global Maritime tells Stevie Knight.

He points out that although the safety requirement grabs people’s attention it doesn’t mean that fuel economy issues go away; however estimating fuel use whilst on DP is notoriously difficult to predict and many older vessels don’t have the necessary flow-meters to accurately record the actual engine consumption, something that would be an eye-opener for quite a few operators.

Mr Barton explains that each DP system is tailored to the individual vessel. A theoretical model – almost a portrait - based on the shape of the vessel both above and below the waterline helps inform the DP system, giving it a good baseline to work from. Layered on top of this is information from the vessel’s environmental reference (gyros, wind sensors, vessel motion response units) plus the position reference systems; these add ‘an up-to-the-minute sketch’ of the real situation which is held in a mathematical model.

Of course it’s a complex interaction; Mr Barton adds “the better the theoretical model, the more accurate the mathematical model becomes” and conversely if the system is left without external input for a while, the value of the mathematical model reduces and the theoretical model is given more importance.

Plug-and-play impact

However, there’s a pattern emerging that’s affecting all of this. Bigger or more flexible vessels are being designed with a variety of plug-and-play equipment options, especially subsea construction vessels which can have different ‘module deployment’ devices such as towers or huge cranes on the deck, affecting windage and overall handling. 

This obviously impacts the fundamental assumptions the DP is making about the vessel. Although Øystein Andreassen of well-known DP manufacturer Kongsberg says that the DP is able to take on the changes to the windage and so on as there is enough latitude in the system to be able to cope, he adds system modifications are unlikely to be carried out “if the additional equipment is looked on as temporary”. However, this will of course have an effect on the overall fuel efficiency.

Now, while fuel saving is not first thing on anyone’s mind when nuzzled up to a rig, it still has some impact overall, and both Marcel Vermeulen of Imtech and Mr Barton point out that the lack of accuracy of the model leaves more corrections for the system to labour under in order to keep it on course, with Mr Vermeulen saying that it would be a good idea in the longer term to have a pattern that could include the add-on equipment in the overall mathematical modelling.

‘Look no hands’ check

There are times where investigation shows that the basic DP model isn’t all it should be. Mr Barton explains that one very useful check is to run a test a bit like taking your hands off a bicycle’s handle bars, in other words, cut off the input from the environmental and positioning systems to see what happens. “The DP system reacts to the continually updated mathematical model and should all reference systems fail, it will continue to manoeuvre the vessel for a short period with the last known set of thruster commands - stored in the updated model - so that any position drop-out will be minimised,” he says. “If all is well, I’d say you are looking at it keeping station to within a few metres for five, maybe ten minutes.

However, it does happen that you find out something is wrong because the vessel will drift off much further than expected when the external signals are cut.” If all’s not well there’s a couple of retests and consistently poor results will raise concerns over the model’s accuracy: in one case a combination of issues led to the boat skidding off by as much as 60m says Mr Barton.

He also points out that included in the control system programs of DP Class 2 and 3 vessels is software that enables the DP Operator to compile accurate capability plots for the vessel in the intact condition (all thrusters working) and reduced capability, (with one or more thrusters out of action). Some programmes automatically compile footprints of the vessel’s actual manoeuvring with different sea states and wind forces over a period of time, though if not, these can also easily be generated manually. They can be very useful: “These footprints demonstrate the vessel’s capability in a variety of situations far more accurately than the model can,” he points out, and can help to check the system is actually working efficiently.

Vessel Operating Envelopes

There are other savings to be had with a few changes to the kit; if offshore vessels are often subject to waiting periods and there’s no real need to have them station keeping with all the constant changes to the thruster demands and readjustments to the power system that requires, plus of course the extra maintenance it eventually entails.

“It’s much better to stand down a notch and move a short distance away where the operating limits can be increased,” explains Mr Vermeulen so some manufacturers are developing a so-called ‘green mode’. While Mr Andreassen points out that it was originally developed for the bigger drill ships which have fuel bills to match the scale of their engines, some relaxed-mode systems are being developed for support vessels. Generally this means alarm limits are opened out and the thruster and related power demands calm down, reducing overall fuel consumption. “They are not abandoned as you still need to stay in range, but the heading can be altered from those required right by the installation to the best fuel economy compromise while still allowing for sea, swell and currents.” 

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