During the latest GREEN4SEA Athens Forum, Dr. John Kokarakis, Technical Director – Hellenic-Black Sea & Adriatic Zone, Bureau Veritas, explained how Vessel Performance Monitoring can open the path of reducing GHG emissions. This, Dr. Kokarakis explained, can happen as Vessel Performance Monitoring answers critical questions, such as what are the operating conditions of the vessel? Also, are company’s procedures properly followed? Answering such questions will eventually lead to a better operation of the vessel, as it will define when corrective actions are needed.
Today’s topic has to do with vessel hull machinery and, in general, performance monitoring with the aim to reduce pollution and of course the fuel bills.
Fouling is bad news because it increases the frictional or drag resistance, the most important component of resistance. Typically, we observe annually a speed reduction of 2-4%, which is quite significant. It is desirable to have an automatic alarm system in place that will tell us when is time for hull cleaning. A 6% reduction of speed is equivalent to 18% increasing of fuel bills or 18% in pollution. And this increase is added every year.
It is not necessary to have a complicated system in place to monitor the fouling and the reduction of speed. The basic system we need consists of a speed log/GPS which is there anyway, shaft torque and RPM meters and, if we want to become fancier, we might add draft gauges and wave radar to monitor the wave height because draft and waves will affect the power.
It is imperative to know the measuring principles of the measuring device. Consider for example the speed log, which measures speed based on the Doppler phenomenon and acoustic wave propagation. It is affected by shallow water, drift velocity, ship motions, water temperature and pressure and the orientation of the acoustic wave emitters. An oscillation of 2 knots in the resulting measurements is a sure sign that we have disturbance and inaccurate results.
It is also imperative to know what affects the measured quantity, for example the specific fuel consumption. Specific fuel consumption increases with worn injectors and liners, broken piston rings, high back pressure, fouled turbocharger and air cooler and leaking exhaust valves. Knowing what affects the measurand can lead to the proper corrective action.
The curve of power versus speed at a fixed draft provides a criterion to decide when hull cleaning is needed. But a word of caution here: Sometimes, it is necessary to produce a power speed curve for a vessel for which sea trials have been performed at ballast draft. This is typically the case in bulk carriers. Sometimes, we utilize the admiralty coefficient. Equality of admiralty coefficient is valid only for draft differences below 2.5 meters. If we have higher draft difference than that, then we have to utilize model tests or CFD in order to go from the trial power speed curve to the one at design or full load draft.
We need to differentiate between speed through water (STW) and speed over ground (SOG). The one that does not change is the STW. It is the speed that the engine develops and it is linked to the fuel consumption. Sea current will affect the speed over ground (SOG) or where the ship will go with respect to an observer not moving with it. GPS based on satellite triangulation defines the SOG. Speed log measures the speed through water. The difference (STW-SOG) yields the current speed, either positive or negative i.e. pushing or resisting. Vectorial sum of STW and sea current speed yields the SOG.
The best KPI is the simple KPI, like annual speed loss or diesel generation operation. We don’t want diesel generators operation at lower than 50% loads. The efficiency of diesel generators is reduced at lower loads. Total Fuel Oil Consumption: the shipowner who pays the bill can tell you which ship is more efficient. Specific Fuel Oil Consumption, light running margin (we don’t want to run a heavy propeller), propeller slip (which is the difference between the actual distance travelled minus the distance traveled in frictionless water. So increased slip is not good. Other KPIs are frequency of calibration, sensor accuracy and amount of leakages.
A damaged propeller may increase consumption by as high as 3%. Machine learning is nothing else, but an experience exercise. For example, CFD can be replaced by study of real time measurements of trim versus fuel consumption in order to pinpoint the optimum trim. Same for the exotic energy saving devices, the VFDs and the condition-based maintenance. At one time, I had been asked to deduce when a liner must be replaced due to wear, on the basis of millions of data points. Machine learning is a more scientific implementation of trend analysis.
What does the future bring? It brings data. Shipping becomes data-centric and data becomes a valuable commodity. We are running in the era of big data. Big data for weather modelling can provide a valuable tool in order to deduce when and where to travel depending on the time of the year. All these have been systematically produced in ISO 19030, which is similar to the common structural rules. ISO 19030 is nothing else but a tool to harmonize on how people are doing their vessel monitoring. It is not mandatory, but it gives unified guidelines on how to utilize the measurands, what sensors to be used, measurement procedures, various filters and corrections for the outliers. 19030 also proposes some KPIs. Of course the enemy of good is the better.
But why do we need to do all this work? The answer is in order to enable us to reply to a series of critical questions. What are the operating conditions of the vessel? Does vessel’s crew responds as required and report properly? Can we apply predictive or planned maintenance? Are sensors working properly? Very importantly, it enables fast and accurate decisions. Locating inefficiencies, failures can be predicted before they occur.
Above article is an edited version of Dr. Kokarakis’ presentation during the 2020 GREEN4SEA Athens Forum.
You may view his presentation herebelow
The views expressed in this article are solely those of the author and do not necessarily represent those of SAFETY4SEA and are for information sharing and discussion purposes only.
About Dr. John Kokarakis, Vice President Technology & Business Development – Hellenic-BS-ME Zone, Bureau Veritas
Dr Kokarakis a 1979 graduate of National Technical University of Athens, holds PhD (1986) and Masters degrees in Naval Architecture (1983) and Mechanical Engineering (1984) from the University of Michigan. He worked for over ten years as a consultant undertaking technical problems worldwide. His specialization was in the area of technical investigation of marine accidents. In his capacity as a forensic engineer he participated in the technical investigation of the Exxon Valdez grounding, the Space Shuttle Challenger disaster, the drillship Sea-Crest capsize, the Piper Alpha fire and explosion, the Aleutian Enterprise foundering in Alaska as well as many other accidents of less notoriety. In addition, he was a technical consultant for SEALAND, APL, MATSON Navigation, Chevron and other companies. The last twenty years he works in Greece, in the area of classification. Having served in the plan approval office of American Bureau of Shipping in Piraeus, he then joined Germanischer Lloyd heading their tanker and bulk carrier team in Greece. He is currently the Technical Director of Bureau Veritas in the Hellenic and Black Sea & Adriatic Zone. In his duties Dr. Kokarakis is responsible for the smooth technical operation in the Zone as well as in the harmonic cooperation with the BV offices worldwide to the benefit of the BV clients. He is a member of SNAME since 1976 and a Fellow of the Society. He is currently the Chairman of the Greek Section since 2014. He has also served as International Vice President of the Society and a Member of the Fellows Committee.