COMADEM2011 Paper.lijzenga (1)

7/29/2019 COMADEM2011 Paper.lijzenga (1)

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Increasing cost effectiveness through a fleet widecondition based maintenance strategy within the

Royal Netherlands Navy

Harry LIJZENGALieutenant Commander Royal Netherlands Navy

1. INTRODUCTION

Condition monitoring of maritime objects has entered a new era (1).Developments in technology, in recent years, have created new possibilities for derivinginformation regarding equipment health affordably. Furthermore, the exploitation of suchinformation is becoming ever easier as communication systems become more distributedand processing power increases. Condition monitoring techniques within the RoyalNetherlands Navy have been in existence for many years now. Among the bestestablished in mechanical systems, are oil and vibration analysis. In recent years, the artof condition monitoring has developed to the extent that automated prediction of failureevents is becoming possible a concept known asprognostics.

The proliferation of (remote) online condition monitoring (ROCM) technologies oncommercial maritime assets has prompted the desire, within the Netherlands Ministry ofDefence (NL MOD),

to exploit this technology on navy ships. The key desired benefits from such a policy are:

a. greater equipment availability

b. whole-life cost savings

c. reduced manning

Essential by introducing a new technology like ROCM is the awareness of the relation

Abstract

Within the Royal Netherlands Navy (RNN) a study is performed for a more cost effectiverealisation of its operational tasks. By changing from an used based maintenance concept toa more condition based maintenance concept a reduction in operation costs can possible beachieved. A realisation of this concept is by 1) Online monitoring and/or 2) outsourcing ofdata analysis. Using remote online condition monitoring (ROCM) could be a logical solution.In this paper it will be outlined how the RNN is using online monitoring systems, but alsowhat next step has to be done to achieve the most optimal (cost effective) maintenanceinterval by combining ROCM with knowledge of failure mechanism.

Keywords: cost effective, outsourcing data analysis, condition based maintenance, online


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between used condition monitoring technique in relation to the physical failuremechanism. In this paper, it will be demonstrated how technical innovations can lead to amore efficient maintenance process.

2. MAINTENANCE CONCEPTS

The main goal of performing maintenance is to obtain the optimal availability of acapital asset, which requires balancing the availability of the asset with the costs ofmaintenance activities. Performing too much maintenance results in high costs andfrequent non-productive periods, whereas too limited maintenance leads to unexpectedfailures and reduced availability. There is a wide variety of maintenance concepts thatcan be applied to maintain an asset. These concepts can be divided into two basic types:

- corrective maintenance

- preventive maintenance

Which maintenance strategy is chosen depends on the criticality of the part and thevariability and predictability of the usage. For non-critical parts, corrective maintenance is

in many cases the optimal strategy of the usage. For non-critical parts, correctivemaintenance is in many cases the optimal strategy, since the full service is utilized andno expensive inspections or monitoring are required. However, for critical parts, failuremust be prevented for safety or economical reasons (large plants in process industry).Then the variation in and predictability of the usage determines which preventivemaintenance concept is suitable. For machines that operated continuously at constantand well-known power settings, the usage is fully known and replacement intervals canbe determined accurately. A calendar time concept is cost-effective strategy, since nomonitoring is required.For systems that are operated in a more variable way (e.g. navy gas turbines, thrusters),the usage must be taken into account when the service intervals are determined.Assuming the usage in the design phase is difficult, and often large safety factors must

be applied. When, on the other hand the usage or loads are monitored, a much moreaccurate prediction of the service interval can be made. The only remaining uncertainty isthe relation between the usage/ loads and the failure mechanism.Therefore, the optimal amount of maintenance, which provides the best balance betweencosts and availability, is ideally obtained with a condition based concept, since it enablesthe execution of maintenance activities exactly on the right moment. However, not everyasset is suitable for a condition based maintenance concept. It must be feasible to assesthe condition of the asset and to relate the condition to the required maintenanceactivities. For many assets that is not possible and a usage or load based approach maythen be a better option.

3. USED CONDITION BASED TECHNIQUES ROYAL

NETHERLANDS NAVY

Within the Royal Netherlands Navy several CBM techniques have been applied andhave proven to be effective. Most of these methods are available as commercial services.

3.1. Vibration monitoring

Vibration monitoring is nowadays a widely accepted CBM method that can be used todetect wear, failures or malfunctioning of rotating or reciprocating machinery. Withregard to the general requirements for CBM systems this technique is based on theknown relation between measured vibrations and the failure mechanisms that are to bedetected. Vibration sensors, that measure displacements, velocities or accelerations,

monitor the dynamic behaviour of the system. The resulting spectra can be used todetect, for example, unbalance, misalignment, bearing wear of gear defects. The


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possibilities of assessing the condition of the system depend on the complexity of thesystem, the number of sensors and the availability of information on the machines orsystem (e.g. stiffness). For rather simple systems like bearings, the method works quitewell. For more complex systems analysing trends in vibration characteristics may indicatea degradation of the system, but quantitative predictions of remaining life may be hard todetermine.

3.2. Condition monitoring of bearings

Since bearings are rather simple systems and also very widely applied in largenumbers, the system knowledge of bearings is well developed. As a consequence therelation between monitored vibrations and bearing wear or failures are known for themajority of the applied bearings. This means that a CM system can be effectively used tooptimize the maintenance tasks on bearings and a lot of manufacturers can deliversensors and analysis software with their products. The software is able to detect the typeof degradation and also predict a value for the remaining life.

3.3. Monitoring of hydraulic and lubrications systems

Another well developed condition monitoring technique is the monitoring ofhydraulics and lubrication oil. By analysing the oil, both information about the quality ofthe oil and the condition of the system can be obtained. The quality of the oil is qualifiedby determination of the viscosity, the total acid / base numbers and the concentration ofadditives. Further, contaminations are metal particles, originating form wear andcorrosion process and water. In most cases, samples must be taken from the oil, whichare then analysed in the Ministry of Defence chemical & analytical laboratory in DenHelder. However, recently also oil monitoring sensors have been developed that may beused to measure the water content in oil or the number of particles.

4. CONDITION BASED MAINTENANCE

The theory of CBM can be explained using the concept of the P-F interval (2). The P-Finterval is shown in figure 1. This concept is based on the fact that most failures givesome sort of warning or deteration in condition before they occur. The condition shouldbe measurable either directly or indirectly via a related parameter, for example pressureor vibrations. The condition should be directly related to the functional capability of thesystem of interest. When a component is new or just maintained the functional capabilityand thus the condition is assumed optimal. As time passes the condition decreases to apoint the deviation from the normal condition becomes measurable, this point is P.Point P is the point of potential failure, which means the developing mechanism can bemeasured, but it has no or no unacceptable effects on the capability yet. As more timepasses the condition decreases further until point F, the point of functional failure. Atpoint F the components ceases to function at the required level of capability. The P-Finterval is the time between the point where a developing failure becomes measurableand the point of actual failure. This interval depends on the type of failure mechanismand the type of condition monitoring technique (when a technique is more sensitive it candetect point P earlier, increasing the P-F interval). The P-F interval determines therequired measuring periodicity to allow detection of potential failure at adequate timebefore functional failure occurs. As a rule of thumb this measuring frequency is set to halfof the P-F interval. In most cases this gives a reasonable amount of time to act on thedeveloping failure, however as with all rules o thumb, there are exceptions.


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Figure 1 P(otential) F(ailure) interval

Figure 2 gives general representations of the condition of a given system as a function oftime. The top left part of figure 2 represents the optimal condition. A refinerement of theP-F interval is the net P-F interval. The net P-F interval is important for logistical reasonsand it is determined by the time between P and F minus the inspection interval. This netP-F interval is the worst case scenario in which there is the least time to react on adeveloping failure. The net P-F interval determines how fast ahead you can planmaintenance actions, thus the longer this interval the less emergency provisioning andrepairs in overtime are expected. Up to a certain level the net P-F interval can beshortened by shortening the inspection interval, in other words increasing measuringfrequency. In figure 2 the left graph represents a P-F interval of 13 weeks monitored atinspection intervals of 8 weeks. The worst case response time is 5 weeks; this is the netP-F interval. In the right graph of figure 2 the same P-F interval is monitored with an

inspection interval of 1 week, resulting in a net P-F interval of 12 weeks. Obviously, thisway of shortening the net P-F interval leads to additional maintenance costs, becauseinspection and analysis must be performed more often. When monitoring online (or inother words continuously) the difference between the P-F interval and the net P-F intervaldisappears, resulting in the maximal possible planning time for maintenance actions.

Figure 2 Nett P-F interval


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4.1. DEVELOPMENTS OF CBM

Over the last fifteen years condition monitoring techniques have developed rapidly.The ever ongoing development of computers has led to software that can perform routineanalysis of condition monitoring measurements at ever decreasing prices. This createsmayor benefits for normal offline condition monitoring measurements, but it also

creates the possibility to perform online measurements without gathering unmanageableamounts of data to be analysed. Furthermore the decreasing prices of broadband satellitecommunication has brought mobile data transmission from anywhere in the world toanywhere else in the world within reach. Several manufactures, enabled by these generaltechnical developments, have created condition monitoring equipment devices enablingremote online condition monitoring (ROCM). An installation enabling ROCM is a devicecontinuously logging condition parameters, which indicates upcoming maintenance tasksbased on warning and alarms levels or on trends. A random example of a ROCM capabledevice is shown in figure 3.

Figure 3 SKF Multilog On-line System IMx-S.

4.2.REMOTE ONLINE CONDITION MONITORING ROYALNETHERLANDS

NAVY

Devices enabling ROCM have become available in the past fifteen years and thegeneral trend in the RNN is to use these devices. Main driver to implement ROCM in themaritime sector is general to decrease exploitation costs, while maintaining or increasingthe effectiveness of the asset, in other words increasing cost effectiveness. There areseveral reasons to assume an increase in cost effectiveness on account of ROCM withinthe RNN, for example:

- Because part or all of the condition monitoring tasks normally done by thecrew are automated a direct decrease in OLM will occur;

- The condition monitoring tasks performed by the condition andperformance

(C&PM) department of the RNN are automated which will decrease thenumber of flights from shore specialists to the ship concerned. This results

inless expenditure on plane tickets and in a reduction needed man hours

from shore specialists. The reduction in needed man hours results directly fromthe

reduced number of flights and indirectly from the reduced number of hoursofobligated compensation for travelling outside regular working hours;


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- When performing offline measurements there is always a chance a certainfailure mode has a P-F interval shorter than the regular periodicity ofmeasurement. By monitoring continuously, rapidly developing failures will

alsobe detected, preventing them to occur including possible accompanyingcollateral damage;

- Because ROCM devices perform continuous measurements, suchmeasurements in a certain operational state no longer have to be planned

inthe programme of the ship (for example propulsion vibration

measurements at120 rpm on both shafts, rudder angle between plus and minus two

degrees).

5. BUSINESS CASE ONLINE VIBRATIONMONITORING

In the RNN the first ships equipped with online monitoring are two Hydro graphicSurvey Vessels, Dutch abbreviation HOVs. The HOVs are two vessels built by Damenshipyards in Rumania and Vlissingen and entered into active service in 2003 and 2004.The ships have diesel-electric propulsion installation powered by three diesel generatorcapable of delivering 620 kW each. The generators feed the bow thrusters and the mainelectric motors and through the main gearboxes these drive the propellers. Soon afterthe HOVs were taken in operations, the bearings of the generators started to causeproblems. These problems were seizing bearings causing unplanned standstill and loss ofoperational capability, since the propulsion of the HOV is diesel electric and all three

generators are needed for optimal surveying.

The failure mechanism of several of the seized bearings was investigated and the rootcause was found to be fretting corrosion resulting in contaminated grease causing badlubrication. Fretting corrosion was detected between the outer ring and housing and innerring and shaft.

Fretting is mechanisms in which small relative movements of two surfaces result in wearparticles between the surfaces. When these particles oxidize, the created metal oxide isharder than the original metals, resulting in fretting corrosion. In the case of the HOVsthese hard metal oxides contaminate the lubricating grease. No other explanation for theshort average lifetime in the beginning could be found than the bearing failure due to

these hard particles. Based on the above findings redesign of the diesel generators wasconsidered, but ultimately not carried out due to the inability to find a properreplacement for the generator in the set time. Therefore, to prevent further collateraldamage and to increase reaction time it was decided to install an online conditionmonitoring installation for the condition parameter vibration. Costs or loss of operationalavailability were not explicitly taken into account in this decision, mainly due to a lack oftime before the installations would re-enter service after revision.

The chosen condition monitoring technique, vibration analysis, cannot detect frettingcorrosion directly. However, vibration analysis is suitable to detect problems with bearinglubrication resulting from fretting corrosion. Because the bearings sometimes failed onlydays after an offline vibration measurement that showed no irregularities, meaning the P-F interval is short, the case demanded for continuous monitoring. After tendering the IMxmonitoring installation (SKF) was chosen. The configuration of the installation consists per diesel generator of four vibration sensors, a tachometer and a load indicator. The

ExternaltachoLoad indicator 4- - Alarms via relay

Alarms stem


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IMx connects with the Platform Management System (PMS) of the HOV to alert crew whenan alarm level is exceeded. Given the use profile of the ships, which consists mainly oftwo weeks sailing and arriving in homeport for crew change of a short period in portremote accessing of data was not deemed necessary. In figure 4 an overview of theonline system is shown.

Figure 4 online vibration monitor system HOV

6. CONCLUSIONS

It is vital that the Royal Netherlands is clear in its aims regarding the use of (remote)online condition monitoring technology. If the exploitation is not approached logically, ithas the potential to be a costly white elephantconsuming resource whilst deliveringlittle useful information. By combining the information and data from conditionmonitoring technology, together with knowledge of physical failure mechanisms, it ispossible to get the optimal maintenance interval. Although further research has to bedone by analysing the data and combining it to an useful and meaningful mathematicaltool it is clear that ROCM will be the next step ahead for the RNN.

References

(2) Jongen, M. 2010 Cost effectiveness of remote online condition monitoring: A studyand a business

case on the Oceangoing Patrol Vessel, Asset Management Centre, Den Helder


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Authors Biography

Lieutenant Commander Harry Lijzenga finished his Institute of Technology (MechanicalEngineering) in 1993 and went to the Royal Netherlands Navy in 1995 (in between hefinished his military service at the Army as soldier). After graduating at the Naval Collegein Den Helder in 1997 he went sailing as a lieutenant on the supplyship Hr.Ms.Amsterdam for 1 year (practical year). After this ship he subsequently went to Hr.Ms.Heemskerck (1998) and Hr.Ms. Philips van Almonde as secondary head of the engineeringdivision.

Lieutenant Commander Lijzenga went to shore as project officer special projects at theNaval College. Here (1999 2001) he was responsible for making education- and trainingprograms when new technical equipment was implemented on ships. After this hebecame Head Condition Monitoring at the Material Logistical Department within theDutch Navy. From 2001 till 2005 he worked on this position. In 2005 he became Head ofthe Engineering Division on Hr,Ms, Van Speijk (Multipurpose-frigate) until 2008. At themoment Harry is Head Condition- and Performance Department (C&PD) in Den Helder.Responsible for condition monitoring, underwater noise measurements (acoustic &

magnetic) and artillery analysing (analysing if guns and fire-control systems of navalships work properly). Besides head C&PD he gives leadership training (Situational Selfleadership and DiSC behaviorstyles) for all rank personnel in the Navy.

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