Evers, Karls-Ulrich

Lu, Wenjun

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Article, Published VersionEvers, Karls-Ulrich; Lu, WenjunIce Load Measurement by tactile sensor in model scale testin relation to rubble ice transport on arctic offshorestructures (RITAS)HydrolinkVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/109249Vorgeschlagene Zitierweise/Suggested citation:Evers, Karls-Ulrich; Lu, Wenjun (2014): Ice Load Measurement by tactile sensor in modelscale test in relation to rubble ice transport on arctic offshore structures (RITAS). In:Hydrolink 2014/3. Madrid: International Association for Hydro-Environment Engineering andResearch (IAHR). S. 74-77. https://iahr.oss-accelerate.aliyuncs.com/library/HydroLink/HydroLink2014_03_Hydrolab_IV.pdf.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.74Studying ice-structure interactions oricebreaking processes of  icebreaking vesselsrequire a good understanding on ice mechanicsand ice failure processes. Physical experiments and numerical simulationsare executed in ice tanks with focus on thespatial distribution of ice loads acting onoffshore structures located in arctic or sub arcticregions. When designing arctic offshore struc-tures or icebreaking vessels not only the globalice load acting on the structure is of relevancebut also the spatial distribution of ice load (localice pressure) is of interest for a safe andeconomic design.   When ice encounters sloping sided offshorestructures it fails sequentially from breaking,rotating to submerging. Current offshorestructure design practice does not differentiateICE LOAD MEASUREMENT BY TACTILE SENSOR IN MODEL SCALE TEST IN RELATION TO RUBBLE ICE TRANSPORT ON ARCTIC OFFSHORE STRUCTURES (RITAS)BY KARL-ULRICH EVERS & WENJUN LUIce is a rather brittle material, strong in compression but weak in tension. The magnitude of ice loadlargely depends on the corresponding ice feature’s dominant failure modes and failure processes.Studying ice-structure interactions or icebreaking processes of  icebreaking vessels require a goodunderstanding on ice mechanics and ice failure processes. such failure process in a procedural manner.Instead, all the ice load contributions from thesefailure processes are conservatively added uptogether to yield the final design load (API_RP2,1995; ISO/FDIS/19906, 2010).In the framework of the I3 Hydralab-IV contract atransnational access project “Rubble IceTransport on Arctic Structures (RITAS) wasexecuted in the Large Ice Tank at the HamburgFigure 1 - Schematic diagram of interaction mechanisms between level ice and a wide sloping structureFigure 2 - Buoyancy box (a), schematic diagram of test set-up (b), tactile sensor mat (c)hydrolink number 3/201475hydrolink number 3/2014IAHRThe operating temperature ranges from -9°C to60°C; the pressure measuring range is within 0to 175 MPa. This specific sensor has a ratherlong tail that ensures that the handle whichconnects the tail to the computer can bepositioned far away from the water.This sloping structure has been tested indifferent ice conditions with different icethickness and different interaction speeds. Theapplication of tactile sensor in all these testsfollows generally three important steps whichare described below:Installation of tactile sensor (step 1)During the installation, great attention has beenpaid to make sure that the tactile sensor iswaterproof and protected against ice abrasion.To achieve this, the sensor was first put inbetween two plastic films adhered by siliconegel so as to make it waterproof. Afterwards, ametallic adhesive layer was applied above thesensor serving as the abrasion protection. Thisstep is finished in the very beginning of the testsand no further repetition was made as long asthe tactile sensor worked properly.Calibration of tactile sensor (step 2)Due to the complexity of ice-structure interac-tions, the ice pressure covers a very wide rangeof possible values. In the current tests, basedon the chosen sensitivity and saturationpressure, tactile sensor tends to capture the icepressure that repeats most often, saying thepressure that would be around the mean icepressure. However, for extreme values, thesensor is prone to underestimate the extremevalues. Even though, the merits of using tactilesensor in the current test should not bedegraded. Tactile sensor will anyhow producethe contact area (i.e. the load’s spatial variation)and comparative pressure irrespective ofpossible errors within its measured maximumvalues. The pertinent choice on tactile sensor’ssensitivity and saturation pressure are made inthe literature (Lu et al., 2013b).Ship Model Basin (HSVA) in Hamburg,Germany. The objective of the study was to investigate thetemporal and spatial distribution of ice loads ona wide inclined structure with the presence ofice rubble accumulation by application of tactilesensors.Interaction mechanismsWide sloping structures have many applicationsin ice covered waters. Due to the relativelylimited ice clearing capability of a wide slopingstructure, the presence of ice rubble greatlyinfluences the whole interaction mechanism(Serré et al., 2013). The interaction mechanismbetween level ice and wide sloping structurecould be categorised into three different stages(Lu et al., 2014).  A brief interaction mechanismis shown in Figure 1. The first stage is the ice breaking stage (Figure1  ➊). In this stage, the incoming ice fails againstthe structure and a large ice breaking load isexpected to be detected at the waterline. The second stage is the ice rotating stage see(Figure 1➋ ). As the broken ice fragment istravelling downwards, the corresponding icerotating load is supposed to be measuredbelow the waterline. The third stage is therubble accumulation stage which occurs simul-taneously with the previous two stages and isshown in both Figure 1 ➊ and  ➋.The questions for the current problem are: (1) Where exactly are these loads spatiallylocated? (2) What are the comparative values of theseloads?(3) How would these loads evolve with time?A tactile sensor mat (sensor type #5513)produced by I-Scan™ Tekscan Inc. wasinstalled on a sloping surface of a structureconfined by two transparent Lexan™ plates.The geometry of the test set-up and the locationwhere the tactile sensor mat is mounted areshown in Figure 2. Karl-Ulrich Evers is a civil hydraulicsengineer graduated at the Leibniz Uni-versity of Hannover (LUH), Germany.After his studies he joined the Ham-burg Ship Model Basin (HSVA) inHamburg where he works as seniorproject manager in the Artic Technol-ogy department. His main focus is onice mechanics, ice model testing ofarctic offshore structures and consult-ing in ice engineering. During his pro-fessional career he participated ac-tively on 7 Arctic Expeditions and fieldcampaigns. From 2004 to 2008 he waschair of the IAHR International IceCommittee where he is still a co-optedcommittee member. Since 2011 herepresents Germany in the POAC In-ternational Committee.Wenjun Lu is currently a PhD candi-date of the research centre ‘Sustain-able Arctic Marine and Coastal Tech-nology (SAMCoT)’ of NTNU,Trondheim, Norway. Having a jointMSc degree in Coastal and Marine En-gineering (CoMEM) from TU-Delft,University of Southampton, and NTNU,his current research interests involveice load calculation/measurements,rubbling process and fracturing of icefloes. In addition, he is also a two-term recipient of Best Student PaperAwards from conferences of ‘IAHR:Symposium on ice, 2012, Dalian,China’ & ‘Port and Ocean EngineeringUnder Arctic Conditions (POAC),2013, Espoo, Finland’. Figure 3 - Sloping structure pushed through level ice (a), the ice failed into fragments along the inclined plate and rotated downwards (left)76 hydrolink number 3/2014Validation of tactile sensor in each test (step 3)Before each test, the tactile sensor is againvalidated against several known weights so asto confirm its functionality (calibration check). Inall the validations, errors between the measuredresults and the known weights were all within15%. With all these three steps implemented,tactile sensor was successfully utilised inmeasuring the ice load (at least its spatial distri-bution). Measurement visualisation During different tests, the sloping structure inFigure 3 was pushed through level ice sheets inthe ice tank.  The ice failed into fragments alongthe inclined plate and rotated downwards(Figure 3 b).The spatial and temporal variation of the iceload is recorded by the installed tactile sensor.A real time ice load evolution can beexemplified as in Figure 4 (different coloursrepresent different pressure magnitude). Thismeasurement illustrates one circle of the iceload development, i.e. the ice breaks at thewaterline and rotates downwards. It takesapproximately 3 seconds for such cycle todevelop in Test 1210. It can be seen that, afterthe initial breaking of the incoming ice, the localpressure did not diminish instantly. Instead, thepressure keeps travelling down at a relativelysmaller yet comparable magnitude.Ice load’s spatial variationWith the measured data as presented in Figure5, it is possible to study how the ice load isdistributed in the vertical direction (i.e., Z-direction or the short edge’s direction of thepanel in Figure 4).Summing all the load recordings along the longedge’s direction in Figure 4, we are able topresent the time averaged ice load’s spatialdistribution as in Figure 5(a) and the maximumice load’s spatial distribution as in Figure 5(b).Since both the elastic foundation beam andplate theory suggest that the tip deflection atflexural failure is minimal comparing to thethickness of the ice, it would be reasonable toassume the ice breaking load (i.e. the loadrequired to bend the incoming intact ice) iswithin the un-deformed level ice’s thicknessregion (i.e. the shaded area in Figure 5). Notethat inside this region, other interaction mecha-nisms such as the initial ice rotating and rubbleeffect also exist (Lu et al., 2014). As shown inFigure 5 (b) the maximum loads are mainlyfound within such shaded area. This agrees withour common sense and previous researchassumptions that the ice breaking load is one ofthe decisive components of the ice load.However, as it is shown in the theoretical model(Lu et al., 2014; Lu et al., 2013a), the ice rotatingload would also become decisive when there issufficient rubble accumulated in front of thestructure. For the time being, it can be simplyconcluded that based on tactile sensor’smeasurement, the maximum load often takesplace around the un-deformed level ice’sthickness region. The numerical simulationconducted by Paavilainen and Tuhkuri (2013)also detected the maximum ice load slightlybelow the waterline for gentle slope angles.Ice load’s spatial and temporalvariationAmong all important findings based on themeasurements from tactile sensor, it is inter-esting to illustrate the ice load’s spatial andtemporal variation as shown in Figure 6.It can be seen from Figure 6 that generally mostof the recorded loads in the vertical directionincrease with time. This underlines the impor-tance of rubble accumulation. Moreover, belowthe un-deformed level ice’s thickness region, i.e.below bin number 3 and 4, the recorded iceload also increases with time and may becomeeven more significant than the process thatoccur within the un-deformed level ice’sthickness region. This further strengthens thepoint that the accumulated rubble together withthe ice rotating process intensifies the ice loadunder the un-deformed level ice’s thicknessregion.Though Figure 5 and 6 are presented withabsolute values of the ice load, caution shouldFigure 4 - Plot of ice load development along the sloping surface77hydrolink number 3/2014IAHRaccumulation;• Generally the recorded maximum load actsat the un-deformed level ice’s thicknessregion (see Figure 6b). The ice breakingoccurs mainly at the waterline region. This isin line with previous experiments andassumptions that the ice breaking load is oneof the decisive loads regarding design.With respect to the utilisation of tactile sensor inthis test campaign, it can be concluded thattactile sensor is beneficial in displaying iceload’s relative spatial distribution while itsmagnitude should be treated with caution. Ameasurement system which combines bothtactile sensor and the conventional measuringtechnique (e.g., measuring by load cell) tendsto offer a better understanding on the ice load’sspatial and temporal variation.AcknowledgementThe work was supported by the EuropeanCommunity's 7th Framework Programmethrough the grant to the budget of theIntegrated Infrastructure Initiative HYDRALAB-IV,Contract no. 261520. The authors would like toacknowledge the Norwegian Research Councilthrough the project 200618/S60-PetroRisk andthe SAMCoT CRI for financial support and allthe SAMCoT partners. The financial supportfrom Statoil AS and the test set-up design ofMulticonsult is also greatly acknowledged.ReferencesAPI_RP2, 1995. Recommended Practice for Planning, Designing, andConstructing Structures and Pipelines for Arctic Conditions.American Petroleum Institute.ISO/FDIS/19906, 2010. Petroleum and natural gas industries - Arcticoffshore structures, International Standard, InternationalStandardization organization, Geneva, Switzerland.Lu, W., Lubbad, R., Serré, N. and Løset, S., 2013a. A theoretical modelinvestigation of ice and wide sloping structure interactions,Proceedings of the 22nd International Conference on Port andOcean Engineering under Arctic Conditions (POAC’13), Espoo,Finland, pp. 1-14.Lu, W., Serré, N., Høyland, K.V. and Evers, K.-U., 2013b. Rubble Icetransport on Arctic Offshore Structures (RITAS), part IV Tactilesensor measurement of the level ice load on inclined plate,Proceedings of the 22nd International Conference Port and OceanEngineering Under Arctic Conditions (POAC’13), Espoo, Finland,pp. 1-14.Lu, W., Lubbad, R., Høyland, K.V. and Løset, S., 2014. Physical modeland theoretical model study of level ice and wide sloping structureinteractions. Cold Regions Science and Technology, 1: 1-33.Paavilainen, J. and Tuhkuri, J., 2013. Pressure distributions and forcechains during simulated ice rubbling against sloped structures.Cold Regions Science and Technology, 85(0): 157-174.Serré, N., Høyland, K.V., Lundamo, T., Bonnemaire, B., Evers, K.-U.and Gürtner, A., 2013. Rubble Ice transport on Arctic OffshoreStructures (RITAS), part I: Scale-model investigations of level iceaction mechanisms, Proceedings of the 22nd InternationalConference on Port and Ocean Engineering under ArcticConditions (POAC’13), Espoo, Finland.be made on the reliability of tactile sensor’smeasurement since it does not have the sameaccuracy in all the measuring range. In this testcampaign, the tactile sensor was calibrated tocapture the mean ice load with a higheraccuracy than the maximum ice load. Therefore,the measured absolute value of the mean iceload can be assigned a higher confidence.Furthermore, tactile sensor’s capability inmeasuring the ice load’s spatial distribution asin Figure 5 and 6 supplied valuable informationin investigating the pertinent questionsregarding spatially load distribution and loadevolution with time.Regarding the sensitivity of measurements withthe loading area size, this test campaign wasnot able to avoid this problem. However, consid-ering the fact that most of the time, the effectiveice load behaves a ‘line-like’ distribution asshown in Figure 5. Significant variation incontact area is not expected. Therefore, we canhave a higher confidence in the measured valueirrespective of this unsolved disadvantage oftactile sensor.In a further analysis of the measured data in theRITAS project (Lu et al., 2014), we have utilisedload cells’ measurement to compensate theinaccuracy of tactile sensor’s measurement inthe maximum ice load range. From experiencesmade we recommend to use a combination ofboth measuring techniques in order to obtainmost reliable results.ConclusionIn the study carried out tactile sensors wereused to investigate the interaction mechanismbetween level ice and a wide sloping structurein particular the temporal and spatial distribu-tions for different test scenarios.Based on these physical experiments we canconclude:• During the interaction, after the breaking ofinitially intact ice, the recorded ice load doesnot diminish instantly. Instead, the ice movescontinuously downward with a relatively lowerload magnitude (see Figure 4). This isconsidered due to the effect of ice rotatingload in combination of the accumulatedrubble effects.• Based on the mean ice load’s (i.e., averagedin time) vertical variation, it is found out thatequally large ice load can be detectedbeneath the un-deformed level ice’sthickness region (see Figure 6a). Asdiscussed above, the contribution of thislarge ice load is mainly due to the combinedeffects of ice rotating load and the rubbleFigure 5 - Time averaged ice load (a) and time history of maximum ice load variation in vertical direction of sensor (b), shaded area is the location of un-deformed level iceFigure 6 - Vertically spatial and temporal ice load distribution (bins 3 and 4 in the red square indicatesapproximately the location of un-deformed level ice)

Ice Load Measurement by tactile sensor in model scale test in relation to rubble ice transport on arctic offshore structures (RITAS)

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Ice Load Measurement by tactile sensor in model scale test in relation to rubble ice transport on arctic offshore structures (RITAS)

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