Interactions between irregular wave fields and sea ice: A physical model for wave attenuation and ice breakup in an ice tank

Irregular, unidirectional surface water waves incident on model ice in an ice tank are used as a physical model of ocean surface wave interactions with sea ice. Results are given for an experiment consisting of three tests, starting with a continuous ice cover and in which the incident wave steepness increases between tests. The incident waves range from causing no breakup of the ice cover to breakup of the full length of ice cover. Temporal evolution of the ice edge, breaking front, and mean floe sizes are reported. Floe size distributions in the different tests are analyzed. The evolution of the wave spectrum with distance into the ice-covered water is analyzed in terms of changes of energy content, mean wave period, and spectral bandwidth relative to their incident counterparts, and pronounced differences are found between the tests. Further, an empirical attenuation coefficient is derived from the measurements and shown to have a power-law dependence on frequency comparable to that found in field measurements. Links between wave properties and ice breakup are discussed

Terveydenhuollon kansalliset laaturekisteripilotit loppusuoralla

Pilottiprojektin tulokset laaturekisterien toiminnan organisoimisesta kannustavat ¬jatkamaan työtä moniammatillisesti ja pitkäjänteisesti. Kansallinen koordinaatio on välttämätöntä laatu- ja vaikuttavuustiedon vertailukelpoisuuden ja hyödyntämisen varmistamiseksi

Aaltovoimarakenteen ja aallokon vuorovaikutuksen mallinnus

In this study a linear model for the interaction of water surface waves and a two dimensional flap-type wave energy converter is constructed. The converter consists of a wide flat base on the seabed in intermediate depth of water and of a thin vertical flap hinged to it at the lower end and extending to the surface. Sinusoidal Airy waves are assumed normally incident on the flap. The hydrodynamic and flap dynamic problems are decoupled by assuming harmonic motion. Flow around the flap is assumed potential and analysis restricted to first order. The wave flow is divided into diffraction and radiation parts, for which known analytical solutions are presented. Flap displacement is taken into account as a linear restoring force. The linearized hydrodynamic reaction forces are included as stiffness, added mass and damping and a wave excitation moment in the linear equation of motion of the flap. Analytical solutions for flap motion response, power absorbed by linear mechanism damping and absorption efficiency are derived. Numerical results are produced for a test case flap over a range of wave periods. Absorption efficiencies very near to the theoretical maximum of 50 per cent are found at the nominal frequency range with constant flap parameters. The efficiency is not very sensitive to flap thickness or the chosen mechanism damping. The bandwidth of high efficiency is found very wide for the flap device in intermediate depth in comparison with other single-mode wave absorbing devices.Tässä työssä luodaan kaksiulotteinen lineaarinen malli pinta-aallokon ja levytyyppisen väliveteen sijoitettavan aaltovoimalan vuorovaikutukselle. Aaltovoimala koostuu matalasta merenpohjalla makaavasta perustasta sekä siihen nivelöidystä pystysuorasta ohuesta voimalalevystä. Sinimuotoisten Airyn aaltojen oletetaan kohtaavan levyn meren puolella. Virtauksen ja levyn liikkeen ongelmat erotetaan toisistaan olettamalla liike harmoniseksi. Virtaus levyn ympärillä oletetaan potentiaaliseksi ja analyysi rajoitetaan ensimmäiseen kertalukuun. Aaltovirtaus jaetaan diffraktio- ja radiaatio-ongelmiksi, joille työssä käytetään tunnettuja analyyttisiä ratkaisuja. Levyn uppouman vaikutus otetaan huomioon lineaarisena palautusvoimana. Virtauksen linearisoidut reaktiovoimat sisällytetään levyn liikeyhtälöön jäykkyytenä, lisättynä massana ja vaimennuksena sekä aaltoherätemomenttina. Levyn liikevasteelle, lineaarisen mekanismivaimennuksen absorboimalle teholle sekä absorptiohyötysuhteelle johdetaan analyyttiset ratkaisut. Numeerisia tuloksia lasketaan esimerkkilevylle merkityksellisille aallon jaksonajoille. Absorptiohyötysuhteen havaitaan valituilla kiinteillä parametreillä olevan hyvin lähellä teoreettista maksimiaan 50 prosenttia nimellistaajuusalueella. Valitun levynpaksuuden ja mekanismivaimennuksen vaihtelun vaikutus hyötysuhteeseen on verrattain pieni. Korkean hyötysuhteen taajuusalueen havaitaan olevan poikkeuksellisen laaja kyseiselle väliveden levyvoimalalle muihin yhden vapausasteen aaltovoimaloihin verrattuna

Wave-induced spill from grounded tankers

The effect of sea waves on the oil release from grounded tankers is unknown and ignored by maritime safety regulations. This study predicts the wave-induced spillage through fluid-mechanical analysis of a damaged double-hulled tanker under wave action. The punctured tanks are identified as Helmholtz resonators for wave pressures, and the induced oil spillage is estimated based on mechanics of fountains and layered fluids. Waves are predicted to increase the spill considerably already in moderate weather: for typical grounding damages, additional spilling initiates at wave heights of 0.5–3 metres and amounts to 5–30% of tank capacity, possibly more than doubling the total release. The results imply that the current hydrostatic outflow predictions may grossly underestimate the spill hazard in moderate-to-heavy weather tanker groundings, and that the environmental risk of oil transport may be larger than thought.Peer reviewe

Laajakaistaläpinäkyvä portti vesiaallokossa

Linear and nonlinear performance of semi-infinite backside domain fluid reaction as an absorption load in active absorption of water waves in experiments is evaluated through the study of plane wave transmission at a single-mode hinged gate in a channel.  An analytical model is constructed for the gate based on a second-order perturbation solution to the potential flow problem on either side. Gate motion and wavefield are solved to second order in incident wave steepness. Passbands of 11/2 –2 octaves of reflections below 5 % are predicted for most gates. The wide-band transparency is explained by reversal of the hydrodynamic added inertia trend. Equiripple and maximally flat tunings are demonstrated. Low first superharmonic free wave emissions at fractions of the incident Stokes component amplitude are predicted over half an octave in the fundamental passband, where superharmonic gate dynamics is shown to reduce the emissions compared to simple-harmonic motion and to enable a coincident near-zero minimum. Most of the dynamic reduction is shown to follow from an idealized resistive response. Prospects for superharmonic tuning are shown to be poor.  Experiments with low- and high-amplitude regular waves in the passband of a test gate are reported. Very low amplitude-averaged reflection coefficients 1–3 % are measured for the gate with two techniques and confirm the theoretical predictions. Estimated first superharmonic emissions show reasonable agreement with predictions at moderate amplitudes. Pure transmission of the Stokes wave seems to take place above the predicted frequency. At the highest test amplitudes between Ka1 = 0.2–0.3, the change in fundamental scattering is negligible, and relative superharmonic emissions are constant or decline. Higher harmonic emissions were found to be negligible.  The experimentally confirmed fundamental-frequency performance of the accurate backside reaction as an absorption load is found to be comparable with existing active absorbers already with a single interface mode. Superharmonic emissions would be critical only when compared to wave-makers with advanced control. Other nonlinear properties of the load are considered favourable–neutral. Improvement is expected with added interface modes.Puoliäärettömän takapuolisen nestealueen reaktiovoiman lineaarista ja epälineaarista absorptiokykyä veden pinta-aaltojen aktiivisessa vaimennuksessa arvioidaan tarkastelemalla tasoaallon läpäisyä yksimoodisella nivelöidyllä portilla kanavassa.  Portille muotoillaan molemminpuolisen potentiaalivirtausongelman toisen kertaluvun perturbaatioratkaisuihin perustuva analyyttinen malli. Portin liike ja aaltokenttä ratkaistaan toiseen kertalukuun aallon jyrkkyydessä. Puolentoista–kahden oktaavin päästökaistoja alle 5 prosentin heijastuksilla ennustetaan useimmille porteille. Laajakaistaläpinäkyvyys selitetään käänteellä hydrodynaamisen lisätyn hitauden taajuuskäytöksessä. Porteille esitetään Tšebyšov- ja Butterworth-viritykset. Alhaisia ensimmäisten vapaiden yliaaltojen emissiotasoja tulevan aallon Stokesin komponentin murto-osien suuruusluokassa ennustetaan puolen oktaavin alueilla päätaajuuden päästökaistalla, jolla porttien yliharmonisen liikevasteen osoitetaan alentavan yliaaltoemissiota sinimuotoiseen liikkeeseen verrattuna ja tuottavan samankeskisen erittäin alhaisen emissiominimin. Pääosan dynaamisesta emissiovähennyksestä osoitetaan selittyvän yksinkertaistetulla resistiivisellä vasteella. Yliharmonisen virityksen osoitetaan olevan hyödytöntä.  Testiportin päästökaistalla matalilla ja korkeilla aallon amplitudeilla tehdyt kokeet selostetaan. Portille kahdella menetelmällä mitatut erittäin alhaiset amplitudikeskiarvotetut 1–3 %:n heijastuskertoimet vahvistavat teoreettiset ennusteet. Yhdenpitävyys kokeellisesti arvioitujen ja ennustettujen ensimmäisen yliaallon emissiotasojen välillä oli kelvollista kohtalaisilla aallonjyrkkyyksillä. Puhdas Stokesin aallon läpäisy vaikutti tapahtuvan ennustettua korkeammalla taajuudella. Päätaajuuden sironta muuttui suurimmilla koeaallon jyrkkyyksillä Ka1 = 0,2–0,3 vain vähän, ja suhteellinen yliaaltoemissiotaso oli tasainen tai laskeva. Korkeampien yliaaltojen emissio oli vähäistä.  Tarkan takapuolisen nestealueen reaktiovoiman kokeellisesti vahvistettu perustaajuuden suorituskyky absorptiokuormana todetaan nykyisiin aktiivisiin aallonvaimentimiin verrannolliseksi jo yhdellä rajapintamoodilla. Yliaaltoemissio on kriittinen ainoastaan edistyneisiin aaltokoneisiin verrattuna. Kuorman muut epälineaariset piirteet todetaan suotuisiksi–neutraaleiksi. Suorituskyvyn odotetaan parantuvan rajapintamoodeja lisäämällä

Load distributions in the ice-structure interaction process in shallow water

Model-scale experiments were performed to explore the process of level ice interacting with a, in model-scale, ten-meter-wide inclined structure in shallow water. During each experiment, an initially intact ice sheet was pushed against the sloping structure, failed against it and accumulated into a grounded rubble pile in front of it. The strength of the model ice used in the experiments was varied. The spatial and temporal ice load distributions were measured at two scales: the structure was divided into ten identical segments along its waterline with the horizontal load on each segment measured, while the local pressure distributions were simultaneously measured with tactile sensors from two of the segments. This paper focuses on the relationships between the loads on different segments and the correlation between global and local loads. The load records of the individual segments during the whole experiment correlated with each other. The Pearson correlation coefficient for the load records of two neighboring segments was about 0.8, and for segments having a distance of 5 m or more, about 0.4. The magnitudes of both the maximum and the mean load on each segment were approximately equal. Even if the segment loads correlated during the whole experiment, the load records of the individual segments showed also transient ice load peaks, which did not correlate. These were used to define local peak load events and related local line loads, defined by dividing the magnitude of a peak load by its width in space. Roughly 2/3 of the local peak load events concentrated on one segment and only about 5% of them covered more than three segments. Narrow local peak load events caused the local line loads of the highest magnitude, with the magnitude and the relative frequency of the events increasing with the ice strength. Narrow events resulted in local line loads up to four times higher than the global line load, which was defined by dividing the total load on the structure by its width. The investigation of the local peak load events also showed that an inclined structure having a width of approximately 5 m (the ice thickness to structure width ratio of 1:100) appeared infinitely wide and gave similar results as a wider structure. Ice pressure measurements showed a line-like pressure distribution at the waterline of the structure with the weakest ice, whereas with stronger ice, zones of high ice pressure were observed at upper parts of the segments.Peer reviewe

Ice channel breakout performance of a double-acting vessel

The capability of an icegoing ship to break out from an ice channel is a main index of its manoeuvrability in ice. Breakout is necessary if a ship needs to change its course when sailing in an ice channel. Breakout can also be used as an evasive action to avoid collision with the leading ship in emergencies during escort and convoy. Although the ice channel breakout test is documented in ITTC guidelines as a standard manoeuvring test, little data is available on the performance of icegoing ships in this context. This paper presents model-scale tests carried out at Aalto Ice Tank with a double-acting vessel with podded propulsion. Parameters including ice thickness and strength, ice channel width, initial speed and propulsive settings are varied between the tests to examine their influence on the capability and distance to break out from ice channels. In addition, stopping tests by reversing propeller revolution were conducted in ice channels to benchmark the breakout operation as an option to avoid collision. The test results reveal the influence of multiple factors on the ship's channel-breakout capability, including channel width, shaft power, initial speed, as well as thickness and strength of ice. With additional complementary numerical simulations at high speeds, it is found that for collision avoidance braking is more efficient at low speed and breakout at high speed. Recommendations on breakout test in model scale are given based on the lessons and experience learnt from the experiments.Peer reviewe

Safety Distance During Escort and Convoy Based on Channel Breakout Simulation and Model Tests

Escort and convoy are typical operations when a merchant ship navigates in ice beyond its own ice-going capability. In such scenarios, a safety distance between the front ship and the following ship is maintained. The leading vessel may suddenly decelerate or even come to a halt if it encounters a heavy ridge or thick ice floe. When this happens, the following ship needs to take actions to avoid collision with the leading vessel. An evasive operation for such collision avoidance is to break out of the channel. The safety distance can be defined as the minimum distance which is needed for successful collision avoidance. This depends on various parameters such as ice thickness, ship speed, channel width, the ship’s icebreaking capability and its manoeuvrability in ice. This paper applies the in-house simulation software package of Aalto University Marine Technology group to simulate ships breaking out of an ice channel. The dependencies of the safety distance on ice thickness, channel width and initial speed are identified via multiple simulation runs. The paper also reports model tests were carried out at Aalto Ice Tank to validate the simulation. The simulation proves to provide accurate estimation on breakout distance through comparison with model test results, and reveals the influence of ice thickness, channel width and initial speed on the breakout distance.Peer reviewe