Ice-structure interaction in shallow water - A study based on laboratory-scale experiments and discrete element simulations

Ida Lemström

Ice-structure interaction in shallow water - A study based on laboratory-scale experiments and discrete element simulations

Julkaisun otsikon käännös: Jään ja rakenteen välinen vuorovaikutus matalassa vedessä

Ida Lemström

Energia- ja konetekniikan laitos

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Ilmastonmuutoksen vuoksi muuttuvat jääolosuhteet ovat lisänneet aktiviteettejä ja merirakenteiden tarvetta arktisilla merialueilla. Merirakenteiden optimointiin jääpeitteisille merialueille tarvitaan lisäymmärrystä jään ja rakenteen välisestä vuorovaikutuksesta. Jään ja rakenteen vuorovaikutuksessa liikkuva jää törmää rakenteeseen ja murtuu jäälohkareiksi, joista syntyy jääkasa rakenteen eteen. Jään kasaantumisprosessi aiheuttaa rakenteelle suuria jääkuormia. Useat arktiset merirakenteet sijaitsevat matalassa vedessä, jolloin jääkasa voi pohjautua. Pohjautuminen puolestaan vaikuttaa jäänmurtoprosessiin ja rakenteeseen kohdistuviin kuormiin. Tässä työssä tarkastellaan jään ja leveän sekä kaltevan rakenteen välistä vuorovaikutusta matalassa vedessä. Työ koostuu mallimittakaavassa tehdyistä jääkokeista ja täysmittakaavassa tehdystä numeerisesta mallinnuksesta. Mallimittakaavakokeet, joissa ehjä jäälautta työnnettiin kymmenmetristä kaltevaa matalan veden rakennetta vasten, suoritettiin Aalto Ice Tankissa. Rakenne koostui kymmenestä identtisestä segmentistä, joista kustakin mitatut kuormat analysoitiin eri resoluutioissa. Kokeet toteutettiin kolmessa sarjassa, jotka erosivat toisistaan jään lujuuden suhteen. Kokeiden uutuusarvo koostui aiemmin tehtyihin kokeisiin verrattuna huomattavasti leveämmästä rakenteesta sekä rakenteen segmentoinnista. Rakenteesta mitatut kuormitusaikasarjat osoittivat, että kuormitusprosessissa on kaksi erillistä vaihetta: (1) Ensiksi jääkuormat kasvoivat lineaarisesti, minkä jälkeen (2) jääkuormat pysyivät suunnilleen vakioina. Ensimmäisen vaiheen aikana kuorman kasvunopeus riippui suoraan rakenteen edessä olevan jääkasan kasvavasta massasta. Jääkuormat eivät olleet suoraan verrannollisia jään lujuuteen, sillä heikoin jää aiheutti rakenteeseen suurempia kuormia kuin jää, joka oli heikkoa jäätä kaksi kertaa lujempaa. Segmenteistä mitatut kuormitusaikasarjat korreloivat voimakkaasti koko kuormitusprosessin tasolla, viitaten siihen, että oletus jään eriaikaisesta murtumisesta rakennetta vasten ei pidä paikkaansa. Numeerinen mallinnus perustui täysmittakaavaisen jäänmurtoprosessin simulointiin kaksiulotteista diskreettielementtimenetelmään perustuvaa mallia apuna käyttäen. Simulaatiot mahdollistivat jäänmurtoprosessin tutkimisen täysimittakaavassa ja simulaatioissa keskityttiin siihen, kuinka veden syvyys vaikuttaa jääkuormiin. Simulaatioissa havaittiin rakenteeseen kohdistuvien jääkuormien kasvavan veden syvyyden pienentyessä. Simulaatiot osoittivat myös, että jääkasan mittasuhteiden skaalaus jään paksuuden neliöjuurella ottaa totuudenmukaisesti huomioon jään paksuuden vaikutuksen mittasuhteisiin. Tämä osoittaa, että matalan veden tarkastelussa usein mittana käytetty vedensyvyyden ja jään paksuuden suhde ei välttämättä toimi. Sekä simulaatiot että mallikokeet osoittivat, että pohja ottaa vastaan vain pienen osuuden kokonaisjääkuormasta.

Julkaisun otsikon käännös	Jään ja rakenteen välinen vuorovaikutus matalassa vedessä
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Polojärvi, Arttu, Vastuuprofessori Tuhkuri, Jukka, Ohjaaja
Kustantaja	Aalto University
Painoksen ISBN	978-952-64-0712-8
Sähköinen ISBN	978-952-64-0713-5
Tila	Julkaistu - 2022
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

jää-rakenne vuorovaikutus
jäämekaniikka
arktiset merirakenteet
jäämallikokeet
diskreettielementtimenetelmä
jääkuormat
painejakauma

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-64-0713-5

Aalto-yliopiston jäätutkimusallas
Puolakka, O. (Manager)
Insinööritieteiden korkeakoulu
Laitteistot/tilat: Facility

Siteeraa tätä

@phdthesis{1c245aa982164cb09dc4ce9497ecc508,

title = "Ice-structure interaction in shallow water - A study based on laboratory-scale experiments and discrete element simulations",

abstract = "The global warming and retreating sea ice cover open up new transportation routes and offer new opportunities for exploratory activities in the Arctic regions. The increasing levels of marine activities in these regions constantly increase the demand for offshore structures in ice-covered sea areas. Optimizing the design of offshore structures still requires new engineering insight on ice-induced loads and the mechanics of ice-structure interaction. During an ice-structure interaction process, ice is drifting against a structure and failing into ice blocks, which form an ice rubble pile. This process may subject the structure to high ice loads. Many offshore structures operate in shallow water, meaning that the rubble pile may ground, which affects the further ice loading process. This thesis studies the ice-structure interaction against a wide, sloping structure in shallow water. The work consists of model-scale experiments and full-scale numerical simulations. The model-scale experiments were performed in the Aalto Ice Tank by pushing an ice sheet against a ten-meter-wide, inclined, structure in shallow water. The structure consisted of ten identical segments and the loads measured on the structure were analysed in different resolutions. The experiments were conducted in three tests series with varying ice strength. The novelty within the experiments was in the very wide structure and the segmentation of the structure. The ice loading process in the experiments showed two distinct phases: the load on the structure (1) first increased linearly with a rate of increase depending on the ice mass above waterline, after which (2) a steady-state phase with an approximately constant load level was reached. The experiments also showed that the magnitude of ice loads was not directly proportional to the ice strength, as the weakest ice yielded higher loads than the ice having twice its strength. The loads on the individual segments correlated strongly on the level of the entire interaction process, suggesting that the assumption of non-simultaneous ice load does not apply. In the numerical modelling, a two-dimensional finite-discrete element method was used to simulate the ice-structure interaction process against a shallow water structure in full-scale. The simulations allowed for investigations on the ice loading process in full-scale and the focus was on the influence of water depth and the ice thickness on the ice loads. The simulations showed that the peak ice loads increase with decreasing water depth. The simulations also indicated that scaling the rubble pile dimensions with the square root of the ice thickness correctly account for the influence of ice thickness on them. This suggests that the water depth to ice thickness ratio, often used in ice action-related problems in shallow water, is dubious when considering the ice-structure interaction process. Both the simulations and the experiments indicated that the loads applied on the bottom, simulating the seabed, are small compared to the loads applied on the structure.",

keywords = "ice-structure interaction, ice mechanics, offshore structures, model-scale experiments, finite-discrete element method, ice loads, ice load distribution, j{\"a}{\"a}-rakenne vuorovaikutus, j{\"a}{\"a}mekaniikka, arktiset merirakenteet, j{\"a}{\"a}mallikokeet, diskreettielementtimenetelm{\"a}, j{\"a}{\"a}kuormat, painejakauma, ice-structure interaction, ice mechanics, offshore structures, model-scale experiments, finite-discrete element method, ice loads, ice load distribution",

author = "Ida Lemstr{\"o}m",

year = "2022",

language = "English",

isbn = "978-952-64-0712-8",

series = "Aalto University publication series DOCTORAL THESES",

publisher = "Aalto University",

number = "29",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Ice-structure interaction in shallow water - A study based on laboratory-scale experiments and discrete element simulations

AU - Lemström, Ida

PY - 2022

Y1 - 2022

N2 - The global warming and retreating sea ice cover open up new transportation routes and offer new opportunities for exploratory activities in the Arctic regions. The increasing levels of marine activities in these regions constantly increase the demand for offshore structures in ice-covered sea areas. Optimizing the design of offshore structures still requires new engineering insight on ice-induced loads and the mechanics of ice-structure interaction. During an ice-structure interaction process, ice is drifting against a structure and failing into ice blocks, which form an ice rubble pile. This process may subject the structure to high ice loads. Many offshore structures operate in shallow water, meaning that the rubble pile may ground, which affects the further ice loading process. This thesis studies the ice-structure interaction against a wide, sloping structure in shallow water. The work consists of model-scale experiments and full-scale numerical simulations. The model-scale experiments were performed in the Aalto Ice Tank by pushing an ice sheet against a ten-meter-wide, inclined, structure in shallow water. The structure consisted of ten identical segments and the loads measured on the structure were analysed in different resolutions. The experiments were conducted in three tests series with varying ice strength. The novelty within the experiments was in the very wide structure and the segmentation of the structure. The ice loading process in the experiments showed two distinct phases: the load on the structure (1) first increased linearly with a rate of increase depending on the ice mass above waterline, after which (2) a steady-state phase with an approximately constant load level was reached. The experiments also showed that the magnitude of ice loads was not directly proportional to the ice strength, as the weakest ice yielded higher loads than the ice having twice its strength. The loads on the individual segments correlated strongly on the level of the entire interaction process, suggesting that the assumption of non-simultaneous ice load does not apply. In the numerical modelling, a two-dimensional finite-discrete element method was used to simulate the ice-structure interaction process against a shallow water structure in full-scale. The simulations allowed for investigations on the ice loading process in full-scale and the focus was on the influence of water depth and the ice thickness on the ice loads. The simulations showed that the peak ice loads increase with decreasing water depth. The simulations also indicated that scaling the rubble pile dimensions with the square root of the ice thickness correctly account for the influence of ice thickness on them. This suggests that the water depth to ice thickness ratio, often used in ice action-related problems in shallow water, is dubious when considering the ice-structure interaction process. Both the simulations and the experiments indicated that the loads applied on the bottom, simulating the seabed, are small compared to the loads applied on the structure.

AB - The global warming and retreating sea ice cover open up new transportation routes and offer new opportunities for exploratory activities in the Arctic regions. The increasing levels of marine activities in these regions constantly increase the demand for offshore structures in ice-covered sea areas. Optimizing the design of offshore structures still requires new engineering insight on ice-induced loads and the mechanics of ice-structure interaction. During an ice-structure interaction process, ice is drifting against a structure and failing into ice blocks, which form an ice rubble pile. This process may subject the structure to high ice loads. Many offshore structures operate in shallow water, meaning that the rubble pile may ground, which affects the further ice loading process. This thesis studies the ice-structure interaction against a wide, sloping structure in shallow water. The work consists of model-scale experiments and full-scale numerical simulations. The model-scale experiments were performed in the Aalto Ice Tank by pushing an ice sheet against a ten-meter-wide, inclined, structure in shallow water. The structure consisted of ten identical segments and the loads measured on the structure were analysed in different resolutions. The experiments were conducted in three tests series with varying ice strength. The novelty within the experiments was in the very wide structure and the segmentation of the structure. The ice loading process in the experiments showed two distinct phases: the load on the structure (1) first increased linearly with a rate of increase depending on the ice mass above waterline, after which (2) a steady-state phase with an approximately constant load level was reached. The experiments also showed that the magnitude of ice loads was not directly proportional to the ice strength, as the weakest ice yielded higher loads than the ice having twice its strength. The loads on the individual segments correlated strongly on the level of the entire interaction process, suggesting that the assumption of non-simultaneous ice load does not apply. In the numerical modelling, a two-dimensional finite-discrete element method was used to simulate the ice-structure interaction process against a shallow water structure in full-scale. The simulations allowed for investigations on the ice loading process in full-scale and the focus was on the influence of water depth and the ice thickness on the ice loads. The simulations showed that the peak ice loads increase with decreasing water depth. The simulations also indicated that scaling the rubble pile dimensions with the square root of the ice thickness correctly account for the influence of ice thickness on them. This suggests that the water depth to ice thickness ratio, often used in ice action-related problems in shallow water, is dubious when considering the ice-structure interaction process. Both the simulations and the experiments indicated that the loads applied on the bottom, simulating the seabed, are small compared to the loads applied on the structure.

KW - ice-structure interaction

KW - ice mechanics

KW - offshore structures

KW - model-scale experiments

KW - finite-discrete element method

KW - ice loads

KW - ice load distribution

KW - jää-rakenne vuorovaikutus

KW - jäämekaniikka

KW - arktiset merirakenteet

KW - jäämallikokeet

KW - diskreettielementtimenetelmä

KW - jääkuormat

KW - painejakauma

KW - ice-structure interaction

KW - ice mechanics

KW - offshore structures

KW - model-scale experiments

KW - finite-discrete element method

KW - ice loads

KW - ice load distribution

M3 - Doctoral Thesis

SN - 978-952-64-0712-8

T3 - Aalto University publication series DOCTORAL THESES

PB - Aalto University

ER -

Ice-structure interaction in shallow water - A study based on laboratory-scale experiments and discrete element simulations

Abstrakti

Tutkimusalat

Pääsy asiakirjaan

Sormenjälki

Laitteet

Aalto-yliopiston jäätutkimusallas

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