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Some Aspects of Arctic Offshore Floating Structures
Get PDFThe present work highlights some aspects related to the analyses of Arctic offshore floating structures. This thesis consists of five papers, which can be divided into two main categories. One category deals with the dynamics of slender structures with an emphasis on the prediction and suppression of vortex induced vibrations (VIV), and the other category examines the process of interaction between sloping structures and sea ice with focus on developing a numerical model to simulate this process in real time.
Slender structures, such as mooring lines and marine risers, are very important for the offshore petroleum industry, which is currently approaching deeper waters. Increasingly, attention has been focused on predicting the susceptibility of these structures to VIV. In this thesis, two asymptotic techniques namely, the local analysis and the WKB methods, were used to derive closed-form solutions for the natural frequencies and mode shapes of slender line-like structures. Both the top-tensioned nearly-vertical configuration and the catenary configuration were considered. The accuracy of the solutions derived was established through comparison with other analytic solution techniques and with results of numerical finite element solutions. The effects of the bending stiffness and the effects of approximating the tension variation as a linear function were discussed. Experimental data on the multi-modal in-line and cross-flow response behaviour of a towed catenary model were analysed to examine the usefulness of the solutions for predicting the response frequencies and envelopes due to VIV.
Helical strakes are often used as a mitigating measure to suppress the VIV of slender structures. This thesis presented an innovative method to fit ropes helically to a riser in the installation phase. Such a procedure will help to overcome the handling problem associated with the use of conventional sharp-edged strakes. Experimental investigations were then performed to verify the efficiency of these ropes (round-sectioned helical strakes) in suppressing VIV. Systematic experimental investigations including twenty-eight configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The effects of varying pitch, the surface roughness and the ratio between the cross-flow and in-line natural frequencies on the efficiency of the proposed configuration of round-sectioned helical strakes were also investigated.
The process of interaction between sea ice and offshore sloping structures (e.g., conical structures and ship-shaped structures) is quite complex. Modelling this process is very demanding and often computationally expensive, which typically hinders the chances for realtime simulations. This kind of simulation can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion was overcome in the present work. This thesis developed a numerical model to simulate the process of interaction between sea ice and sloping structures in real time. In this model, only level- and broken-ice features were studied. New analytical closed-form solutions were established and used to represent the ice breaking process. PhysX was used for the first time to solve the equations of rigid body motions with six degrees of freedom for all ice floes in the calculation domain. The results of the simulator were validated against experimental data from model-scale and full-scale tests.
Accurate predictions of ice actions are also vital to optimise the design of the structures in the Arctic regions. A good understanding of the role of seawater in the process of interaction between the sloping structures and level ice will help to establish reliable models to estimate the ice forces. This work formulated both the static and dynamic bending problems for a floating wedge-shaped ice beam interacting with an offshore sloping structure. For the dynamic interaction, the effects of the water foundation on the bending failure of the ice were studied by comparing the results of an elastohydrodynamic approach with a model of a Winkler foundation. The thesis also investigated the breaking lengths of the ice wedges (i.e., the frequency of the ice loads) as a function of the ice thickness, the compression in the ice and the acceleration of the interaction.PhD i bygg, anlegg og transportPhD in Civil and Transport Engineerin
Assessment of Impact of Dredging on Sediment Transport at Borg Port
Get PDFMålet for denne masteroppgaven har vært å modellere hvordan sedimenter spres under mudringsoperasjoner. En numerisk modell for et utvalgt prosjekt, Borg Havns planlagte mudringsprosjekt, er blitt utarbeidet ved å bruke MIKE21 fra DHI. Den hydrodynamiske modulen i MIKE21 ble brukt til å beregne strømningshastigheten og ble validert ved å sammenligne med lokale målinger. Partikkelsporingsmodulen i MIKE21 ble brukt til å simulere de suspenderte og sedimenterte sedimentene under mudringsprosessen. Modulen benytter resultatene fra den hydrodynamiske modulen. Vår numeriske modell ble validert ved å sammenligne våre resultater med SINTEF (forskningsorganisasjon) sine resultater fra deres simuleringer i programvaren DREAM.
Tidsrommet, strømningen fra elva, vindhastigheten og vindretningen ble undersøkt og det ble funnet at strømningen fra elva er den viktigste parameteren for strømningshastigheten. Undersøkelsene av mulig tidsrom, basert på strømning fra elva og strømningshastigheten, ekskluderer juni og deler av mai.
Effekten av størrelsen på mudringsområdet ble undersøkt og det har kun påvirkning på resultatet hvis det endres i en størrelsesorden på 100 eller mer. En sensitivitetsanalyse ble utført for to parametere, fallhastighet og utslippsrate. Undersøkelsene viste at utslippsraten har stor effekt på resultatene og det er derfor viktig å forsikre seg om at utstyret som brukes har den utslippsraten som er beskrevet i denne oppgaven eller lavere. Sensitivitetsanalysen viste også at fallhastigheten har stor effekt på resultatet, hvis den er lavere enn hva som er brukt tidligere (0.1 mm/s) vil sedimentene spre seg over store områder og være vanskelig å kontrollere.
Videre arbeid bør fokusere på å utvikle den tredimensjonale modellen videre slik at effekten av strømningslagdeling blir inkludert. Dette er viktig da området ligger slik til at det dannes en estuarinsirkulasjon ved innløpet til elva, som deler vannkolonnen inn i lag med ulik salinitet.The purpose of this research was to model sediment transport during dredging operations. A numerical model for a selected case study, which is a dredging project at Borg Port, has been created using MIKE 21, powered by DHI. The hydrodynamic module of MIKE21 was used to calculate the current speed and it was validated using measurements at site. The particle tracking module of MIKE21 was used to simulate the suspended and sedimented sediments during the dredging process. It uses the output from the hydrodynamic module. Our numerical model was validated by comparing our results to results from SINTEF (research organisation) using their in-house software "DREAM".
The operating window, the discharge from the river, the wind speed and the wind direction were investigated. It was found that discharge from the river is the governing parameter for the current speed. The operating window, based on the discharge data and current speed, excludes June and parts of May.
The effect of the size of the release area were investigated and it only has an effect on the results if the area is changed with a magnitude of 100 or more. A sensitivity analysis for two parameters, namely the settling velocity and the sediment release rate, was performed. It showed that the release rate has a large effect on the results. This shows the importance of knowing with certainty that the release rate for the equipment are not higher than what is described in this report. The sensitivity analysis also shows the significant influence of the settling velocity on the results, as with a lower settling velocity than what has been used in the previous modelling, the sediments will spread over a large area and be difficult to control, especially in May.
For further work, attention should be given to three-dimensional (3D) modelling, to account for effects of flow stratification, which is important at Borg Port, as a estuarine circulation is formed at the river inlet, dividing the water colon in layers with different salinity levels
Assessment of Impact of Dredging on Sediment Transport at Borg Port
No full textMålet for denne masteroppgaven har vært å modellere hvordan sedimenter spres under mudringsoperasjoner. En numerisk modell for et utvalgt prosjekt, Borg Havns planlagte mudringsprosjekt, er blitt utarbeidet ved å bruke MIKE21 fra DHI. Den hydrodynamiske modulen i MIKE21 ble brukt til å beregne strømningshastigheten og ble validert ved å sammenligne med lokale målinger. Partikkelsporingsmodulen i MIKE21 ble brukt til å simulere de suspenderte og sedimenterte sedimentene under mudringsprosessen. Modulen benytter resultatene fra den hydrodynamiske modulen. Vår numeriske modell ble validert ved å sammenligne våre resultater med SINTEF (forskningsorganisasjon) sine resultater fra deres simuleringer i programvaren DREAM.
Tidsrommet, strømningen fra elva, vindhastigheten og vindretningen ble undersøkt og det ble funnet at strømningen fra elva er den viktigste parameteren for strømningshastigheten. Undersøkelsene av mulig tidsrom, basert på strømning fra elva og strømningshastigheten, ekskluderer juni og deler av mai.
Effekten av størrelsen på mudringsområdet ble undersøkt og det har kun påvirkning på resultatet hvis det endres i en størrelsesorden på 100 eller mer. En sensitivitetsanalyse ble utført for to parametere, fallhastighet og utslippsrate. Undersøkelsene viste at utslippsraten har stor effekt på resultatene og det er derfor viktig å forsikre seg om at utstyret som brukes har den utslippsraten som er beskrevet i denne oppgaven eller lavere. Sensitivitetsanalysen viste også at fallhastigheten har stor effekt på resultatet, hvis den er lavere enn hva som er brukt tidligere (0.1 mm/s) vil sedimentene spre seg over store områder og være vanskelig å kontrollere.
Videre arbeid bør fokusere på å utvikle den tredimensjonale modellen videre slik at effekten av strømningslagdeling blir inkludert. Dette er viktig da området ligger slik til at det dannes en estuarinsirkulasjon ved innløpet til elva, som deler vannkolonnen inn i lag med ulik salinitet
Numerical Simulation of Ice-Rubble Mound Breakwater Interactions
Get PDFOffshore activity in energy production, fishing, shipping, and tourism is projected to increase in the Arctic and Sub- Arctic. This projected increase in offshore activity means the supporting coastal infrastructure needs to be expanded. All human activity in ice-prone regions requires a specialized knowledge and understanding of ice mechanics and how to properly design against ice forces and ice-structure interactions. Analyzing ice-structure interactions is a prerequisite for any successful venture into areas where sea ice can occur. This thesis studied numerically modeling the interaction between pre-broken, rigid ice sheets and wide, sloping structures. The thesis focused on adapting a numerical model and validating the base phenomena of the simulated ride-up and pile-up.
The numerical model used in this study is the Simulator for Arctic Marine Structures (SAMS). SAMS has previously been validated for ship-shape structures, and individual modules within SAMS has been validated for a wider range of applications. However, coastal structures with wide, upward slopes were previously unanalyzed, and SAMS required a few modifications before simulations could proceed. Due to limitations in the version of SAMS used for this study, level sheet ice was approximated with a section of pre-broken, rigid bodies being driven by a significantly larger, unbroken sheet. Ice was driven by simulated current and wind, thus a limit-force scenario could theoretically be reached. Side confinement was used to reduce the three dimensional (3D) effects introduced in the rigid-body approximation.
For ride-up, 36 test conditions were used with ice thicknesses between 0.5 and 1 m, velocities between 1.0 and 2.0 m/s, and the slopes of 1:4 to 1:6. Each test combination was repeated 40 times for a total 1440 simulations. Results from the tests show ride-up is both qualitatively and quantitatively well represented with an 18.9% difference between the simulations and Christensen's analytical model for ride-up.
For pile-up, 22 tests conditions were used with values similar to those found in the North Caspian Sea. The ice thickness was 0.15 m, the velocity was 0.5 m/s, and the slope was 1:3. Simulations were designed to test the effects of ice-ice friction, ice-structure friction, and rubble geometry on the pile-up behavior of SAMS. Qualitatively, pile- up simulations showed 5 distinct stages of simulation: 1) initial ride-up, 2) initial pile-up, 3) rubble-pile development, 4) ice-sheet failure away from the pile, and 5) the unbroken ice sheet directly influencing the pile. Stages 1-3 correspond with the expected behavior of pile-up, but stages 4 and 5 represent unrealistic behavior caused by the rigid-body approximation. Quantitatively, ice loads ranged between 3 and 9 kN/m, porosity between 0.35 and 0.65, the pile sail from 1-3 m, and the pile keel was not consistently grounded. Sensitivity tests have shown pile-up in SAMS is: 1) sensitive to changes in ice-ice friction for lower friction coefficients and relatively insensitive for higher friction coefficients, 2) small increases in ice-structure friction can exaggerate the aberrant stages, and 3) triangular rubble geometry can exaggerate the aberrant stages
Vortex Induced Vibrations of Slender Catenary Structures
No full textA wide range of offshore science and engineering applications utilize slender catenary-shaped structural elements in their design. These applications include moorings and steel catenary riser designs. The susceptibility of these slender catenary-shapes to vortex-induced vibrations remains the subject of theoretical, numerical and experimental studies.
This thesis studies the measured responses from the 2001 MARINTEK experiment with the purpose of trying to understand the relation between analytical models, numerical models, and experimental measurements.
The time domain decomposition (TDD) method is implemented in a Matlab program, considering three velocity cases of cross-flow displacements in the catenary plane. The results show that in each of the cases, one of the frequencies has a higher degree of contribution than other frequencies. However, multiple frequencies are present throughout the measurements. Also, the frequencies corresponding to each mode varies. According to the MAC-criterion, the obtained mode shapes have a high degree of correlation with the numerically calculated mode shapes.
For verification of the TDD results, the poly-reference least squares complex frequency method and the data- and covariance-driven stochastic subspace methods of operational modal analysis (OMA) are used for studying the data in the commercial Matlab toolbox MACEC. As for the TDD method, the mode shape estimates have high MAC-values, and the corresponding natural frequencies vary. The damping estimates show a great amount of scatter. Also, there is a varying degree of certainty in the results according to result processing guidelines.
The effect of accelerometer masses and first order added mass effects are studied using a numerical model in Matlab. The effects of the masses are concluded to be negligible.
A comparison between the operational modal OMA obtained eigenfrequencies and the corresponding numerically calculated modes show a great amount of agreement in how the natural frequencies increase with mode number. However, for this case study, the analytical and numerical calculations appear to underestimate magnitudes of the frequencies. Also, there is an indication that the frequencies corresponding to each mode shape vary, possibly resulting from higher order added mass effects
Expansion of the port in Ustka. Simulation of wave conditions.
No full textAlong with the development of port cities and conducting new market research and analysis it often turns out that the current port infrastructure is insufficient to exploit the full potential of the location. Then a modernization and expansion of the current port is needed. This is also the case of Port in Ustka, Poland, which is a small harbour handling mainly small fishing boats and yachts. With the ensuing need for a port being able to handle larger vessels, such as military units or vessels used during the construction and service of offshore wind farms, a decision for the port expansion has been made.
Designing a new harbour or a harbour expansion is always a very complex task, where a vast variety of complex phenomena and aspects have to be taken into account. One has to consider such aspect as technical requirements, environmental impact and financial feasibility and cost effectiveness of such an investment.
The research carried out in this thesis aims to choose the best harbour layout of new Harbour of Ustka out of four concepts provided by Maritime Office in Słupsk. As the decisive criterion wave climate inside the designed harbour was chosen. The best breakwater layout should ensure waves as small as possible in order not to disturb berthing of vessels and cargo handling operations inside the port. Wave propagation towards and inside harbour was modelled using MIKE 21 - numerical software developed by DHI. It is comprehensive tool containing numerous models and toolboxes that help to prepare all input data needed for the simulation. It includes also a module called MIKE C-MAP that contains global bathymetry and tidal data. As scarce data on waves and tides is available in the nearest proximity of the site of interest, the nesting approach has been used. Firstly, using all available information Spectral Wave model was prepared. It is a phase averaging model, that calculates wave statistics and allows for wave calculations in large scale. It was used to obtain reliable information about wave climate near the harbour of Ustka. Period of two months, January and February, was simulated, as major and most severe storms occur in this time. Output from Spectral Wave simulation was used to determine the input condition for a models prepared in much greater resolution. The four proposed breakwater layouts were tested using Boussinesq Wave models. Boussinesq Wave model is a phase resolving model, commonly used to investigate wave propagation and transformation in coastal area and wave interaction with the structures, as it is capable of resolving such wave phenomena as diffraction, refraction and reflection. Every layout was tested against waves coming from main direction, which is north-west, and most unfavourable direction of incoming waves, that is north-east.
After successful completion of numerical calculation, the results were summarized, compared and discussed. Conducted analysis allowed for choosing the best proposed breakwater layout in terms of provided wave conditions inside harbour basins and at the entrance to the harbour. In conclusion, limitations of the prepared models are listed, and ways of improvement are suggested. What is more a scope of future research is outlined
Erosion in the Arctic: A Thermoabrasion Model to Predict Shoreline Change After an Extreme Event
Get PDFErosion process in the Arctic is broadly classified into two categories: thermoabrasion and thermodenundation. The study completed under this MS framework is focused on thermoabrasion process. A numerical model is developed to predict the shoreline erosion during an extreme event like a storm.
A conceptual model identified the thermoabrasion erosion as a combination of three separate physical processes (1) storm surge flooding, (2) wave cut niche growth and (3) bluff collapse. For each physical process, a separate numerical module was developed. The numerical modules dynamically interact with each other and estimate erosion rate. The model developed under the study is 1D and uses a probabilistic approach by considering distribution patterns of the input parameters. The results were found to be in good agreement with field measurements of Baydara Bay, Russia.
A sensitivity analysis was performed to demonstrate the relative impact of the input parameters on the model. Thermoabrasion erosion rate was found to be greatly dependent on sustained wind speed during the storm, inundation depth at the base of the bluff from storm surge and ice wedge polygon size. In conclusion, the limitations of the model and the scope of future research are outlined
Experimental Investigation of an Inclined, Floating Breakwater with a Porous design and Bio-enhanving features
No full textVerden går i møte risikoer ved den globale oppvarmingen som er betydelig større enn det som tidligere er beskrevet, herunder økning av sjønivå og ekstreme værfenomener. Klimaendringene har også medført betydelige og irreversible tap i det marinbiologiske mangfoldet og det marine økosystemet. Tilpasninger som bremser trenden, er avgjørende. Flytemoloer er et etablert alternativ til den konvensjonelle massive moloen, hvor bygging av sistnevnte ikke er hensiktsmessig grunnet økonomiske, miljømessige eller praktiske hensyn. Typiske bruksområder er områder som hovedsakelig er utsatt for skarpe, vindgenererte bølger, ettersom flytemoloer generelt har en dårlig effekt mot dønninger med lange perioder.
Denne studien presenterer et nytt design for en flytemolo. Konseptet for moloen kan brytes ned i én bølgedempende del og én biodiversitetsforbedrende del, hvor det fysiske designet spiller inn på begge deler. Moloen vil være vinklet mot vannplanet med hensikt å etterligne en strand, hvor innkommende bølger skal brytes. Overflaten vil være porøs og delvis gjennomtrengelig, med hensikt å forstyrre vannets strømning langs modellens overflate i tillegg til å skape luftig turbulens i, og rundt, modellen. Potensialet for biodiversitetsforbedrende egenskaper vil bli diskutert og testet for å undersøke om hvorvidt moloen kan fungere som et flytende kunstig rev. Eksperimentelle studier på modellen utføres i en bølgetank, hvor hovedparameterne som undersøkes er overflateporøsitet, vinkling mot overflaten og festemekanismer. Testene gjennomføres ved bruk av regulære bølger over forskjellige frekvenser og et JONSWAP irregulært bølgespekter, på totalt 8 porøse konfigurasjoner og én referansemodell. Bølgetransmittering, bølgerefleksjon, bevegelser, fortøyningskrefter og hydrauliske mekanismer undersøkes. Resultatene peker på svake egenskaper i forhold til bølgetransmittering for lange bølger, sammenlignet med andre flytende moloer. Bevegelser grunnet innkommende bølger er den dominerende begrensende faktoren for moloens prestasjon. Lavere vinkling fører til mindre transmittering av bølgeenergi grunnet økning i bølgebrytning. Porøsitet for festemekanismer som tillater bevegelser gir ingen endring i bølgetransmittering, økning i bølgerefleksjon, reduserte fortøyningskrefter, reduserte bevegelser og økt oksygentilførsel i vannet på innsiden og utsiden av modellen. For fastspent festemekanisme er bølgetransmittering og -refleksjon redusert ved porøse konfigurasjoner. Videre undersøkelser av design er anbefalt, og flere forslag til endringer og framtidig testing er presentert
Expansion of the port in Ustka. Simulation of wave conditions.
Get PDFAlong with the development of port cities and conducting new market research and analysis it often turns out that the current port infrastructure is insufficient to exploit the full potential of the location. Then a modernization and expansion of the current port is needed. This is also the case of Port in Ustka, Poland, which is a small harbour handling mainly small fishing boats and yachts. With the ensuing need for a port being able to handle larger vessels, such as military units or vessels used during the construction and service of offshore wind farms, a decision for the port expansion has been made.
Designing a new harbour or a harbour expansion is always a very complex task, where a vast variety of complex phenomena and aspects have to be taken into account. One has to consider such aspect as technical requirements, environmental impact and financial feasibility and cost effectiveness of such an investment.
The research carried out in this thesis aims to choose the best harbour layout of new Harbour of Ustka out of four concepts provided by Maritime Office in Słupsk. As the decisive criterion wave climate inside the designed harbour was chosen. The best breakwater layout should ensure waves as small as possible in order not to disturb berthing of vessels and cargo handling operations inside the port. Wave propagation towards and inside harbour was modelled using MIKE 21 - numerical software developed by DHI. It is comprehensive tool containing numerous models and toolboxes that help to prepare all input data needed for the simulation. It includes also a module called MIKE C-MAP that contains global bathymetry and tidal data. As scarce data on waves and tides is available in the nearest proximity of the site of interest, the nesting approach has been used. Firstly, using all available information Spectral Wave model was prepared. It is a phase averaging model, that calculates wave statistics and allows for wave calculations in large scale. It was used to obtain reliable information about wave climate near the harbour of Ustka. Period of two months, January and February, was simulated, as major and most severe storms occur in this time. Output from Spectral Wave simulation was used to determine the input condition for a models prepared in much greater resolution. The four proposed breakwater layouts were tested using Boussinesq Wave models. Boussinesq Wave model is a phase resolving model, commonly used to investigate wave propagation and transformation in coastal area and wave interaction with the structures, as it is capable of resolving such wave phenomena as diffraction, refraction and reflection. Every layout was tested against waves coming from main direction, which is north-west, and most unfavourable direction of incoming waves, that is north-east.
After successful completion of numerical calculation, the results were summarized, compared and discussed. Conducted analysis allowed for choosing the best proposed breakwater layout in terms of provided wave conditions inside harbour basins and at the entrance to the harbour. In conclusion, limitations of the prepared models are listed, and ways of improvement are suggested. What is more a scope of future research is outlined
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