Long-term statistics for wave power describes the waves that will occur over years and their energy. This can be found by putting together short-term statistics for individual waves and statistics from sea states. All the calculations are done both by the use of wave scatter diagrams and analytical expressions. It is generally seen that the two different methods gives similar results.For the long-term statistics the quality of the results is dependent on both the resolution of the calculations, and the size of which the calculations span over. Most of the waves have low values for wave energy, wave height and wave period. It is seen that the wave power varies with the square of the wave height which is expected. The long-term marginal pdf of wave power is very steep and resembles an exponential curve, but it is not an exact match. The CDF of wave energy is also very steep, and here the differences between the two methods become visible.To optimize the motion of a WEC in waves a control system is necessary. The goal of the control system is to change the WEC's parameters so that it increases the motion of the moving parts. The key control variable, the machinery force, can be divided in three categories: resistive loading, reactive loading and non-linear loading. Different concepts that uses reactive loading is approximate complex conjugate control, tracking of approximate optimal velocity and model predictive control. These give the highest power output, but they use some energy in order to control the motion. These methods use prediction because the machinery force is dependent on future values. Resistive loading, i.e. no machinery force, is presented by phase control by latching and clutching. Latching is a method keeping the absorber fixed during parts of the cycle, and clutching is a method which is coupling and decoupling the machinery at intervals. These methods give smaller energy output, but do not use energy in order to control the phase.A good wave energy converter is a device that has low cost of electricity per kilowatt hour. This criterion is dependent on the cost of the device, which is generally not known, and the power absorption that can be estimated. The power absorption has been estimated for four different concepts: a heaving buoy, a heaving two bodies system, a pitching device and a floating OWC. The power absorption varies for different sea states and is generally increasing as the total present wave power increases, but the WEC's have different optimal values that give the best power absorption and best efficiency. The efficiency can for some areas be very large, even larger than 100% mainly because of focusing techniques and limited accuracy on the raw data used in the calculations. However most of the efficiency results are much lower than that. The efficiency of the WEC's is combined with the long-term wave power statistics to obtain long-term power absorption. The differences between the concepts are large, as they have efficiency between 11% and 47%.

Calculations have been made with the commercial CFD-software Ansys Fluent, with dynamic mesh.

Viscous flow around circular cylinders is a classical research topic in fluid dynamics with a vast amount of practical applications in the field of offshore marine technology. In the flow around cylinders of finite length, complex wake behaviours and coherent structures occur even at relatively low Reynolds numbers. An understanding of the nature and dynamics behind such behaviour could form a basis for improved designs and innovative solutions for offshore and subsea constructions.In the present study, flow around long finite cylinders at Re = 100 is investigated numerically using the incompressible Navier-Stokes Equations solver MGLET. To study the isolated flow near the free end, a cylinder with aspect ratio L/D = 50 is chosen. The flow over the free end gives rise to a wake consisting of two vortex shedding cells with different shedding frequencies; one small near the free end and one larger in the central region of the span. It is found that each vortex shed in the end cell bends horizontally and connects with the upstream vortex shed from the opposite side of the cylinder. The horizontal vortex shedding is found to give rise to a pair of trailing vortices in the time averaged flow.When a vortex is shed with a large phase difference between the two cells, the vortex is split and connects with other surrounding vortices. This phenomena is commonly referred to as vortex dislocations and occurs with the beat frequency, i.e. the difference between the two vortex shedding frequencies. It is found that this frequency can be detected in time histories of $u$ in the wake at the spanwise centre.A second configuration, consisting of a wall mounted cylinder with aspect ratio L/D = 25 is simulated in order to study the effect of introducing a no-slip surface. The effect on the end cell is found to be minimal, while the central cell shedding frequency is reduced. Comparisons with published data on a cylinder with aspect ratio L/D = 25 and two free ends shows that both the reduction of aspect ratio and the introduction of the no-slip boundary condition contributes to the reduced shedding frequency.

The ITTC78 method was originally designed for conventional single screw ships, but has later been modified to adapt twin screw ships and podded propulsors. Nowadays even more unconvensional propulsors are introduced, and the need for a new powering performance method is increasing. This thesis covers the load varying self propulsion method, and looks into how this corresponds to the standard ITTC78 method. The load varying method uses data from a self propusion test only, and uses a predefined increment value to change the revolutions during each run. In this way there are no need for an open water test or a resistance test, and time can be saved. In addition the vessel is tested as a unit, and not broken down into separate pieces like with the ITTC78 method.

AbstractThe wind power business has in recent times changed its focus from land-based installations to offshore installations. This has presented challenges both technological and financial, mainly related to construction and maintenance. To optimize the availability of the offshore wind turbines it is important to have support vessels and boarding systems that can handle as rough sea conditions as possible, and the relative motions between these vessels and the wind turbines become increasingly important to predict, as the offshore wind business expands.For this purpose, a need has been expressed for a simple tool for quick estimation of such motions. In this thesis, a MATLAB program has been developed for this purpose. It takes various input from the user, such as information on the sea state and the physical situation to be considered, as well as limiting criteria. The program provides the user with information on the local vessel motions and the relative motions between a point on the vessel and a fixed point on the wind turbine, and then compares it to the given criteria. It also gives out various plots to illustrate the motions and the relevant transfer functions.The final version of Relative Motion Calculator, RMC 2.3, features the following options:•Two types of wave spectra•Arbitrary placement of the moving coordinate system•Arbitrary placement of the considered points•Long- or short-crested wave theoryRMC 2.3 has undergone thorough testing to prove its validity, and all test results are reasonable and according to expectation. Although the program is a bit difficult to use, it can be used as intended, for calculating relative motions between a moving point on a vessel and a fixed point. Furthermore, the program might provide a good platform for further development.

In this thesis the response variables (RAOs) of a semi submersible unit are inspected. Both operational and survival condition as well as a shallow draft are inspected. The survival condition is inspected with respect to an element analysis. And both operational- and shallow draft condition are case studies, where the operational condition is inspected for two different damage cases. The unit in question is a four column semi submersible, based on the GG5000 design. This is a relatively new design, and the first vessel to get this design is in its final engineering stage. Construction start is planned to be in August this year (2012). This unit will get the name COSLProspector and will be built in CIMC Yantai Raffles shipyard in China.The unit is symmetrical about the centre line and close to symmetrical about the vertical transverse plane, only pontoon tips are different. Because of this, no significant simplifications have been necessary in order to simplify the calculation due to computational time. Another reason for not doing any simplifications to the geometry is due to the fact that the results are desired to be the most realistic. However, to reduce computational time, only half the unit has been modelled due to symmetry about centre line.To find the appropriate element size for the mesh, an element analysis has been carried out. The results from this analysis resulted in a chosen element size of 2.5m. This element size both gives accurate results, and requires a relatively short computational time. The units’ resonance periods has been investigated, and verified by help of hand calculations and comparison with RAOs done by Global Maritime (2011). However not all the values were identical to each other, but many factors can influence on that result. The GM value was not changed in this thesis, but was in Global Maritime (2011), in addition the additional damping was in this thesis taken as 3% of critical damping, while in Global Maritime (2011), Morrison elements were taken into account. These factors, and perhaps a few shortenings are assumed to be the reason for the small difference in the responses, they are however small differences for most of the periods.Two damage cases have been modelled by flooding two different water ballast tanks. These damages will give an angle of list for the unit. Damage case 1 gives an angle of list of 13.18o with a rotation of heel axis of 7o forward. Damage case 2 gives a list angle of 11.68o with a rotation of the heel axis of 39o forward. An earlier study like this one is done by Henriksen (2011), found in Grenland Groups internal archive. AQWA does not give out the tilt angles in damage cases as this is not the main purpose of this program. Therefore, the list angles for the different cases have been obtained from the report done by Henriksen (2011). However, AQWA will be used to obtain the RAOs for both cases, as well as confirm floating equilibrium in such conditions.It is assumed that the tanks are completely emptied for air, and that seawater is filling the entire volume. A table showing the different tanks flooded and its weight with seawater is shown in table 1.UnitVolume [m3]Sea water weight [MT]BWT ST-2692.51709.82BWT ST-8616.83632.25Table 1: Weight of water for flooded tanksWhen it comes to the RAOs in the damage cases, they are very hard to read. This is mainly due to the fact that the motions in these cases are highly dependent on each other due to coupled motions. Due to an angle of list, the unit is no longer symmetric. As a consequence of this a RAO for a specific degree of freedom can no longer be read like it is only this degree of freedom which is affecting the responses, but one or more of the other degrees of freedom are strongly influencing. This makes some of the peaks appear where not normally expected.It is also noticeable that the highest motions are encountered for damage case 1, which is natural because this case has the highest list angle. The resonance periods are lower in the damage cases compared to the normal operational condition, however not to a degree which is dangerously low. The lowest resonance period is still in heave.From the RAOs in the shallow draft case, it can be noticed that the highest responses in heave are encountered for the shallow condition (14.5m) compared to the survival condition (15.5m) and operational condition (17.5m), however only up to about 18s, where after that it has the smallest response, and the operational condition has the highest.In roll and pitch the trends are fairly similar. The graphs are a little uneven until the first cancellation period, and then the shallow draft gives a higher response until reaching the resonance period. In the resonance region, the operational condition has the highest response for both roll and pitch, same as for the heave.As a conclusion, the optimal approach in a situation where the unit is heavily tilted is to try to ballast the unit to an even keel. But of course risks of doing this are a possibility, such as slamming problems and the fact that the resonance periods will be shorter.

Increasing demand for clean and effective energy production turns interests of the world on floating offshore wind technology. To establish floating wind farms, a wind turbine have to be mound on a floating structure. The floating structure has to be carefully design according to sea environmental condition and kept in precise position. Different types of floating structure and stationkeeping systems have been proposed for floating wind turbines. This project deals with design a spar floater with tension leg mooring system, where the vertical fairlead position located between center of buoyancy and center of gravity. In this project a details study about floater design and tension leg concept was presented. Further, a model was established in computer program RIFLEX and static and dynamic analysis was carried out for two different environmental conditions, one for an operation condition to understand the model behaviour in normal sea state. The second one was for extreme condition to estimate the extreme tether forces and find out slacking possibility.

The lattice Boltzmann method has become a valuable tool in computational fluid dynamics, one of the reasons is due to the simplicity of its coding. In order to maximize the performance potential of today’s computers, code has to be optimized for parallel execution. In order to achieve parallel execution of the lattice Boltzmann method, the data dependency has to be solved. And to get good performance, the memory has to be organized for unit stride access. Here we investigate the most known algorithms for lattice Boltzmann, and implement a code which runs on a parallel graphics processor, using a library for parallelization called C++ AMP. Furthermore, we show how the code compares to known solutions of fluid flows to verify the numerical results. The optimized parallel code achieves a speed up of 650 times the un-optimized code, on a current generation high-end graphics card.

As the worlds demand for energy is increasing mostly due to the increase in population, and coal, oil and gas deposits are limited, it is desirable to gather energy from renewable energy sources. Wind energy is a form of renewable energy. Wind turbines have been common on land and near shores for some time, but now one wants to take advantage of the wind resources further away from the coast. As the length from the coast increases, so does the water depth making it necessary to use other foundations than the well-known monopile. In this thesis the “Federal Maritime and Hydrographic Agency” in Germany, also known as Bundesamt für Schifffarth und Hydrographie requires in the standard “Design of Offshore Wind Turbines” that an offshore wind turbine needs to be risk evaluated against a ship tanker of more than 500 MJ in collision energy in order to be classified as collision energy. This is to prevent environmental pollution in form of oil spill. Whether the offshore wind turbine models used in this thesis are collision friendly or not relies on different factors. When the given soil properties are used the analyses show in all cases except one that the offshore wind turbine models can be called collision friendly. The case were it could not be called collision friendly was a case with the small jacket at a water depth of 27 m getting hit by a loaded ship at a column of the jacket, but installing a horizontal brace on this jacket would make it also collision friendly. The effects of soil, water depth and a horizontal x-brace are looked further into in this thesis. If the soil had been stronger it is not certain that the outcome would be the same. When the jackets were fixed to the sea bed, several of the models could collapse in the dangerous direction over the ship. A horizontal brace was seen to have a positive effect when installed on the different jacket models. It seems also that it is more favorable to use jackets at deeper water.

Det hydro-pneumatiske strekksystemet til stigerørene virker som en passiv hivkompensator med en olje/gass-akkumulator for a utligne de relative bevegelsene mellom semien og stigerørene. Semiresponsen fører direkte til at stempelet beveger seg i sylinderen, noe som gir kompressjon eller ekspansjon av gassen basert på ideell gasslov. Dette gir en ekstra ikkelineær stivhet, som også er avhengig av gassvolumet i systemet. Effekten av semiresponsen for et strekksystem med variabelt gassvolum har blitt evaluert.Strømmen av hydraulisk væske mellom sylinderen og akkumulatoren i kombinasjon med strømmen av gass mellom akkumulatoren og gassreservoaret, gir viskøs dempning i strekksystemet. Disse friskjonskreftene er bestemt av lengde og diameter i rørsystemet, ventiler og andre komponenter i det hydraulisk-pneumatiske systemet. I denne masteroppgaven er det brukt forenklede beregningsmodeller som gir gode, men ikke eksakte resultat. En numerisk metode har blitt skrevet for å beregne trykktap basert på Bernoullis ligning og konservering av masse og moment i systemet.En numerisk metode har også blitt skrevet for å løse svingeligningen i tidsdomenet med ikkelineær stivhet og dempning fra det hydro-pneumatiske strekksystemet. En sensitivitetsanalyse basert på forskjellige perioder og bølgehøyder ble gjort for å separere de forskjellige bidragene fra stigerør-strekksystemet, og lettere finne trender og sammenhenger som definerer dette. I forprosjektet ble transferfunksjoner regnet ut for noen gitte senarioer. Disse gir viktig og mye brukt informasjon i tidlig designfase for prosjekter. Nye transferfunksjoner er tatt ut ved å plotte maksimum respons i stabil fase, basert på tidsserier fra de forskjellige bølgehøydene og periodene.Det finnes friskjonskrefter i ulike kontaktpunkter mellom bevegelige deler på semien, noe som er spesielt viktig for små sjøtilstander. Semiresponsen blir påvirket av friksjonskreftene mellom stigerørene og kjølguiden, og i pakninger i strekksystem-sylinderene. Når friskjonskreftene blir større enn eksitasjonskreftene fra bølgene, vil semien bli «forankret» til stigerørene. Dette fører til at bølgekreftene blir overført direkte til stigerørene, og denne belastningen kan redusere forventet levetid. Analyser har blitt gjort for hvilke regulærbølger dette vil skje. Det er også sett på hvordan den økte stivheten ved en «forankret» tilstand reduserer egenperioden til systemet.