Traditionally ships have been optimized for minimizing the fuel consumption in calm water. For slow, large ships like tankers and bulk carriers this has resulted in very blunt bows with high added resistance due to waves. The objective of this thesis has been to investigate if the optimal bow shape, when realistic wave conditions are taken into account, should be more slender than the current blunt bows. The added resistance is also highly dependent on the actual wave conditions the vessel experiences. Thus a question has been if the optimal bow changes with the operational area, or route, of the vessel.Five designs have been investigated representing a range of waterlines from blunt to sharp. They are based on the MOERI Tanker KVLCC2. KVLCC2A is the original design of the MOERI Tanker with no flare. KVLCC2B has the same water line curve as KVLCC2A, but with straight sides and small bilge radius in the bow. KVLCC2C has a more slender bow by moving volume from the shoulders to above the bulb. KVLCC2D is a blunter design than KVLCC2A and KVLCC2E has been elongated by 8 m compared to KVLCC2C to get a more slender bow.Four routes have been chosen to represent trades and ocean areas. The routes are; Arabian Gulf (AG) to the Gulf of Mexico (GM), AG to Japan, Brazil to China and Norway to the East Coast of US.Calm water resistance has been calculated and verified against experimental data. The wave resistance was calculated numerically using Shipflow. These calculations were not satisfying and should be taken a closer look at. Modification of the results had to be done. The results show that KVLCC2A, KVLCC2C and KVLCC2E have very similar calm water resistance. They have slightly lower values than KVLCC2D. KVLCC2B has the greatest calm water resistance. The added resistance was calculated by ShipX. The sharper bow designs have significantly lower resistance in the diffraction regime, as intended. KVLCC2E has slightly a slightly lower added resistance coefficient in the short wave regime than KVLCC2C.The speed-loss calculations were performed by combining wave statistics for the routes, calm water resistance, added resistance and engine and propulsion characteristics in ShipX. The result is an attainable speed at a given power input, 27 000 kW.The results show that KVLCC2C and KVLCC2E have the lowest speed-loss. The attainable speed is highest for KVLCC2C and it can thus be concluded that a sharper design is more optimal when realistic wave conditions are taken into account. The relative speed loss on different routes between KVLCC2C and KVLCC2A shows that the speed loss of KVLCC2C is 14.2% lower for the AG to GM, 13.8%, 16.2 % and14.9 % for respectively AG to Chiba, Mongstad to East coast of US and Brazil to China. Thus, a small difference can be seen, but not enough to change the best design in this case. A review of innovative bow shapes dealing with added resistance was performed and an evaluation based on working principles and applicability to a large, slow vessel was discussed. The designs reviewed were X-bow (Ulstein Design), a new bow from STX OSV and Beak-bow, Ax-bow and LEADGE-bow designed in Japan especially for larger ships. The two first bows are designed primarily with offshore service vessels in mind and focus more on the long waves. The LEADGE-bow, which is based more or less on the same principles as KVLCC2C, shows that a simple sharpening of the bow is an easy and effective measure. This seems like the most promising bow for large, slow ships of those evaluated.

In this thesis, the effects on regular waves when propagating from deep to shallow water have been investigated, assuming linear wave theory. The effects of shoaling, refraction, reflection and diffraction have been studied. The process and types of breaking waves has also been reviewed.Linear wave theory, second and higher order Stokes theory, Stream function theory, Solitary wave theory and Cnoidal theory have been reviewed. The relative validity of the different wave theories was also assessed.The effects on the wave spectrum as a sea state travels from deep to shallow water, described by the energy balance equation, has also been addressed. The effects addressed were wind, nonlinear wave-wave interactions, white-capping, bottom friction and surf-breaking. Two computer models for numerically solving the energy balance equation was mentioned, being SWAN and STWAVE where SWAN was chosen for use in the analyses.The NORA10 hindcast was believed not to properly take the change in water depth into account at the Dogger Bank Zone. At a point north of the location of interest, the water depth was of such a magnitude (81m) that the NORA10 hindcast was believed to yield credible data for the significant wave height and spectral peak period. A long term estimate of the 50 year significant wave height was performed from the NORA10 data at the point north of the Dogger Bank Zone, by the environmental contour method. This resulted in an estimated 50 year significant wave height of 11.74m and spectral peak period of 15.68s.The SWAN model was used on a test case from Svangstu (2011) to get familiar with the program, and investigate how the different physical effects influence the solution. With the knowledge acquired from the test case, the Dogger Bank case was analyzed in SWAN, to obtain the 50 year sea state parameters at the location of interest. The 50 year sea state north of the Dogger Bank Zone, the Dogger Bank bathymetry, as well as a constant wind of 23.3 m/s was used as input. The 50 year sea state at the location of interest was found to be characterized by a significant wave height of 7.34m and a spectral peak period of 15.56s. SWAN was found to result in a significant wave height of some 15-27% lower than what was found in NORA10.By performing a short term analysis on the 50-year sea state at the location of interest, the 50-year design wave height was found to be 12.5m, assuming the individual wave heights to be modeled by the Gluhovski distribution. The 90% confidence interval of the design wave period was found to be 9.6s <T< 16.3s. This was estimated from studying the ratio between the period of the three largest waves in a time series, and the spectral peak period of the sea state in 95 time series from Svangstu (2011) By evaluating only the limiting values of the period range, the longest and shortest design wave was found to be 80% and 97.4% of the breaking wave height respectively, assuming linear theory.The wind turbine structure geometry was simplified to be modeled by a cylinder with a diameter of 6m. By computing the kinematics from the Stream Function theory, the maximum base shear and overturning moment using Morison’s equation was found to be 3.67MN and 65.93MN respectively. The effects of the rate of change of added mass momentum was also assessed.From a simplified and conservative approach, the impact loads from a breaking wave was estimated. This resulted in a base shear and overturning moment of such a magnitude that en extensive analysis is recommended on this topic in the future.

The conventional way of transporting personnel from shore to offshore platforms is done by helicopter. For large distances of transportation this results in high costs due to the limitations in the maximum number of persons each helicopter can transport per trip and due to the high prices on helicopter fuel. In this thesis is an alternative solution to this conventional transportation proposed by the utilization of a logistical HUB. The concept is based ferries doing the transportation of personnel from shore to the HUB and helicopters doing the remaining, relatively short, transfer from the HUB to the respective platforms. The HUB evaluated is based on the characteristic Sevan 650 design, having a cylindrical shape with diameter D=78m in the waterline.The models that have been analyzed in this thesis were modeled in GeniE, the hydrodynamic analyses were done in Wadam and the post processing was performed using Postresp.Models of a platform with a single tunnel cut out with varying tunnel length L have been evaluated, and the motions of these models as well as the surface elevation inside the tunnel were studied in detail. It was recorded a two peaked response in heave for the models with tunnel lengths ranging from 30m<L<40m. This unexpected behavior was found to be due to diverging values for the added mass in heave for increasing tunnel lengths for models with intact tunnel bottoms. This was adjusted for by removing the bottom of the tunnel to add damping to the system, resulting in the usual one peaked response in heave being retrieved. The reason the two peaked response in heave occurred was concluded to be due to Wadam neglecting viscous effects including the viscous damping. This leads to the system having little or no damping and the added mass to diverge towards negative infinity giving unphysical motion representations.Since critical situations for the platform-ferry interaction will occur during loading and unloading of personnel from the ferry to the platform and during entry of a ferry into a tunnel, the wave pattern inside and on the immediate outside of this tunnel have been studied and evaluated. A total of 4 different designs for the layout of the tunnels have been proposed and evaluated to find the design that results in the least surface elevation inside the tunnel and at the tunnel entrance. A window of acceptance for the incoming wave headings were established with the intention of minimizing the surface elevation. The designs were also evaluated regarding their ability to resist large motion for a variety of incoming wave periods. It was concluded that a three tunnel solution with the tunnels being shifted 120 degrees relative each other would result in the smallest platform motions for wave periods smaller than 18s. A design consisting of 4 tunnels, where three of the tunnels are shifted 30 degrees relative each other and the last tunnel being located opposite of these three would result in the smallest surface elevations. An operability study was done for all 4 designs proposed based on the elevations inside and at the tunnel entrance. It was found that the 4 tunnel design described above would result in the largest operability for the platform. It was also found that this 4 tunnel solution would be unstable in roll due to an unsatisfactory low transverse metacentric height. This low metacentric height could be adjusted for by installing a vertical wall in the waterline in the transverse direction of the tunnels orientation.

This thesis considers a foil propulsion system on a supply vessel. In analysing the potential of a foil propulsion system, it is imperative to establish a rigid mathematical model. In that respect, modelling of the dynamic system is emphasised, and a comprehensive study is presented on the matter. The equations of motion for an oscillating foil and a vessel are derived, separately. The two systems are then combined, to form the coupled vessel-foil structure. For the vessel, a time-domain model based on Cummins' equation is proposed. Cummins' equation has proven efficient in assessing a unified seakeeping and manoeuvring problem (Fossen [2011]). In line, the vessel-foil system will be exposed to both vessel oscillatory motion due to waves and forward speed effects, i.e seakeeping and manoeuvring. Moreover, the efficiency of the foil is directly dependent on the two.Additionally, aspects of foil control is looked into. In theory, active control could maximise thrust while preventing stall, which would be ideal. The validity of simulations with active control, however, is highly dependent on the accuracy of the emph{basic} vessel-foil model. Consequently, effort has been focused on presenting a rigid mathematical foundation.

A stepped cylinder could be a desired design for an offshore buoy or SPAR platform. The geometry of a stepped cylinder consists of a small diameter cylinder (d) placed on top of a large diameter cylinder (D). This master thesis has investigated numerically the flow around a stepped cylinder with different diameter ratios (d/D) for a Reynolds number, ReD = 150. The commercial software Fluent v13.0 by Ansys was used for the numerical investigation.The aim of the study has been exploring the nearby wake flow as well as the region where the two cylinders are joined. The hydrodynamic forces and vortex shedding frequency have been analysed and compared for the four different diameter ratios: d/D = 0.3, 0.5, 0.8 and 0.9The major part of the published papers on the topic is based on experimental studies. Only two papers are based on numerical studies, having considered d/D = 0.5 exclusively. Earlier studies have focused mainly on vortex shedding in the wake flow, omitting the forces acting on the stepped cylinder.The stepped cylinder has been modelled using the software GAMBIT. A convergence study investigating the domain size and element density was conducted to ensure a grid independent solution. Special attention was directed at the step region to fully resolve the complex flow in this region. The numerical model was verified to be in good agreement with previous experimental- and numerical studies.For the stepped cylinders significant spanwise velocity was detected in the step region. For d/D = 0.3 and 0.5, upflow was detected over the leading edge of the step whereas downwash characterised the trailing edge of the step. Similarities to the flow around a finite length cylinder could be drawn for d/D = 0.3 and 0.5, whereas d/D = 0.8 and 0.9 resembled that of a straight cylinder.The step was found to affect the wake flow ≈ 10D into D independent of diameter ratio. The wake flow behind the small diameter cylinder was less affected by the step than the large for d/D < 0.5. In the step region two distinct streamwise vortices were detected. A pair of edge vortices as well as a junction vortex were readily detected for d/D = 0.3 and 0.5. The junction vortex was not detected for d/D = 0.8 and 0.9 due to the small step change in diameter.The drag force on D was found to increase as d/D increased. Similarly, the amplitude of the lift force was also found to increase as d/D increased. The mean drag-coefficient varied along the span with peaks in the local drag-coefficient observed in close vicinity of the step.Regular spanwise vortex shedding was detected away from the step at a frequency similar to that of a straight cylinder. In the step region, located mainly on D, a cell of lower vortex shedding frequency was detected for d/D = 0.3 and 0.5. As d/D increased this cell seemed to disappear. Suppression of regular vortex shedding close to the step for d/D = 0.3 was observed for ReD = 150, 300 and 600. For ReD > 150 the large spanwise vortex structures were still discernible, but the presence of small-scale streamwise vortices complicated the flow.

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.