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.

The main objective of this thesis is to investigate and present the most relevant techniques and topics within the field of robot modeling and control. The studies will then be used to develop a working control system for the 'Raptor' manipulator stationed on the ROV 'SubFighter 30K'. Due to insufficient information, a simplified model is made to resemble the actual manipulator. This model forms the foundation of all subsequent actions, including the model based control design. The dynamic model is developed by the well known method of Euler-Lagrange. Since this is an energy based method, both the kinetic and the potential energy of the system must be calculated. Systematic procedures are given to clarify the process of these calculations. In this thesis, a sliding-mode controller is derived and proposed as a suitable controller for the given manipulator. The control objective is to force the manipulator to track a time dependent, desired path in the joint space. However, since it is inconvenient for the operator to specify joint space trajectories, several inverse kinematics algorithms are suggested. Due to the kinematic structure of the manipulator, no closed-form solutions can be obtained. The focus is thus directed towards numerical Jacobian based methods. A full-scale implementation requires a working interface between the developed control system and the manipulator system. For that reason, the main concepts of digital communication are presented. Although no communication data is logged from the Raptor, this presentation will pose an advantage if the work is continued. When no control forces are applied to the dynamic model, the manipulator model is expected to behave like a multi joint, three dimensional pendulum. The simulations corresponds to this assumptions, thus the model is assumed to be correct and valid. Simulations of the complete system shows that the sliding-mode controller works as intended. Two chosen IK algorithms are then implemented and compared through simulations. The DLS method proves to be superior to the simple inverse Jacobian method. Finally, the control system is implemented in LabVIEW and thus prepared for full-scale testing.

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.

In view of the increasingly popular climate change debate worldwide, maritime transport is under pressure to create sustainable solutions for a cleaner future.One of these solutions is using liquefied natural gas (LNG) as ship fuel. LNG has a higher hydrogen-to-carbon ratio compared with oil-based fuels, which results in lower specific CO2 emissions (kg CO2/kg fuel). Other benefits of LNG are the total elimination of SOx emissions and particulate matter, and 85-90 % reductions in NOx emissions. DNV has estimated a net global warming benefit of 15 % with the use of LNG. However there is still a need to map the greenhouse gas emissions throughout the life cycle of LNG. This thesis is a Screening Life Cycle Assessment of LNG as fuel, with the main objective to carry out a life cycle analysis of the process “Bunkering of LNG”. Screening is a simplified LCA which aims at identifying the important parts of a life cycle, or so-called hotspots. Bunkering of LNG and the bunkering facility have been chosen as the hotspots in this analysis for their uniqueness. The processes related to bunkering have not been analysed at this level of detail in published literature. The LCA software GaBi Educational has been used to implement the bunkering model and analyse the life cycle inventory results. Processes related to energy use, manufacturing and direct emissions were included in the GaBi model. The CML 2001 method was used to assess the Global Warming Potential (GWP). This is the main characterisation factor of the environmental issue climate change, which was in focus in this analysis.The impact assessment showed that emissions related to manufacturing are the greatest contributors to the GWP, with a total GWP of 75 917 [kg CO2-Equiv]. Energy use contributes the least, with only 0,36 ‰ of the total impact, which is considered negligible. Direct emissions stand for 7 777 [kg CO2-Equiv] and is the only area of the bunkering life cycle where emissions can be considerably reduced. In fact, all direct emissions can be omitted by the use of BOG recovery strategies, such as vapour return. The conclusion drawn is that the emissions associated with bunkering of LNG is perhaps not the main issue. A more pressing issue at the moment is the low fuelling possibilities for LNG. If LNG is to become the fuel of the future, fuelling must be made more accessible and available.

The number of collisions between ships and icebergs are increasing in correlation with the increase of the number of ships sailing in Arctic areas. Due to the exploitation of gas fields in Arctic areas, the LNG shipping in these areas is expected to increase. The consequences of an iceberg impact can be critical for ship structure, the crew and the cargo, and it is important to be able to predict the result of such a scenario.Different relevant impact scenarios have been discussed, and it has been concluded that the most critical scenario for a spherical type LNG carrier is an impact at the bow shoulder where the distance between the outer hull and the cargo tanks is at its shortest. Based on this study, a bow shoulder impact has been chosen as the scenario investigated in the thesis.To investigate the influence of the surrounding water, an ALE-model representing this was therefore supposed to be modelled in LS-DYNA. After several unsuccessful tests this work was postponed to further work, but a detailed description of the ALE-modelling and the different options tested has been carried out.A detailed FE model of the bow shoulder section of an ice strengthened spherical type LNG carrier has been carried out. The mesh in this section has been modelled sufficiently fine to capture the governing deformation mechanisms and the total number of elements in the model are therefore large. This will lead to increased computation time, but this is acceptable based on the accuracy of the results obtained. In addition to the local bow shoulder model, a FE model of a global 125 000 dwt. LNG tanker has been established. The outer shell and coarse geometry has been provided by Ph.D. candidate Martin Storheim, and the detailed bow shoulder model has been included in this global model. The global model is meshed with large rectangular four-node elements, and transitions to the detailed model have been created. Properties have been added to the global model in terms of getting a model which represents the real ship to the best extent possible.The steel material model used is developed by Hagbart Alsos and includes a RTCL fracture criterion. The ice material model is developed by Zhenhui Liu and is modelled to represent a physical approach to the crushing of the ice. The aluminium material is based on studies performed by Moss Maritime and simple power law calculations.The added mass and weight of the LNG cargo has been included as increased density in the outer hull below the water line and in the tank shells respectively. The aim of the simulations has been to investigate similarities and differences in different collision approaches. Three different approaches have therefore been simulated for two different impact angles. For the local model, two simplified approaches have been simulated:•A prescribed displacement has been applied to the iceberg. This is the most common approach in collision simulations due to lower requirements to computer performance.•An initial velocity has been given to the iceberg. This approach requires more computer resources, but will probably yield less conservative results.Pinned boundary conditions have been applied to the bow shoulder model for all the local analyses. In addition to the local analyses, a full global analysis has been performed for the two different impact angles. In these simulations no boundary conditions have been applied to any of the colliding bodies, and initial velocity is given to the ship structure. The mass of the iceberg is approximately 2500 tons and for the global analyses no velocity of the iceberg has been assumed.The results obtained from these simulations have shown that both of the local simulations tends to over-estimate the indentations. The prescribed displacement analyses yields over-conservative results for long duration simulations, but at large impact angles the results obtained are reasonable for longer simulated time than for small impact angles. The initial velocity method yields more reasonable results, and this approach is therefore better to apply when the damage from a collision scenario is estimated by simplified methods.To investigate the effect of the iceberg size, two simulations including an iceberg of approximately 19 000 tons have been carried out. These simulations have shown that the damage in such collision is significantly larger than for the small iceberg. It has also been seen that there is less difference between the simplified approach and the full analysis for the large iceberg than for the small iceberg approaches. This is remarkable, since the boundary conditions have been assumed to give larger contribution to the result for the large iceberg.Unreasonable large accelerations have been found in collision simulations described in different papers. To investigate this in the simulations performed in this thesis, the accelerations and motions have been observed in different spots which is not directly impacted by the collision. No extreme values of either accelerations or motions have been found, and it is concluded that this is not a problem for the collision scenario simulated in this thesis.

The calculation of ice-induced loads on ships is still mainly based on empirical models. In order to gain a better physical understanding of the loading on ice-going vessels, Det Norske Veritas launched an <i>ice load monitoring<i> project involving full scale trials with the coastguard vessel KV Svalbard during the winters 2006, 2007 and 2011. The results from the full scale measurements conducted with KV Svalbard has been topic of several earlier master's thesis at NTNU,The master thesis consists of four parts. The first part is a literature review of the mechanical and physical properties of sea ice.The second part is a review of the rule sets developed by DNV and the IACS regarding vessels operating in ice infested waters. Both design principles and numerical values have been evaluated. The main difference between the designs principles used, is that IACS base their rules on a plastic method of approach, while DNV uses an elastic method. Despite the difference in the design principles, when comparing their numerical values turned out to be quite similar. The DNV rules are in general most conservative for the smaller vessels and the IACS rules the most conservative for large vessels. The third part consists of a finite element study of a part of the bow on KV Svalbard. A systematic load scheme is used, consisting of 102 load cases. For each of the stress factors there where made graphs that showed the stress at the sensor location when moving the patch load. The sensor mounted on the frame were able to measure load that was within the frame loading area and sensor mounted on the stringer could measure stress for all of the load cases in the horizontal directions. One of the explanations for this is that the stringer transfers stress from the load patch area that could be measured by the sensors.The last part consists of a comparison between measurements from the full scale trials and the results from the 102 load cases. This comparison is done through a weighted summation method where 5 different load cases are combined to represent the measured result, and a load factor is calculated for each load case for its contribution of the measured results. The stress component used in this comparison is the shear stress tau xy The load cases were tested against the 11 measurements from the full scale trials. There were in total 11 load cases that gave positive factors for all of the 11 measurements at the same time. A figure was made to show which load cases were likely to contribute in the solution of the load cases. Load cases inside the frame loading area have the largest load factors for the solution of measurements.This load decision scheme is very sensitive to the selection of load cases and boundary conditions. A change of the boundary conditions for the model was tried out for 7 load cases, and with changed boundary conditions, only 5 gave positive load factors.The results of this thesis shows that is possible to find many solution to the measured result by combining many load case, but is it not possible to decide <i>the<i> solution.

The exible pipelines has been widely used by the oil and gas industry the lastdecades and this trend is expected to continue as the operating depths are increasedfurther. For deep water applications the radial and lateral buckling modes can becritical. Many studies has been carried out using nite element models to studythese failure modes.In this thesis a nite element model is created with the aim to recreate resultsobtained in a recently published article by Vaz and Rizzo. The model is built upusing pipe, beam, contact and spring elements to represent the complex behaviourof the cross section. The loading is carried out by rst applying the dry mass, thenthe external pressure and nally end compression.When comparing buckling loads generated in this thesis by the ones in thearticle by Vaz and Rizzo the observation made is that the buckling loads from thisthesis are signicantly higher. When comparing only the inclination of the endshortening versus buckling load curves it was seen that the curves from article hada only slightly larger inclination than the results from the analyses in this thesis.This indicates that there is a small dierence in the stiness used. By modifyingthe stiness it should be possible to get the same inclination of the curve.Analyses were also carried out on how the slip distance aects the bucklingloads. By increasing the slip distance by 50% and 100% it was observed thatthe buckling loads were reduced drastically. In the article by Vaz and Rizzo noinformation is given on the slip distance. By tuning the slip distance and stinessof the springs it should therefore by possible to obtain the exact same results as inthe article. This clearly illustrates the importance of stating all assumptions andinput parameters when describing models used for analysis.

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.