Transfer functions are often used together with a wave spectrum for analysis of wave–ship interactions, where one application addresses the prediction of wave-induced motions or other types of global responses. This paper presents a simple and practical method which can be used to tune the transfer function of such responses to facilitate improved prediction capability. The input to the method consists of a measured response, i.e. time series sequences from a given sensor, the 2D wave spectrum characterising the seaway in which the measurements are taken, and an initial estimate of the transfer function for the response in study. The paper presents results obtained using data from an in-service container ship. The 2D wave spectra are taken from the ERA5 database, while the transfer function is computed by a simple closed-form expression. The paper shows that the application of the tuned transfer function leads to predictions which are significantly improved compared to using the transfer function without tuning.
This PhD theis focuses on identifying the opportunities and challenges that on-board maintenance and practical operation of vessels poses in the development of autonomous ships. Inspired by the rapid development of autonomous vehicles considerable effort and interest is now invested in the development of autonomous ships. So far however, most of the research has focused on the legal aspect of unmanned vessels and on developing a system enabling a vessel to operate within the maritime collision regulation without human interaction. Specifically, the theisi looks into three research questions: (1) How is autonomous technology going to affect the workload required for operating and maintaining modern cargo vessels? (2) How is autonomous technology going to affect the operational patterns of the vessels? And (3) How is autonomous technology going to affect the reliability and utilization rate of the vessels?
The study is planned in cooperation between Svendborg International Maritime Academy (SIMAC) and University of Southern Denmark.
The power output from many wave energy converters (WECs) is limited by a finite stroke length in the power take-off (PTO) mechanism. As the PTO approaches its maximum stroke length, an end-stop system needs to be engaged to avoid damage to the machinery. Still the on-set of the end-stop is a nonlinear trigger force, a stiff point in the system. In this respect it is similar to how snap loads in the mooring cables affect the system after a period of cable slack. This paper presents a detailed study into the dynamics of end-stop events and snap loads for a WEC. The WEC is a bottom-mounted linear generator connected to a surface buoy via a steel wire. By comparing a linear spring model with three dynamic mooring line models we conclude that large differences are observed in the low-tension and slack regions of the cable during moderate wave loads, while minor differences are seen in the estimated peak tension. By further varying end-stop parameters we observe that the peak tension in the line changes mildly with the axial stiffness for moderate wave heights. The peak tension is surprisingly unaffected by the introduction of a critical damping level to the end-stop system, despite the significant increase in end-stop force which causes the translator to come to a sudden stop. We discuss how the connection between maximum line force and end-stop parameters is highly dependent on the buoy position in the wave at the instant of end-stop onset.
The levelized costs of energy (LCoE) of wave power is still not fully competitive with other sources of renewable energy. However, wave energy is partly in a different phase than other renewable energy types and could thus contribute to a better predictability and smoothed power output. This work focuses on co-location of wave and wind power by investigating the intermittency of wind and waves power based on measured historical data from several hundreds of locations worldwide. Employing wind power curves and wave power matrices, the sites are evaluated based on several different metrics. The results indicate that there are several spots where wave power has a much lower intermittency than wind power providing reliable energy supply. Best sites for co-location in terms of energy yield were found in North-Western Europe. However, both wind and wave production have the same seasonal variability in these sites. Only a handful of sites found in California showed the possibility of seasonal power smoothing using the combination of wind and wave.
The present paper deals with separation of long-crested regular waves into incident and reflected components. Such methods have been available for several decades for linear waves, but have recently been extended to cover nonlinear waves over horizontal foreshores. The overall goal of the present paper is to extend the separation method for nonlinear regular waves to also cover sloping foreshores. This requires the combination of the existing method with a nonlinear shoaling model. A nonlinear shoaling model was very recently found valid for the shoaling of the primary and bound components in regular waves when the slope angle is positive and mild. In the present paper this shoaling model is utilized and assumed valid also for the de-shoaling of the reflected waves, ie on a negative mild slope angle. However, if the reflected waves are nonlinear the de-shoaling process is much more complicated and will for example cause the release of free waves. Interactions among those free reflected wave components may cause nonlinear interactions not included in the mathematical model. For that reason, the applicability range is limited to mildly nonlinear reflected waves. Using numerical model data with various foreshore slopes, wave nonlinearities and reflection coefficients the reliability of the developed model is examined in detail.
Highly accurate and precise heave decay tests on a sphere with a diameter of 300 mm were completed in a meticulously designed test setup in the wave basin in the Ocean and Coastal Engineering Laboratory at Aalborg University, Denmark. The tests were dedicated to providing a rigorous benchmark dataset for numerical model validation. The sphere was ballasted to half submergence, thereby floating with the waterline at the equator when at rest in calm water. Heave decay tests were conducted, in which the sphere was held stationary and dropped from three drop heights: a small drop height, which can be considered a linear case, a moderately nonlinear case, and a highly nonlinear case with a drop height from a position where the whole sphere was initially above the water. The precision of the heave decay time series was calculated from random and systematic standard uncertainties. At a 95% confidence level, uncertainties were found to be very low — on average only about 0.3% of the respective drop heights. Physical parameters of the test setup and associated uncertainties were quantified. A test case was formulated that closely represents the physical tests, enabling the reader to do his/her own numerical tests. The paper includes a comparison of the physical test results to the results from several independent numerical models based on linear potential flow, fully nonlinear potential flow, and the Reynolds-averaged Navier–Stokes (RANS) equations. A high correlation between physical and numerical test results is shown. The physical test results are very suitable for numerical model validation and are public as a benchmark dataset.
This report provides a current assessment on the prospects for aerial drone applications onboard ships. Three use cases are each forecasted to their time to implementation and evaluated as an opportunity for the maritime and offshore industries. The report's findings are based on respondents' answers to surveys about the three use cases. The data for this report is based on desk research and an analysis of survey responses. The report is produced by the PERISCOPE network.
We present the results of a numerical model which has been developed for estimating the contribution to the methane slip from different sources in a four-stroke dual-fuel marine engine running on natural gas. The model is a thermodynamic three-zone zero-dimensional full engine cycle model and considers methane slip contributions from short-circuiting, crevices and wall quenching. The model is applied to analyze the methane slip from a four-stroke dual-fuel medium speed marine engine using natural gas as primary fuel. At low loads, wall quenching is found to be the dominant contribution to the methane slip. At full load, the wall quenching contribution is comparable to the level of the short-circuiting and crevice contributions which only vary relatively little with load. At 75% load, the contribution from short-circuiting is highest. In addition, we found that in-cylinder post-oxidation of unburned fuel remaining after the main combustion is negligible.
When a ship navigates at sea, the slamming impact can generate significant load pulses which move up along the hull plating. The effect of the moving pressure has so far not been explicitly considered in the Rules and Regulations for the Classification of Ships. Based on a modal superposition method and the Lagrange equation, this paper derives analytical solutions to study the elastic dynamic responses of fully clamped rectangular plates under moving pressure impact loads. The spatial variation of the moving slamming impact pressure is simplified to three types of impact loads, i.e. a rectangular pulse, a linearly decaying pulse and an exponentially decaying pulse. The dynamic responses of fully clamped rectangular plates under the moving slamming impact pressure are calculated in order to investigate the influence of the load pulse shapes and moving speed on the plate structural behaviour. It is found that the structural response of the plate increases with the increase of the moving speed. The response of the plate subjected to a moving pressure impact load is smaller than the case when the plate is subjected to a spatially uniform distributed impact load with the same load amplitude and load duration. In order to quantify the effect of the moving speed on the dynamic load, a Dynamic Moving Load Coefficient (DMLC) is introduced as the ratio between the dynamic load factor for the moving impact load and that under the spatially uniform distributed impact load. An expression for DMLC is proposed based on analyses of various scenarios using the developed analytical model. Finally an empirical formula which transforms the moving impact loads to an equivalent static load is proposed.
ECOPRODIGI (2017-2020) is an Interreg Baltic Sea Region flagship project, which links research organisations, enterprises, associations and business support organisations. Altogether, 21 partners jointly investigate the most critical eco-inefficiencies in maritime processes in the Baltic Sea Region as well as develop and pilot digital solutions for improving the eco-efficiency by focusing on three specific cases: 1) digital performance monitoring of vessels, 2) cargo stowage optimisation at ports and 3) process optimisation at shipyards. Furthermore, looking towards the future, the project partners, on one hand, create a digitalisation roadmap and training modules for future decision makers in the maritime industry but also reach out to policymakers to engage them in discussion regarding how they can support the digital change. This report provides an overview of the project and main findings achieved to date, describes the main eco-inefficiencies identified and presents the potential of digital technologies and new concepts for improving them. Also, as the current digital transformation relates to the way how changes are managed in organisations, this report presents the main challenges and requirements identified in the process of moving towards more digitalised business operations. Finally, the last section looks at the maritime sector from a broader perspective and provides some ideas about the most likely future developments. The main findings of the project so far indicate that major improvements in eco-efficiency can be carried out in the maritime industry. They can be summarised as follows: 1) In the first case, ‘digital performance monitoring’, the project partners estimate, for instance, that fuel consumption and emissions can potentially be reduced by 2-20% based on data and analysis from distinct ship segments, routes and their baseline situations. The reductions are possible to achieve by taking such actions as capitalising on the latest digital technologies, utilising and analysing real-time operational data and vessel performance, anticipating operating conditions and maintenance of the ship and its components, changing working methods and improving practices as well as placing a focus on the training of personnel. 2) In the second case, ‘cargo stowage optimisation’ the project partners identified a set of eco-efficiency bottlenecks in the cargo stowage processes at ports that can be subject to improvement. The use of advanced digital technologies can contribute to more efficient utilisation of vessels and terminal operations. The port stays can be reduced, and, thereby, vessels can sail more slowly and reduce fuel consumption and emissions. Moreover, when stability calculations improve due to further digitalisation of cargo unit data, the ship can be loaded more optimally and the amount of ballast water can potentially be decreased without compromising safety, which again reduces fuel consumption on the sea leg. It is estimated that fuel consumption and emissions can potentially be reduced by 2-10% per route and ship and that additional benefits can be gained on the landside due to future digital decision support tools applied for the end-to-end stowage process. In addition, improved cargo unit pick up time estimates can be provided to customers waiting for the cargo to be handled at port, whereby the service improves. 3) In the third case, ‘process optimisation at shipyards’, improved situational awareness and process management, including the use of new technologies, such as 3D and solutions for managing the complex supply chain, have potential for improving the shipyard processes aimed at increased eco-efficiency. For example, in block building phase 3D technology reduces lead-time and potentially saves hundreds of man-hours in rework due to the fact that more efficient processes and proactive actions are enabled.