There have been a number of recent papers in the literature that investigate the relationship between ship speed and required power, or between ship speed and fuel consumption. Using regression analyses for selected case studies these papers show that in many cases the traditional “cube law” is not valid, and exponents lower than 3 (and in some cases lower than 2 or even below 1) are more appropriate. Perhaps more important, they use these results to derive implications on the validity (or lack thereof) of policies to reduce greenhouse gas (GHG) emissions from ships through slow steaming. This paper reviews some of these papers and shows that their results are partially based on pitfalls in the analysis which are identified. Policy implications particularly on the quest to reduce GHG emissions from ships are also discussed.
Traditionally, most ship hulls are optimized for ideal conditions, where the ships are sailing in calm water with full speed in full load. In the last decade, some ships have been designed for a range of draughts and speeds in calm water. However, there is still a large gap between the ideal conditions the ships are designed for and conditions (waves, wind, currents, hull roughness ets.) the ships will operate in. The target for the thesis is to develop accurate numerical models that can help ship designers narrow a part of this gap.
The main body of this thesis is three papers. The first papers compares the speed/power performance of full-scale CFD simulations, towing tank predictions, and high quality speed trial measurements from six sister vessels. Much research have been conducted comparing model- scale CFD with towing tank results. However, very few studies have compared full-scale CFD with speed trial measurements. The study includes both a ro-ro vessel and a general cargo vessel. The present study finds that including the hull and propeller roughness directly into the CFD simulations by modifying the wall-functions provides more accurate results than the traditional approach of estimating the effect of roughness using an empirical formula.
Today, most ships are designed for sailing in calm water. However, very few ships sail entirely in calm water. Before numerical simulations can be used to predict added resistance in waves and seakeeping responses, a systematic verification and validation is required to ensure the accuracy. The second paper presents such a systematic verification and validation for the KCS container ship in oblique waves. Five wave headings and six wavelengths are studied. The estimated spatial and temporal discretization errors are found by an extensive verification study to be less than 5 %. Results from the verified CFD model are compared with existing potential flow and CFD results from the literature, as well as up to three experimental data sets. The comparison shows that the present CFD results in general show significantly better agreement with the experiments than previously published CFD results.
This CFD set-up is used in the third paper to study how sailing in oblique regular waves influences the nominal wake field of the KCS ship. Five different headings are studied and the waves have a steepness of 1/60 and a wave length equal to the ship length. The present study finds that the studied incident waves make the nominal wake field highly transient. Especially the transient bilge vortex and shadow from the skeg have a significant influence on the nominal wake field. The results show that the nominal wake fraction fluctuates up to 39 % of the mean nominal wake fraction for the studied waves. The mean nominal wake fraction is higher than in calm water for all headings besides head sea waves. It is found that the stern quartering sea waves has the maximum mean nominal wake fraction, with a 16 % higher mean nominal wake fraction than in calm water. Finally the study finds that the modified advance angle on the r/R = 0.7 circle in the propeller plane varies 3.5 degrees more in stern quartering than in calm water. This increases the risk of cavitation leading to potential vibrations and loss of propulsive efficiency.
The three papers show that CFD simulations can deliver highly accuracy results, when the CFD simulations are set-up very carefully and systematic verification and validation are conducted. The results from the three papers shows that numerical simulations have a massive potential as useful tools when designing ships for the conditions, the ship will operate in.
This paper addresses the connection between added wave resistance and required propulsion power of ships, having focus on the early stage of new ship designs, notably tankers and bulk carriers. The paper investigates how mean added wave resistance affects the required torque of a fixed pitch propeller and thus also the operational conditions of a directly coupled main engine. The interest of the study has its background in the assessment of minimum propulsion power, and the study considers the prescriptive guidelines of the IMO as basis. Specifically, the study focuses on an assessment of the minimum forward speed attainable under consideration of the propeller light running margin and static load limits of engines in the early phase of new ship designs, where details of hull geometry are not available. The study considers three semi-empirical methods for predicting mean added wave resistance. All methods are known to be applied in the industry, emphasising that only methods relying solely on main particulars, together with information about sea state and advance speed, are of interest. The paper contains a case study used to illustrate the importance of the added wave resistance prediction with respect to the loading of the main engine. It is shown that, despite small absolute differences, the consequence in relation to the loading of the propeller and hereby the directly coupled main engine can be relatively large. Furthermore, the study illustrates that the propeller light running margin of a fixed pitch propeller directly coupled to the main engine has crucial influence on the attainable speed during adverse weather conditions.
Marine flexible pipe/cables, such as umbilicals, flexible pipes and cryogenic hoses, are widely adopted in ocean engineering. The reinforcing armor layer in these pipe/cables is the main component for bearing loads, which is a typical multi-layer helically wound slender structure with different winding angles for different devices. There has been no general theoretical methodology to describe the tensile performance of these flexible pipe/cables. This paper first introduces a theory to solve the tensile mechanical behavior of a helically wound structure. Based on the curved beam theory, a solution of the tensile stress of a helically wound slender is derived. Then, the deformation mechanism of the marine flexible pipe/cables structure with different winding angles is studied. Through comparing theoretical and numerical results, the deformation characteristic of the helically wound slender structure is further explained. It is found that a sectional torsional deformation generates when the structure with a larger winding angle is under tension condition, while the sectional deformation of the structure with a smaller winding angle is mainly tension. Finally, a couple types of marine flexible pipe/cables under the tension condition are provided to analyze the mechanical performance and compare the difference between different theoretical models. The research conclusion from this paper provides a useful reference for the structural analysis and design of marine flexible pipe/cables.
The power system of an all-electric ship (AES) establishes an independent microgrid using the distributed energy resources, energy storage devices, and power electronic converters. As a hybrid energy system (HES), the power system of an AES works as a unified system where each part can affect the reliability of the other parts. The systemic reliability centered maintenance (SRCM), which efficiently enhances the reliability and safety of the AES by identifying optimal maintenance tasks of the AES, is considered in this article to apply to the entire system. In order to calculate the reliability and optimal maintenance schedule, the Markov process and Enhanced JAYA (EJAYA) are utilized. A layer of protection analysis (LOPA), which is a risk management technique, is adopted to assess the safety of the system. A hybrid molten carbonate fuel cell, photovoltaic (PV), and lithium-ion battery are considered as energy sources of the AES. Based on two common standards, DNVGL-ST-0033 and DNVGL-ST-0373, the suggested maintenance planning method can be used in industrial applications. Eventually, in order to validate the proposed method, a model-in-the-loop real-time simulation using dSPACE is carried out. The obtained results show the applicability and efficiency of the proposed method for improving reliability and safety.
This paper examines the statistical properties and the quality of the speed through water (STW) measurement based on data extracted from almost 200 container ships of Maersk Line’s fleet for 3 years of operation. The analysis uses high-frequency sensor data along with additional data sources derived from external providers. The interest of the study has its background in the accuracy of STW measurement as the most important parameter in the assessment of a ship’s performance analysis. The paper contains a thorough analysis of the measurements assumed to be related with the STW error, along with a descriptive decomposition of the main variables by sea region including sea state, vessel class, vessel IMO number and manufacturer of the speed-log installed in each ship. The paper suggests a semi-empirical method using a threshold to identify potential error in a ship’s STW measurement. The study revealed that the sea region is the most influential factor for the STW accuracy and that 26% of the ships of the dataset’s fleet warrant further investigation.
Maintaining the condition of a vessel and its equipment guarantees the scheduled completion of voyages and the safety of the crew.
This paper presents condition monitoring techniques for early detection of faults related to piston rings in remote cylinders of two-stroke marine diesel engines. Operational sensor data from the main engine of a container ship are provided by a shipping company.
A graphical approach complimented by correlation heatmaps and feature importance from gradient boosting trees are used for feature selection. Support Vector Machine, Random Forest and Extreme Gradient Boosting Trees are tested for residual generation from the nominal behavior.
The residual time series gives a good indication of the degradation of the system and can be used for alarm raising under strict rules. It is proven that the proposed method could alert the engine crew of a change in the condition of the piston rings much earlier than existing methods.
In a number of experiments and field tests of point absorbers, snap loads have been identified to cause damage on the mooring cables. Snap loads are basically propagating shock waves, which require special care in the numerical modeling of the mooring cable dynamics. In this paper we present a mooring cable model based on a conservative formulation, discretized using the Runge-Kutta discontinuous Galerkin method. The numerical model is thus well suited for correctly capturing snap loads. The numerical model is verified and validated using analytic and experimental data and the computed results are satisfactory.
A generic point-absorbing wave energy converter is modeled in CFD as a vertical cylinder, moored with a single catenary chain that is fully coupled through a dynamic mooring code. The method of choice is very complete and takes much of the non-linearities in the highly coupled system of the moored body into account. The paper presents numerical results compared with experimental data for surge, heave and pitch motion in both decay tests and regular waves. Further, the wave motion response of the cylinder is computed using both a viscous and a non-viscous formulation as a first attempt to quantify viscous effects. Results show a good match between numerical and experimental results in heave, while the surge and pitch motion are more difficult to reproduce. The mooring load cycle appearance compares well with the experiments in shape but gives higher peak values. Although made at low Keulegan-Carpenter numbers, the simulations show vortical structures due to the heave motion, and the resulting motions are clearly affected by the inclusion or exclusion of viscosity. More test-cases and detailed experimental results are needed for further quantification of the viscous impact on floating point absorbers.
Normal flow past flat plates at high Reynolds numbers appears in various engineering contexts. To accurately model such flows for slender plates in Computational Fluid Dynamics requires scale-resolving rather than scale-modelling methods. The present paper uses Detached-Eddy Simulation to investigate the influence of plate corner curvature on global flow quantities such as the time-averaged drag coefficient. The effect of corner curvature is mapped and collated with the literature. Solution verification is carried out to quantify the numerical uncertainty. The time-averaged drag coefficient increases significantly between semi-cylindrically rounded (〈𝐶〉=2.28) and sharp-cornered (〈𝐶〉=2.42) plates.