ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V07BT00A001. doi:10.1115/OMAE2018-NS7B.

This online compilation of papers from the ASME 2018 37th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2018) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Metocean

2018;():V07BT06A001. doi:10.1115/OMAE2018-77092.

Offshore wave conditions can be measured using wave buoys, which are generally designed for wind waves. Longer waves (swell or bound second-order waves) are very relevant for certain maritime structures. The accuracy of the instrumentation in a typical wave buoy in long and short waves was therefore studied, and it was investigated if the buoy can be applied in longer waves.

A Waverider buoy was placed on a hexapod, which applied regular and irregular prescribed motions in multiple degrees of freedom. The hydrodynamic response of the buoy in waves and the effect of its mooring system were not evaluated; the buoy was assumed to follow the orbital motions of a wave and to rotate with its slope.

The tests showed that the buoy sensors measure accelerations and rotations with periods between 1.5 and 35 s very well. Vertical displacements derived from the accelerations by the buoy are accurate for the period range of 2 to 20 s. In longer waves, the motions are significantly underestimated, even though the accelerations are accurately measured. This will not lead to large errors in normal operation, as the energy of such long waves is generally low. This explains why the buoy also performs well when it is subjected to irregular motions (less than 2% error in the significant wave height of a half-hour measurement in realistic irregular sea states with peak periods between 5 and 20 s can be expected).

It can be concluded that the buoy accurately measures accelerations. The accuracy of the derived displacements decreases when very long swell wave energy (> 20 s) is present. Review of the internal integration procedure may be considered when there is a specific interest in measuring longer waves.

Topics: Waves , Buoys
Commentary by Dr. Valentin Fuster
2018;():V07BT06A002. doi:10.1115/OMAE2018-77170.

Here we present a wealth of comparison data from acoustic backscatter current meters against current meters that use an active acoustic path (the RPS CM04). The data cover a wide range of environmental conditions. Very poor agreement between current meters frequently occurs, presenting as both bias and noise. The causes of the differences remain unproven, although for acoustic profilers deployed in deep water, failure of the assumption of the same flow in all beams is a possible source of significant error. As a result of decade’s worth of comparisons plagued with unexplained differences, we have reduced confidence in the accuracy of current data collected by acoustic backscatter sensors. We therefore avoid (where practical) using these instruments when the application of the current data is for engineering design, where uncertainty in accuracy can have significant cost or risk consequences.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A003. doi:10.1115/OMAE2018-77219.

Simultaneous data of metocean parameters, such as wind, waves and current, of sufficient quality and duration are necessary to establish reliable, joint models of metocean loads and load effects on marine structures. In lack of such joint models, the Norwegian design standard, NORSOK N-003, recommends combinations of metocean parameters for load estimations assumed to be conservative. However, the degree of conservatism is rather uncertain.

The possible conservatism in NORSOK N-003 for combinations of wave and current conditions in the northern North Sea has been assessed. To perform such an assessment, precise knowledge about the wave and current conditions is required, as well as simultaneous wave and current data of high quality and long duration.

Available measured wave and current data during nearly five years, at selected locations in the northern North Sea, are described. A thorough assessment of the current conditions at these locations is given, with the following important findings; (1) the quality of measured current data is poorer than anticipated, (2) the dominating current conditions at some locations is wind-generated inertial oscillations, (3) the seasonality of current conditions at these locations is very distinct due to the inertial oscillations and (4) significant inter-annual variations in current conditions is found.

For waves in the northern North Sea, both measured and hindcast data are found to be of appropriate quality and duration for joint considerations, but neither current measurements nor hindcast have the required quality or duration. To generate adequate current data, a simple model for wind-generated inertial oscillations is applied and validated at one location in the northern North Sea. With that, simultaneous wave and current data of sufficient quality and duration for joint modelling are available and a joint conditional model for waves and currents is proposed.

The anticipated conservatism in NORSOK N-003 for load estimations is assessed by a case study. A simplified model for a generic static load on a jacket, caused by waves and currents, is assumed. For the northern North Sea, metocean loads are estimated first according to the NORSOK N-003 recommendation, and then directly from a load times series. Comparison of the two different approaches gives a clear indication that the NORSOK recommendation is not necessarily conservative in the northern North Sea. Due to several simplifications in the steps leading up to the load estimations, this result is intended to be illustrative.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A004. doi:10.1115/OMAE2018-77400.

Liaodong Bay is located at the northern part of the Bohai Sea. There are four blocks where oil and gas are found. In order to exploit these resources in these connected four blocks, extreme value statistics is used to estimate the storm-surge design parameters. Long-term series of storm surge are hindcast by numerical models. Using data from series of extreme storm surge caused by tropical cyclones in the Liaodong Bay, statistical analysis of extreme storm surge is carried out with Poisson-generalized extreme value distribution by considering the occurrence frequencies of tropical cyclones. The 100-year return storm surge is 195cm and the 50-year storm surge is 164cm. Both of them occur in Block 1. Due to the influence of topography of the total four blocks, the intensity trends of storm surges in Blocks 1, 2 and 3 are similar, whereas the intensity of storm surge in Block 4 is relatively small. These results are reference for the structural design in these blocks.

Topics: Design , Storms , Surges
Commentary by Dr. Valentin Fuster
2018;():V07BT06A005. doi:10.1115/OMAE2018-77581.

The study of very long-term ocean climate is of great interest in a number of different applications. In a climate change perspective, estimations of return values of wind and wave parameters to a future climate are of great importance for risk management and adaptation purposes. However, there are various ways of estimating the required return values, which introduce additional uncertainties in extreme weather and climate variables pertaining to both current and future climates. The different approaches that are considered in the present work include the annual maxima approach, the block maxima approach, and the MENU method which is based on the calculation of return periods of various level values from nonstationary time series data. Furthermore, the effect of different modelling choices within each of the approaches will be explored. Thus, a range of different return value estimates for the different data sets is obtained for a field of datapoints. Long-term datasets for an area in the North Atlantic Ocean are used in the present study, derived for project ExWaCli, comprising of 30 years in the present (historic period) and two sets of 30 years in the future (future projections). The comparison between the results of the various approaches reveals a variability of the return period estimates, and an assessment of this is given. Moreover, it seems that a slight shift towards higher extremes in a future wave climate might be possible based on the particular datasets that have been analysed.

Topics: Waves , Wind
Commentary by Dr. Valentin Fuster
2018;():V07BT06A006. doi:10.1115/OMAE2018-77674.

In the present paper, we propose a novel decision analytical framework for systems modeling in the context of risk informed integrity management of offshore facilities. Our focus concerns the development of system models representing environmental loads associated with storm events. Appreciating that system models in general serve to facilitate the optimal ranking of decision alternatives, we formulate the problem of systems modeling as an optimization problem to be solved jointly with the ranking of decision alternatives. Taking offset in recent developments in structure learning and Bayesian regression techniques, a generic approach for the modeling of environmental loads is established, which accommodates for a joint utilization of phenomenological understanding and knowledge contained in databases of observations. In this manner, we provide a framework and corresponding techniques supporting the combination of bottom-up and top-down modeling. Moreover, since phenomenological understanding as well as analysis of databases may lead to the identification of several competing system models, we include these in the formulation of the optimization problem. The proposed framework and utilized techniques are illustrated on a principal example. The example considers systems modeling and decision optimization in the context of possible evacuation of an offshore facility in the face of an emerging storm event.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A007. doi:10.1115/OMAE2018-77769.

The duration of current measurements is often short, ranging from a few weeks up to a year. Application of extreme value statistics to derive design levels requires relatively long time series. To mitigate the lack of long-term measurements, the Norwegian standard NS9415 for fish farm design requires the design level of 50-year return period to be derived by multiplication of the current maximum in month-long current measurements by a prescribed conversion factor of 1.85. Here we use twelve data sets of yearlong coastal current measurements to explore the validity of this factor. For each yearlong time series, a design level of 50-year return period is calculated by extreme value statistics and used to calculate estimates of the conversion factor. The mean value of the resulting conversion factor is close to that of NS9415, 1.85 and 1.80 at 5 and 15 m depth, respectively. However, the spread in values is great, both geographically and between months. A conversion factor ranging from 1 to 4 reflects different relative dominance of the driving forces at different coastal regions and different seasons. The absence of a significant seasonal cycle in the conversion factors calculated here, illustrates the difficulty in adjusting for season. The results illustrate and quantify the uncertainty and — often — the lack of conservatism in design levels derived from month long current observations.

Topics: Design
Commentary by Dr. Valentin Fuster
2018;():V07BT06A008. doi:10.1115/OMAE2018-77810.

Environmental contours are commonly applied in design of marine structures to identify extreme environmental conditions that may give rise to extreme loads and responses. Recently, there has been some focus on the fact that different methods exist for establishing such contours, and that in some cases significant differences may be obtained from the different methods.

In this study, we address another uncertainty related to the calculation of environmental contours, namely the uncertainty due to sampling variability when constructing environmental contours based on metocean data of finite sample size. The uncertainty of environmental contours for joint distribution of significant wave height and wave period for different sample sizes (10, 20, 30 and 100 years of data) are investigated considering different underlying datasets. Both cases where samples are drawn from a known joint distribution of wave height and periods and cases where samples are drawn from a real hindcast dataset and fitted to the joint distribution are considered. The uncertainty of the estimated contours is quantified and discussed in light of differences that can be anticipated from the different methods used to calculate the contours.

Topics: Uncertainty
Commentary by Dr. Valentin Fuster
2018;():V07BT06A009. doi:10.1115/OMAE2018-77863.

In widely used metocean terminology, solitons are large amplitude, often highly nonlinear, internal waves. They are responsible for complex vertical profiles of rapidly varying ocean currents. These current profiles need to be reliably quantified for a wide range of offshore engineering applications, often with very limited suitable data.

Some recent advances in this field of applied research were described at OMAE2017 by Jeans et al (2017) [1]. Vertical displacements, derived from temperature measurements, were the primary input for soliton quantification. Associated current speeds were estimated from relevant theory and validated using available measured current data. This represents a notable development, because soliton current profiles are traditionally quantified via direct measurements of velocity. However, reliable current measurements can be a challenge, so the new approach is considered more reliable in some circumstances.

Jeans et al (2017) [1] applied one simple and elegant theory for relating vertical displacement to velocity. This theory performed well, considering its limitations. This paper further evaluates different theoretical options, using a new dataset with much larger amplitude solitons. Theories with higher order nonlinearity are required for estimation of soliton current profiles in such challenging conditions.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A010. doi:10.1115/OMAE2018-77869.

Internal waves in the ocean occur in stably stratified fluids when a water parcel is vertically displaced by some external forcing and is restored by buoyancy forces. A specific case of such internal waves is internal tides and their associated currents. These currents can be significant in areas where internal waves degenerate into nonlinear solitary waves, known as solitons.

Solitons are potentially hazardous for offshore engineering constructions, such as oil/gas pipelines and floating platforms. The most efficient mechanism of soliton generation is the tidal energy conversion from barotropic to baroclinic component over large-scale oceanic bottom obstructions (shelf breaks, seamounts, canyons and ridges).

In this paper, a methodology is provided to compute diagnostics and prognostics for soliton generation and propagation, including the associated currents. The methodology comprises a diagnostic tool which, through the use of a set of theoretical and empirical formulations, selects areas where solitons are likely to occur. These theoretical and empirical formulations include the computation of the integral body force (1), the linear wave theory to compute the phase speed and the empirical model proposed by (2). After the selection procedure, the tool provides initial and boundary conditions for non-hydrostatic numerical models. The numerical models run in 2D-V configuration (vertical slices) with horizontal and vertical resolutions ranging from 50 to 200 m and 5 to 10 m, respectively. Examples are provided for an open ocean location over the Mascarene Plateau in the Indian Ocean. Validation of diagnostics and prognostics are provided against ADCP and satellite data.

Topics: Solitons , Oceans
Commentary by Dr. Valentin Fuster
2018;():V07BT06A011. doi:10.1115/OMAE2018-77926.

The marine environment represents a large and important resource for communities around the world. However, the marine environment increasingly presents hazards that can have a large negative impact. One important marine hazard results from storms and their accompanying surges. This can lead to coastal flooding, particularly when surge and astronomical high tides align, with resultant impacts such as destruction of property, saline degradation of agricultural land and coastal erosion.

Where tide and storm surge information are provided and accessed in a timely, accurate and understandable way, the data can provide:

1. Evidence for planning: Statistics of past conditions such as the probability of extreme event occurrence can be used to help plan improvements to coastal infrastructure that are able to withstand and mitigate the hazard from a given extreme event.

2. Early warning systems: Short term forecasts of storm surge allow provide early warnings to coastal communities enabling them to take actions to allow them to withstand extreme events, e.g. deploy flood prevention measures or mobilise emergency response measures.

Data regarding sea level height can be provided from various in-situ observations such as tide gauges and remote observations such as satellite altimetry. However, to provide a forecast at high spatial and temporal resolution a dynamic ocean model is used. Over recent decades the National Oceanography Centre has been a world leading in developing coastal ocean models. This paper will present our progress on a current project to develop an information system for the Madagascan Met Office. The project, C-RISC, being executed in partnership with Sea Level Research Ltd, is translating the current modelling capability of NOC in storm surge forecasting and tidal prediction into a system that will provide information that can be easily transferred to other regions and is scalable to include other hazard types The outcome, an operational high-resolution storm surge warning system that is easy to relocate, will directly benefit coastal communities, giving them information they need to make effective decisions before and during extreme storm surge events.

Topics: Storms , Surges , Tides
Commentary by Dr. Valentin Fuster
2018;():V07BT06A012. doi:10.1115/OMAE2018-77945.

Numerical investigation on storm surge characteristics would benefit the planners and designers of coastal structures and offshore platforms along the Krishna-Godavari (K-G) basin. The adjoining coastline has a wide range of geomorphological features and varying geometries due to the sediment deposition from the two major rivers, Krishna and Godavari. Two severe cyclonic storms (SCS) Laila (2010) and Helen (2013) that approached the basin from two different directions and made landfalls closer to each other were analyzed for determining the storm surge heights and currents along the K-G river basin. The maximum water elevations and maximum currents during the storm event and evolution of storm surge heights at different locations were studied. It could be concluded from the study that when a SCS event approaches K-G basin, in addition to the tide and wave effect, a maximum storm surge height and current of 1 m and 1.2 m/s can be expected along the coast, respectively. Similarly, the surge and current in the offshore regions were found to be 0.3 m and 0.8 m/s, respectively. These values may be considered while deriving design parameters for the offshore installations. The critical regions in the basin were identified where high surge heights and currents are expected.

Topics: Surges
Commentary by Dr. Valentin Fuster
2018;():V07BT06A013. doi:10.1115/OMAE2018-77989.

The complex nature of the energy industry across extraction, transportation, processing, delivery and decommissioning creates significant challenges to how the sector responds, adapts and mitigates against risks posed by the changing future climate. Any disruption in this interconnected system will affect both industry and society. For example, in the summer of 2005 Hurricane Katrina and a month later Hurricane Rita had wide reaching impacts on the US offshore Oil and Gas industry which resulted in an increase in global oil prices due to loss of production and refinery shutdowns in the Gulf of Mexico.

Preparing, mitigating and adapting to these climate changes is dependent upon identifying appropriate climate indicators as well as the associated critical operational thresholds and design criteria of the identified vulnerable assets. The characterization and understanding of the likely changes in these climate indicators will form the basis for adaptation plans and mitigating actions.

The Met Office in collaboration with energy industry partners, under the Copernicus Clim4energy European project, has developed a Climate Change Risk Assessment tool, which allows the visualization and extraction of the most recent sea level and wave climate information to evaluate their future changes. This study illustrates the application of this tool for evaluation of the potential vulnerability of an offshore infrastructure in the North Sea.

The analysis shows that for this asset there is a small increase in sea level of 0.20–0.30 m at the location of interest by 2050. However, there is a small decrease or no consistent changes projected in the future wave climate. This wave signal is small compared to the uncertainty of the wave projections and the associated inter-annual variability. Therefore, for the 2050s time horizon, at the location of interest, there is no strong impact of climate change at the annual scale on the significant wave height, the sea level and thus the associated climate change driven extreme water level. However, further analysis are required at the seasonal and monthly scales.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A014. doi:10.1115/OMAE2018-77994.

Many approaches exist to build environmental contours, i.e. the ensemble of joint metocean parameters corresponding to a N-year return level. However, these contours may provide rather different curves according to their calculation process. In this work, the use of response meta-models is introduced for the inter-comparison of environmental contours for two application cases: the roll of a FPSO and the tension in a mooring line of a semi-submersible.

Some state-of-the-art methods available for modelling multivariate extremes are applied for two or more variables and in a directional context. Among the conditional models, joint parametric laws (such as Weibull, log-normal...), Heffernan and Tawn simulations and several copula are investigated. Two ways of contour building are considered in 2D and 3D: Inverse First Order Reliability Model (I-FORM) and physical-space Huseby contouring method.

The comparison between the resulting design points are presented for both applications. The combined wind, wave and current parameters statistics (including directions) are presented, in addition to the extreme loads obtained from the meta-models. The advantages and drawbacks of each method are reviewed. Some key findings are finally presented, pointing out the interest of a meta-model to determine the more realistic set of design points in function of the structure response of interest.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A015. doi:10.1115/OMAE2018-78335.

The marine environmental conditions of typhoon affected area in South China Sea (SCS) in China are complicated. The parameters of wind, wave and current speed of different return period are the basis of the selection of oilfield project and the important role of development cost evaluation. Based on the deduction method of API specification, the design criteria of the typhoon in SCS oil and gas regions with the two-layer nested model combined with typhoon numerical model is proposed in this paper. The statistical analysis of typhoon characteristics were presented, then wind speed of all the blocks in north part of SCS induced by typhoon with long return period was calculated. The prediction accuracy of the design parameters of long return period is improved by this model.

Topics: China , Seas
Commentary by Dr. Valentin Fuster
2018;():V07BT06A016. doi:10.1115/OMAE2018-78755.

Prediction of extreme wave heights has always been a challenge in both the naval and energy industries. The survivability and safe operation and design of marine vehicles and devices are highly dependent on the probability distribution of the wave heights of extreme waves. In traditional linear approaches, researchers use various probability distribution functions mostly generated from field measurements and are usually modified with some statistical methods to account for the distribution of wave heights. These approaches do not take into account nonlinearity and instability in wave train behavior and solely relies on linear wave theory assumptions and perhaps some second order effects in more advanced probability models. This study emphasizes the application of modulation wavelengths and periods, resulting from modulational instability analysis of the nonlinear Schrödinger equation (NLS). In this study, state-of-the-art nonlinear Fourier analysis (NLFA) based on NLS is employed to calculate the unstable wave components. The resulting rise time and travel distance for such unstable modes and their maximum possible growth amplitudes are used to derive a range of probable occurrences. Numerical simulation results from CFD computations are used to examine the capability of such an approach in predicting the magnitude and location of extreme wave occurrence. It is shown that application of the proposed NLS-based analytical procedure enables a more accurate prediction of the extreme wave field.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Model Tests

2018;():V07BT06A017. doi:10.1115/OMAE2018-77090.

When conducting model tests in a water tank, available model sizes and wave conditions are determined for each tank, depending on measurement accuracy and tank specifications. For deep-water mooring of a floater, a mooring extent in model scale is presumably over 10 meters in depth, making it difficult to be conducted in small-sized tanks without mooring line truncation. The purpose of the research is to develop a device, which could be used as deep-water mooring system in small-sized tanks. Although the law of geometrical similarity is compelled to quit because of the line truncation, the law of mechanical similarity can be maintained by keeping the same restoring, damping and inertia characteristics as those of the full-scale mooring system obtained by numerical simulations.

The mooring device consists of a cylinder, a piston, an orifice, springs, pulleys and weights. A spring attached to the mooring line is to generate required restoring force. The orifice, together with the piston, is to generate required damping forces. Inertia forces are generated by the motions of hanged weights, also by the motion of the fluid inside the cylinder. Even negative inertia forces can be given by adjusting natural frequencies of the weight-spring system. With all these examined elements, the mooring device works like the full-depth mooring system. Particulars of the elements of the device have been determined by numerical simulations of the floater moored in the full-depth condition. It has been confirmed that the mooring device behaves as expected in comparison with forced oscillation tests, where prescribed motions were given to the floater-side end point of the mooring line.

A tank test has been conducted of a floater with a turret multipoint-moored with the devices and has been satisfactorily compared with numerical simulations of the full-depth system. With the present research it is verified that the mooring device can well simulate actual deep-water mooring system, which makes it possible for small water tanks to deal with deep water mooring experiments.

Topics: Mooring , Water
Commentary by Dr. Valentin Fuster
2018;():V07BT06A018. doi:10.1115/OMAE2018-77184.

Green water generated by random waves on a fixed, simplified geometry model structure was measured in a large wave basin. The velocity field of the flow that is aerated and highly turbulent was quantified using the bubble image velocimetry (BIV) technique. BIV utilizes shadow textures created by air-water interfaces as tracers in backlit images recorded by a high speed camera. The tracers in consecutive images are then cross-correlated to obtain the corresponding two-dimensional velocities. Random waves were generated by the JONSWAP spectrum with a significant wave height close to the freeboard. An image-based triggering method was employed to detect the green water events and trigger image acquisition. A total of 179 green water events were collected and categorized into three different types, based on the flow behavior. That includes the collapse of overtopping wave, fall of bulk water, and breaking wave crest. Statistical distributions of maximum green water velocities under random waves were developed, while the lognormal distribution was found as the best fit. By modeling the green water as a dam break flow, the Ritter solution was found to be able to capture the horizontal velocity distribution for the random green water events. A prediction equation for the green water velocity distribution under random waves was also obtained.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A019. doi:10.1115/OMAE2018-77329.

Vortex-induced motions (VIM) of large marine structures have been emerged as hot issues both in engineering and academic fields. Aiming to investigate the effect of the appendages on the VIM phenomenon of the tension leg platforms (TLP), experimental studies were carried out in a towing tank., The tendon pretensions and mass ratio were modeled exactly by using an air-bearing system. Additionally, the motions in vertical plane were almost restricted and the TLP model was allowed to move freely in horizontal plane. The 6-degrees of freedom (6-DOF) motions of the model were recorded by the motion acquisition system, consistent with an acceleration sensor installed at the deck as well as four load cells employed to acquire the restoring forces of the four mooring lines respectively.

This paper presents the VIM responses of the TLP model with bare hull and the model with appendages located at the surface of columns with different relative positions to the current (i.e., 0°, 45°, 90° and 135°, respectively). In addition, the dynamic behaviors are also processed. The results show that the amplitudes of VIM response in the transverse direction are mitigated significantly for the configuration of 45°-appendages, lower than that of the bare hull model about 65 percent but slightly subsided at other locations. Moreover, the appendages at all locations can obviously reduce the amplitudes of the yaw motions, with the maximum decrease of 52 percent of the yaw responses for the configuration of 0°-appendages and 45°-appendages, compared with that of the bare hull model.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A020. doi:10.1115/OMAE2018-77502.

Potential liquefaction of some cargoes (Nickel ore, iron ore, ...) is a major risk for the maritime industry. The difficulties to simulate accurately the behaviour of these materials as well as their interaction with a bulk carrier model leaded us to use a non-Newtonian highly viscous fluid to model a liquefied ore. An experimental approach is presented in this paper. Roll responses of a ship model as well as details on the internal free surface behaviours are investigated for different loading conditions: solid cargo, fresh water and viscous fluids.

Topics: Ships
Commentary by Dr. Valentin Fuster
2018;():V07BT06A021. doi:10.1115/OMAE2018-77509.

As the demand for offshore wind energy continues to grow, the strive to understand the wave forces acting on the substructure of the wind turbines continues. In regard to wind turbine design, it is vital to consider not only the total wave force, but also the local wave forces. Local forces are particularly important for the design of secondary structures as e.g. mooring platforms. Typically, however, experimental studies mainly concern total forces or idealized local forces. We present here a rather simple way to measure local forces along a model monopile. The study is conducted in a wave flume of 28 m in length, in which waves are generated by a piston-type wave maker at a water depth of 0.515 m and shoal onto a bed of slope 1:25. A model monopile is installed and subjected to forcing from a series of both regular and irregular waves. In the experimental set-up, the model monopile is fixed at the bottom and the top and consists of seven independent cylindrical sections. The cylindrical sections are connected by force transducers which measure local shear, and so the associated local forces may be determined. The measured local forces are compared to the force distribution given by Morisons equation combined with linear theory and Wheeler stretching, which is a force estimate commonly used in the industry. This study shows that the total force is rather well captured by Morison’s equation. The force distribution estimated from Morison’s equation, however, shows larger discrepancies from the measured forces. This encourages for further measurements. In this study, we show that it is possible to measure force distribution on a model monopile in a simple and cost-effective manner. The aim is here to demonstrate the method and we will later present a larger body of work associated with the outcome of the measurements.

Topics: Wave forces
Commentary by Dr. Valentin Fuster
2018;():V07BT06A022. doi:10.1115/OMAE2018-77637.

Investigating the hydrodynamics of a ship manoeuvring is a continuing concern for researchers. In recent years, with the growth of the operations complexity, understanding the hydrodynamics of a ship manoeuvring has become a central issue for supply vessels operating in close proximity of oil platforms. One of the main obstacles in understanding this problem is the difficulty of reproducing the manoeuvring on real ships with acceptable measurements, uncertainties and environmental control, not to mention the cost involved. A natural approach to address the issue is to run model tests, where it is possible to control a great number of variables.

In the desire to develop new methods to evaluate hydrodynamic coefficients and to improve the understanding of the phenomena, this paper proposes different types of free-running tests to be conducted in an experimental ocean basin. An Anchor Handling Tug Supply Vessel (AHTS) scaled model was used to perform the classic turning circle and the novel turning eight, a substitute to the zigzag that fits in limited manoeuvring facilities. Optimizations of mathematical manoeuvring models were applied to estimate the hydrodynamic coefficients with a new proposed metric of manoeuvres comparison. Simulations results were compared with the experimental measurements of the model during the tests. The experimental tests were performed at LabOceano, the Ocean Technology Laboratory of the Federal University of Rio de Janeiro.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A023. doi:10.1115/OMAE2018-77718.

Truncated mooring system is the foundation of hybrid model test of deepwater floating platform with spread mooring system. This paper presents supervised learning method using linear regression model as the learner to generate empirical formulas which can determine the properties of truncated mooring system given the properties of full-depth mooring system. Thousands of completed truncation tasks are used to train the leaner, and then empirical formulas determining the length, axial stiffness and wet weight of the truncated system are established. The formulas are tested using a set of new truncation tasks. The results indicate that the truncated mooring system can be properly predicted by simply using the formulas.

Topics: Design , Mooring
Commentary by Dr. Valentin Fuster
2018;():V07BT06A024. doi:10.1115/OMAE2018-77859.

This paper presents an experimental study using Real-Time Hybrid Model (ReaTHM) testing of a moored floating cylindrical buoy, conducted in a wave basin. ReaTHM testing is a method for studying the dynamics of marine structures by partitioning the system into numerical and physical substructures that are then coupled in real-time using a control system. In this study, the floating cylinder buoy is modelled physically, and the mooring system modelled numerically. In this paper, the effect of selected controller parameters on the performance of the control system is studied, for both wave frequency and low-frequency ranges. The architecture/design of the control system is presented in the first part of the paper, while results from experimental tests with wave excitation on the physical substructure are presented in the second part of the paper.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A025. doi:10.1115/OMAE2018-77879.

In this paper, rigid ship hull and elastic ship hull were respectively designed. For the former, the hull is divided into two segments, while for the latter, into five segments. And a series of experiments of ship model navigating in regular wave were carried out. Time histories of motion and vertical bending moment of the two models were recorded and analyzed. By comparing results of the two models, the impact of elasticity of hull grider on motion and vertical bending moment can be analyzed.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2018;():V07BT06A026. doi:10.1115/OMAE2018-77980.

The paper refers on a research activity, focused at DREAMS Lab of the University of Genoa (Italy) and still under development, oriented to experimental application of air-bubbling techniques on flat plates and hull models. In this study the reduction in the frictional resistance by air bubbling generated by customized pneumatic circuits is tested, both on the lower surfaces of flat plates characterized of different geometries of holes and on a hull model tested in towing tank. The effective shape of air bubbles is observed, and changes in the local frictional drag are measured, using flexible and low cost thin sensors at different levels of flow rate and pressure of injected air. In towing tank tests the experiments compare hull without and with holes on the bottom, modifying the characteristics of speed, pressure, flow rate and areas interested to the air injection.

Systematic tests campaign has been developed, using also actuation pneumatic workbenches expressly designed for the experiments.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Ocean Engineering Technology

2018;():V07BT06A027. doi:10.1115/OMAE2018-77157.

In this paper, we report both experimental and computational studies of hydrodynamic cavitation generated by accelerating liquid through a series of constrictions. The detailed process of cavitation generation is visualized using a high-speed photography. The cavitation is initiated when a gas bubble moves towards the constrictions. The gas bubble initially accelerates, expands and then splits into smaller bubbles when it moves along the constriction. As these bubbles migrate into a large liquid compartment, they collapse violently to form a bubble cloud, owing to a sudden jump in liquid pressure in the compartment. The experimental observation is further confirmed using computational fluid dynamics (CFD) simulations. We also present experimental evidence showing a significant reduction in gram-negative Escherichia coli concentration after it passes through the constrictions.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A028. doi:10.1115/OMAE2018-77354.

Fibreglass reinforced flexible pipe (FRFP) is regarded as a great alternative to many bonded flexible pipes in the field of oil or gas transportation in shallow water. This paper describes an analysis of the mechanical behavior of FRFP under torsion. The mechanical behavior of FRFP subjected to pure torsion was investigated by experimental, analytical and numerical methods. Firstly, this paper presents experimental studies of three 10-layer FRFP subjected to torsional load. Torque-torsion angle relations were recorded during this test. Then, a theoretical model based on three-dimensional (3D) anisotropic elasticity theory was proposed to study the mechanical behavior of FRFP. In addition, a finite element model (FEM) including reinforced layers and PE layers was used to simulate the torsional load condition in ABAQUS. Torque-torsion angle relations obtained from these three methods agree well with each other, which illustrates the accuracy and reliability of the analytical model and FEM. The impact of fibreglass winding angle, thickness of reinforced layers and radius-thickness ratio were also studied. Conclusions obtained from this research may be of great practicality to manufacturing engineers.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A029. doi:10.1115/OMAE2018-77472.

The dolphins’ cruising, generally, with an extremely high thrust efficiency and low drag, which attracted many researchers’ wide attention. It is hoped that we can improve the hydrodynamic performance of underwater vehicle by studying the thrust characteristics of dolphin’s kick and the relationship between the formation of vortex and the thrust generation. However, previous work is mostly focused on investigation of hydrodynamic performance of dolphin fluke motion with a rigid tail which means that the locomotion of caudal fin is defined only by the oscillating motion, without the chordwise deformation. In this paper, the dolphin’s fluke motion is realized by a flexible caudal fin which is defined by a combination of oscillating motion and chordwise deformation.

The simulation of the dolphin fluke motion is achieved by STRA-CCM, and dynamic moving mesh is implemented for different stroke functions. This paper primarily analyzed the thrust characteristics and the formation of vortex of dolphin fluke motion, then compared with the available data from previous work with rigid tail. It can be found that the structure of the vortex generated by the dolphin fluke motion with flexible caudal fin is different from a rigid one. Finally, by analyzing the instantaneous flow condition behind the dolphin caudal fin, it can get the reason why the thrust generated by the flexible caudal fin is larger than rigid one.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A030. doi:10.1115/OMAE2018-77795.

The paper studies the path following of a ship sailing in restricted waters based on an output feedback control, which consists of a state feedback control law and an extended updated-gain high-gain observer. According to the separation principle, the state feedback control and the extended updated-gain high-gain observer are designed separately. The state feedback control law is designed based on a robust guaranteed cost control method assuming that system states are measurable. Sufficient conditions are given for the control based on a linear uncertain system. The extended updated-gain high-gain observer, whose gains are updated according to the nonlinear functions of available evaluation errors, is used to reconstruct system states. Then the output feedback control is obtained by replacing states value in the state feedback control law with its estimation yielded by the state observer. Numerical simulations confirm the effectiveness of the proposed control method for the path following of a ship sailing in restricted waters.

Topics: Ships , Water
Commentary by Dr. Valentin Fuster
2018;():V07BT06A031. doi:10.1115/OMAE2018-78003.

Holistic energy management in the shipping industry involves reliable data collection, systematic processing and smart analysis. The era of digitisation allows sensor technology to be used on-board vessels, converting different forms of signal into a digital format that can be exported conveniently for further processing. Appropriate sensor selection is important to ensure continuous data collection when vessels sail through harsh conditions. However, without proper processing, this leads to the collection of big data sets but without resulting useful intelligence that benefits the industry. The adoption of digital and computer technology, allows the next phase of fast data processing. This contributes to the growing area of big data analysis, which is now a problem for many technological sectors, including the maritime industry. Enormous databases are often stored without clear goals or suitable uses. Processing of data requires engineering knowledge to ensure suitable filters are applied to raw data. This systematic processing of data leads to transparency in real time data display and contributes to predictive analysis. In addition, the generation of series of raw data when coupled with other external data such as weather information provides a rich database that reflects the true scenario of the vessel. Subsequent processing will then provide improved decision making tools for optimal operations. These advances open the door for different market analyses and the generation of new knowledge. This paper highlights the crucial steps needed and the challenges of sensor installation to obtain accurate data, followed by pre and post processing of data to generate knowledge. With this, big data can now provide information and reveal hidden patterns and trends regarding vessel operations, machinery diagnostics and energy efficient fleet management. A case study was carried out on a tug boat that operates in the North Sea, firstly to demonstrate confidence in the raw data collected and secondly to demonstrate the systematic filtration, aggregation and display of useful information.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A032. doi:10.1115/OMAE2018-78037.

The workability of various types of operations offshore are largely affected by waves and wave induced motions. Examples are crew transfer from crew transfer vessels or service operation vessels to offshore wind turbines for maintenance, landing of helicopters in (navy) vessels and various crane operations. Over the recent years quite some effort has been put in technology aiming to provide a real time on-board prediction of approaching waves and wave induced vessel motions some minutes in advance. Enabling crew to anticipate, thus enhancing the safety and operability of these operations. This paper addresses the performance during a field test of the system as being under development by Next Ocean enabling such predictions, based on using an off-the-shelve (non-coherent) navigation radar system as a remote wave observer. Briefly summarizing (earlier publications on) the technical approach, focus will be on results obtained from a field test where the system was validated. Good agreements between ship motions as measured by an on-board motion reference unit and predictions obtained by the wave and motion prediction system during a field test on the North Sea near the Dutch coast on a 42 m patrol vessel will be shown in the results section, from which the usefulness of the system for operational decision support can be concluded.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2018;():V07BT06A033. doi:10.1115/OMAE2018-78171.

Economic factors together with protection laws and policies pertaining to marine pollution drive research for improved power generation. Fuel cells, being fuel efficient and environmentally friendly, could provide a desirable option and suitable alternative to conventional propulsion systems based on fossil fuels or even nuclear fission. Fuel cells are becoming fast a mature technology and employed in many various other areas. Flexibility of special purpose watercraft, power autonomy and modularity can all benefit from the use of fuel cells. Specifically, proton exchange membrane fuel cells are considered among the most promising options for marine propulsion applications. Switching converters are the common interface intermitted between fuel cells and the load in order to provide a stable regulated voltage. DC-DC converters have been widely used since the advent of semiconductors. These devices are typically adopted to accomplish voltage regulation tasks for a multitude of applications: from renewable energy power-plants to military, medical and transportation systems. Nonetheless voltage regulators exhibit the need for consistent closed- and open-loop control. Most common approaches are PID controllers, sliding mode controllers and artificial neural networks that are considered in this work. An artificial neural network (ANN) is an adaptive, often nonlinear system that learns to perform a functional mapping from data. In our approach, a typical example of a fuel cell, a power converter outfitted with an ANN controller, and a resistive load configuration is investigated. Simulation studies are crucial in power electronics to essentially predict the behavior of the device before any hardware implementation. General requirements, design specification together with control strategies can be iteratively tested using computer simulations. This paper shows the simulation results of the full system behavior, as described above, under dynamic conditions. Initially, an open-loop simulation of the system is performed. Next, an appropriately trained ANN is incorporated to the switching model of the DC-DC converter to perform simulations for validation. Conversely, during design and calibration of the ANN controller, instead of the switching model of the DC-DC converter, a trained ANN equivalent is employed.

Topics: Fuels
Commentary by Dr. Valentin Fuster
2018;():V07BT06A034. doi:10.1115/OMAE2018-78639.

This paper mainly studies the numerical analysis of the Pendulous Installation Method of a pump module in 1,500 meter water. A nonlinear time-domain coupled vessel and the pump installation system is established using ORCAFLEX riser program to simulate the pendulous free-fall motion of the pump module. The analysis studies the effect of environmental loads and vessel’s dynamic response on the entire installation system. Different directions of environmental loads and a series of sea-states defined using significant wave heights are taken into consideration. The results show that the optimum direction of the environmental loads is taken as 180 degrees (aganist vessel length), and the dynamic response of the vessel has a great impact on the entire system and should not be neglected.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Towed and Undersea Cables and Pipes, Mooring, and Buoy Technology

2018;():V07BT06A035. doi:10.1115/OMAE2018-77128.

As the demand of marine resources is continuously growing, more and more people are focusing on the study of underwater towed system for marine survey, in which mastering and predicting the dynamic characteristic of the system is the key problem. Based on the parameters of a certain underwater system, combined with the lumped mass method, the underwater cable-body 3D motion mathematical model has been established by OrcaFlex, in which the variation of the tension of the towed cable and the variation of the depth of the towed body have been given and the effect of the towed speed on critical radius in the process of the 360° rotary motion has been discussed. The results show a good agreement with previous research results. At the same time, we also make a research on the effects of the change of the hydrodynamic coefficients and the parameters of towed cable on variation of the tension and towed depth. These studies can provide a basis for the selection of cable in the system.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A036. doi:10.1115/OMAE2018-77163.

Survey optimization and cost reductions are ongoing challenges for the marine seismic petroleum exploration industry. The present research intends to improve seismic streamer steering as well as to reduce time on line-changes, where the streamer spread is turned 180 degrees. Sea current estimation and prediction of future streamer state are important aspects of improved streamer steering, which will increase survey efficiency and quality.

A seismic streamer simulator has been validated against full scale proprietary data from two seismic survey line-changes. The streamer spread has 12 cables of slightly more than 4 km length each, with 13 control birds for each streamer. Interpolated Acoustic Doppler Current Profiler (ADCP) measured sea current is introduced at the streamer nodes. Two streamer tension cells are installed at each streamer, giving validation of simulated tension.

Secondly, a sea current estimation algorithm has been validated against the interpolated ADCP current data for the two line-changes.

Topics: Seas
Commentary by Dr. Valentin Fuster
2018;():V07BT06A037. doi:10.1115/OMAE2018-77661.

Mooring systems constitute an important element to secure the stability and survival of floating structures. In the last years, its use has increased potentially linked to the growth of marine renewable energies, about all, offshore wind and waves. Traditionally, fixed foundations such as gravity, monopiles or jackets have been installed. However, new alternatives have been developed based on floating structures looking to take advantage of the potential resource in deep waters.

This paper involves an exhaustive mooring system study based on catenary configuration from different points of view. Physical modelling was performed by means of different experimental tests under different loading conditions including prescribed movements and natural forcing (waves and currents) considering two different types of seabed: a rigid seabed using the glass of the flume simulating a rocky bottom and a deformable seabed through sandy seabed. Different types of numerical approaches were implemented and they were validated with laboratory tests. Quasi-static and dynamic models were included. Also, a commercial software called SESAM was used to verify the results. Finally, the effect of wave groupiness on extreme loads was analysed using a Floating Offshore Wind Turbine (FOWT).

Topics: Stress , Waves , Modeling , Mooring
Commentary by Dr. Valentin Fuster
2018;():V07BT06A038. doi:10.1115/OMAE2018-77734.

Large underwater equipment used in deep-sea engineering, which needs mooring line in order to hang down while using pendulous installation method in 1500m deep-water installation. During the procession, the mooring line will suffer the vessel’s pull, large equipment’s traction, fluid force leading to elongation, bending and other deformation, so need to analyze its motion. Aiming at the numerical simulation problem of underwater equipment installed by pendulous installation method in deep-water environment, this paper combines lumped mass method, three-dimensional potential theory and Morison equation to establish the analysis model of the whole installation system. Under the different horizontal dropping offsets of the equipment, this paper computes not only tension of mooring line, but motion performance of underwater equipment and working vessel during the pendulous lowering phase.

The Pendulous Installation Method (PIM) puts a working vessel, a mooring line and a underwater equipment installed in succession, and there is a coupling interaction among them, so it is essential to do time-domain coupling analysis. So the Orcaflex Software is used to simulate the entire deep-water installation system. Using different horizontal dropping position as a variable, we can get the contrast about the different movement states of the mooring line and underwater equipment, and finally we will get the conclusion.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A039. doi:10.1115/OMAE2018-78525.

In order to predict the coupled motion and external wave load for the design of deepwater floating structure system, based on the three-dimensional time-domain potential flow theory, this paper present the indirect time-domain dynamic coupling method and the body nonlinear dynamic coupling method. The perturbation expansion theory is adopted to evaluate hydrodynamic on the fixed mean wetted body surface for the former method. The transient free surface Green function has been extended and applied to calculate the nonlinear hydrodynamic on the instantaneous wetted exact body surface for the latter method. The finite element model is employed to solve dynamic response of mooring line. Then asynchronous coupled method is adopted to achieve the coupled dynamic analysis of platform and mooring lines. The time-domain motion responses and spectrum analysis of Spar platform are verified and compared with the traditional indirect time-domain coupling dynamic method when the mooring system is completed. Also the time-domain motion responses and statistical characteristic of Spar platform are investigated with one mooring line broken in extreme sea condition. Some conclusions are obtained, that is, dynamic coupling effects are significant and transient position hydrodynamic calculation of platform has a great influence on the low frequency motion. The results also show that the influence on the global performance of mooring system is different when the broken line is in different place. A remarkable influence occurs when the broken mooring line is in the head-wave direction.

Commentary by Dr. Valentin Fuster

Ocean Engineering: Unsteady Hydrodynamics, Vibrations, Acoustics, and Propulsion

2018;():V07BT06A040. doi:10.1115/OMAE2018-77363.

The viscous flow past a prolate spheroid is always complicated and often three-dimensional, thereby fascinating scientists in fluid dynamics and aerodynamics. The objective of this paper is to investigate the asymmetric wake behind a 6:1 prolate spheroid at 45-deg angle of attack by means of Large-Eddy simulations(LESs). The LES was performed at a Reynolds number of 10000, based on the free stream velocity U0 and minor axis diameter D. Results in the paper were compared with the previous LES results for different Re. The simulation revealed that there was a pair of counter-rotating vortex in the near wake field, which developed from a pair of vortex sheet separating from the sides of prolate spheroid. The vortex broke and the coherence of vortex tube lost because of the influence of vortex shedding from the tip of the prolate spheroid and energy dissipation. The asymmetry could be observed evidently from several physical quantities, such as velocity, vorticity, pressure and side force. The results showed an obvious skew of vortex structure, the direction of which was random and could be explained by pitchfork theory. The comparison between near field and far field was present to show the development and features of the flow characteristics. The asymmetric wake should be given more attention as it may be detrimental to underwater vehicle performance and submarine maneuverability.

Topics: Wakes
Commentary by Dr. Valentin Fuster
2018;():V07BT06A041. doi:10.1115/OMAE2018-77921.

The need of continuously improving propulsive efficiency encourages the development of energy saving devices, the understanding of their underlying principles and the validation of their effectiveness. In this work, a design by optimization of Propeller Boss Cap Fin (PBCF) devices is carried out using Computational Fluid Dynamics analyses. RANS calculations (by the OpenFOAM library) are applied in an automatic optimization design approach involving a parametric description of the main characteristics of PBCFs. The optimization is carried out with multiple purposes: identify a reliable design strategy necessary to customize the PBCF geometry based on the propeller functioning and evaluate the influence of alternative configurations and of main geometrical parameters in achieving higher efficiency. The use of high-fidelity RANS calculations confirm that the decrease of the hub vortex strength, the reduction of the net torque and the influence of the additional fins on blades performance are the major contributors to the increase of efficiency. Results of detailed analyses of optimal PBCF configurations show model scale increases of efficiency of about 1%.

Topics: Optimization
Commentary by Dr. Valentin Fuster
2018;():V07BT06A042. doi:10.1115/OMAE2018-78209.

This paper aims to perform a numerical analysis of application effects of a superhydrophobic paint by completely coating the blades of a model-scale marine propeller in order to make it a superhydrophobic surface (SHS). First, a two-dimensional study was conducted. Two foils were analyzed for several hydrophobic conditions, varying the slip length. Pressure and skin friction distributions were shown. There is an increase of lift-to-drag ratio with hydrophobicity, but followed by an increase in suction pressure. In three-dimensional case, a propeller was simulated for several hydrophobic conditions, comparing thrust, torque and efficiency coefficients and pressure and friction distribution. Results with propeller showed that an increase in slip length is not always followed by an increase in efficiency, with an apparent efficiency gain limit. For the imposed simulation conditions, from the limit of gain, efficiency no longer increases with hydrophobicity, but its area of low pressure continues to grow.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A043. doi:10.1115/OMAE2018-78213.

Simulations of propeller E1619 of two models with different scales are presented using an in-house numerical code based on the solution of the Reynolds averaged Navier-Stokes equations for the purpose of analyzing the scale effect on propellers. Propeller open water performance at given advance coefficient was obtained and compared against experimental data, showing good agreement. In aspect of CFD results, scale effect is not obvious. ITTC’78 Performance Prediction Method is applied to correct both experimental and computational open water performance of model 1. Computational KT of model 2 and corrected KT of model 1 agrees well, but the difference between computational KQ of model 2 and corrected KQ of model 1 is not neglectable. The locations of the tip vortex core of the two models are similar to each other, and so is the pressure and fluid velocity distribution. The absolute value of pressure on the blades of the smaller model is higher than the bigger model. The fluid axial velocity around the smaller model is higher than the bigger model.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A044. doi:10.1115/OMAE2018-78600.

Energy saving devices (ESD) such as propeller ducts, pre-swirl stators, pre-nozzles, etc have been explored as a more economic and reliable approach to reduce energy consumption for both in-operation and newly design ships over the past decades. Those energy saving devices work in the principle of reducing ship resistance and improving propulsion efficiency as well as hull-propeller interactions. Potential saving from various types of ESD have been reported in literature from the range of 3–9% [1] for propulsion efficiency dependent on different measures. Deployment of those devices on actual full-scale ships has been limited over the past years. One of the key obstacles in application of ESD is the lack of confidence in measuring its efficiency on full-scale ships in actual operational conditions. Advances in computational fluid dynamics (CFD) has provided an alternative approach from model scale test to better understand uncertainties in prediction of ESD efficiency in full-scale ship operations [Shin et al, 2013]. In this work a high fidelity CFD model is presented for investigation effects of pre-nozzles on propulsion efficiency and ship resistance. The model is based on the Reynolds Average Navier-Stokes (RANS) solver with different turbulent models including a hybrid detached eddy simulation (DES) approach for predictions of complex near body flow features as well as in the wake regions from hull and propeller. The model is validated with model test for both towing and self-propulsion conditions. Finally a study of pre-nozzle effects on propeller efficiency as well as hull-propeller interaction is presented and compared with available experimental data (Tokyo 2015 Workshop). The current work constitutes a fundamental approach towards designing more efficient ESD for a specific hull form and propeller.

Topics: Propellers , Ships , Hull
Commentary by Dr. Valentin Fuster

Ocean Engineering: Wave Mechanics and Wave Effects

2018;():V07BT06A045. doi:10.1115/OMAE2018-77069.

Excitation of steep unidirectional broad-banded wave trains is studied numerically and experimentally. Iterative method is developed to adjust the driving signal of a paddle-type wavemaker to generate wave train with a prescribed free waves’ spectrum. Analytical post-processing procedure based on the Zakharov equation is applied to separate complex amplitude spectrum of the surface elevation into free and bound components, as required for the proposed method of the adjustment of the wavemaker driving signal. Numerical wave tank in the simulations was based on application of the Boundary Element Method. The results of numerical simulations were supported by measurements in a wave tank. The measured and the designed shapes of the surface elevation variation with time, as well as of the corresponding amplitude spectra were found to be in a good agreement.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A046. doi:10.1115/OMAE2018-77186.

Recent work by McAllister et al. (2018) [1] has experimentally confirmed that the set-down of the wave-averaged free surface, first described by Longuet-Higgins and Stewart (1962) [2], can turn into a set-up when wave groups are sufficiently spread or cross at large angles. Experimental results were shown to agree well with second-order theory, including frequency-sum and frequency-difference terms, where the latter are responsible for the wave-averaged free surface. In this paper, we review these experimental results and examine theoretically the magnitude of the wave-averaged free surface in realistic extreme North Sea conditions. Specifically, we examine the role of the shape of the spectrum, water depth, and the relative magnitude of the peak frequencies of the two crossing groups. We find that having a realistic spectrum (JONSWAP vs. Gaussian) considerably enhances the magnitude of the second-order contribution, the total second-order signal increases with decreasing depth and can display a maximum provided the water depth is shallow enough for small to moderate degrees of spreading or crossing angles and is larger for spectral peaks that are further apart.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A047. doi:10.1115/OMAE2018-77504.

We conducted experimental and theoretical studies on Benjamin-Feir (BF) instability and revealed a number of new features of the development of instability on the late stages of wave’s evolution. We employ the reduced (truncated) version of Zakharov equations — the multi-wave near-neighbor resonance model (NN model), which takes into account the most effective quasi-resonances with minimum detuning from exact resonance conditions.

We show that near-neighbor model for wave interactions can adequately describe the number of new prominent features of BF instability observed in experiments and it is much simpler than Zakharov equation for computation and analysis. Numerical simulations of the full Zakharov equations confirm the main predictions obtained by the NN modeling and both reasonably correspond to the results of available physical experiments.

Strong permanent downshifting of spectral maximum for gentle waves without wave breaking is revealed for twice as narrow side band spectral width in comparison with the most unstable case. Regime of multiple downshifting accompanied by wave breaking is discovered for steep waves. Discrete energy flow to higher spectral components takes a place in breaking and no breaking regimes. Results of numerical simulations of Zakharov and NN models reasonably correspond to each other and to our experimental and field observations on wave modulation.

Topics: Waves
Commentary by Dr. Valentin Fuster
2018;():V07BT06A048. doi:10.1115/OMAE2018-77624.

Marine structures are subjected to wave forces whether they are stationary or moving. Such wave forces play a significant role in the design and operation of marine structures. The aim of this study is to understand and predict the unsteady hydrodynamic loads experienced by a submerged body near the surface. Both numerical and experimental studies were conducted. For the experimental work, a newly constructed wave maker inside a tow tank was utilized while a computational fluid dynamics model was developed for the numerical study. Both experimental and numerical studies can complement each other. First, the computational model was validated against experimental wave data so as to understand what parameters in numerical modeling influence the reliability of the numerical results. The second aim was to understand the force and moment that a submerged body would experience for different wave lengths.

Topics: Stress , Waves
Commentary by Dr. Valentin Fuster
2018;():V07BT06A049. doi:10.1115/OMAE2018-77726.

This paper concerns the estimation of wave run-ups on a fixed surface-piercing square column. Experiments and numerical simulations were carried out under waves of different scattering parameters and steepnesses. The results of the run-up height ratio, force coefficient, velocity field, and scattered wave profile were shown and discussed. The reasonable agreement with the experimental results indicates the capability and reliability of the numerical model in the wave run-up prediction. The nonlinearity under short waves is mainly due to the interaction between the scattered waves and the next incident wave crest, while the wave-induced flow around the column becomes more influential under long waves. These nonlinearities are further intensified under steeper waves, and the run-up height ratio increases consequently. A correction factor of 1.2–1.3 can be applied to estimate the run-up height based on the linear potential prediction, but a higher factor of 1.3–1.4 is necessary under long and steep incident waves.

Topics: Waves
Commentary by Dr. Valentin Fuster
2018;():V07BT06A050. doi:10.1115/OMAE2018-77819.

A laboratory study of turbulent boundary layers over wind-generated waves using Particle Image Velocimetry (PIV) in a wind-wave flume at the University of Melbourne is presented. The experiments are taken at two different wind speeds of 5.5 and 8.5 m/s at a fetch length of 3.5 m. Two types of multi-camera measurement are specifically tailored to capture the flow behaviours.

The first is a measurement with high spatial resolution, with aims of characterizing the mean velocity, surface drag and Reynolds stresses over the non-stationary surface. The second type is a large field-of-view measurement, designed to capture the large-scale turbulent motions which are directly associated with the surface-wave topography. Although the turbulent flow is developed over a non-stationary surface (i.e. wind-generated waves), it embodies similarities in both integral parameters and Reynolds stress behaviours to the turbulent flows over stationary rough surfaces. This observation could open a possibility to develop an important turbulence model as well as drag prediction over wind-generated waves, which could be closely related to stationary rough-wall boundary layers.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A051. doi:10.1115/OMAE2018-77925.

Typical offshore structures are designed as tension-leg platforms or gravity based structures with cylindrical substructures. The interaction of waves with the vertical cylinders in high sea states can result in a resonant response called ringing. Here, the frequency of the structural response is close to the natural frequency of the structure itself and leads to large amplitude motions. This is a case of extreme wave loading in high sea states. This understanding of higher-order wave forces in extreme sea states is an essential parameter for obtaining a safe, reliable and economical design of an offshore structure. The study of such higher-order effects needs detailed near-field modelling of the wave-structure interaction and the associated flow phenomena. In such cases, a Computational Fluid Dynamics (CFD) model that can accurately represent the free surface and further the wave-structure interaction problem can provide important insights into the wave hydrodynamics and the structural response. In this paper, the open source CFD model REEF3D is used to simulate wave interaction with a vertical cylinder and the wave forces on the cylinder are calculated. The harmonic components of the wave force are analysed. The model employs higher-order discretisation schemes such as a fifth-order WENO scheme for convection discretisation and a third-order Runge-Kutta scheme for time advancement on a staggered Cartesian grid. The level set method is used to obtain the free surface, providing a sharp interface between air and water. The relaxation method is used to generate and absorb the waves at the two ends of the numerical wave tank. This method provides good quality wave generation and also the wave reflected from the cylinder are absorbed at the wave generation zone. In this way, the generated waves are not affected by the wave interaction process in the numerical wave tank. This is very essential in the studies of higher-order wave interaction problems which are very sensitive to the incident wave characteristics. The numerical results are compared to experimental results for higher-order forces on a vertical cylinder to validate the numerical model.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A052. doi:10.1115/OMAE2018-77998.

There has been speculation that energy input (wind) can play an important role in the formation of rogue waves in the open ocean. Here we examine the role energy input can play by adding energy to the modified non-linear Schrödinger equation. We consider NewWave type wave-groups with spectra which are realistic for wind waves. We examine the case where energy input is added to the group as the wave-group focuses. We consider whether this energy input can cause significant non-linear effects to the subsequent spatial and spectral evolution. For the parameters considered here we find this to have only a small influence.

Topics: Waves , Ocean waves , Wind , Seas
Commentary by Dr. Valentin Fuster
2018;():V07BT06A053. doi:10.1115/OMAE2018-78204.

Deformations and drag forces of fish cages are two main problems in engineering design and optimization. To precisely predict these parameters, a combined numerical method, which combined the beam and truss elements, was used to study the deformation and drag forces of SPM (the single point mooring) fish cage in this paper. Flume model experiments were carried out to validate the accuracy of the numerical model. It is found that the numerical results agreed well with the experimental data, with the maximum and averaged errors of 13% and 5.9%, respectively. It can be conducted that this combined numerical model was useful in the designing and development of fish cages.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A054. doi:10.1115/OMAE2018-78367.

We investigate a nonlinear phase-resolved reconstruction algorithm and models for the deterministic prediction of ocean waves based on a large number of spatio-temporal optical measurements of surface elevations. We consider a single sensor (e.g., LIDAR, stereo-video, etc.) mounted on a fixed offshore structure and remotely measuring fields of free surface elevations. Assuming a uniform distribution of measurement points over the sensor aperture angles, the density of free surface observation points geometrically decreases with the distance from the sensor. Additionally, wave shadowing effects occur, which become more important at small viewing angles (i.e., grazing incidence on the surface). These effects result in observations of surface elevation that are sparsely distributed. Here, based on earlier work by [1], we present and discuss the characteristics of an algorithm, aimed at assimilating such sparse data and able to deterministically reconstruct and propagate ocean surface elevations for their prediction in time and space. This algorithm could assist in the automatic steering and control of a variety of surface vehicles. Specifically, we compare prediction results using linear wave theory and the weakly nonlinear Choppy Wave Model [2, 3], extended here to an “improved” second order formulation. The latter model is based on an efficient Lagrangian formulation of the free surface and was shown to be able to model wave properties that are important to the proper representation of nonlinear free surfaces, namely wave shape and celerity. Synthetic datasets from highly nonlinear High Order Spectral simulations are used as reference oceanic surfaces. Predicted results are analyzed over an area that evolves in time, using the theoretical amount of information assimilated during the reconstruction of the wave field. For typical horizons of prediction, we discuss the capabilities of our assimilation process for each wave model considered.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A055. doi:10.1115/OMAE2018-78393.

An extremely large (‘freak’) wave is a typical though quite a rare phenomenon observed in the sea. Special theories (for example, the modulational instability theory) were developed to explain the mechanics and appearance of freak waves as a result of nonlinear wave-wave interactions. This paper demonstrates that freak wave appearance can be also explained by superposition of linear modes with a realistic spectrum. The integral probability of trough-to-crest waves is calculated by two methods: the first one is based on the results of a numerical simulation of wave field evolution, performed with one-dimensional and two-dimensional nonlinear models. The second method is based on the calculation of the same probability over ensembles of wave fields, constructed as a superposition of linear waves with random phases and a spectrum similar to that used in nonlinear simulations. It is shown that the integral probabilities for nonlinear and linear cases are of the same order of values. One-dimensional model was used for performing thousands of exact short-term simulations of evolution of two superposed wave trains with different steepness and wavenumbers to investigate the effect of wave crests merging. The nonlinear sharpening of merging crests is demonstrated. It is suggested that such effect may be responsible for appearance of typical sharp crests of surface waves, as well as for the wave breaking.

Topics: Waves
Commentary by Dr. Valentin Fuster
2018;():V07BT06A056. doi:10.1115/OMAE2018-78560.

We analyze surface waves generated by a translating, oscillating surface disturbance atop a horizontal background flow of arbitrary depth dependence, with a focus on determining the Doppler resonance. For a critical value of the dimensionless frequency τ = ωV/g (ω: oscillation frequency, V: source velocity, g: gravitational acceleration) at which generated waves cannot escape. In the absence of shear the resonant value is famously 1/4; the presence of a shear current modifies this. We derive the theoretical and numerical tools for studying this problem, and present the first calculation of the Doppler resonance for a source atop a real, measured shear current to our knowledge. Studying graphical solutions to the (numerically obtained) dispersion relation allows derivation of criteria determining the number of far-field waves that exist in different sectors of propagation directions, from which the criteria for Doppler resonance follow. As example flows we study a typical wind-driven current, and a current measured in the Columbia River estuary. We show that modeling these currents as uniform or with a linear depth dependence based on surface measures may lead to large discrepancies, in particular for long and moderate wavelengths.

Commentary by Dr. Valentin Fuster
2018;():V07BT06A057. doi:10.1115/OMAE2018-78576.

Fiber reinforced polymeric laminated materials are suitable for risers in deep-sea applications due to their superior strength, corrosion and fatigue resistance, light weight, low maintenance cost, low transportation cost, and ability for continuous manufacturing. However, due to their anisotropic material properties, the modeling of the dynamic response due to interaction with the internal flow and the sea water is more complicated. In the present work a model for flow induced instability analysis of long, multi-layered, fiber reinforced risers is performed. The motion equations take into account the elastic flexural restoring force of the anisotropic material, the centrifugal force of the fluid flowing in curved portions of the pipe, the Corriolis force, the inertia force of the mass of pump, pipe, and fluid, and the effect of the surrounding water. Combination of the motion equations yields a fourth order partial differential equation in terms of flexural displacements. The transfer matrix method is implemented to the above equation for the critical flow velocities calculation. The “global stiffness matrix” of the pipe-pump system containing the boundary conditions, the anisotropic material properties and the flow parameters, is derived. The condition for non-trivial solution is solved numerically yielding the values of the critical flow velocity, i.e. the internal flow velocity causing flow induced pipeline instability. The results are affected by the anisotropic properties of the material, the mass of the hanged pump, the drag coefficient, and the flow parameters. The results are commented and discussed.

Commentary by Dr. Valentin Fuster

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