Experimental study on a breaking-enforcing floating breakwater

Floating breakwaters are moored structures that attenuate wave energy through a combination of reflection and dissipation. Studies into floating breakwaters have been generally restricted to optimising the attenuation performance. This study presents a novel floating breakwater type that was developed to have good attenuation performance while keeping wave drift loads as small as possible. The floating breakwater was designed as a submerged parabolic beach that enforces wave energy dissipation through breaking. The design was tested in a 3D shallow-water wave basin in captive and moored setups for regular and irregular wave conditions. Results are presented in terms of attenuation performance, motions, and (mooring) loads. The results show that the breaking of waves improves the attenuation performance of the floater in captive setup. However, in moored setup, the attenuation performance was dominated by diffraction and radiation of the wave field, with breaking being of secondary importance. This shows that breaking-enforcing floating breakwaters have potential, but require a high vertical hydrostatic and/or mooring stiffness in order to enforce intense breaking. Mean wave drift loads on the object showed significant difference between breaking and non-breaking waves in both setups, with breaking waves leading to lower normalized loads. This is attributed to breaking-induced set-up and set-down of the water level. As a result, the new breakwater design has a more favourable balance between wave attenuation and drift loads than common (i.e., box-, pontoon-, or mat-type) floating breakwater designs. Tests with varying surface roughness showed that floating breakwaters may benefit from dual-use functions that naturally increase the roughness (e.g., shellfish, vegetation), which have a marginal effect on the attenuation performance, but increase the added mass and hydrodynamic damping and as such, reduce mooring line loads

Evaluation of Directional Analysis Methods for Low-Frequency Waves to Predict LNGC Motion Response in Nearshore Areas

Because LNG terminals are located increasingly close to shore, the importance of shallow-water effects associated with low-frequency (LF) waves increases as well. The LF wave spectrum in these areas is generally complex, with multiple frequency peaks and/or directional peaks due to LF wave interaction with the shore. Both free and bound LF waves at the same frequency can be present. Since LF waves are potentially very significant for moored vessel motions, it is important to include their effect in an early stage of the terminal design. This requires an efficient and relatively simple tool able to estimate the LF wave spectrum in nearshore areas. The benefit of such a procedure with respect to state-of-the-art response methods is the ability to include the LF free wave distribution in a local wave field in the vessel response calculation. The objectives of the present study are to identify such a tool, and to evaluate the use of its output as input for a vessel motion calculation. Three methods, designed for the determination of wave spectra of free wave-frequency (WF) waves, were applied to artificial LF wave fields for comparison of their performance. Two stochastic methods, EMEP (Hashimoto et al., 1994) and BDM (Hashimoto et al., 1987) and one deterministic method, r-DPRA (De Jong and Borsboom, 2012) were selected for this comparison. The foreseen application is beyond the formal capabilities for which these three methods were intended. However, in this study we have investigated how far we can take these existing methods for the determination of directional LF wave spectra. Sensitivity analyses showed that the EMEP method is the most suitable method of the three for a range of LF wave fields. The reconstructed LF wave spectra using EMEP resembled the input spectra most closely over the whole range of water depths and frequencies, although its performance deteriorated with increasing water depth and wave frequency. Subsequently, a first effort was made to use the information in the reconstructed EMEP LF wave spectrum of a representative shallow-water wave field for a first estimate of the motions of a moored LNG carrier. The results were acceptable. This is a first indication that EMEP output might be used to calculate the motions of an LNG carrier moored in shallow water.</jats:p

Computational analysis of wind environmental conditions in an entrance channel in Galicia, Spain

An analysis of wind environmental conditions in the entrance channel of Ria de Ferrol in Galicia, Spain has been conducted with Computational Fluid Dynamics (CFD). The aim of the study is to provide input for real-time manoeuvring simulations to evaluate accessing the LNG terminal with larger LNG carriers. The entrance channel is enclosed by irregular hilly terrain, which is expected to yield complex wind environmental conditions in the channel and complex forces on the LNG carriers. The simulations are performed with the 3D steady Reynolds-averaged Navier-Stokes (RANS) equations and the realizable k-ε model. The simulation results of mean wind speed and direction are generally within 10% of the on-site measurements. Both the simulation and the measurement results illustrate the complex wind-flow patterns and the funnelling effect by the topography on the wind

CFD simulation of wind flow over natural complex terrain: case study with validation by field measurements for Ria de Ferrol, Galicia, Spain

Accurate and reliable Computational Fluid Dynamics (CFD) simulations of wind flow over natural complex terrain are important for a wide range of applications including dispersion of pollutants, wind energy resource assessment and ship manoeuvring in channels and near harbours. In the past 50 years, a very large number of CFD studies of wind flow over hills have been performed. However, a detailed review of the literature shows a lack of CFD studies including validation by field measurements for natural complex terrain beyond the case of isolated hills. Therefore, this paper presents a CFD study with field measurement validation for natural complex terrain that consists of an irregular succession of hills and valleys surrounding a narrow entrance channel. The aim of the study is twofold: (1) to evaluate the accuracy of 3D steady Reynolds-averaged Navier-Stokes (RANS) simulations with a revised k- model for calculating mean wind-velocity patterns over this type of natural complex terrain; and (2) to provide mean velocity data that can be used as input for real-time ship manoeuvring simulations to evaluate accessing the LNG terminal with larger LNG carriers. The irregular hilly terrain is expected to yield complex wind environmental conditions in the channel and complex forces on the LNG carriers. The study focuses on high wind speed conditions, for which the atmospheric boundary layer exhibits neutral stratification. The simulations are performed with 3D steady RANS and the realizable k- model for 12 wind directions. Special attention is given to surface roughness parameterisation and specification. The simulation results of mean wind speed and wind direction are generally within 10-20% of the corresponding measurement values. The results show that for wind directions 60° and 90°, the funnelling effect leads to an increase of wind speed in the channel compared to the wind speed over open sea. For other wind directions, the topography leads to a reduction of the wind speed in the channel, but also to strong wind speed gradients along the channel axis, which are important for ship manoeuvring. The study shows that for the present application, the 3D steady RANS approach with the realizable k- model can provide an accurate assessment of the complex mean wind-flow patterns and the funnelling effect by the natural complex topography on the wind.status: publishe

Computational analysis of wind environmental conditions in an entrance channel in Galicia, Spain

An analysis of wind environmental conditions in the entrance channel of Ria de Ferrol in Galicia, Spain has been conducted with Computational Fluid Dynamics (CFD). The aim of the study is to provide input for real-time manoeuvring simulations to evaluate accessing the LNG terminal with larger LNG carriers. The entrance channel is enclosed by irregular hilly terrain, which is expected to yield complex wind environmental conditions in the channel and complex forces on the LNG carriers. The simulations are performed with the 3D steady Reynolds-averaged Navier-Stokes (RANS) equations and the realizable k-ε model. The simulation results of mean wind speed and direction are generally within 10% of the on-site measurements. Both the simulation and the measurement results illustrate the complex wind-flow patterns and the funnelling effect by the topography on the wind

Innovative Mooring in Locks Using Shoretension: Density and Mooring Force Measurements in the North Lock Ijmuiden

AbstractA field measurement campaign was conducted in the North Lock of IJmuiden, in which the density distribution in the lock chamber and the resulting mooring forces were measured. The innovative mooring system ShoreTension was applied in a navigation lock for the first time to investigate possible ways to increase safety of the mooring process. Density measurements provided valuable insight in the governing hydraulic processes during levelling and lock-exchange and most influential external drivers for density differences over the lock complex could be identified. Potential benefits and limitations of the ShoreTension system were identified, and valuable practical experience was gained during the test period.</jats:p

Global Mapping of Seaport Operability Risk Indicators Using Open-Source Metocean Data

Seaport operability is key to the economic viability of ports. Metocean conditions (e.g., wind, short waves, and infragravity waves) affect this operability when certain thresholds are exceeded. This paper describes a method for the global mapping of seaport operability risk indicators using open-source metocean data. This global-scale assessment provides a geographic overview of operability risks and first-order insights into the most relevant metocean risk indicators at each location. The results show that locations around the equator and inland seas have lower operability risk than locations farther away from the equator. “Hotspots” are mainly located along the southern capes (Cape of Good Hope, Leeuwin, Horn), around the ‘Roaring Forties’, and at exposed locations along the oceans. Of the metocean parameters considered, short waves are found to be the most critical risk indicator for port operability at most locations. Using (the insights of) this study, port authorities, operators, and designers can prepare for metocean risks at an early stage and effectively respond with mitigation measures and layout adjustments to improve port operability.</jats:p

Methodology for the Design of LNG Terminals in a Nearshore Environment

Nowadays, more and more nearshore LNG terminals are being built as it offers easy access to vessels coming from deep water and mitigates the risk by isolating regasification units from the cities. However, designing these terminals can be challenging in shallow water, as it is exposed to low-frequency waves which can excite the moored vessels at their natural periods. By lack of knowledge and adequate numerical simulation techniques, the effect of these low-frequency waves on the motions of moored vessels are unfortunately often ignored in the design. This is likely to result in an underestimation of the vessel motions and terminal downtime. In this paper, a methodology for the design of terminals in a nearshore wave climate is presented. The methodology consists of six steps which guide the engineer from the definition of the deep-water sea states to the calculation of the vessel motions and terminal downtime. In an initial stage, computational efficient tools are used, with the limitation that several approximations need to be made. In a later stage, more detailed but expensive methods are applied. The objective of this paper is to show how the developed methodology can give insight in the expected downtime due to the low-frequency waves in any nearshore mooring location. As an example, the methodology is applied on a fictive but realistic case, for which the motion response of a LNG carrier moored to a jetty on a sloping bottom is calculated. From seven years of deep-water sea states, the terminal downtime is estimated. The application of the methodology to the design case confirms that the terminal downtime can be significantly underestimated if shallow water effects are not taken into account. So the influence of the water depth, bathymetry, wave directionality and low-frequency waves on the vessel motions should be investigated with care. However, the results obtained in the design case also show that the spectral shape of the low-frequency waves predicted by the wave models are sensitive to the tuning of numerical parameters. Tuning the wave models against model tests or full scale data is therefore highly recommended, because the motion response of a low-damped moored vessel can be dominated by the amount of low-frequency free wave energy at its natural periods.</jats:p

Field Measurements of Flow Velocities in Propeller Jets

AbstractPropellers of ships generate high velocities adjacent to quay walls, jetties and locks. Generally, a bottom protection is installed in order to prevent instability due to scour. Although design guidance exist, propeller-induced loads are far from fully understood and have predominantly been derived on the basis of model tests. The validation of the existing design methods is lacking, especially for specific types of bow thrusters. In this research, field measurements of flow velocities induced by a 4-channel bow thruster system against a vertical quay wall have been performed. Test results showed a flow characterized by low mean velocities and large fluctuations, with the extent of reflected flow limited to few meters from the quay wall and inflow beneath the suction points playing a role.</jats:p