Wave Drift Research Articles

Prediction of the mean heading of a turret moored FPSO in bi-modal and bi-directional sea states

Ship-shaped platforms like Floating Production Storage and Offloading (FPSO) vessels are deployed in many offshore locations where the sea states are dominated by wind driven local seas combined with long period swells that are incident from different directions. Many of these vessels are held in station by a mooring system that is connected to the platform by a rotating turret. The heading stability of the vessel and its ability to self-align with respect to oncoming environmental forces is studied in this paper. Experiments at 1:120 scale conducted in a wave basin facility for two storm conditions offshore Brazil and West of Africa show that the heading angle is self-limiting to a range of ±20° with respect to head sea direction. Time domain simulations conducted using industry-standard software by modeling the wind-sea and swell with suitable spectrum models approaching from different directions show almost double the heading range seen in experiments. If similar numerical analyses were part of a design process, it is conceivable that this could result in incorrect predictions of the weathervaning of the platform, and hence the latter’s global motions in response to design storm conditions.Previous studies by the authors have confirmed that the delicate balance between various mean wave drift forces and moments is the main contributor to the heading stability of a single-point moored ship shaped vessel. It is shown here that in a bi-directional seastate, the interaction between the two wave components can significantly alter the drift loads on the platform. A simple cross-wave correction is added to the time domain simulations to account for the interaction. Implementing this correction brings the numerical results more in line with the experimental findings. The cross-wave correction term is quantified and recommendations are made for better simulations and experiments of such platforms.

Ocean wave characteristics prediction and its load estimation on marine structures: A transfer learning approach

Statistical and stochastic approaches, commonly used to estimate wave induced loads in the marine and coastal structural analysis and design of offshore vessels and platform, require accurate sea state or wave characteristics, which are usually estimated using computationally intensive numerical wave models. While advances in machine learning helps to overcome the computational challenges of these approaches, they still need measurement of historic data at the region of interest to help prospective estimation of wave loads. This calls for deployment of sensors and allied resources for continuous monitoring and collection of streaming data. Transfer learning, on the other hand, provides a framework to derive representations from an existing region to estimate the characteristics in the region of interest. Deep Belief Networks are excellent candidates for latent representation of the relationship between the different sea state characteristics and the induced loads. In this paper, we first train a DBN based wave characteristic prediction model to predict the sea state characteristics (namely, significant wave height Hs, dominant or peak wave period Tp and average or zero crossing period Tz) using data from 12 stations at 3 geologically different regions (the Gulf of Mexico, the Korean Region and the UK region), using data between the period Jan 1, 2011 and Dec 31, 2014. We validate the model using data in these regions between Jan 1, 2015 and Aug 30, 2015. We then compute the effect of the wave characteristics, namely, the wave-drift force and moment on 12 different marine structures with both regular and irregular waves. Finally, we transfer the DBN representations to predict the sea state characteristics in Irish region for the same period. Evaluations against ground truth and comparisons with state-of-the-art methods show the superior transfer learning performance of the DBN and accurate computations of the wave drift force and moments.

Wind generation of waves: Energy and momentum transfer - An overview with physical discussion

The study considers the generation of surface water waves by wind through pressure and shear stress acting on the surface. The situations of temporal growth of an infinitely long uniform wave train and of down-wind growth of a stationary non-uniform wave field are treated. The mechanisms of energy and momentum transfer between the wind and the waves are considered and it is explained how the ‘wave Reynolds stress’ and the wave set-down are contributing to a force balance over the water column. The role of the wind forcing for the wave drift and a mean current is shown. The analogy between energy dissipation by wave breaking and wave generation by a harmonic surface shear stress is discussed together with the transfer of momentum to the ocean currents by breaking waves.

파랑 표류력을 고려한 선박의 파랑 중 선회성능 해석

This paper performs a numerical computation of ship maneuvering performance in waves. For this purpose, modular-type model (MMG (Mathematical Modeling Group) model) is adopted for maneuvering simulation and wave drift force is included in the equation of maneuvering motion. In order to compute wave drift force, two different seakeeping programs are used: AdFLOW based on Wave Green function method and SWAN based on Rankine panel method. When wave drift force is calculated using SWAN program, not only ship forward speed but also ship lateral speed are considered. By doing this, effects of lateral speed on wave drift force and maneuvering performance in waves are confirmed. The developed method is validated by comparing turning test results in regular waves with existing experimental data. Sensitivities of wave drift force on maneuvering performance are, also, checked.

Yaw motion analysis of a FPSO turret mooring system under wave drift forces

In order to evaluate the dynamic behavior of a FPSO-turret system, experimental tests in engineering wave basin have been performed. During the realization of experimental test large slow yaw rotations were detected in the dynamic behavior of the vessel during regular and irregular wave conditions tests. In order to study this phenomenon, an analytical method was developed in order to predict the maximum yaw angle reached under the slow yaw varying oscillations of a single point moored FPSO-turret system dependent of the regular mean wave drift forces and the turret position. The results for the proposed analytical method were verified by comparing it with experimental and simulated results. Then, the proposed method allows to identify the critical regular wave periods than can provoke large yaw rotations as an alternative for the stability tests by bifurcation analysis. On the other hand, the performed yaw motion simulations take into account the following aspects: the turret position, and the force and momentum associated with the mooring system variations (in function of the FPSO center of gravity positions and regular mean wave drift forces). Analyzing the time domain simulations it is possible to identify that large yaw motion is reached faster when the turret is near the CG and so, as the turret moves away, the yaw motions become slower.

Hydrodynamic Heat Transport Regime in Bismuth: A Theoretical Viewpoint.

Bismuth is one of the rare materials in which second sound has been experimentally observed. Our exact calculations of thermal transport with the Boltzmann equation predict the occurrence of this Poiseuille phonon flow between ≈1.5 and ≈3.5 K, in a sample size of 3.86 and 9.06mm, consistent with the experimental observations. Hydrodynamic heat flow characteristics are given for any temperature: heat wave propagation length, drift velocity, and Knudsen number. We discuss a gedanken experiment allowing us to assess the presence of a hydrodynamic regime in any bulk material.

Experimental study on hydrodynamic characteristics and operational feasibility of floating LNG bunkering terminal moored side-by-side with LNG Carrier and two LNG bunkering shuttles

ABSTRACTIn this study, hydrodynamic characteristics and operational feasibility of a floating liquefied natural gas (LNG) bunkering terminal moored side-by-side with an LNG carrier and two LNG bunkering shuttles were investigated, during the LNG transferring process. Experiments consisted of two configurations, namely the soft spring moorings individually and the side-by-side mooring system. Motion response amplitude operators (RAOs) and wave drift force RAOs in regular waves were measured from the former configuration while the latter configuration was used to obtain global motion responses, structural loads acting on ship-to-ship mooring lines and relative displacements between LNG loading arms and their manifolds. The gap flow effect was considered by conducting comparisons of the numerical and experimental results with various radiation body boundary damping values. The results provided insights into the structural reliability of side-by-side mooring lines and the applicability of a conventional rigid LNG loading arm.

New approaches and mathematical models for environmental risk management in seaports

Abstract Seaports and surroundings areas environmental conditions are depending on meteorological and oceanographic conditions that affect their infrastructure and also their operations. The increase of marine traffic in the Black Sea Basin has raised issues concerning the environmental quality of coast areas and the sea water. Therefore, accurate and timely predictions of extreme weather and sea state are important for efficient port management in terms of safety, while they are also significant for the sustainable development and environmental quality of seaports and their adjacent coastal areas. The aim of this paper is to present an innovative, fully automated marine monitoring system for the Black Sea Basin, which is able to provide real time monitoring and analysing historical data of wind, wave, storm-surge and oil spill drift. The system is based on advanced numerical models and ICT tools for simulation and visualization of essential information, including early warnings for marine extreme phenomena and hazards. Within ECOPORTIL project new ICT tools will be developed to provide online training and capacity building of stakeholders and decision makers in order to improve their knowledge on legislation issues and adopt good environmental practices.

A quasi-cloaking phenomenon to reduce the wave drift force on an array of adjacent floating bodies

The mooring of offshore floating structures, such as offshore platforms, in large waves against drift forces and rotational moments is a challenging problem in offshore engineering. To accurately investigate such problems, called positioning problems, the time-averaged steady forces of the second order known as the wave drift forces must be taken into account. Fortunately, a cloaking phenomenon occurs under certain conditions and dramatically reduces the wave drift force acting on such a floating body, as previously reported by several researchers. In the diffraction problem of water waves, cloaking refers to the condition where there is no scattering in the form of radial outgoing waves. The reduction of wave drift force on a truncated cylinder with the occurrence of cloaking phenomenon has been numerically and experimentally confirmed. In this paper, the arrangement of several small circular cylinders at regular intervals in a circle concentric with a fixed floating body is considered as an effective means of reducing the wave drift force. Using a combination of a higher-order boundary element method (HOBEM) and wave interaction theory, the influences of the geometric parameters of the outer surrounding cylinders on the wave drift force and the total scattered-wave energy are systematically investigated and discussed. A quasi-cloaking phenomenon is first found and reported in the present study, which is beneficial and flexible for application in practical engineering. More than one quasi-cloaking trigger (where a trigger is an occurrence condition) can be found simply by varying the distance between the inner and outer floating bodies.

Measurement Calculation System “Solid-state Resonator Gyroscope”

В статье идет речь об измерительном приборе - твердотельном волновом гироскопе. Работа прибора основана на эффекте инертности стоячих волн. Твердотельный волновой гироскоп рассматривается с двух сторон: как самостоятельный измерительно-вычислительный комплекс и как датчик некоторой инерциальной системы. Структура твердотельного волнового гироскопа представлена в виде взаимосвязанных элементов: резонатор, измерительная компонента, управляющая компонента, вычислительная компонента. Анализируется структура прибора, рассматриваются его подсистемы и их связи. Исследуются различные типы конструкций резонатора, вопросы математического моделирования резонатора. Рассматриваются явления, ухудшающие точность прибора, такие как дрейф волны, нелинейность и шум выходного сигнала. Анализируются причины появления дрейфа. Одна из основных причина дрейфа - конструкционное демпфирование. Изучаются вопросы моделирования резонатора с учетом конструкционного демпфирования. Описываются различные типы датчиков и систем съема на их основе. Рассматривается измерительная компонента как система для наблюдения эффекта Брайана. Обосновывается необходимость формирование сигналов в системе съема для дополнительного наблюдения деформации резонатора в месте крепления. Рассматриваются вопросы управления динамикой резонатора. Приводятся различные контуры управления, которые позволяют получить необходимые характеристики прибора. Описываются точностные параметры. Анализируются зависимости точностных параметров твердотельного волнового гироскопа от различных факторов. Описываются направления повышения точности твердотельного волнового гироскопа и систем на его основе. Ставится общая задача повышения точности прибора за счет учета конструкционного демпфирования в модели резонатора, измерения величин, характеризующих конструкционное демпфирование, и решения задачи редукции ТВГ к идеальному измерительному прибору.

損傷船舶の曳航に関する研究 : その3 各種船型の損傷・折損時の運動と波漂流力

損傷船舶の曳航に関する研究 : その3 各種船型の損傷・折損時の運動と波漂流力

Effect of a modified sinusoidal forcing on spiral wave in a simulated reaction-diffusion system.

Spiral waves are often found in excitable media. In the hearts, they are abnormal forms of action potential propagation. Under an external forcing, the spiral waves drift and are subsequently terminated at the boundary. Spiral waves can be studied in simulations using a discrete reaction-diffusion system; thereby the time step must not exceed a numerical stability limit (ts). In this article, we present the dynamics of spiral waves in a simulated system under an external forcing as a modified sinusoidal function of time. The spiral waves are forced to drift along a straight line with a velocity and an angle depending on the time step. An optimization study provides the optimal time step of 0.2ts, where further reductions of the time step do not alter the drifting of the spiral waves.

A theory for spiral wave drift induced by ac and polarized electric fields in chemical excitable media

Spiral waves are shown to undergo directional drifts in the presence of ac and polarized electric fields when their frequencies are twice of the spiral frequencies. Here, we propose a quantitative description for the spiral wave drift induced by weak electric fields, and provide the explicit equations for the spiral wave drift speed and direction. Numerical simulations are performed to demonstrate the quantitative agreement with analytical results in both weakly and highly excitable media.

Wave-free characteristic of heave motion response for new sandglass-type model

ABSTRACTThis paper mainly studies the wave-free characteristic of heave motion of a sandglass-type floating body to overcome the limitations of traditional ship-type and cylindrical floating, production, storage and offloading units. In the first instance, the theoretical and numerical methods are used to determine the added mass and damping wherein the heave motion can be decoupled. The existence of wave-free characteristic of heave motion is theoretically and numerically verified. Moreover, the heaving quadratic transfer function of second-order wave drift force is calculated and its performance has been discussed. Finally, the relationship between wave-free frequency and wave spectrum is selected as the design principle of the shape parameters. On this basis, some experiments of new sandglass-type and cylindrical models are carried out, which coincide with the numerical predictions very well and show that the new design based on wave-free frequency can significantly improve the heave motion performance.

Passive Optical Sensing of the Near-Surface Wind-Driven Current Profile

AbstractEstimation of near-surface current is essential to the estimation of upper-ocean material transport. Wind forcing and wave motions are dominant in the near-surface layer [within O(0.01) m of the surface], where the highly sheared flows can differ greatly from those at depth. This study presents a new method for remotely measuring the directional wind and wave drift current profile near to the surface (between 0.01 and 0.001 m for the laboratory and between 0.1 and 0.001 m for the field). This work follows the spectral analysis of high spatial (0.002 m) and temporal resolution (60 Hz) wave slope images, allowing for the evaluation of near-surface current characteristics without having to rely on instruments that may disturb the flow. Observations gathered in the 15 m × 1 m × 1 m wind-wave flume at the University of Miami’s Surge-Structure-Atmosphere Interaction (SUSTAIN) facility show that currents retrieved via this method agree well with the drift velocity of camera-tracked dye. Application of this method to data collected in the mouth of the Columbia River (MCR) indicates the presence of a near-surface current component that departs considerably from the tidal flow and may be steered by the wind stress. These observations demonstrate that wind speed–based parameterizations alone may not be sufficient to estimate wind drift and to hold implications for the way in which surface material (e.g., debris or spilled oil) transport is estimated when atmospheric stress is of relatively high magnitude or is steered off the mean wind direction.

Ocean Wave Model and Wave Drift Caused by the Asymmetry of Crest and Trough

It follows from the review on classical wave models that the asymmetry of crest and trough is the direct cause for wave drift. Based on this, a new model of Lagrangian form is constructed. Relative to the Gerstner model, its improvement is reflected in the horizontal motion which includes an explicit drift term. On the one hand, the depth-decay factor for the new drift accords well with that of the particle’s horizontal velocity. It is more rational than that of Stokes drift. On the other hand, the new formula needs no Taylor expansion as for Stokes drift and is applicable for the waves with big slopes. In addition, the new formula can also yield a more rational magnitude for the surface drift than that of Stokes.

Analysis of tether anchored floating suspension bridge subjected to extreme environmental loads

The Norwegian public road administration wants to replace eight ferry crossings on the main road of western Norway with fixed straight crossings. Many of these crossings are too long, and too deep for conventional bridges, requiring novel technologies and new methods. One of the challenges these bridges face is environmental loading.This paper studies the response of a tether anchored floating three span suspension bridge over Bjørnafjorden subjected to environmental loads with a return period of 100 years. The crossing has a length of over 4000 m and depths in excess of 500 m. With eigenperiods spanning from 100 s and downwards, the bridge is susceptible to severe dynamic amplification from many types of environmental loads. The two largest load contributions are due to wind and waves. Wind loads are included with a stochastic wind field over the middle span of the bridge. This includes both temporally - and spatially varying wind speeds in all three directions. Second order slowly varying wave drift forces on the floaters are also taken into account.The analysis is performed by time domain dynamic simulation using the computer program USFOS. The bridge girder, pylons cables and hangers are modelled with beam elements. The key response quantities studied and reported in the paper are horizontal displacement of both floaters and the midspan, accelerations of the floaters, both at the bridge girder and at the top of the superstructures and at the midspan, and the tension force both in the main cables and in the tethers. The simulations are based on the contour line method, which is commonly adopted by the offshore industry. Thirty one-hour time domain simulations were conducted to estimate the 100-year response of the bridge to environmental loads.

A review of iceberg and bergy bit hydrodynamic interaction with offshore structures

This paper establishes the current state of the art in the subject of iceberg and bergy bit motion during interaction and impact with offshore structures. Our particular interest in the present case is the influence of hydrodynamic interaction on change in velocity as small ice masses approach a larger structure in open water. Iceberg impacts are a design driver for structures located in offshore regions subject to drifting glacial ice. It is found that although many studies note that near-field effects influence the velocity and trajectory of drifting bergs prior to impact, the understanding of near-field hydrodynamic effects such as negative wave drift force, fluid cushioning, shadowing, reduction in impact velocity, and hydrodynamic damping, is incomplete. These effects are more obvious for small ice masses and bergy bits and although they have been identified qualitatively, there is very little quantitative information available. The inability to properly account for these phenomena generally leads to overestimation in impact velocity and consequently input impact energy. Better understanding of hydrodynamic interaction between small icebergs and offshore structures in very close proximity could improve load predictions by improving the estimation of input energy.

CONNECTION BETWEEN THE SHOCK WAVE SPEED AND II TYPE RADIO BURSTS DRIFT VELOCITY

The substantial arguments of strong connection between shock wave speed and drift velocity of II type radio bursts in 25-180 MHz range are presented. The studied sample has included 112 proton events that were accompanied with coronal shock waves. To evaluate drift velocity and shock wave speed there was used original records of dynamic spectra from radio spectrograph in 25- 180 MHz range. The velocities of shock waves were evaluated with the power mode model of solar corona density falloff.

Determination of ion mobility collision cross sections for unresolved isomeric mixtures using tandem mass spectrometry and chemometric deconvolution

Ion mobility (IM) is an important analytical technique for determining ion collision cross section (CCS) values in the gas-phase and gaining insight into molecular structures and conformations. However, limited instrument resolving powers for IM may restrict adequate characterization of conformationally similar ions, such as structural isomers, and reduce the accuracy of IM-based CCS calculations. Recently, we introduced an automated technique for extracting “pure” IM and collision-induced dissociation (CID) mass spectra of IM overlapping species using chemometric deconvolution of post-IM/CID mass spectrometry (MS) data [J. Am. Soc. Mass Spectrom., 2014, 25, 1810–1819]. Here we extend those capabilities to demonstrate how extracted IM profiles can be used to calculate accurate CCS values of peptide isomer ions which are not fully resolved by IM. We show that CCS values obtained from deconvoluted IM spectra match with CCS values measured from the individually analyzed corresponding peptides on uniform field IM instrumentation. We introduce an approach that utilizes experimentally determined IM arrival time (AT) “shift factors” to compensate for ion acceleration variations during post-IM/CID and significantly improve the accuracy of the calculated CCS values. Also, we discuss details of this IM deconvolution approach and compare empirical CCS values from traveling wave (TW)IM-MS and drift tube (DT)IM-MS with theoretically calculated CCS values using the projected superposition approximation (PSA). For example, experimentally measured deconvoluted TWIM-MS mean CCS values for doubly-protonated RYGGFM, RMFGYG, MFRYGG, and FRMYGG peptide isomers were 288.8 Å2, 295.1 Å2, 296.8 Å2, and 300.1 Å2; all four of these CCS values were within 1.5% of independently measured DTIM-MS values.