Search

Along with the technology development of ocean resources, offshore platforms are gradually becoming larger and more complex. Recent development of the oil and gas field in the deep-water region often involves multiple floating platforms adjacent to each other to perform more complex functions for oil and gas production. This paper describes the investigation carried out on the dynamic responses of a two platforms system containing a Tension Leg Platform (TLP) and a tender assisted drilling (TAD) with a flexible connection between the two platforms. The mooring lines and tendons are taken into consideration in the coupled analysis of the multi-body platform's system. The motion responses and wave load characteristics on the two platforms in multi-body coupled model are investigated in the numerical simulation. Compared to the situation of the two platforms in isolation, it is revealed that the motion responses for the two platforms in coupled model are altered by the combined effects of the platforms' interaction and constrain by the connection. The interaction between platforms can increase both the first- and second-order wave force on platforms in the arrangement direction of two platforms. Quantitative analysis demonstrates their relative importance and thus provides much-needed guidance in practical design of the coupled system.

The deployment of offshore wind turbines has focused primarily on shallow seas (such as the North Sea, Chinese coastal waters, and the New England coast) using bottom fixed foundations. However, much of the world’s offshore wind resource lies in deeper waters where bottom-fixed foundations are not suitable, and floating platforms must be utilised. To date, the majority of floating concepts have been developed to support a single wind turbine. This leads to a high capital cost for each individual platform and consequently a high levelised cost of energy. The W2Power platform (developed by EnerOcean S.L, Spain) currently supports a pair of 6 MW wind turbines on outward-leaning towers. The design significantly reduces the cost per installed MW, increases the structure’s natural period, added mass, and radiation damping. The platform, patented in 2009, was the world’s first twin-turbine platform and the first to be demonstrated at sea (2019). This paper presents the hydrodynamics of a 1:40 scale model of the W2Power platform using the well-known OrcaFlex software. The analysis has been carried out under extreme and operational conditions, and the resulting hydrodynamic loads and motion response are presented. The mooring system was found to be sensitive to wave direction, particularly when propagating along the current direction. Furthermore, the results showed advantages in the hydrodynamic responses for the W2Power platform as an innovative floating system.

The operation and maintenance (O&M) of an offshore wind turbine consumes large percentage of cost in the whole life of the offshore wind project. In most maintenance strategies, the service operation vessels (SOV) are widely employed in the maintenance activities. So far, the SOVs are large vessels to satisfy the requirement of equipment and functions. Some regular inspections for the wind farm are carried out by remotely operated underwater vessels (ROV). The offshore support vessel can be smaller in regular inspections. An attempt of small service operation vessel with ROV operating regular inspections is discussed in present research. The single point mooring method between offshore wind turbine and offshore support vessel is applied in the case. The relative distance between the vessel and the tension in the connecting lines are discussed under different environmental conditions. The numerical results are validated by the full-scale measurements on the vessel. It is found that the tension in the connecting lines is highly determined by the wind and wave directions. The wind-wave misalignments are also considered in the present study. It is found that the wind-wave misalignment may lead the sharp increase of relative distance and tension in connecting lines between offshore wind turbine and support vessel.

The integration of wave energy devices and coastal structures may be an innovative and sustainable way to achieve energy production purposes with a secondary benefit of coastal protection, which can increase accessibility and reduce the costs of wave energy technology. In this paper, a 3-D theoretical model was developed to investigate the hydrodynamic efficiency and breakwater function of a periodic array of oscillating buoys embedded in a caisson breakwater. The generalized radiation problem was solved to derive generalized radiation force. The theoretical model was validated using Haskind's relation and energy flux conservation law. The influences of wave/geometrical parameters and PTO damping were revealed. In particular, hydrodynamic phenomenon of multiple orders reflected and transmitted propagating waves and their influence on wave power extraction and coastal protection was examined. Results show that a satisfactory hydrodynamic efficiency and coastal defense are realized simultaneously under oblique waves for this proposed system. A decline of hydrodynamic efficiency is found beyond a critical wavenumber, accompanied by the occurrence of the strong reflection phenomenon. The findings of this paper contribute towards the preliminary design of the hybrid breakwater-WEC system for the synergy effect between the wave energy devices and breakwaters.

Flow and heat transfer of a nanofluid through a horizontal microchannel in the presence of the magnetic field effects and electric double layer (EDL) is investigated theoretically. For a microchannel with a large aspect ratio, the flow problem is treated as a two-dimensional nonlinear system. The body force generated by the EDL and magnetic field is considered in momentum equation. In order to study the mechanism of nanofluid heat transfer, the nanoparticles distribution and the heat transfer process of nanofluid flow are represented by the Buongiorno’s nanofluid model with the passively controlled nanoparticles distribution at the boundary, which has not been considered in previous microchannel studies. Compared to the so-called active control of nanoparticle volume fraction at the boundary, the current approach makes the model physically more reliable by taking into account of the effect due to varying temperature. The analytical approximations obtained by the homotopy analysis method reveals that both the magnetic field effects and the EDL play significant roles on altering the flow and heat transfer in microchannels. It is also found that the heat enhancement is significantly depend on the Brinkman number and the temperature applied to the wall.

Oscillating water column (OWC) device has the potential application in the field of harbor agitation mitigation/coastal protection. In this paper, we present a theoretical study on the hydrodynamic performance of a novel coastal structure, which combines a conventional harbor/coast-based single-chamber OWC device and a surface-piercing perforated barrier. Based on the linear potential flow theory, a semi-analytical model for wave interaction with the novel coastal structure is developed. The influence of the geometrical/physical parameters on hydrodynamic characteristics of the novel coastal structure is revealed. The results show that, by implementing the perforated barrier, a remarkable reduction of wave reflection from the OWC device can be achieved, without the compromise of the hydrodynamic efficiency. The incident wave energy is absorbed by the OWC device in the low frequency region and is dissipated by the perforated wall in the high frequency region. Comparisons of the conventional OWC structure and the proposed structure indicate that the frequency bandwidth for effective wave attenuation is broadened. In addition, the location of the perforated wall is identified as a key parameter influencing the hydrodynamic performance of the novel coastal structure.

Fatigue failure is an important issue for offshore wind turbine support structures. Especially, fully coupled time domain analysis will lead to high computational burden. Therefore, accurate and efficient fatigue evaluation methods have always been the common goal pursued by the industry and researchers. This paper evaluates fatigue damage under combined wind and wave loading, using a fixed jacket offshore wind turbine as an example. A half coupling model (HCM) with distinct advantage by considering aerodynamic damping and structural damping is developed for dynamic response analysis of offshore wind turbine. The precision and effectiveness of the developed HCM is verified to be reasonable by comparing with numerical simulated results. A linearized Morison equation is proposed to achieve frequency domain analysis of wave response which is embedded as a programming module in ANSYS. Based on the developed model, dynamic response analysis is carried out under wind and wave loading, and then combined stress spectra at the location of different tubular joints (X-joint and K-joint) are obtained. To consider the interaction of wind and wave, a spectral fatigue combination rule based on Han and Ma's method is used to calculate fatigue damage. The developed combination rule is assessed by compared with time domain fatigue result based on rainflow cycle counting. Analysis using the newly developed model and method demonstrates a high accuracy and efficiency for fatigue damage prediction.

Surface elevation around multiple column offshore structure is an important phenomenon crucial to air gap design of offshore platforms. This paper investigates the competing hydrodynamic phenomena, i.e., wave run-up of surface elevation rising along the column and near-trapping – the increase of surface elevation due to near-resonance among the columns. Both wave run-up and near-trapping have the characteristics of generating surface elevation peak and often impact the offshore structures with nonlinear wave loads and potentially cause slamming to platforms. With the free-surface Keulegan-Carpenter number Kc

The bow shape is the most critical factor to determine the icebreaking performance of an icebreaker. Mechanism study on the icebreaking process for different bow types is necessary for the initial design of the icebreaker hull form. This paper proposed an ice-ship interaction model based on the meshfree method, Peridynamics, in which the geometric mathematics concept is embedded to detect the contact between material points and ship hull. Furthermore, a fast contact detection algorithm based on Massage Passing Interface (MPI) solver is built to improve the computational efficiency of the developed numerical method. Two typical icebreaker bows, the conventional bow and the unconventional bow, breaking the level ice with constant speed is numerically studied by the above model. The results of the conventional icebreaker bow are compared with the experimental results, which verifies the simulation accuracy of the model developed in the present work. Afterwards, the icebreaking modes and icebreaking loads of two different shapes of icebreaker bows are compared and analysed. The results show that the developed ice-ship interaction model effectively predicts differences of icebreaking processes between different icebreakers, such as ice damage pattern, ice loads, and channel, despite their common point in domain bending failure mode. Moreover, this research significantly improves computational efficiency and provides theoretical guidance for designing the icebreaker bow.

A ring-shaped spar-type Very Large Floating Structure (VLFS) is proposed as an intermediate base for supporting deepwater resource exploitation far away from the coast line. The proposed VLFS is composed of eight rigidly connected deep-draft spar-type modules and an inside harbor. A double-layered perforated-wall breakwater is vertically attached to the VLFS and pierces through the water surface to attenuate the wave energy in the inside harbor. The hydrodynamic performance characteristics of the ring-shaped VLFS was experimentally evaluated in the present study, focusing on the motion responses, wave elevations, and wave run-ups. The natural periods of the motions in vertical plane were determined to be larger than 40 s, which is much larger than common wave periods. This enhanced the motion performance in vertical plane and afforded favorable habitation and operation condition on the VLFS. A large surge damping was induced by the vertical breakwater, which tended to significantly affect the surge and pitch motions, but had a negligible effect on the heave motion. The component frequencies of the wave elevations in the inside harbor and the wave run-ups were identical with those of the incident waves. The wave attenuation was frequency-dependent and effective for the common wave frequencies, with a smaller loss coefficient observed in higher sea state. The wave attenuation and wave run-ups tended to improve in the absence of the leeward walls.

The existence of a moonpool may affect motion characteristics and stability of a drillship. This paper presents numerical and experimental study on the influence of the configuration of a moonpool on the motion performance and stability of a drillship under the condition of a constant displacement. Ship hydrostatics and three-dimensional (3D) potential flow theory are employed to calculate the static stability curves (GZ curves) and motion response amplitude operators (RAOs) of the drillship with conjoined double moonpools, separated double moonpools and single rectangular moonpools, respectively. Numerical model is validated by sea-keeping model tests of the drillship with a conjoined double moonpool carried out in towing tank. It is found that there is a critical heeling angle for the static stability curve of the drillship. The variation of the opening size of the moonpool is found to merely have significant effect on the heave RAOs of the drillship near the piston mode resonance period, while slightly influence on the pitch RAOs near the sloshing mode resonance period. It is demonstrated that smaller or larger openings of the moonpool under a constant displacement can reduce dramatically roll RAOs of the drillship around the natural roll period under bow quartering waves and beam waves, respectively. The coupling effect of roll and heave leading to the heave RAOs decreasing around the natural roll period in beam waves, as well as coupling effect in heave, pitch and roll motion RAOs around the natural roll period in bow quartering waves are observed in the experimental results. The results presented in this study can provide references to the design of the moonpool of a drillship.

The climate change may affect the long-term wave statistics and consequently affect the cumulative fatigue damage. This paper aims to project the trend of annual fatigue damage of offshore floating structures and to detect the climate change impact on the future fatigue damage by coupling a conventional fatigue design method with climate and wave models. Firstly, climate scenarios are selected to project the global radiative forcing level over decadal or century time scales. Secondly, climate models are used to simulate atmosphere circulations and to obtain the wind field data. Thirdly, wave conditions are simulated by coupling wind driven wave models to climate models. Fourthly, stress analysis and fatigue assessments are conducted to project the annual fatigue damage. At last, control simulations are carried out in order to identify the range of natural variability and to detect the human-induced change. A case study is presented in the Sable field offshore South Africa. The results indicate that the significant wave height is considerably influenced by the human-induced climate change. However, this change induced by human activities is still partially masked by the dominant natural variability. In addition, both the significant wave height and the annual fatigue damage increase over century time-scales.

An iterative time-marching scheme is developed to investigate the hydrodynamic interactions between multiple ships. Such an unsteady interactive effect could be magnified in restricted waterways, e.g., a channel or harbor area. To the author’s knowledge, nearly all the research on the ship-to-ship interaction neglecting the free surface effects. The free surface is usually treated as a rigid wall. This assumption is only reasonable when the speed of the ships is very low in deep water condition, due to the hydrodynamic interaction between the ships is mainly induced by near-field disturbances. However, when the moving speeds are moderately higher, especially with a small lateral separation between ships, the far-field effects arising from the ship waves become important. The main objective of the present paper is to develop an iterative time-matching algorithm to solve the hydrodynamic interaction between high-speed ships taking into account the nonlinear free surface boundary condition in time domain.

Offshore floating wind turbines (FWTs) in deep water experience cyclic loadings from both environment and mechanical operations. For FWTs, the upper turbine and tower are mainly subjected to wind loading; and the floater is subjected to wave forces. It has been widely accepted that there is a strong coupling between the floater motions and the turbine forces. As the tower is placed between the upper wind turbine and the floater, both wind and wave loadings affect the cyclic forces on the tower. The construction of towers makes use of prefabricated segments. These prefabricated segments are bolted together with flanges at either end. The paper aims to investigate the axial hotspot stress on FWT’s tower base and analyze its induced fatigue damage at the welding joints around the flanges. A coupled aero-hydro-servo-elastic analysis is conducted to simulate the motion of FWTs. Then, the local welding joint along the reverse-balanced flange connection is modeled to consider the influence of local geometry. At last, the hourly fatigue damages at four locations over the tower base section are compared.

The computational investigation on the ice cover fragmentation under extreme loadings, such as a shock wave, is still a poorly understood research topic. In this process, the influence of the temperature change on the crack evolution, and the damage of the ice cover can be significant since the temperature is one of the critical factors affecting ice behaviour. Thus, it is necessary to analyze the coupled case of mechanics and thermotics. Based on ordinary state-based Peridynamic theory, a fully coupled model of mechanics and thermodynamics is employed to study the failure process and crack evolution of the ice sheet subjected to explosive loading in this paper. The load of the explosion, which is defined by the empirical formula, is non-uniformly distributed on the bottom surface (underwater side) of the ice layer. By comparing the damage modes to the observation from the existing field test, the present Peridynamic model is rigorously validated. The exponential decay constant, which is a coefficient that determines the distribution of shock waves acting on the bottom of the ice layer, is numerically examined. Since the fracture toughness (FT) of the ice contributes to the failure criterion and has no clear value, it is discussed according to the ice damage with various FT values. Finally, ice fragmentation with thermal effects is analysed and compared to uncoupled cases, and the crack propagation paths influenced by the thermal field are further investigated.

Purpose The purpose of this paper is to investigate the immiscible two-layer heat fluid flows in the presence of the electric double layer (EDL) and magnetic field. The effects of EDL, magnetic field and the viscous dissipative term on fluid velocity and temperature, as well as the important physical quantities, are examined and discussed. Design/methodology/approach The upper and lower regions in a horizontal microchannel with one layer being filled with a nanofluid and the other with a viscous Newtonian fluid. The nanofluid flow in the lower layer is described by the Buongiorno’s nanofluid model with passively controlled model at the boundaries. An appropriate set of non-dimensional quantities are used to simplify the nonlinear systems. The resulting coupled nonlinear equations are solved by using homotopy analysis method. Findings The present work demonstrates that increasing the EDL thickness and Hartmann number can restrain the fluid flow. The Brinkmann number has a significant role in the enhancement of heat transfer. It is also identified that the influence of EDL effects on microflow cannot be ignored. Originality/value The effects of viscous dissipation involved in the heat transfer process and the body force because of the EDL and the magnetic field are considered in the thermal energy and momentum equations for both regions. The detailed derivation procedure of the analytical solution for electrostatic potential is provided. The analytical solutions can lead to improved understanding of the complex microfluidic systems.

Sea-keeping model tests of ships based on transient waves have been widely applied over the past several decades. In order to obtain response amplitude operators (RAOs) of a ship, most of the post-processing of the experimental data uses the fast Fourier transform (FFT) to obtain the wave spectrum and the corresponding response spectrum. However, for transient waves related model tests, FFT may produce larger errors due to its characteristics. Hilbert-Huang transform (HHT) is a newly developed signal analysis tool which is suitable for nonlinear and non-stationary data. The application of HHT to the post-processing of the experimental data of sea-keeping model tests of ships has not yet been investigated. In this study, the transient wave packets satisfying a Gaussian wave spectrum were generated in a large towing tank to conduct the sea-keeping model tests of a drilling ship under the condition of head waves, oblique waves and beam waves, respectively. Then the marginal Hilbert spectrum (MHS) in the framework of HHT is introduced to obtain the motion and the acceleration RAOs the drilling ship. In order to demonstrate the effectiveness of the approach, the results based on FFT and regular waves are also presented. It is found that in most cases, in comparison to that by means of FFT, the RAOs of the ship based on the transient Gaussian wave packets by means of MHS agree better with the results based on regular waves, especially for roll motion with significant nonlinear characteristics. Due to the advantages of HHT, the MHS approach employed in this study is expected to play a vital role in more sea-keeping related model tests of ships.

Along with the development in arctic region, the icebreaking technologies are gradually becoming the focus. As one of the most powerful and effective way to breaking ice, especially in the ability to solve ice jams, the study of the behaviour of the sea and river ice under dynamic loads is an urgent subject of scientific research and it attracts extensive attention. In addition, the temperature change in the process of ice failure cannot be neglected since that temperature plays an important role in the mechanical properties of the ice. In this study, a fully coupled thermoelastic ordinary state-based Peridynamic model is employed to investigate fragmentation of ice cover subjected to an underwater explosion. Both the deformation effect on the thermal effects and the thermal effects on deformation are taken into consideration. The pressure shocks generated by the underwater explosion are applied to the bottom surface of the ice cover for non-uniform load distributions. Crack propagation paths are investigated, the damage is predicted and compared with existing experimental results. The corresponding temperature distributions are also examined. Furthermore, the ice failure mode in both the top surface and the bottom surface of the ice sheet is investigated.

Plunging breakers could induce one of the most devastating forces on an offshore structure. As compared to non-breaking wave, a plunging breaker would induce an additional slamming load on an offshore structure. Current industry guidelines for design breaking wave slamming coefficient do not take into account of the geometric cross section of the offshore structures and the breaking wave parameters. The slamming load is affected by a few factors; the capability of the structure to entrap air upon the onset of plunging breakers, breaking wave parameter – rise time, just to name a few. This research paper attempts to expand the correlation of the slamming load coefficient to the geometric cross section of the section and the rise time of the impact breaking wave. It was found out that the geometrical cross section of cylinders do have significant impact on the slamming coefficient. The rise time of the plunging breaker does have an impact on the slamming load itself, but the impact the rise time had on the slamming coefficient is indistinctive.

Vortex flow around multiple columns of finite length is ubiquitous in engineering. The present work focuses on the basic fluid physics in terms of the vortex shedding flow patterns and their dependence on structural configurations and flow parameters. Though widely documented in the literature, there is no consensus on certain aspects of the wake characteristics immediately behind the obstacles for a multi-column structure at a relative high Reynolds number range. A comprehensive set of numerical simulations has been conducted to investigate the flow interactions with four square section shaped columns in a diamond configuration, which is complimented by experiments using particle image velocimetry and force measurements in a physical model with Reynolds numbers varying from 3.7 × 10 4 to 6.0 × 10 4 . Horizontal structural members called pontoons were added near the end of the columns to alter the interactions with the surrounding fluid. This work reveals further insights of the fluid physics including the interactions of the vortex shedding processes due to the multi-columns and pontoons. The pontoons are seen blocking the vortices shed from the free end of the column by pushing the recirculation region further away from the free end of each column. In addition to the vortex shedding period being increased, further examination of the wake region indicates that the vortex street tends to be tidier and more structured by adding the pontoons to a basic multi-column structure. The findings will lead to better understanding in vortex shedding fluid physics and improved design in new offshore structure development such as deep-draft semi-submersibles and tension leg platforms.

A comprehensive numerical study on flow-induced motions (FIMs) of a deep-draft semi-submersible, a typical multiple cylindrical structure in offshore engineering, was carried out to investigate the energy transformation of the vortex shedding process. In addition, the corner shape effect on the flow characteristics, the hydrodynamic forces, and the FIM responses are presented for a multiple cylindrical structure with various corner shapes (sharp, rounded, and chamfered) under 45° current incidence. Different energy transformations, hydrodynamic characteristics, and FIM responses were observed due to the slight variation of the corner shape. The galloping at 45° incidence for a square-section shape column was observed when the corner shape modified as a chamfered corner. A “re-attached vortex shedding” phenomenon is discovered when the “lock-in” happened for a chamfered corner design. Further insights of the fluid physics into the flow characteristics due to the difference of the corner shape are revealed. In addition, the energy transformation and the mechanism for reducing the hydrodynamic forces and the FIM responses are analyzed.

For the successful deployment of large scale tidal turbine arrays occupying a large part of tidal channels, understanding the effects of wake interaction in densely spaced arrays is of importance. A comprehensive set of experiments has been conducted with scaled tidal turbines to investigate the resulting wake characteristics in a number of different staggered array configurations with up to four turbines on a designated support frame. Wake velocity deficits and turbulence intensities at a number of locations within and downstream of the array are presented and in addition the flow field recordings from Particle Image Velocimetry (PIV) measurements are presented for visual investigation of the resulting wake field and wake characteristics along the array centre line. The experiments show that lateral and longitudinal spacing variations of the individual devices vary the resulting flow field downstream of the array section significantly. Lateral spacing can be optimised to result in beneficial flow effects that accelerate the downstream wake recovery. Very close spacing however leads to significantly reduced velocity recovery. Longitudinal spacing shows less significant influence, especially for configurations with wide lateral distances. Differences in wake velocity deficit of up to 10% have been identified and suggest array wake recovery in and downstream of staggered sections, in areas of lower ambient turbulence levels, to be more significantly influenced by the lateral spacing especially towards the front rows of the array. With every additional array section the increasing turbulence intensity within the array is anticipated to reduce this effect.

The behaviour of microchannel flow of a nanofluid between two parallel flat plates in the presence of the electrical double layer (EDL) is investigated in this paper. The problem is formulated based on the Buongiorno nanofluid model with the electrical body force due to the EDL being considered in the momentum equation. As one of the highlights of the present investigation, the unphysical assumption introduced in previous studies often leading to the discontinuities of flow field that the electrostatic potential in the middle of the channel has to be equal to zero is eliminated. In addition, the inappropriate assumption that the pressure constant is treated as a known condition is also rectified. The new model is developed with the governing equations being reduced by a set of dimensionless quantities to a set of coupled ordinary differential equations. Based on the analytical approximations, the influences of various physical parameters on the flow field and temperature field, and the important physical quantities of practical interests are analysed and discussed in detail.

The air flow may take effects on the responses of the damaged ship in the dynamic flooding process. It not only relates to the amount of inflow but also the stability of the ship. In order to accurately predict the responses of a damaged ship, it is essential to take the air into account. In this study, a multi-phase SPH model combined with a dummy boundary method is proposed. One of the advantages of the new SPH model in solving this nonlinear problem is that, it does not rely on other algorithms to track the interface of different phases but can easily deal with breaking, splashing and mixing. The stability and accuracy of the numerical model are verified by comparing with experimental and published numerical results. The air captured in the flooding process is further studied with focus on the exchange of air and water near the opening. Finally, the effects of the sizes and number of the deck openings on the air flow are analyzed. It is found that the air flow can reduce the kinematic energy of inflow water, leading to decreases in the dynamic moment formed by the flooding water and sinking rate of damaged cabin.

The flow field characteristics in the wake of an isolated tidal turbine and tidal turbine arrays of up to four devices is investigated by numerical simulations solving the Reynolds Averaged Navier Stokes (RANS) equation in the Open Source OpenFOAM CFD solver to make use of the significantly increased availability of computational resources and multi-core processing. Transient simulations utilizing the k – ω SST turbulence closure model were used in combination with a dynamic mesh interface to account for the rotation of the three bladed tidal turbine at constant tip speed ratio, the applicability of dynamic mesh simulations for the investigation of array wakes has been shown. The velocity and turbulence characteristics are compared to experiments previously conducted with a number of small scale tidal turbine devices arranged in staggered array formations and tested within a low ambient turbulence circulating water channel. Results showed good agreement between simulations and experiments and further insight to the flow field within an array is provided, however further improvements to predict the wake characteristics in array are required.

Numerical benchmark studies on drag and lift coefficients of a marine riser have been organized by the 27th ITTC Ocean Engineering Committee. The purpose of the studies was to benchmark the capabilities of CFD methods through quantitative comparisons and validation studies against the model test results of a circular cylinder by MARIN. Studies were focused on the drag crisis phenomenon for the stationary smooth cylinder in the critical Reynolds number regime. Eight organizations have participated in the studies by using RANS, DES and LES methods. An overview of the model test results, test cases, submissions and comparison results are presented in this paper. Conclusions and recommendations are made for future studies.

The vortex-induced motion (VIM) is a critical issue in mooring and riser system design for column-type deepwater platforms. As regards to deep-draft semi-submersibles (DDS), even though VIM is mainly excited by vortex shedding around columns, the large-volume pontoons beneath the columns are also responsible for the wake interference, implying a non-negligible influence on VIM behavior. An experimental study and three-dimensional numerical simulations were performed to analyze the pontoon effect on the VIM of two semi-submersibles and a four-column structure without pontoons. The numerical results using Detached Eddy Simulation (DES) are in good agreement with the experimental measurements obtained from the towing model tests. The present investigations indicate that the resonant phenomenon is observed for all configurations. However, the four-column structure without pontoons shows the most significant transverse responses and yaw motions at both 0°- and 45°-incidences owing to the largest fluctuating lift forces induced by the well-established wake. Additionally, the negative values of work done by the pontoons at all reduced velocities confirm their damping effect on the VIM response.

The design of fixed and compliant offshore platforms requires the reliable estimation of extreme values with small probabilities of exceedance based on an appropriate probability distribution. The Weibull distribution is commonly utilised for the statistical analysis of wave crests, including near-field wave run-ups. The parameters are estimated empirically from experimental or onsite measurements. In this paper, the data set of wave crests from a Spar model test was statistically analysed by using the method of LH-moments for parameter estimation of the Weibull distribution. The root-mean-square errors (RMSEs) and the error of LH-kurtosis were used to examine the goodness-of-fit. The results for the first four LH-moments, the estimated parameters, and the probability distributions showed that the level of the LH-moments has a significant influence. At higher levels, the estimation results gave a more focused representation of the upper part of the wave crest distributions, which indicates consistency with the intention of the method of LH-moments. The low tail RMSE values of less than 2.5% demonstrated that a Weibull distribution model estimated by using high-level LH-moments can accurately represent the probability distribution of large extreme wave crests for incident waves, wave run-ups, and moon pool waves. Goodness-of-fit test on the basis of comparison of sampling LH-kurtosis and theoretical LH-kurtosis was recommended as a procedure for selecting an optimum level.

Energy dissipation mechanisms of bubble pulsation in compressible fluids have always been a significant aspect of research into bubble dynamics. In this paper, bubble dynamics in compressible fluids are investigated numerically with the Eulerian finite-element method (EFEM), and the energy dissipation due to the wave effects of the compressible surrounding fluid is analyzed. The present model is validated by comparing with experimental results. Results from both the simulation and experiment show that bubble fragmentation also contributes to the energy dissipation, which has seldom been discussed before. It is also shown that the initial discontinuity is significant to the energy dissipation which is non-trivial to simulate in 1-dimensional bubble dynamics equations like the Gilmore equation. Then, the relationship between dissipated energy and bubble maximum radii in adjacent pulsating cycles is formulated to quantitatively evaluate the energy dissipation during a pulsating cycle. At last, based on the linearized theory of the energy conservation of the bubble system, a new non-dimensional parameter M a is modified from the Mach number to represent the energy dissipation due to wave effects. With simulation and discussion on cases with different initial pressure and sound speed, it is found that the dissipated energy is related linearly to M a , which can be used to predict the energy dissipation of a new case.

The spatial localized influence of wind on wave induced load on a flexible cylinder has been assessed throughout a test series conducted in a wave-wind-current flume at Newcastle University. The tests are motivated from other experimental and numerical investigations showing air flow separation on the leeward side of steep waves that can lead to added wind energy transfer, which could suggest an increase in the impulsive wave loading. The waves are generated as focused waves, resulting in a plunging breaker, leading to an impulsive wave load. The test model was equipped with a load cell measuring the connection load. Due to the flexibility of the cylinder, the measured force response shows oscillations and dynamic amplification of the load. The maxima of the force responses are compared for the tests with and without wind. Another measure for comparison is the local and short-lived impulse, which is responsible for the amplification. This impulsive load is estimated from the load cell and acceleration measurements. For the tests in this study, the introduction of wind over the breaking waves does for some cases lead to a slight increase in the peak of the impulsive load and thereby the load response, although large scattering is present. Further investigations are needed to verify this effect. Some differences in the time series of the free surface elevation are observed when wind is present, but the maximum of the surface elevation does not change notably, and the slope is only minimally changed, meaning that this should not give basis for the differences in the loads.

Understanding the dynamics of oscillating bubbles beneath a free surface is crucial to many practical applications including airgun-bubble clusters, underwater explosions, etc. In this paper, an experimental and numerical study of the dynamic behaviors of a coalesced bubble near a free surface is conducted, which shows quite different physical features from single bubble dynamics. Firstly, two similar sized underwater discharge bubbles are generated simultaneously beneath a free surface and their complex interactions are experimentally studied with high-speed photography imaging. A strong interaction between two bubbles and the subsequent coalescence are observed when the initial distance between two bubbles is smaller than the maximum equivalent bubble radius. Secondly, both axisymmetric and three-dimensional (3D) boundary integral models are used to simulate the pre-coalescence and post-coalescence of two bubbles. The results obtained by the two models agree well in axisymmetric conditions. The essential physical phenomena in representative experiments are well reproduced by the present 3D model. The pressure field is calculated by the auxiliary function method, which helps to reveal the underlying mechanisms of bubble collapse patterns and jetting behaviors. A parametric study reveals the dependence of the coalesced bubble dynamics and free surface motion on the governing dimensionless quantities.

A simple mathematical model is developed based on the single-degree-of-freedom analogy and principle of conservation of energy evaluating various modes of Vortex-Induced-Vibration (VIV) of a jack-up with cylindrical legs in regular waves. Similar to uniform current, mass ratio, damping ratio and mode factor are found to be the important parameters controlling the cross-flow VIV and radius of gyration also for the yaw VIV. Criteria for the initiation of the mentioned VIV modes are developed for the cases of a single 2D cylinder experiencing planar oscillatory flow, four rigidly coupled 2D cylinders in rectangular configuration experiencing planar oscillatory flow and jack-up experiencing regular waves. The newly developed VIV model is validated by a set of experiments conducted in a wind, wave and current flume. The importance of mass damping parameter is further demonstrated in suppressing VIV in regular waves. The mathematical method will equip engineers to consider the effect of VIV due to regular waves in jack-up designs.

Leading-edge tubercles have been investigating widely on the performance of foils in the last decade. In this study, the biomimetic tubercle design has been applied to the corner shape on a deep-draft semi-submersible. A numerical study on flow over a deep-draft semi-submersible (DDS) with a biomimetic tubercle corner shape was carried out to investigate the corner shape effects on the overall hydrodynamics and motion responses. The hydrodynamic performance of the biomimetic tubercle corner is compared with a traditional round corner design platform. It is demonstrated that, as the corner shape design changed, the motion responses alter drastically. In addition, the flow patterns were examined to reveal some insights into fluid physics due to the biomimetic tubercle corner design. The comprehensive numerical results showed that the three-dimensional effect, which causes spanwise flow, can be reduced by a continuous spanwise (column-wise) variation of the shear-layer separation points.

Plunging breakers, unlike non-breaking waves, impose additional slamming load on the offshore structures. This additional slamming load is considered an extreme event and is one of the most devastating forces that an offshore structure could encounter during its operational lifecycle. Whilst there are design guidelines for offshore structures pertaining to breaking waves, however it is limited to only cylindrical shape. The amount of slamming load contribution by the plunging jet is also dependent on the cross section geometries of the offshore structures. Different geometries would give rise to different air entrainment phenomenon during wave breaking and therefore affecting the slamming load contributions. In this research, JONSWAP spectrum was used to create plunging breakers via the focusing method at Newcastle University’s Wind Wave and Current tank. The crux of this research is to investigate the wave-breaking impact load on cylindrical structures with different cross section geometries commonly used in the offshore industry.

In this study, numerical analysis has been carried out to investigate the hydrodynamic interactions of two multi-columns platforms. The objective of the work is to preliminarily evaluate the feasibility of a tension-leg-platform (TLP) dry tree unit (DTU) with tender assisted drilling (TAD) from the aspect of vortex-induced motions, characterized by the current. Two multi-columns floating platforms with a small gap ratio (28% of the overall platform width) are numerically simulated with the scenario of 3 degree-of-freedom (DOF) on the horizontal plane (including transverse, in-line and yaw motions). A comprehensive numerical simulation was conducted to examine the hydrodynamics interactions due to the flow over two floating platforms. Horizontal plane motions including transverse, in-line and yaw motions as well as drag and lift forces on both structures are discussed. The numerical data on the multi-body VIM interactions within the “lock-in” region will serve as a preliminary study for future coupled motions analysis of the TLP-TAD system design.

Along with the development in offshore technology, the offshore platforms are gradually becoming larger and more complex. Recent development of oil and gas field in the deepwater region often involves multiple floating platforms adjacent to each other. Wave free surface associated with the air-gap design is one of the most important issues as the interaction between the platforms can complicate the hydrodynamics further. In this paper, the nonlinearity of incident wave and scattered wave are considered in diffraction analysis based on the potential theory. In addition, the nonlinear incident wave is considered to capture the nonlinear features of free surface due to wave diffraction and radiation. The wave surface amplitude around a multi-body platforms system is numerically analyzed in the frequency domain and compared with the numerical results of a single platform. The distribution of wave surface amplitude with different scatter parameter at different wave steepness is investigated to examine the relationship between the two parameters critical to the nonlinear wave surface elevation.