Abstract A semi-submersible drilling unit model was tested to estimate horizontal wave impact loads on vertical side of deckbox following the procedure recommended by DNVGL OTG-14. The present model test data show that there is clear difference in the relationships between upwell and horizontal wave impact pressure between near column/pontoon and around centerline. Near column and pontoon, not only is the maximum pressure much lower but the pressure increases more smoothly to its maximum value, compared to those of centerline. CFD simulations with focusing breaking waves have been made to examine the effect of wave-body interaction on horizontal wave impact on deck-box. The present CFD simulation results clearly show that the flows in front of column are strongly accelerated in vertical direction by blocking effect of column and pontoon, eventually producing strong run-up jets. The run-up jets in the present study are so strong that the direct impact of the incoming breaker on the wall does not occur, which leads to much smaller peak pressures, compared to those of centerline.

Vortex-induced motion (VIM) poses a serious challenge in the design and operation of deep-draft offshore platforms subjected to ocean current flow. The objective of the present work is to investigate the suppression of VIM by blowing jet flows at the wake side of a floating platform. To begin with, we first perform a systematic investigation of 2D four-column configuration with and without steady jet flows at Re = 200 and m * = 1. The numerical investigations are carried out for various jet flow velocity ratios ( V jet /U ) ranging from 0 to 1 at reduced velocity U r = 5. From the 2D studies, we observe approximately 30% reduction of forces and displacements for the system with steady jet flow compared to the system without jet flow. We also investigate the effect of angle of blowing jet flow (θ) on the effectiveness of VIM suppression. The idea of jet implementation is extended to 3D model of single square column. The effects of jet positions at the back surface of the single square column are discussed. From the investigations, it is observed that the jet flows at center portion is more effective than the jets at the corners of the column. Based upon the 2D and 3D studies of canonical configurations, the VIV suppression technique by jet flows is employed in a 3D deep-draft floating platform at Re = 20,000.

Many offshore constructions and operations involve water impact problems such as water slamming onto a structure or free fall of objects with subsequent water entry and emergence. Wave slamming on semi-submersibles, vertical members of jacket structures, crane operation of a diving bell and dropping of free fall lifeboats are some notable examples. The slamming and water entry problems lead to large instantaneous impact pressures on the structure, accompanied with complex free surface deformations. These need to be studied in detail in order to obtain a better understanding of the fluid physics involved and develop safe and economical design. In the special case of free-fall lifeboats, model testing can be expensive and time consuming. Here, numerical modelling can make useful contributions to the design process. The slamming of a free falling body into water involves several complex hydrodynamic features after its free-fall such as water entry, submergence into water and resurfacing. The water entry and submergence lead to formation of water jets and air cavities in the water resulting in large impact forces on the object. In order to evaluate the forces and hydrodynamics involved, the numerical model should be able to account for the complex free surface features, the instantaneous pressure changes around the lifeboat and accurately evaluate the loads on the lifeboat. As a step towards simulating free-fall lifeboats, water entry of a free-falling wedge into water is studied in this paper using a CFD model. The vertical velocity of the wedge during the process of free fall and water impact are calculated for different cases and the free surface deformations are captured in detail. Numerical results are compared with experimental data and a good agreement is seen. The open-source CFD model REEF3D is used in this study. The model solves the Reynolds-Averaged Navier-Stokes equations to evaluate the fluid flow. The convective terms are discretized using a 5th-order conservative finite difference WENO scheme. Time discretization is carried out using a 3rd-order Runge-Kutta scheme. Pressure discretization is carried out using Chorins projection method. The Poisson pressure equation is solved using a pre-conditioned BiCGStab algorithm. A sharp representation of the free surface is obtained using the level set method. The falling wedge is represented using the level set paradigm as well, avoiding the need for re-meshing during the simulation. Turbulence modeling is carried out using the k-ω model. Computational performance of the numerical model is improved by parallel processing using the MPI library.

The use of the Lattice Boltzmann Method (LBM) for fluid-flow simulation has been the subject of several recent studies where it was reported that the method offers many advantages such as high accuracy coupled with computational efficiency. In this paper, we report on the use of this method, in conjunction with Large-Eddy Simulations, to study an interesting phenomenon related to the suppression of vortex shedding from circular cylinders. Specifically, it has been observed in experiments that vortex shedding from a cylinder can be drastically reduced by the injection of a fluid jet into the approach flow. We first present results for a cylinder without jet injection in order to quantify the suitability of the LBM for use in such flows. Thereafter, we present results for the case with jet injection where we consider the case where Re = 55,440. Preliminary results conclusively demonstrate that the presence of jet injection does indeed lead to substantial reduction in the magnitude of lift forces on the cylinder. This strongly argues in favor of further research to understand the dynamics of this phenomenon, and the range of parameters needed to maximize its beneficial effects.

This paper presents a CFD simulation to study the ventilation phenomenon of tunnel thrusters in dynamic positioning (DP) mode of a typical North Sea shuttle tanker consisting of two main propellers, two rudders, and two bow tunnel thrusters. Measurements of blade thrust and moment for the propeller shaft (corresponding to propeller torque) with ventilated propellers at different submersion positions are presented and discussed. Additionally, in order to obtain insight into the effect of waves on ventilation which further affects propeller loading and dynamic fluctuations, simulations of a tunnel thruster are performed at different immersion ratios. This paper also presents and discusses the factors in the evaluation of thruster performance, such as, the extent of present knowledge for tunnel thrusters as related to ahead ship speed, and interaction between thruster jet flow and the mainstream with various drift angles. Moreover, thrust degradation of tunnel thrusters is considered in the thrust allocation algorithms for the DP capability calculation. The objective of this study is to understand the dynamics of thruster forces. In addition, results of the study provide knowledge for a robust thrust allocation algorithm for dynamic positioning capability assessment.

The consequence analysis is used to define the extent and nature of effects caused by undesired events being of great help when quantifying the damage caused by such events. For the case of leaking of flammable and/or toxic materials, effects are analyzed for explosions, fires and toxicity. Specific models are used to analyze the spills or jets of gas or liquids, gas dispersions, explosions and fires. The central step in the analysis of consequences in such cases is to determine the concentration of the vapor cloud of hazardous substances released into the atmosphere, in space and time. With the computational advances, CFD tools are being used to simulate short and medium scale gas dispersion events, especially in scenarios where there is a complex geometry. However, the accuracy of the simulation strongly depends on diverse simulation parameters, being of particular importance the grid resolution. This study investigates the effects of the computational grid size on the prediction of a cloud dispersion considering both the accuracy and the computational cost. Experimental data is compared with the predicted values obtained by means of CFD simulation, exploring and discussing the influence of the grid size on cloud concentration the predicted values. This study contributes to optimize CFD simulation settings concerning grid definition when applied to analyses of consequences in environments with complex geometry.

Sea-keeping characteristics of a wave-adaptive modular catamaran, developed by Marine Advanced Research Inc., is studied using at-sea experiments and dynamic modeling. The vehicle is based on the Proteus design and is supported by two inflatable pontoons with each pontoon attached to a center payload tray using an independent suspension system connected in such a way that the demi-hulls perform somewhat independent motions through a wave field. The suspension system is designed to isolate the center payload tray from the motions induced by waves incident on the demi-hulls. The vehicle is propelled by two water jets and is capable of achieving speeds of 8–15 knots. Accelerometers have been mounted on the pontoons and the center payload tray of the vehicle and field tests in the open ocean have been carried out to study sea-keeping properties of the vehicle. Heave and surge motions for the cases of following seas, beam seas and head seas have been recorded. In each case the center payload tray is found to experience attenuated heave accelerations compared with the two pontoon hulls. The reductions in surge and sway motion are, however, not evident. The associated spectra suggest that heaving motion of the center payload tray over a range of frequencies is attenuated by the suspension system. No attenuation is apparent in the surge and sway motions. The WAM-V dynamics is modeled as a three-body problem with the pontoons corresponding to bodies 1 and 2, and the central tray to body 3. Results of the analysis are compared with the sea trials.

Large reserves of natural gas exist worldwide, particularly in areas in which there is no market or where the resources exceed the demand; this natural gas is liquefied for shipping to areas where there is a demand. As the liquefied natural gas is a flammable substance, a leakage in this process may cause undesired events like fires and explosions. The consequence analysis is used to define the extent and nature of effects caused by undesired events and thus to quantify the damage caused by such events. Specific models are used to analyze the spills or jets of gas and liquid, gas dispersion, explosions and fires. The central step in the analysis of consequences is to determine the concentration of the vapor cloud of hazardous substances released into the atmosphere, in space and time. Gaussian and integral tools are extensively used in risk analysis, mainly to develop analysis about gas flow over flat terrain, providing fast dispersion estimations. Recently, with the computational advances, computational fluid dynamics (CFD) tools are used to short and medium range gas dispersion scenarios. However the advantages and disadvantages of each approach according with the scenario evaluated have not been widely discussed. This paper evaluates, using a CFD model, the cloud dispersion of a LNG leakage. Then the results are compared with the results previously obtained by UDM (Unified Dispersion Model) and some advantages and disadvantages of each model are discussed. This study contributes to the decision making about the choice of most appropriated model to evaluate the consequences analysis.

Typically, study of hydrodynamic interaction between vessels navigating in close proximity to each other is limited to hydrodynamics of bare hulls. Meanwhile, ship propulsors, especially heavily loaded, which may happen in accelerating motion, can alter substantially the flow and distribution of pressure on the hulls which can be viewed as generalization of the thrust deduction phenomenon. The 3D doubled body potential interaction code based on the source panel method developed earlier by the authors has been enhanced to include the effect of a propeller on each of the interacting ships under the assumption that the propeller jets (slipstreams) are not involved into the interaction. Each propeller is simulated by a disk of sinks further approximated with a polygon composed of identical triangular panels with identical constant sink density linked to the thrust of the propulsor according to the actuator disk theory. Comparative computations were carried out for two identical tanker vessels in the close-proximity overtaking manoeuvre at various values of the loading coefficient of each propeller. The loading coefficient is not supposed to be necessarily defined by the steady propulsion point. Numerical results demonstrate that a heavily loaded propeller substantially modifies the pressure distribution on both hulls resulting in alteration of the hydrodynamic interaction loads, especially of the surge force and yaw moment.

Squid and jellyfish generate propulsive forces by successively taking in and expelling high momentum jets of water. This method of propulsion offers several advantages to underwater vehicles/robots. The driving mechanism can be placed internal to the vehicle, reducing the drag associated with an abundance of external thrusters and control surfaces. The thrusters can generate accurate predictable forcing in the low thrust range, while still generating thrust nearly instantaneously over the entire force range. Vortex ring formation dynamics play an important role in creating thrust. It is observed that squid and jellyfish eject fluid jets which are not exactly parallel, and have a contracting velocity in the radial direction. A prototype thruster was developed which generates both parallel and converging propulsive jets. The total impulse of the jet is determined from DPIV techniques to determine the effect a non-zero radial velocity had on thrust production. The radial velocity was observed to increase the total impulse of the jet by 70% for low stroke ratio jets, and 75% for large stroke ratio jets.

The Brazil Current (CB) flows southwestward as vertically stratified and organized western boundary jet in the Brazilian shelf-break region ranging from 20–40° S, where the CB’s mass transport grows vertically. This geographical band show intense mesoescale activity due to passageway of eddies and meanders, superimposed over oceanic large-scale recirculation features which influence the oceanic circulation in the Santos Bight Pre-salt cluster area 300 km offshore. Based on 2-year observed data series of an oceanographic mooring array at Lula Field, and based on repeated hydrographic data (seawater temperature, salinity and N 2 profiles) we used statistical and dynamical orthogonal modes in order to approach the local vertical current profile variability. We verified that it is 85 % explained by EOF-1. This variability is essentially of 1 st baroclinic mode. Great part of it occupies the first 400–600 m water depth, with no predominant direction. We also found remarkable water column seasonal stratification. Albeit of relative weaker mean flows (0.1–0.2 m s −1 ), the study area is eddy dominated which are geostrophically adjusted to the 1 st baroclinic mode. Furthermore, we observed that the significant directional variability over the São Paulo Plateau occurs far away from the mean current jets that flow parallel to the continental shelf-break geometry.

Wetdeck slamming can be defined as a large vertical acceleration event that occurs when ship motions cause an impact between the cross deck and the ocean’s surface. The use of Computational Fluid Dynamics (CFD) and other simulation tools to accurately predict wetdeck slamming loads and ship motions has become the objective of a number of efforts (Hess, et al, 2007; Lin, et al, 2007; Faller et al, 2008; for example). The Sea Fighter , FSF-1, is a high-speed research vessel developed by the U.S. Office of Naval Research (ONR). Christened in 2005, she is an aluminum catamaran propelled by four steerable water jets capable of speeds up to 50 knots. In 2006, Sea Fighter underwent a series of rough water trials to assess its operational profile in high sea states (Fu, et. al., 2007). Along with this assessment, ONR sponsored an effort to obtain full-scale qualitative and quantitative wave slamming and ship motion data. One of these rough water trials took place 18–20 April 2006 as the ship transited from Esquimalt, British Columbia, Canada to San Diego, California, USA. During this trial, the significant wave height ranged from 1.5 to 2.7 m and the ship speed ranged from 20 to 40 knots. This paper describes the results of the effort to characterize the Sea Fighter’s motion in waves. To provide suitable full-scale validation data, the incoming ambient waves had to be characterized. A Light Detecting and Ranging, (LiDAR) system, an array of ultrasonic distance sensors, and several video cameras were used to characterize the incoming wave field. In addition, three fiber optic gyro motion units were deployed to record ship motions. Additionally, a GPS unit was utilized to measure ship speed, pitch, roll, and heading. Several slam and near slam events are discussed over the range of ship’s speed, heading, and sea states tested. Similarities and differences between these events are also noted. Additionally, this data was used to develop a simulation of the Sea Fighter ’s motion in waves similar to previous work done utilizing model test data (Hess, et al, 2007; Faller et al, 2008).

This paper provides an overview of the development of abrasive-slurry-jet methods for completion and stimulation applications. Abrasive particles added to the fluid dramatically reduced the system pressure requirements. The paper discusses the technical capabilities of cutting through various materials and formations and also discusses improvements and proven applications. Abrasive fluid-jet systems are capable of cutting through rocks of all types, and with greater location control that is not susceptible to the geologically induced deviations encountered with mechanical methods. Abrasive fluid-jets drill rock through the erosion induced by very small particles which individually remove only small fragments but are in such numbers that the drilling rate is at or above that of conventional tools. The particles are powered by the velocity of the supporting fluid, generated in turn by pumps on the surface. The cutting occurs ahead of the nozzle body allowing the nozzle assembly to be fed in the tunnel to create drain-holes. Fluid-jet methods have the potential of improving completion and stimulation efficiency in heterogeneous formations such as fractured and/or vuggy carbonate reservoirs. Application for completion, stimulation and, even, well/platform abandonment have been successful.

The hydro-elasticity effect of sloshing loads in LNG cargo tank has been studied through experiments and numerical simulations regarding the fluid-structure interaction between sloshing impact pressures and tank structures. Sloshing model tests with 1/50 scale membrane type tanks were carried out for 1-D regular harmonic motion to investigate variations of impact pressures due to elasticity differences of the tank structure. Numerical simulations were performed and validated for the same case. Additionally, wall impinging jet flow was simulated by numerical simulation to verify the relation between elasticity of structure and impact pressure. It was commonly observed that the elasticity of the tank structure had significant influence on the height and shape of the impact pressure peak. Numerical study showed that the ratio between the structural natural period and the duration of the impact pressure is important for the influence of impact pressure on the tank structure.

For oilfield developments in deep water, it is of utmost importance to accurately predict vortex induced vibrations (VIV) of risers and free span pipelines subjected to ocean current. In order to prolong operational life of such structures, VIV suppression devices such as helical strakes or shrouds are often employed. Such devices will, however, imply certain disadvantages such as drag amplification and increased operational costs. Therefore the quest for effective suppression devices with a minimum of such drawbacks is still ongoing. The present paper presents a novel approach for VIV suppression based on radial water jets from a prescribed pattern of circular openings in the cylinder wall. Jet flow will introduce a disturbance that will change VIV amplitudes. The alternation of the flow pattern must be understood to have both 2-dimensional (2-D) and 3-D effects. 2-D effects will influence the local pressure on the cylinder surface by altering the separation point as well as creating a general disturbance to the flow, while the 3-D effects involve changes in correlation of the vortex shedding process along the span of the cylinder. The disturbance to the flow pattern is expected to yield reduced VIV amplitudes. Results will be presented from experiments in a towing tank testing three 2m long spring supported cylinders with diameter 0.1m and two different patterns of radial water jets. The volume flow rate and reduced velocity have been varied in the tests. Oscillation amplitudes, frequencies, added mass and lift and drag force coefficients are presented and compared to a smooth cylinder.

Deep-water oilfield developments demand accurate predictions of vortex induced vibrations (VIV) of risers and free span pipelines subjected to ocean current. In order to prolong operational life of such structures, VIV suppression devices such as helical strakes or shrouds are often employed. Such devices will, however, imply certain disadvantages such as drag amplification and increased operational costs. Therefore the quest for effective suppression devices with a minimum of such drawbacks is still ongoing. The present paper presents a novel approach for VIV suppression based on radial water jets from prescribed patterns of circular openings in the cylinder wall. Jet flow will introduce a disturbance that will change VIV amplitudes. The alternation of the flow pattern must be understood to have both 2-dimensional (2-D) and 3-D effects. 2-D effects will influence the local pressure on the cylinder surface by altering the separation point as well as creating a general disturbance to the flow, while the 3-D effects involve changes in correlation of the vortex shedding process along the span of the cylinder. Results will be presented from experiments in a towing tank testing four 2m long spring supported cylinders with diameter 0.1m and three different patterns of radial water jets. One cylinder has three straight rows of holes at angular positions 0° and ±120° with respect to the ambient flow. The second has two straight rows of holes at positions ±120° with respect to the ambient flow, while the last has one straight line of holes blowing directly upstream. Since the first can be said to consist of the two latter, comparing them to each other gives valuable information in order to understand the physics of the first. The volume flow rate and reduced velocity have been varied in the tests. Oscillation amplitudes, frequencies, added mass and drag force coefficients are presented and compared to a smooth cylinder.