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A great deal of work has been developed on the spar and monocolumn vortex-induced motion (VIM) issue. However, there are very few published works concerning VIM of semi-submersible platforms, partly due to the fact that VIM studies for this type of platform recently became interesting particularly due to the increasing semi-submersible dimensions (columns diameter and height. In this context, a meticulous experimental study on VIM for this type of platform concept is presented here. Model test experiments were performed to check the influence of many factors on VIM, such as different headings and hull appendages. The results comply with in-line, cross-flow and yaw motion amplitudes, as well as with combined motions in the XY plane.Copyright © 2011 by ASME

The objective of the present paper is to carry out numerical simulations on the coupled transient processes of oil leakage, water flooding and study of the stability in a damaged crude oil carrier. For this purpose, numerical approach based on Moving Particle Semi-Implicit (MPS) method is applied to model the complex fluid-solid interaction problem with free surface and oil-water multiphase flow. Changes on the modeling of the towing tank utilized in a previous study on oil leakage carried by the author are done to reduce the undesirable effects of the wave reflection. As a consequence, the improved results of the transient behaviors of hull motions are obtained for the cases with oil leak, as well as the final list angle and volume inside the tank in cases of water flooding. The results of the simulations show that the volume of flooded water is inversely proportional to the filling ratio. Also, the height of the opening has not significant effect on the final list and flooded volume.

Vortex-Induced Motion (VIM) is a highly non-linear dynamic phenomenon. Usual spectral analysis methods, using the Fourier transform, rely on the hypotheses of linear and stationary dynamics. A method to treat non-stationary signals that emerge from non-linear systems is denoted Hilbert-Huang transform method (HHT). The development of an analysis methodology to study the VIM of a MPSO (Monocolumn Production, Storage and Offloading System) using HHT was presented. The purposes of the analysis methodology are to improve the statistics characteristics of VIM. The results showed to be comparable to results obtained from the traditional analysis (mean of the 10% highest peaks) principally for the motions in the transverse direction, although the difference between the results from the traditional analysis for the motions in the in-line direction showed a difference of around 25%. The results from the HHT analysis are more reliable than the traditional ones, owing to the larger number of points to calculate the statistics characteristics. These results should be used to design the risers and mooring lines, as well as to obtain parameters of the VIM to calibrate numerical predictions.Copyright © 2010 by ASME

A parametric analysis which investigates the effects of the hull main dimensions on the seakeeping comfort was proposed to improve a standard SWATH vessel. This study was numerically carried out involving some comfort parameters and worldwide sea conditions. Some types of SWATH and their non-dimensional ratios were considered. The main dimensions of a standard SWATH vessel are: length (L), beam (B), pontoons radius (R), draft (T) and strut thickness (t). Some secondary dimensions were assumed so that a parametric model of wet surface could be designed using MULTISURF, which later was used on WAMIT to evaluate the hydrodynamics coefficients. The TPN receives these coefficients as a basic tool to provide the dynamic response of each SWATH case. The parametric comfort analysis was performed using three usual methodologies applied to evaluate the ship behavior: The ISO2631/3 Severe Discomfort, the MSI (Motion Sickness Incidence) and the VI (Vomiting Incidence).Copyright © 2010 by ASME

In the present paper, a moving device to suppress sloshing is proposed and analyzed. The effectiveness of the suppressing device is evaluated by numerical simulations based on MPS (Moving particle semi-implicit) method. As the parameter of study, lateral forces on the tank walls are used. The results shows that the device reduces remarkably the lateral sloshing forces when filling ratio is low and it eliminates the occurrence of hydrodynamic impact on the tank ceiling when the filling ratio is high.Copyright © 2010 by ASME

This paper presents the experimental results obtained from the model tests performed at the IPT (State of Sa˜o Paulo Technological Research Institute) towing tank, located in Sa˜o Paulo, Brazil. The model used was the semi-submersible ITTC-SR192, at a scale 1:105. The main focus of this study is the analysis of the risers influence on the floating unit dynamics, taking into account damping effects caused by the lines. In order to establish basis of comparison the tests were performed with and without risers. These were modeled as a group of large diameter hoses that are able to provide a significant increment in the damping of the system. By such approach, it was possible to evaluate the changes on the dynamic behavior of the unit induced by the presence of risers. As an additional investigation tool, these results were analyzed with the aid of the software Numerical Offshore Tank (Tanque de Provas Nume´rico - TPN), which is a dynamic simulator of offshore systems based on parallel-processing developed by means of a partnership between the University of Sa˜o Paulo, Petrobras and other Brazilian research institutions. By means of the comparisons between the model tests results with and without risers and of the results from the numerical tool, TPN, the dynamic behavior of the floating unit was investigated in terms of the effects caused by the presence of risers, which is a very relevant subject as offshore industry walks towards ultra-depth waters.

The objective of the present paper is to carry out a study on coupled transient process of the oil leakage and the damaged stability of a crude oil carrier. For this purpose, numerical simulations based on MPS (Moving particle Semi-Implicit) method are carried out considering a two dimensional small scaled model and the oil-water multiphase flow with free surface. The results are compared with that obtained by the stability analysis software SSTAB, which provides the final list angle in case of the damage, and show good agreement.

This paper presents the new active absorption wave basin constructed at the University of Sao Paulo (USP), in the Numerical Offshore Tank (TPN) Laboratory. The square (14m × 14m) tank is able to generate and absorb waves from 0.5Hz to 2.0Hz, by means of 148 active flap-type wavemakers. An independent mechanical system drives each flap by means of a 1HP servo-motor and a ball-screw based transmission system. A customized ultrasonic wave probe is installed in each flap, and is responsible for the measurement of wave elevation in the flap. These sensors do not require constant calibration, differently from the capacitive or resistive sensors normally used in similar tanks. A complex automation architecture was implemented, with 3 Programmable Logic Computers (PLC), and a low-level software is responsible for all the interlocks and maintenance functions of the tank. Furthermore, all the control algorithms for the generation and absorption are implemented using higher level software (MATLAB®/Simulink block diagrams). These algorithms calculate the motions of the wavemakers both to generate and absorb the required wave field by taking into account the layout of the flaps and the limits of wave generation. The experimental transfer function that relates the flap motion to the generated wave is used for the calculation of the motion of each flap. Absorption tests were conducted with a prototype wave generator in a 2D wave flume with regular waves. Two different algorithms were tested. The first one is the frequency domain method based on Maeda et al. (2004), in which the commanded variable is the motor velocity. Furthermore, the time domain algorithm proposed by Schaffer (1996) was also tested. It is based on a digital filter and uses the position of the motor as the commanded variable. Both algorithms have hydrodynamic feedback based on the measurement of surface elevation at each flap. The first algorithm needs an extensive test procedure to calibrate its control parameters while the second one, after optimizing the digital filter, is ready to use. Both algorithms presented similar results with reflection coefficient smaller than 10.7% for regular waves in the frequency range of 0.5 to 2.0 Hz. The paper also presents the first results obtained in the tank.Copyright © 2010 by ASME

The damping evaluation of floating offshore systems is based on the viscous effects that are not considered in numerical models using the potential theory. Usually, different techniques for different systems are used to evaluate these hydrodynamic coefficients. The total damping is separated by potential and viscous damping, the first one is evaluated numerically and the second through experiments at reduced scale model. Common techniques considering linear motion equations cannot be applied to all degrees of freedom. Some methods were compared for results of decay test, such as: exponential and quadratic fit. Fourier transform (FT) spectral analysis and Hilbert Huang transform (HHT) can be used to evaluate the signal natural frequency and with HHT this can be done during the time domain. Also, analysis through the Random Decrement Technique (RDT) is presented to demonstrate the damping evaluation for irregular waves. The method to obtain external damping was presented for the different techniques in an ITTC semi-submersible model. The linear method is not sufficient to predict the damping coefficient for all the cases, because in most of them, the viscous damping was better represented by a quadratic fit. The HHT showed to be a good alternative to evaluate damping in non-linear systems.Copyright © 2010 by ASME

The motion of floating bodies linked elastically to the bottom of seas and waterways is of great interest in the analysis of the wave suppressing devices, such as wave breakers, and the behaviors of the floating structures, such as buoys and tension leg platforms (TLP). For the modeling of the dynamics, the coupling between the hydrodynamic loads due to waves and the restoring forces due to the elastic link must be considered. In some simpler cases, the analytical approaches are available. However, in case of large amplitude waves and floating bodies with complex geometries, the analytical solutions do not give accurate results. In the present study, a numerical model based on MPS (moving particle semi-implicit method) for the hydrodynamic loads coupled with the Hook’s Law for the restoring force is adopted to analyze the motion of floating bodies with one or several elastic links to the bottom of shallow water under large amplitude waves. Initially, the results of 2D numerical simulation of simple oscillating buoys are compared with the analytical and experimental ones to validate the numerical approach. After that, the approach is applied to the study of the shallow water wave supressing devices. Heave, surge and pitching motions of the floaters are assessed as well as the hydrodynamic coefficients to show the effect of the elastic links in the nonlinear wave hydrodynamics.Copyright © 2009 by ASME

Vortex-Induced Motions (VIM) of floating structures is a very relevant subject for the design of mooring and riser systems. In the design phase, Spar VIM behavior as well as Semi Submersible and Tension Leg Platform (TLP) flow-induced motions are studied and evaluated. This paper discusses flow-induced behavior on the Monocolumn concept by presenting a phenomenological model and comparing its results with a set of experiments that took place in the IPT Towing Tank - Brazil (September 2008). The experimental results have shown some fundamental differences from previous VIM tests on other units such as Spars. This numerical model attempts to identify these disparities in order to better understand the mechanics of this phenomenon. The model is based on a time-domain, two degree-of-freedom structural model coupled with a van der Pol type wake oscillator. The comparison was performed in order to calibrate the model, to study and better understand the tests results, and finally to identify important aspects to investigate in further experiments.Copyright © 2009 by ASME

This paper presents a new analysis and a comparison of results obtained from Vortex-Induced Motion (VIM) model tests of the MonoGoM platform, a floating unit designed for the Gulf of Mexico. The choice of scale between the model and the platform in which the tests took place was a very important issue that took into account the basin dimensions and mooring design. The tests were performed in three different basins: the IPT Towing Tank in Brazil (September 2005), the NMRI Model Ship Experimental Towing Tank in Japan (March 2007) and the NMRI Experimental Tank in Japan (June 2008. The objective of this work is to discuss the most relevant issues regarding the concept, execution and procedures to analyze comparatively the results obtained from model tests. The approach employed in the tests was designed to build a reliable data set for comparison with theoretical and numerical models for VIM prediction, especially that of Monocolumn platforms.

A great deal of works has been developed on the Spar VIM issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, the present paper presents a meticulous study on VIM for this type of platform concept. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are inline and cross-flow motion amplitudes, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction.Copyright © 2009 by ASME

The recent escalade of the oil prices encourages the search and exploration of new oil fields. This represents a challenge to engineers, due to more difficult conditions of operation in harsh environments and deeper reservoirs. The offshore industry faces, in the edge of technology with new necessities and limiting conditions imposed by the environment, an increase in the cost of production. It is, therefore, of vital importance to have the equipments operating at the most optimized conditions in order to reduce these costs. VIV software developed in the frequency domain was successful in designing risers and pipelines using large safety factors and making conservative assumptions. These tools only predict single-mode vibrations. In this perspective, the present paper describes the results obtained from a new time-domain code developed to assess the vortex-induced vibrations of a long flexible cylinder. A time-domain analysis was chosen because this suits the problem well, since it is able to predict and calculate different modes of vibrations. In the model, a cylinder is divided into elements that can be exposed to an arbitrary current profile. Each of these elements is free to oscillate parallel and transversely to the flow, and is coupled to a pair of van der Pol’s wake oscillators. This simulates the vortex shedding and, therefore, the fluctuating nature of drag and lift coefficient during the occurrence of VIV. The governing equations are solved by 4th-Order Runge-Kutta schemes in time domain. The new time-domain model is compared with small scale model test data from benchmarking.Copyright © 2009 by ASME

Although in most vessels the usual function of a moonpool is a passage for production lines, equipments and divers, the moonpool may be used as a device to reduce the vertical motion of floating structures as it can be observed in published studies. From these published works one may say that changes of the moonpool’s internal geometry modify the behavior of vertical oscillation of the water inside the moonpool. Looking for a “hydrodynamic design of the monocolumn-moonpool”, Petrobras has carried out a set of numerical and experimental tests with monocolumn hulls provided with moonpool. Some of these results have been published by the authors in the literature. To increase the comprehension of the dynamics of the joint motion, fluid and structure, numerical results were obtained using a commercial code based on the potential theory and a commercial code based on the solution of Navier-Stokes equations. The paper presents results of these experiments and analyzes the behavior of the coupled motions of mono-columns and the vertical motion of the water inside the moonpool, comparing the experimental and the numerical results.Copyright © 2008 by ASME

The aim of this paper is to present validation studies of a CFD code based on MPS (Moving Particle Semi-implicit Method). In MPS method the fluid is represented by particles, and the particle interactions are governed by continuity and Navier-Stokes equations. It is a meshless method to simulate incompressible flow and it is able to simulate large surface distortion, fluid fragmentation and non-linear dynamics. For the validation studies, two cases with complex hydrodynamic phenomena were selected for experimental measurements in towing tank. The first one is the dynamics of a floating body in waves with an internal tank partial filled with water. In this way sloshing effects on the motion of the model can be evaluated. Usually, dynamics of the floating body and sloshing are calculated separately, by neglecting their coupling effects; the body’s motion is determined without sloshing and that motion is used to excite the liquid tank. Since the sloshing generates forces and moments, which may change the movement of the hull, sloshing forces and moments may act as a roll absorption device or can enlarger it. In MPS this coupled phenomena can be easily simulated, just by using particles representing water of the internal tank, water of the towing tank and structural particles representing the hull, the walls and the wave maker. The second phenomenon is the motions of a damaged hull from the moment soon after suffering damage until reaches the equilibrium position. This is an initial step of a validation study of the motion of a damage hull in waves, which will be compared with physical experiments. The comparisons between the numerical results obtained by the MPS with the experimental and theoretical ones show very good agreement, reinforcing the potential of MPS.Copyright © 2008 by ASME

This paper introduces a new concept and a preliminary study for a floating unit aimed at exploring and producing oil in ultra deep waters. This platform, which combines two relevant features — great oil storage capacity and dry tree production capability — comprises two bodies with relatively independent heave motions between them. A parametric model is used to define the main design characteristics of the floating units. A set of design alternatives is generated using this procedure. These solutions are evaluated in terms of stability requirements and dynamic response. A mathematical model is developed to estimate the first order heave and pitch motions of the platform. Experimental tests are carried out in order to calibrate this model. The response of each body alone is estimated numerically using WAMIT® code. The paper also includes a preliminary study of the platform mooring system.Copyright © 2008 by ASME

Oil discovery in ultra-deep-waters is carrying out the ocean engineering to develop new conceptions of offshore exploration and production systems. A promising alternative is a monocolumn platform with moonpool. Concept design has shown its good stability make feasible the use as offshore solution. In order to solve the problem brings by increase on water deep, the use of steel catenary risers and wellhead dry completion has been shown a good solution because it decreases the maintenance cost. However, this solution needs a very low vertical motion. In general, platform like the monocolumn present resonance on vertical motion (15–20 sec to heave motion, 20–25 sec to roll/pitch motion) close to typical wave energy. This fact is so that designers introduce mechanisms to leave the natural periods out of wave energy. The moonpool as passive absorber motion was a solution found by designers. The water inside moonpool acts out-of-phase with platform motion. This difference on phases can attenuate the platform dynamic response. Other solution is water-line-area (AWL) reduction that reduced the restoring forces and consequently increased the value of vertical motions natural periods. These two alternatives showed very efficient to improve the monocolumn performance. This paper presents the gain, in terms of motions, achieved as the appendages introduction on the platform. These appendages consist of two plates, one in form of a disk and other around it. It can be located on the bottom at platform and inserted inside at the moonpool. Model test conduced on the scaled platform shows that heave and pitch natural period increased approximately 4 seconds when this appendages is allocated on the bottom. Practically any change is noted when it is allocated only inside of the moonpool.Copyright © 2008 by ASME

An important path to reviewing technology is through failure investigation, which enables understanding of appropriate levels of safety and assurance of design criteria. In the occurrence of extreme environmental conditions in which most units respond well or at least satisfactorily, the assessment of exception cases can be taken as a good starting point for the failure investigation mentioned above. Hence, especially on account of hurricanes Rita and Katrina, the 2005 hurricane season, which had a notable impact on offshore industry at Gulf of Mexico region, can be taken as a promising scenario to assess the losses and bring out cases to be more carefully appraised. It is the case of the TLP Typhoon found upside down after the passage of Rita. Being the only unit located at a significant water depth which followed recent project standards, the case naturally came to the fore. This paper addresses an investigative analysis held out in order to establish and analyze the possible causes that resulted on the capsizing of the referred unit. The investigation focuses on two major paths which were selected as being the most probable and also had feasibility to be carried out — one that analyzes a possible lack of displacement due to the passage of a hurricane wave and the consequent loss of stability of the TLP and the other, concentrated on a dynamic analysis of the unit under the impact of the hurricane metocean conditions. This second analysis focuses on obtaining the tension on the tethers stating if either they suffered excessive loads causing them to break or lack of it, causing compression and possible buckling.Copyright © 2008 by ASME

This paper presents a study on the design and dynamic behavior of hose lines for offshore oil transfer. Moreover, a new configuration of floating hose line that minimizes the dynamic effect in the connections of the line of oil transfer to the Offshore Terminal and to the Shuttle Tanker is analyzed. This conception reduces the damages in the elements of the extremity due to the great accelerations and the small bending radius, which the structure of the hoses is submitted. Although the benefits of the present configuration are applied for abandoning and stowing conditions and a complete set of numerical analysis and model tests were performed, only the results for the offloading operation were presented.

Usually, the hydrodynamic loads due to sloshing are considered in the design of liquid cargo ship or floating units concerning the structural. Owing to the increasing size of these structures, resonant sloshing motions may occur and result in the amplification or attenuation of motion of the vessel. In order to assess the effect of sloshing, traditionally the motion of the vessel is calculated at first without considering the dynamic of the liquid inside the tank. After that, this motion of the vessel is inputted as excitation motion acting on the tank and, finally, the sloshing effect is evaluated. In the other words, the coupling effects of sloshing and sea wave in the vessel’s motion are ignored. A bibliographical survey shows that there are few studies that consider the effect of sloshing on the ship motion, acting as a passive device of absorption of the movements. The main goal of this research is to investigate experimentally the roll motion amplification and reduction due to sloshing. The coupling effects of sloshing and sea wave in the vessel’s motion are taking into account by recording the motions, in regular waves, of a free floating model with a partially filled liquid tank. For this purpose, a two-dimensional model is designed to carry out measurements with fixed cargo and partially filled liquid cargo. The experimental results are evaluated by comparing the measured motion of the free-floating model with fixed cargo against the results obtained by traditional approach. Then, the effects of sloshing on floating units are shown by comparison of the measurements from free-floating model with fixed cargo and liquid cargo. The results shown herein provide data for the validation of new numerical approaches for the study of the coupled motions of the floating units and sloshing.Copyright © 2008 by ASME

This paper presents the study of the dynamic behavior of an onshore pre-assembled 8" SCR (Steel Catenary Riser) being transported to the installation place by two tugboats with DP system. The system is composed by two tugboats similar to a Normand Neptun (LOA 80m and displacement of 4200t), one in front of the riser (Lead) and the other at the back of the riser (Trail) controlled by DP system to keep the trajectory for installation location. During the transportation, the system passes through 3 different configurations: Offbottom, BuoyCatenary and Catenary. Offbottom configuration is composed by many buoys (effective buoyancy of 30kN) and chains distributed along the riser to guarantee the relative distance between the riser and the seabed, independently of seabed irregularities. When the sea gets deeper, the riser assume the BuoyCatenary configuration, where the riser forms a catenary with buoys; when it achieves a water depth up to 500 meters, all the system buoy+chain are removed and the riser assume the form of a free catenary until it arrives at the installation place. Two approaches was used to evaluate transport analysis of a onshore pre-assembled SCR. In the first approach (uncoupled analysis), the transportation was simulated using the Anflex software to calculate the dynamic response of the line composed by beam elements, imposing, at the top of the riser, the tugboat motion obtained from spectral analysis. In the second approach (coupled analysis), the transport was simulated using the TPN software to calculate the dynamic motion of the tugboats considering the dynamic positioning system and the Anflex software to calculate the dynamic response of the lines. The main propose of this paper is show the necessity of do coupled analysis for the SCR transport. This study are focus in a parametric analysis of the tugboat viscous damping influence, that were performed (1, 5 and 10% of the critic damping), as well as the damping generated by the riser in the tugboat motion, the relative motion influence in risers tensions and the tugboat’s bollard pull.Copyright © 2008 by ASME

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to protect drilling equipment from environmental forces, and its design aims the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies have been carried out to predict the water dynamics inside the moonpool. At most, analytical tools have been used with experimental results, to obtain a good evaluation of viscous effects. Another line of development uses the moonpools as a device to reduce motions of ships or oil platforms. In his context, the use of moonpools in monocolumn type platforms was studied during the development of the concept, through the partnership between PETROBRAS and University of Sao Paulo–USP. An alternative that became viable in the last years is the use of numerical methods to evaluate potencial parameters, being only necessary simple experiments to obtains viscous data to complete the model. This work, that is a continuation of articles about the issue written before, intends to consolidate the calculation method of moonpool to monocolumn units.Copyright © 2006 by ASME

The MonoBR is a MPSO — mono-column floater, production, storage and offloading unit — a unique platform designed to handle steel catenary risers (SCR) in a depth of 1800 m in Gulf of Mexico oil fields whose target scenario is the ultra-deep water of the Walker Ridge. In this project, special concern was given to sea keeping behavior, constructability and security. Stability Analyses were carried out to ensure the system security and reliability. This paper describes the development carried out by The University of Sao Paulo and PETROBRAS team, in order to analyze the main stability parameters of this new conceptual design for oil production and storage, the MonoBR. The main topics referred on the text are the damaged and intact stability analyses, the tank arrangement, wind influence, rules discussions, the AVCG (Allowable Vertical Center of Gravity) and damage compensation through ballast rearrangement.Copyright © 2006 by ASME

The aim of this paper is the study of the Allowable VCG in monocolumn offshore platforms. As study case the MonoBR — an innovative conceptual unit developed by a partnership of PETROBRAS/CENPES and University of Sa˜o Paulo — was analyzed. The studies were carried out during the design of MonoBR for the Walker Ridge area, Gulf of Mexico. The effect on the stability of the unit caused by a damaged tank depends on its loading condition, since there is lost of both, buoyancy and mass, modifying unit’s displacement and center of gravity. In other words, depending of the tank loading, the amount of water that enters or leaves the unit in a damage case may vary widely. In this paper is presented the methodology adopted in the study of influence of the ballast tanks loading in the Allowable VCG curve in MonoBR, introducing this other variable beyond the draft, the usual single variable in AVCG curves. At the end are presented the main results for the study case, whose AVCG can vary by 50% for the same draft depending of the tank loading.

After the boom of converted floating, production, storage and offloading systems, based on the old VLCC design, many engineering institutes started thinking about optimum dimension for new units. However, these new FPSOs designs carried out worldwide concerns about good seakeeping behavior when considering general weather conditions, in order to apply their project to different locations around the globe. Analyzing the Brazilian specific conditions, it was verified that, considering waves, current and wind characteristics, the dimension proportions found in the projected units were not the best options, mostly because of the swell waves influence. Thus, in a cooperative project between University of Sao Paulo and PETROBRAS, the best dimensions for a specific case were studied, based on real premises from Campos Basin. During the study, the roll motion, which usually creates operational limits during hard environmental conditions, was focused. It was possible not only to evaluate the best breath and draught relations, but also the inclusion of a structured skirt in the ship bilge. The influence of different shapes in the ship’s bow and stern were also evaluated, showing interesting results regarding the forces applied on the vessel. All the analyses were conducted considering numerical analysis, and the final dimensions were applied to a scaled model, which allowed to verify the real behavior of the projected unit in a test basin. As a conclusion, it was possible to define an optimized hull for the PETROBRAS premises, giving them a real design to be used in future explorations.Copyright © 2005 by ASME

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to have easy installation process and protect drilling equipment from environmental forces, and it is desirable the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies, refs. /7,10,11,12,13/, have been carried out to predict the water dynamics inside the moonpool using analytical and experimental tools, this last due to the high viscous influence in the responses, and the study of coupled dynamics of floating units with moonpools have been presented in previous works (refs. /1,2,3,8,9,17/), with especial focus on FPSO’s and Monocolumn type units. In the present study, the objective is to verify the reliability of numerical approach in heave dynamic evaluation of monocolumns. With this approach the objective is to tune the natural period the floater and moonpool. Two monocolumn example units were numerically calculated using WAMIT software, making possible to verify the 6 degree of freedom of the unit and the 3 of the moonpool. As the software does not evaluate viscous forces, decay tests were performed in order to verify the external viscous damping to be considered in the numerical calculations. The calibrated numerical model transfer functions (RAO) were compared with regular and transient wave tests, where heave and moonpool heave amplitudes could be compared.

The ship based Floating Production Storage and Offloading system (FPSO) has been largely used in the recent offshore oil exploration. In most of the cases the oil stored in FPSO is offloaded to a shuttle ship that is connected by a hawser in tandem configuration. Although the tandem mooring is a common procedure in the offshore oil industry, there are few publications related to the theme. Among these, the papers concerned with the current flow interference due to the presence of FPSO on upstream of shuttle ship, here called current shadow effect, are even more uncommon. Neither among the commercial simulators that analyze tandem systems there is none that considers the shadow effect of environmental conditions, providing an analysis different from the real world. In previous studies, the authors presented some experimental results showing that the dynamic behaviors of shuttle ships subject to current were strongly influenced by the presence of a FPSO in an upstream position. This phenomenon was expected since the FPSO disturbs the downstream current field acting on the shuttle ship. Observing this fact, a tentative empirical model based on the disturbed current field was developed. The disturbed current field was obtained through a 3D CFD program and the hydrodynamic forces acting on the shuttle ship were computed using the local cross flow method. At that time, a particular experimental case was analyzed and the proposed model showed to be effective. However, during the model evaluation, it was realized that the proposed model wasn’t able to reproduce some experimental cases. This paper presents the last developments on the shadow effect model and a more comprehensive comparison between experimental and numerical results.Copyright © 2004 by ASME

Almost all simulators of offshore systems evaluate wave forces using frequency-domain potential analysis. A wave analysis software is previously executed, considering several incidence angles and generating a data base which is used by the simulator. This database contains drift coefficients, RAOs and exciting forces for all incidences. During the simulation, the simulator only performs queries depending on the actual heading of the vessel related to waves. The simulation of multi-body systems cannot be performed in the same way, since the relative positions of the bodies change during simulation time, altering the influence of each ship on the overall wave field and its effect on the others. So, for each time step a new frequency-domain analysis should be performed, considering the vessel positions at that instant and updating the database used by the simulator. However, the computational effort necessary to perform such simulation turns this approach prohibitive for conventional computers and workstations. The wave forces are evaluated only in a predefined configuration of the vessels, or considering the vessels independently. Such approaches neglect all interactions and the complex dynamics behavior that results from it. The University of Sao Paulo has been developing since 2001, the Numerical Offshore Tank (NOT), a 120 processor cluster with the ability to perform detailed simulations considering bodies and lines dynamics very efficiently. In order to simulate offloading operations accurately, the commercial wave analysis software WAMIT was integrated to the NOT simulation code, allowing the “update” of wave coefficients data base every time the vessels configuration changes significantly. WAMIT code was adapted to perform parallel processing, greatly reducing the execution time and allowing the simulations to be performed in approximately 7 hours for the analysis of a 3-hour operation. Without WAMIT parallelization, this time would increase by a factor approximately equal to the number of wave frequencies considered in the simulation. This paper discusses the results obtained by the simulations of offloading operations in the presence of waves, detailing the effects that arise when the bodies interaction is taken into account. Effects are sensed in the equilibrium position, in the first order vertical motions and low-frequency oscillation.Copyright © 2004 by ASME

The authors started this research on estimation method of responses for floating structures using Computational Fluid Dynamics (CFD). This paper is first step, which has potential to be very valuable to the engineering community. The numerical method used in this study is the Moving Particle Semi-implicit (MPS) method 1) 2), which is based on particles and their interactions. MPS method that was developed recently is one of particle method. The particle number density is implicitly required to be constant to satisfy incompressibility. A semi-implicit algorithm is used to for two dimensional incompressible non-viscous flow analysis. The particles whose particle number densities are below a set point are considered as on the free surface. In this paper, first, the authors explain the outline of MPS method that Kshizuka et al. developed. You can see more details of this method by references. And the calculation test of column collapse problem is carried out to check our code that we developed. Second, the modeling of free surfaces is explained. And the authors proposed the way to skip irregular data of wave height. Validation of wave profiles was carried out comparing Stokes theory. Third, calculation results of floating body are showed. This method of calculating response of floating structure is by Koshizuka et al. 3). Finally, the authors show calculation results of wave exciting forces by MPS method comparing singular point distribution method (SDM).

The steel catenary risers (SCR) have been studied to be used in ultra-deep oil fields. In Brazil, the oil field is going to deeper water. The geographical characteristics impose this type of production risers, and the necessity of these risers is real. However, in order to avoid risers fatigue problems, some premises related to low vertical motions have to be respected. To reach the necessary good seakeeping responses, new type of hull shape has been proposed, the monocolumn hull, to which some motion minimization devices have been developed. There are several conceptions of monocolumn type hull, but the concept presented in this paper is different from the existing monocolumn hull. To have good seakeeping capability, three main minimization devices were proposed: the structural skirt, the inclined wall, and the moonpool. The inclined wall, a variation of a section area near the water-plane, works together with the skirt, an enlarged bilge keel at the bottom of the platform, modifying the effect of damping, added mass and inertia. The moonpool is the old tank usually found in drilling units, properly modified to work as a motion minimization system. It works as a passive heave motion reduction system, shifting the natural period far from the wave spectrum. Some numerical analyses were made to present the effects obtained from these devices. These numerical results from WAMIT® were compared to experimental showing to be according to them.Copyright © 2004 by ASME

To be classified and approved by a classification society, all offshore structures shall be submitted and analyzed according to standard rules. The stability criteria are based on the IMO–MODU (International Maritime Organization–Mobile Offshore Drilling Units) Code which has reference to almost all types of floating units such as surface, column-stabilized and self elevating, but problems were found when dealing with monocolumn concept due to differences between this concept and those presented by the rules. The monocolumn studied is a floating production system (FPS) platform designed to handle steel catenary risers (SCR) in a depth of 1800 m in Brazilian oil fields. In this project, special concern was given to sea keeping behavior, constructability and security. This paper discusses the last item, focusing on subdivision and stability analysis. In order to overcome difficulties in finding the appropriate criteria, the work was developed using a special criteria, discussed with Det Norske Veritas (DNV) and PETROBRAS, which could be implanted in future classifications for this type of hull.Copyright © 2004 by ASME

There are several developments concerned to simulate the behavior of floating bodies under waves in the restricted boundary conditions so called numerical wave tank. The main feature of these tanks is to calculate full Navier-Stokes equations taking account the viscosity and free surface conditions. However, the dynamic behavior of oil floating exploitation units in actual ocean environmental condition, in waves, wind and current, is more complex and very difficult to simulate using full non-linear Navier Stokes equations. In addition, in the ultra-deep water, it is the primer importance to consider the more precise mooring line and riser’s dynamics in the analysis. The present numerical simulator laboratory called Numerical Offshore Tank is a development that takes account almost all physical phenomena acting on the floating bodies and mooring and risers lines. Since full non-linear solution is not available, the several numerical, empirical and analytical models are being considered and integrated to numerical simulator. The time domain potential problem is solved to wave forces acting on the bodies and empirical models are used to simulate current and wind forces. To represent mooring & riser lines, the finite element model with more realistic hydrodynamic force models is used. Even the simulator is using the full hydrodynamic equation, the calculation time of the simulation for floating bodies with several risers & mooring lines is very high. Therefore, special cluster with 60 PC based computer was built running the code in the parallel processing. Since the preparation of all data set for numerical experiment is very tedious work, the special pre-processor PREA3D was developed for this purpose. This pre-processor allows the fast change of the environmental and system conditions to run several test conditions. Another important feature is the visualization of the results of the simulation tests. The entire 3D view of the system is presented in the Virtual Reality room with stereoscopic projection of the Numerical Tank Laboratory.

The ship based Floating Production Storage and Offloading system (FPSO) has been largely used in the recent offshore oil exploration. In most of the cases the oil stored in FPSO is offloaded to a shuttle ship that is connected by a hawser in tandem configuration. The problem of dynamic instability that arises in several ship mooring systems, like SPM and SMS subjected to the environmental forces, may also be present in the tandem system. Although the tandem mooring is a common procedure in the offshore oil industry, there are few publications related to the theme. Among these, there are none concerned with the environmental forces interference caused by FPSO on shuttle ship, here called as shadow effect. It is well known that the dynamic behaviour of a moored ship, in particular SPM system, is hardly affected by the environmental forces. Therefore, it is expected that shadow effect on the environmental forces acting on the shuttle ship will cause great influence in its dynamic behaviour, and consequently in the dynamics of whole FPSO-shuttle system. These phenomena could be observed in experiments with single point moored shuttle ships with and without the FPSO in upstream position. Therefore, the shadow effect should be considered in analysis of dynamic behaviour of two ships connected in tandem. Among the commercial simulators that analyse tandem systems there are none that consider shadow effect, making their analysis different from the real world. This paper presents an empirical model of the current shadow effect. The model was implemented in a numerical simulator, named DYNASIM. The comparison between numerical results and experimental one showed that the proposed model is effective.