Fabio Tadao Matsumoto

Damping Coefficient Analyses for Floating Offshore Structures

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

Conceptual Design of Monocolumn Production and Storage With Dry Tree Capability

A new concept and a preliminary study for a monocolumn floating unit are introduced, aimed at exploring and producing oil in ultradeep 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 the WAMIT ® code. This paper also includes a preliminary study on the platform mooring system and appendages. The study of the heave plates presents the gain, in terms of decreasing the motions, achieved by the introduction of the appropriate appendages to the platform.

The Influence at Vertical First Order Motions Using Appendages in a Monocolumn Platform

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

Influence of the Ballast Tanks Loading on the Allowable VCG in Damage Case

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 Sao 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.© 2006 ASME

Stability Analysis of a MPSO (Mono-Column Floater, Production, Storage and Offloading Unit): MonoBR

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

A Design Procedure for Evaluating the Air Gap on a Large-Volume Semi-Submersible Platform

This paper presents a design procedure for the evaluation of the air-gap response on semi-submersible platforms subjected to irregular sea conditions. The suggested procedure takes into account both first and second order (low frequency) effects on hull motions when evaluating the air gap. As a first step of the procedure, a large range of sea conditions with different returning periods and directions of incidence are simulated using a frequency domain model. This first step is intended to determine the critical sea conditions regarding the air gap response of that particular floating unit. For those conditions, it is suggested to be performed a more complete analysis of the problem, including time-domain CFD simulations, in order to improve the results, especially for areas that may be susceptible to intense wave run-up effects. Experimental results for some typical sea states of Campos Basin have been employed to validate the procedure using as an example a large displacement four column semi-submersible platform operating at Campos Basin, Brazil. Results have confirmed that the sea state with the highest significant wave height, or peak period, may not lead to the worst air-gap situation. It’s also shown that, although for the critical sea conditions the first order effects were dominant in the air gap response, at many non-critical sea states the second order effects presented magnitudes comparable to those of first order, indicating that the resonant response of the unit should not be disregarded a priori when dimensioning the air-gap of similar deep-draft semi-submersibles.Copyright

Wave Run-up Simulations with a Moving Large Volume Semi-Submersible Platform

Since July 2008, the Numerical Offshore Tank (TPN) of the University of Sao Paulo and Petrobras have been working on a research project intended to improve knowledge and modeling of advanced hydrodynamics topics, such as the wave run-up phenomenon. Among other activities, wave basin tests were performed with small-scale model of a large volume semi-submersible designed to operate in Campos Basin. These tests evidenced significant run-up effects on its squared-section columns for the steepest waves in several design conditions. In order to evaluate the difficulties involved in modeling the wave run-up phenomenon, simplified tests were also carried out with the model fixed and moored in regular waves with varying steepness. Previous studies using a 2nd order BEM model and a VOF CFD code to predict free-surface elevations below the deck under regular waves were presented in Matsumoto et al. (2010). The studies illustrated considerable differences between the wave elevation results in fixed and moored model setup; however, by that time, the analysis of the moored model by a VOF CFD code was not yet complete. This paper, therefore, presents wave run-up estimations with a moving large volume semi-submersible platform performed with the CFD code ComFLOW, which solves the Navier-Stokes equations employing a local height function to the free surface displacement. The phenomenon is investigated by simulating the flow around the semi-submersible model under the influence of high steepness regular waves on a non-uniform grid. Platform motions, derived from a first order BEM code, are imposed and synchronized with the incoming wave. Aiming at avoiding numerical wave reflections, a damping zone is also applied and positioned downstream the platform model. Predicted results are compared to experimental data, measured by seven vertical wave probes located in different positions below the model deck. Although considerably time-consuming, it will be shown that simulations present very good agreement with the experimental results.Copyright

PARAMETRIC COMFORT ANALYSIS OF A STANDARD SWATH VESSEL

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

Conceptual Design of Floating Production and Storage With Dry Tree Capability

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