Allan C. de Oliveira

The Nonlinear Roll Damping of a FPSO Hull

The ship-rolling problem is a subject that has been studied for a long time. Since Froude’s time (in the 19th century) to nowadays, this subject was revisited several times in order to adjust the theory to changes in ship hulls, dimensions, materials, appendages, etc. On the other hand, ship analysis technological resources, including both experimental techniques and computational capacity (that did not exist in Froude’s time), have also amazingly improved. But despite all those technological developments, the assessment of the nonlinear roll damping of some types of hulls still is a challenging problem. The floating production storage and offloading (FPSO) hull fitted with larger bilge keels, for instance, has behaved in such a way that it is impossible to obtain results from nowadays industry standards via decaying tests. This paper discusses an alternative way to assess the nonlinear damping behavior of FPSO hulls with large bilge keels. Since it is fairly easy to perform decaying tests, the paper also proposes an alternative way to analyze the FPSO properties through this kind of testing by grouping multiple results instead of using only a single test. This artifice brought improvements, such as an increased agreement between the alternative model and the experimental data. The paper also compares the more traditional approaches with the alternative method and finally shows the latter’s applicability. [DOI: 10.1115/1.4025870]

An Empirical Nonlinear Model to Estimate FPSO With Extended Bilge Keel Roll Linear Equivalent Damping in Extreme Seas

Allan C. de Oliveira PETROBRAS R&D Center Rio de Janeiro, Brazil allan_carre@petrobras.com.br Antonio Carlos Fernandes LOC/ COPPE – UFRJ Rio de Janeiro, Brazil acfernandes@peno.coppe.ufrj ABSTRACT Although FPSO roll damping is well known nonlinear, most of the analysis which depends on platform motion evaluations, as riser, mooring and structural analysis are based on frequency domain approach results and transfer. Due to the nonlinearity, RAOs for roll, for instance, are dependent on FPSO motion amplitude, being different for each sea state of interest. Recent researches, however, have detected a saturation level in roll damping with extended bilge keels, which means a constant damping level for larger rolling amplitudes, where a linear coefficient (viscous based) can be used in platform motion analysis. Based in model tests where this saturation occurs, a nonlinear model was fitted to this damping data in order to predict the roll damping in extreme sea conditions. Those tests have taken into account aspects which have strong influence on viscous damping, loading conditions, hull form and bilge keel characteristics. The nonlinear model implemented could provide a safe level of damping comparing the results with extreme irregular wave model tests, becoming interesting for early design phases of such structures.

Experimental Study on Vortex-Induced Motions (VIM) of a Large-Volume Semi-Submersible Platform

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

Minimum Energy DP Heading Control: Critical Analysis and Comparison of Different Strategies

DP offshore operations require that the vessel reference point is controlled in relation to a fixed reference frame. In some cases, the heading is allowed to change, searching for an adequate angle in which the environmental conditions induce small loads in the hull and minimum DP power is required to keep the position. This DP mode is usually referred as “weathervane control”.There is no exact method to determinate the minimum energy heading of a vessel in real-time with only the sensors available in a DP system. Therefore, some control strategies were developed in order to control the vessel heading, and estimate the optimal value.This paper addresses the analysis of the final equilibrium heading of several published weathervane control strategies for two different DP vessels: a typical DP tanker and an asymmetrical DP crane-barge. A static procedure is used to calculate the final heading and the DP power and thrust demand for each controller.Several environmental combinations of wind, current, local sea-waves and swell are considered, with a systematic variation of the intensities and the direction of the environmental agents.For non-aligned environmental agents, differences in the final heading are verified for the zero yaw and zero sway strategies. These headings are also compared to the exact minimum power heading, obtained by an optimization procedure considering the DP thrust allocation.Copyright

A Comparison Among Alternatives to Assess FPSO Roll Damping via Model Testing

The prediction of the nonlinear rolling motions of a FPSO with extended bilge keels stills a challenging problem. Despite recent advantages in CFD computations, the use of model testing is considered the standard strategy for roll damping assessments. There are different ways to assess the roll damping via model testing and the three most common are the use of decay tests, tests in regular and irregular waves. The comparison among those different kinds of tests may present incoherent results, sometimes, introducing the question of which methodology is the most appropriated in those scenarios, with impact in model test specifications.Decay tests are the easiest and the most economical way for roll assessments, but they are usually considered inaccurate compared to the other test types. Recent researches, however, have shown that a statistic approach which utilizes several decay data from the same model and loading condition improves the damping predictions. This paper provides comparisons of damping predictions from different test types based in extensive model test campaigns. It also addresses the issues and the lessons learned during the campaigns to obtain the FPSO damping database.Copyright

A Comparison of Different Approximations for Computation of Second Order Roll Motions for a FLNG

The use of FLNG units for gas exploration and production offshore is a subject in study by some oil companies. More complex and sophisticated than a FPSO production plant, a gas production plant has strict motion criteria in order to have an optimal operational performance. Due to this, designers have been trying hull concepts with small initial stability and higher roll motion periods in order to reduce the unit motions and improve the plant performance. Indeed, the increase of roll natural period dramatically reduces the first order roll motions. However, the unit still responds at its resonance due to second order excitation. These kinds of loads are also more complex and require a great computational power to be evaluated. Due to its complexity, which would involve the solution of a non-homogeneous free surface boundary condition, some approximations are used in order to assess the second order loads and motions. In this paper, the different formulations for the first part of QTF, contributed by first order quantities, are revisited and the differences are highlighted. Furthermore the approximations for the computation of the second part of the QTF, contributed by the second order potential, are benchmarked for the case of a FLNG operating in deep water depth.Copyright

The Influence of Vortex Formation on the Damping of FPSOs With Large Width Bilge Keels

The roll damping of a FPSO assessment is a different subject than the ship case. The fact that the FPSO is not moving changes the flow hydrodynamics in such a way that the well established understanding is no longer applied. This is so at least for certain particularities such as flat bottom, no lift effect due to zero velocity, and so on. Recent researches have proven the strong effect of the vortex shedding on the roll damping of a FPSO mainly when large width bilge keel are present. Although these effects are known by a long time for ships, the increase of the vortex magnitude due the large width bilge keels on a FPSO has let to uncertainties about the behavior of the structures and the situation is challenging. It has been understood that the vortex can modify deeply the pressure distribution along the FPSO hull in such way that the final roll dissipation is higher. Surprisingly, under certain conditions the memory effects are small. The use of visualization techniques allied to the analysis of several decay tests for the same hull can help the understanding of several aspects such as the uncertainty in the measurements and the vortex behavior.Copyright

WAVE EFFECTS ON VORTEX-INDUCED MOTION (VIM) OF A LARGE-VOLUME SEMI-SUBMERSIBLE PLATFORM

Aiming to complete the results presented before by Goncalves et al. (2011) – Experimental Study on Vortex-Induced Motions (VIM) of a Large-Volume Semi-Submersible Platform, OMAE2011, the present work brings new experimental results on VIM of a large-volume semi-submersible platform, particularly concerning its coexistence with waves in the free surface. The VIM tests were performed in the presence of three regular waves and also three different conditions of sea state. According to the results, considerable differences between the presence of regular or irregular waves were observed. The motion amplitudes in the transverse direction decreased harshly when the regular waves were performed and no VIM was observed. In the case of sea state condition tests, the amplitudes decreased slightly but a periodic motion characterized by the VIM was observed. The results herein presented concern transverse and yaw motion amplitudes, as well as spectral analyses.© 2012 ASME

An Alternative to Model the Non-Linear Roll Damping of a FPSO Hull

The ship rolling problem is a subject that has been studied from a long time. From Froude (in the XIX century) to nowadays, this problem was revisited several times in order to adjust the theory to the changes in ship hulls, dimensions, materials, appendages and others. On the other hand, the ship analysis resources technologies including both the experimental techniques and the computational capacity (that not exist in Froude’s time) also have amazingly increased. Although all this technological apparatus, that allows testing a model for several conditions a large number of times, the standard way to estimate the non linear roll damping have not been significantly modified (since Froude’s time!). As a result, some ship applications (as a FPSO, for instance) that require more effective roll suppressor appendages have evidenced behaviors that are not in accordance with the ship industry methodology to predict them. The roll standard theory for ship rolling is, however, a well established theory, based on a solid knowledge about viscous effects in fluids. Hence the question is would it be possible to use all the technological development to get a better approach and estimative of roll damping? This paper will discuss how this may be possible, showing an alternative method to estimate the roll damping and testing for a FPSO with strong non-linear behavior observed in model tests suggesting among other thing alternative ways to perform model testing.Copyright

Calibration of Mooring Line Damping for Taut-Leg FPSOs Model Testing

The paper discusses alternatives to represent the MLD (Mooring Line Damping) in models tests with truncated mooring lines. The work has performed both numerical experiments and reduced model tests. The results for stiffness and damping have been compared. This allows further considerations for future designs.© 2008 ASME