Alexandre N. Simos

2nd Order Hydrodynamic Effects on Resonant Heave, Pitch and Roll Motions of a Large-Volume Semi-submersible Platform

During the last decades, as oil production offshore Brazil moved to deeper waters, technical and economical constraints led to a new generation of floating platforms. Nowadays, in the Brazilian offshore scenario, design trends concerning hull form, size and mooring configurations bring novel characteristics of wave-induced dynamics, including non-linear resonant effects. As part of an extensive study on new semi-submersible configurations for Campos basin, recent model tests have shown that their hulls may be subjected to second-order slow motions in heave, pitch and roll. These resonant motions are directly related to the large dimensions and relatively low natural frequencies of the floating systems. The unexpected effects caused great concern, since, in some cases, the low-frequency motions presented amplitudes comparable to those of the first-order response. This paper discusses the evaluation of the 2nd order wave-induced motions of a large-volume semi-submersible platform using WAMIT® second-order module. It is shown that the hydrodynamic forces induced by the 2nd -order potential represent the prevailing effect in the resonant response. Important aspects concerning the numerical model are addressed, such as the parameters involved in the hull and free-surface panelization. Numerical predictions are directly compared with experimental results obtained with a 1:40 model of the platform. A very good agreement is obtained both for heave and angular (pitch or roll) motions, attesting that the numerical code is able to predict the 2nd order forces accurately. Finally, a simplified procedure for dealing with the slow vertical motions is evaluated, aiming to reduce the substantial computational effort required by the 2nd order calculations. Such procedure takes advantage from the fact that the resonant response spectra of the vertical motions are usually narrow-banded (due to the low damping levels) to propose a “white-noise” approach. According to this approach, 2nd order forces need to be calculated only for one frequency difference, corresponding to the natural frequency of the particular motion. Computational time is, therefore, greatly reduced. It is shown that resonant motions calculated through the simplified approach match those predicted through the “full” analysis perfectly, making it an interesting choice for the evaluation of 2nd order effects, especially in the early stages of the design.Copyright

Influence of Wave Induced Second-Order Forces in Semisubmersible FOWT Mooring Design

The main focus of this paper is on hydrodynamic modelling of a semisubmersible platform (which can support a 1.5MW wind turbine and is composed by three buoyant columns connected by bracings) with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 seconds, accurate computation of the low-frequency second-order components is not a straightforward task. As methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as Newman?s approximation, have their errors increased by the relatively low resonant periods, and as the effects of depth cannot be ignored, the wave diffraction analysis must be based on full Quadratic Transfer Functions (QTF) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software WAMIT®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.

Experimental Evaluation of the Dynamic Air Gap of a Large-Volume Semi-Submersible Platform

Definition of air gap is an extremely important issue in the design of floating offshore systems such as semi-submersible or TLP platforms. For these systems, any unnecessary increase in the static value of air gap generally demands the payload to be decreased or leads to a larger buoyant hull, which, in any case, has a negative effect on the project economics. Designers face a difficult challenge since there is no well-established methodology for predicting the air gap demand in the early stages of the design. This is a consequence of the inherent complexity involved in the problem of predicting the free-surface elevation around large structures in steep-waves, such as the largest wave expected during a design storm-sea spectrum. Non-linear diffraction models are usually called for a more consistent evaluation of the wave field under the deck and the wave run-up upon the columns, but even second-order analysis is not free of uncertainties. Therefore, air gap evaluation still relies heavily on experimental analysis. This paper presents some towing-tank results performed for the evaluation of the dynamic air gap of a large-volume semi-submersible platform. Regular wave tests were performed for the small-scale model in both restrained and moored configurations and results were confronted with numerical predictions. Air gap response at different locations of the hull was evaluated under three different sea states and results were compared to some semi-analytical models proposed in literature for preliminary air gap estimation. The role of dynamic coupling provided by a taut-leg mooring system on the air gap results is also discussed based on the experimental results.Copyright

Model Scale Analysis of a TLP Floating Offshore Wind Turbine

This paper discusses the experimental campaign conducted by CEHINAV-UPM research group as request of IBERDROLA within the OCEAN LIDER RD project framework. The purpose of the campaign has been the hydrodynamic performance of a tension leg platform to support a wind energy turbine (TLPWT). The turbine chosen in this study has been the 5MW reference WT of NREL, the location depth has been 80m and the operation area is Estaca de Bares in the north-west of Spain. Regular waves, operational, survival, failure and transport experiments have been conducted. All tests have been performed in CEHINAV (UPM) model basin, except survival tests performed in CEHIPAR ocean basin. This document presents the experimental setup and results from decay tests, regular wave motion RAOs, irregular wave responses, tendon loads and accelerations. Experimental results are compared with available in-house numerical simulations and other numerical and experimental results found in literature.Copyright

Current Wake Effects on DP System of a Shuttle Tanker

This paper reports the main results of an investigation on the effects of hydrodynamic interactions on current forces on a typical shuttle tanker, when offloading a FPSO moored in SMS configuration. In this situation, large angles between the direction of environmental agents and the platform centerline may exist and cause intense disturbance on the wind, waves and current fields that reach the shuttle tanker. A procedure for incorporating the FPSO wake on the current forces was proposed and validated by means of a series of towing-tank tests. In this article, the methodology is applied to evaluate how the disturbances in the incoming flow may affect the DP thrust allocation and power consumption during typical offloading operations. Wake effects on the tanker behavior after DP failures are also investigated. It is shown that current wake effects may contribute to an increase in power demand even for typical distances between ships, usually adopted for the safety reasons. They might also influence the ship course after a drive-off failure, with consequences on maneuvering decisions.Copyright

Dynamic Simulation of Offloading Operation Considering Wave Interaction Between Vessels

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

Influence of Wave Induced Second-Order Forces in Semi-Submersible FOWT Mooring Design

AZIMUT project (Spanish CENIT R&D program) is designed to establish the technological groundwork for the subsequent development, of a large-scale offshore wind turbine. The project (2010–2013) has analysed different floating offshore wind turbines (FOWT): SPAR, TLP and Semi-Submersible platforms were studied. Acciona, as part of the consortium, was responsible of scale-testing a Semi-submersible platform to support a 1.5MW wind turbine. The floating platform geometry considered in this paper has been provided by the Hiprwind FP7 project and is composed by three buoyant columns connected by bracings. The main focus of this paper is on hydrodynamic modelling of the floater, with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 seconds, accurate computation of the low-frequency second-order components is not a straightforward task. As methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as Newman’s approximation, have their errors increased by the relatively low resonant periods, and as the effects of depth cannot be ignored, the wave diffraction analysis must be based on full Quadratic Transfer Functions (QTF) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software WAMIT®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.Copyright

On the Estimation of Directional Wave Spectrum Based on Stationary Vessels 1st Order Motions: A New Set of Experimental Results

The practicability of estimating directional wave spectra based on a vessel 1st order response has been recently addressed by several researchers. The interest is justified since on-board estimations would only require only a simple set of accelerometers and rate-gyros connected to an ordinary PC. The on-board wave inference based on 1st order motions is therefore an uncomplicated and inexpensive choice for wave estimation if compared to wave buoys and radar systems. The latest works in the field indicate that it is indeed possible to obtain accurate estimations and a Bayesian inference model seems to be the preferable method adopted for performing this task. Nevertheless, most of the previous analysis has been based exclusively on numerical simulations. At Polytechnic School, an extensive research program supported by Petrobras has been conducted since 2000, aiming to evaluate the possibility of estimating wave spectrum on-board offshore systems, like FPSO platforms. In this context, a series of small-scale tests has been performed at the LabOceano wave basin, comprising long and short crested seas. A possible candidate for on-board wave estimation has been recently studied: a crane barge (BGL) used for launching ducts offshore Brazil. The 1:48 model has been subjected to bow and quartering seas with different wave heights and periods and also different levels of directional spreading. A Bayesian inference method was adopted for evaluating the wave spectra based on the time-series of motions and the results were directly compared to the wave spectra measured in the basin by means of an array of wave probes. Very good estimations of the statistical parameters (significant wave height, peak period and mean wave direction) were obtained and, in most cases, even the directional spreading could be properly predicted. Inversion of the mean direction (180° shift), mentioned by some authors as a possible drawback of the Bayesian inference method, was not observed in any case. Sensitivity analysis on errors in the input parameters, such as the vessel inertial characteristics, has also been performed and attested that the method is robust enough to cope well with practical uncertainties. Overall results once again indicate a good performance of the inference method, providing an important additional validation supported by a large set of model tests.© 2008 ASME

Dynamic Compression of Rigid and Flexible Risers: Experimental and Numerical Results

Dynamic compression and buckling are critical issues in the viability analysis of rigid and flexible risers developed for offshore applications, especially concerning deep-water operations. Those subjects have been addressed both numerically and analytically. However, few experimental data for validation purposes is found in literature. This paper presents a set of experimental results on the dynamic compression of rigid and flexible risers in catenary configurations, obtained by means of towing-tank tests. Two small-scale models have been built, the first one emulating the dynamic behavior of a steel catenary riser (SCR) and the other representing a much more flexible line. Uniform circular motion has been applied to the top of the models, emulating the floating system first-order oscillations. Different amplitudes of top motion have been considered, each one of them imposed with different frequencies of oscillation. Tension has been measured at the top of the models. The influence of current velocity has also been evaluated. Dynamic tension estimations obtained through finite element analysis are compared to the experimental results. Tension amplitude and critical compression load values are evaluated and compared for both, the steel catenary (SCR) and the flexible models. Comparisons show, in general, a fair agreement between simulations and experiments, reassuring the reliability of numerical models. Results also demonstrate that finite element code provides good predictions of maximum tension loads even when the risers are subjected to high levels of dynamic compression and buckle. Nevertheless, it is clearly noted that difficulties arise in the treatment of flexible structures under severe buckling and torsion. The accuracy of analytical methods proposed for the estimation of critical compression loads is also discussed, based on the experimental results.

Evaluation of the Dynamic Behavior of the P50 FPSO System Using Dynasim: Comparison With Experimental Results

The P50 system is a Floating Production Storage and Offloading System under construction for future operation at Brazil’s Campos Basin, in a water depth of approximately 1200 m. The system is based on a VLCC vessel, moored in DICAS (Differential Compliance Anchoring) system and presents a reasonably large riser porch on the portside for 77 lines. In this paper the dynamic behavior of the offshore system is evaluated using Dynasim, a time-domain simulation code for moored offshore systems, developed by the University of Sao Paulo and Petrobras. Simulations are compared with experimental results. Two kinds of tests were performed: “Calibration” tests were carried out in order to obtain static coefficients of the hull under isolated current and wind loads. “Validation” tests were conducted to evaluate the dynamic behavior under extreme environmental conditions combining current and wave excitation. First and second-order motions were measured as well as mooring line tensions for three different drafts of the ship. A generally good agreement was observed between numerical simulations and experimental results, reassuring the reliability of the numerical code.Copyright