Eric Oliveira Ribeiro

Reconstituição do clima de ondas no sul-sudeste brasileiro entre 1997 e 2005

A criterious hindcasting study of the South Atlantic Ocean wave field was made using the model WAVEWATCH III including tests to evaluate which numerical grid would provide the best configuration for the wind and other atmospherics forcing regarding the calibration of the respective fields. It was observed that the wind forcing values were slightly underestimated for speeds below 7.5 m/s and overestimated for speeds above this value for specific sites at Campos and Santos Basins (Brazil). Thus, it was proposed a small correction to compensate any trends in the wind forcing values for the region. The evaluation of resultant wave fields was made using extreme measured sea states at PETROBRAS platform P-25, northern Campos Basin. The comparison of measured and modeled values shows that the adjusted atmospherics forcing provided good results for both significant wave heights (Hs) as well as spectral peak periods (Tp). Although, it was observed that the modeled sea states tend to slightly underestimate the local sea component at the initial storm buildup when a second spectral peak is present. It was also observed that the used numerical grid tends not to capture very compact atmospheric systems due to poor resolution of such small scale features. On a general view, the results reproduce the wave field characteristics of the western South Atlantic Ocean.

Directional Extreme Current Profiles Based on Complex Empirical Orthogonal Functions (C-EOF) for Offshore Design

This paper presents a methodology to estimate deep water design current profiles using Complex Empirical Orthogonal Function (C-EOF) and a structural reliability response based model. The advantage of C-EOF is the capability of directly obtaining directional extreme current profiles. It is estimated that most of the variability of the southeast Brazil current system can be explained by the first two EOF modes. The first mode associated with the southwestward Brazil Current (BC) and the second mode with the northeastward Intermediate Western Boundary Current (IWBC). Thus, only two series of C-EOF amplitudes can be used in the response based technique to estimate the 100-y extreme current values. The methodology can also be used with more EOF modes if required to properly represent the current data. The probabilistic cumulative functions are based on extreme value distributions such as Gumbel or Weibull, and Lognormal for conditional distributions. The evaluation of estimated distribution parameters are carried out using Kolmogorov-Smirnov goodness-of-fit hypothesis tests and correlation coefficients for each directional sector.© 2009 ASME

2nd Order Pitch and Roll Slow Motions of a Semi-Submersible Platform: Full Scale Measurements and Theoretical Predictions Comparative Study

In October 2007, the semi-submersible platform PETROBRAS 52 (P-52) was installed in Campos Basin (Roncador Field) offshore Brazil. This Unit is moored through 16 lines in taut-leg configuration in a water depth around 1.800m. Its displacement at the operational draft (T = 27.5m) is 80.986t. The maximum production capacity is 180.000bpd. During the design phase of this floating system, a model test campaign was performed in a wave basin and slow drift motions in the vertical plane (heave, roll and pitch) were identified. It is known that resonant responses vary considerably with the damping loads. As these loads are affected by scale effects, by that time, it was a doubt if this phenomenon would happen during the platform operation. Since June 2008, PETROBRAS has been monitoring P-52 motions with the use of accelerometers and rate-gyros. Through spectral analysis of the measured signals, it was possible to verify the presence of slow motions with frequencies around the natural frequencies of roll and pitch during almost the whole monitoring period. Sometimes, the 2nd order amplitudes were even grater than the 1st order ones. Furthermore, the environmental conditions have also been monitored through wave radars, ADCPS (current) and meteorological stations (wind) in the vicinity of P-52 location, making the excitation loads identification possible. A comparative study confronting full-scale measurements and theoretical predictions was performed. First and second-order forces and responses were calculated using WAMIT® second order module. This study permitted to estimate the full scale damping values of this offshore system (hull plus mooring and riser lines) for one of the environmental conditions measured. This work demonstrates the importance of considering the resonant roll and pitch motions in the seakeeping analysis of large-volume semi-submersible platforms, contributing with an important feedback to future designs.Copyright

Full Scale Measurements and Theoretical Predictions of 2nd Order Pitch and Roll Slow Motions of a Semisubmersible Platform

In Oct. 2007, the semisubmersible platform PETROBRAS 52 (P-52) was installed in Campos Basin (Roncador Field) offshore Brazil, in a depth around 1800 m through 16 lines in taut-leg con. The maximum production capacity is 180.000 bpd with a displacement of 80,986t at the operational draft of 27.5 m. Slow drift motions in the vertical plane (heave, roll, and pitch) were observed in a model test performed in a wave basin during the design phase. As resonant responses vary considerably with the damping loads, slow motion could be affected by scale effects. To observe the phenomena, by that time, it was a doubt if this phenomenon would happen during the platform operation. Since June 2008, PETROBRAS has been monitoring P-52 motions with the use of accelerometers and rate-gyros. Through spectral analysis of the measured signals, it was possible to verify the presence of slow motions with frequencies around the natural frequencies of roll and pitch during almost the whole monitoring period. Sometimes, the 2nd order amplitudes were even greater than the 1st order ones. Furthermore, the environmental conditions have also been monitored through wave radars, ADCPS (current) and meteorological stations (wind) in the vicinity of P-52 location, making the excitation loads identification possible. A comparative study confronting full-scale measurements and theoretical predictions was performed. First and second-order forces and responses were calculated using Wamit® second order module. This study permitted the estimation of the full scale damping values of this offshore system (hull plus mooring and riser lines) for one of the environmental conditions measured. The results indicate the importance of considering the resonant roll and pitch motions in the seakeeping analysis of large-volume semisubmersible platforms, contributing with an important feedback to future designs.

Climatologically Modeled Wave Field Analyses in the Western South Atlantic

Wave height, period and direction are basic parameters for designing off-shore structures. Besides this direct application, knowledge of the regional characteristics of a wave field can also help in the selection of optimal regions for wave power energy plant design and installation. A wave climatology based on data generated by a WAVEWATCH III model simulation (NOAA WW3) for the Brazilian coast was analyzed and validated against statistical values derived from opportunity vessel measurements. The hindcast covered the period from January 1997 to December 2005 in a region between 5°N – 40°S and 10°W – 65°W. The grid used was uniform with a 0.25° spacial resolution. The boundary conditions were obtained from NOAA WW3 operational model and the atmospheric forcing from NOAA GFS model. The model results were calibrated with field data and detailed information about the simulation can be obtained in Alves et al. (2008) and Alves et al. (in press). Monthly averages of significant height, period and wavelength were calculated using 3 hour time resolution fields. Since a simple mean direction has small physical representativeness, the predominant direction (moda) and associated persistency were obtained from the data. The results were then compared with values from the U.S. Navy Marine Climate Atlas of the World. This Atlas has four points located within the selected model grid region. These points showed good agreement with wave period, height and direction persistency based on the WW3 simulation results. The wave climatology showed that the predominant wave direction from April to July was from S and SE in southern Brazil, associated with swells related to cold fronts. The S and SE swells were also responsible for the largest mean wave height (2.1 m) observed in the climatology. Another result that was validated with the literature was the E and NE predominant wave direction during the austral summer. This phenomenon is associated with winds originated from the South Atlantic High Pressure Center, which is a semi permanent high pressure center near Trindade Island. The wave climate in northern Brazil showed a predominant direction from the N during January to March, associated with the northern hemisphere winter storms. During the remaining months of the year, the predominant wave direction is E and NE associated with trade winds. The model results are still in a processing phase to produce extreme values, which will be more useful for coastal and off-shore structure design.Copyright

2 – Wave climatology

This chapter describes the wave climatology in the Campos Basin, establishing classification criteria for typical wave situations in the region from measured data of winds and waves. For each established category a description with examples and general statistics is presented indicating typical values of significant wave height (Hs), spectral peak period (Tp) and dominant wave direction (θp). The most severe sea state situations for the region are also assessed, besides the situations of seas developed or limited by fetch or duration.

CLIMATOLOGIA DE ONDAS

RESUMO Este capitulo descreve a climatologia de ondas na Bacia de Campos, estabelecendo criterios de classificacao de situacoes tipicas de ondas para a regiao a partir de dados medidos de ventos e de ondas. Para cada categoria estabelecida e apresentada uma descricao com exemplos e estatistica geral, indicando valores tipicos de altura significativa (Hs), periodo de pico espectral (Tp) e direcao dominante de onda (θp). Sao tambem avaliadas as situacoes mais severas de estados de mar para a regiao, alem das situacoes de mares desenvolvidos e limitados por pista ou duracao.

A Brazilian Northeast Coast Wave Data Comparison: Radar vs Buoy

Wave buoys are well known by its accuracy in measuring sea surface elevations while wave radars were developed later. Buoy’s motions are considered direct wave measurements while microwave radars use remote sensing for tracking the sea surface. Due to sensitivity to winds, sea surface roughness, rain, etc, it is a good practice to check radar data against buoys records. Even both systems installed being 6 km apart in a deep water region offshore northeast Brazilian shelf, there were differences in wave directional spectral parameters calculated by both equipments. The wave measurements at Sergipe and Alagoas basin were in good agreement in terms of significant wave height. Hs correlation was 91% and bias was only 0.06 m. A particular behavior of radar Hs when compared to buoy Hs was overestimation in high sea states and underestimation in low ones. Tp1 correlation was 69%, bias was 0.02 s and linear regression coefficient was 0.64. The worst correlated parameter was peak direction Dp1 with 60% of correlation for E-ESE directional band. Time series matched each other and showed the typical regional wave climate with Hs 1.0–2.0 m, Tp 6–10 s and Dp1 from E-ESE. The distinction between radar and the buoy directions is related to radar susceptibility to SE trade winds during Spring and Summer seasons. However, the difference did not diverge from prevailing wave regional climate.Copyright

Determination of Infragravity Wave Conditions at a Nearshore Location With a Non-Uniform Coastline: Case Study of Baía de Todos os Santos, Brazil

The Petrobras energy company plans to build a new terminal in the Baia de Todos os Santos, Brazil. This paper describes a study in which an estimate has been provided for infragravity (or low-frequency, LF) wave conditions at this future LNG Regasification Terminal location. To aid the design of the terminal, numerical simulations of the propagation of these waves into the bay have been performed.At the proposed terminal location, within a large coastal bay, LF waves can be related to long waves bound to the local primary sea and swell waves (bound LF waves) and to low-frequency waves that propagate into the bay after being made free near the bay entrance (free LF waves). While most of the primary waves will not fully penetrate into the bay, the long LF waves can. Due to the large area of the bay (approximately 40 km × 30 km), detailed modelling of all complex coastal processes related to infragravity waves was not efficient. Therefore, a practical approach has been followed in this study.For a first evaluation, an estimate has been made of bound LF wave conditions at the terminal location inside the bay and at the mouth of the bay. This estimate is based on an equilibrium method using the local primary wave climate and assuming a flat bottom. In a second stage, free LF wave penetration into the bay has been simulated using the shallowwater model Delft3D-FLOW.By applying the practical approach described in this paper, infragravity wave conditions (bound and free) have been efficiently estimated at the future terminal location for yearly-averaged and extreme primary wave conditions. The results show that the LF wave climate at the terminal will be very mild, corresponding to beneficial conditions for terminal operations. The outcome of this study will be taken into account in the further design of the terminal.Copyright

Ocean Eddies’ Influence on Lula Field, Santos Basin, Brazil

Current data measured by an instrumented mooring line deployed at the Santos Basin, in a water depth of 2200 m, show a mean velocity of 0.20 m/s with no preferential direction. Extreme current events detected were associated with the presence of a pair of eddies with a dipole configuration. The jet stream created by the interaction of the cyclone-anticyclone pair was able to generate speeds up to 1.2 m/s at the first hundred meters of the water column. An investigative study with sea surface height anomaly fields processed from environmental satellites clearly shows the eddy structure during extreme current events.Copyright