Vinícius L. F. Matos

Torpedo Anchor Installation Hydrodynamics

The installation procedure of a torpedo anchor is the release of the torpedo from a high enough position from the sea bottom to allow the device to reach the terminal velocity. A sufficient kinetic energy at the bottom is essential for the penetration. Besides this, the anchor has to reach the bottom in an upright position to maximize the final holding power in all directions. The present work addresses two hydrodynamic aspects for the installation design and analysis. The first is the drag evaluation and the second is the directional stability. If the drag is to be kept small, then the terminal velocity should be high. The work shows that parameters like the mass and the shape are essential for this. On the other hand, the shape and mass distribution have a strong influence on the directional stability. One important parameter is the rear line length connected to the anchor. This line is necessary for further connection with the final mooring line and influences both the terminal velocity and the directional stability. The work addresses all these aspects under the light of an innovative model test setup to be performed in a deep ocean basin. This kind of model testing has been conceived specifically to attend the torpedo anchor evaluation.

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

Ultra-Deepwater Model Testing of a Semisubmersible and Hybrid Verification

Model test verification of floater systems in ultra-deep water meets limitations when it comes to available laboratory sizes. Systems in depths beyond 1000–1500 m cannot be tested at reasonable scales without the truncation of the mooring and riser system. The development of methods and procedures to overcome this problem has been addressed through extensive research programs at MARINTEK (VERIDEEP, VERIDEEP Extension, NDP, DEMO2000). This led to a hybrid verification procedure which combines reasonable truncation principles, model tests of the truncated system, and numerical simulations, to estimate the system’s response at full depth. There is, however, still a need to address the actual influence from the truncation procedure, and from the integration with simulations, on the final extrapolated full depth results. This paper presents a case study for the validation of the procedure, that compares full depth model test results of a semisubmersible in water depth 1250m against the extrapolated full depth results obtained from a truncated system of 500m. Results are presented for line tension and vessel responses in 3 seastates. In general the extrapolated full depth results were found to be in good agreement with the full depth model tests. However, the results confirmed expectation that the low frequency response has the greater uncertainties and presents the greatest challenge for the procedure.Copyright

Hydrodynamic Aspects of the Torpedo Anchor Installation

The torpedo anchor is a novel kind of device to moor floating offshore structures. It has been proved in practice that this kind of anchoring may be used for both drilling and production offshore activities. For drilling, it is indeed easily recoverable and for large production, it has enough holding power even for large production platforms. There are a lot of soil-interaction aspects to be considered and the installation is one of them. The installation procedure is to release the torpedo from a high enough position from the sea bottom to allow the device to reach the terminal velocity: A correct amount of kinetic energy at the bottom is essential for the penetration. Besides this, the anchor has to reach the bottom in a vertically up right in order to maximize the final holding power in all directions. Therefore, the work addresses two hydrodynamic aspects for the installation design and analysis. The first is the drag minimization and the second is the directional stability. If the drag is be kept to a minimum (without compromising, later on, the soil interaction) then the terminal velocity is higher. The work shows that parameters like the mass and the shape are essential for this. On the other hand, the shape and mass distribution have a strong influence on the directional stability. One important parameter is the rear line length connected to the anchor, which is necessary for further connection with the final mooring line: this parameter influences both the terminal velocity and the directional stability. The presence of the rear line and its role is a novel problem and it seems to have no parallel in other filed applications. The work addresses all this aspects under the light of a novel model testing performed in a model basin that is 15 m deep. It is important to say that this model testing procedure has been conceived to attend specifically the torpedo anchor evaluation. For that matter, the work presents an extrapolating mathematical model. Besides that, an analytical model is shown for the directional stability, together with time domain numerical evaluation. Different model have been used in the tests performed with and without the rear line. Finally, the work presents the model testing design including the use of imaging processing to get the anchor tracking during the launching.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

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

Seakeeping Tests With Gaussian Wave Packets

An important property used in the design stage of floating systems is the RAO (Response Amplitude Operator), the transfer function, for motions, forces and so on. This importance has motivated the development of several analytical, numerical and experimental tools to obtain the hydrodynamic behavior of platforms and ships. Experimental model tests in wave tanks are advisable for the accurate evaluation of the body movements. Three known techniques are used to obtain the RAO curves: tests with regular, irregular and transient waves. In the present work, special attention is given to the technique used to perform model testing with one type of transient wave: the Gaussian Wave Packet. The advantages of using such technique are discussed and results are also presented for a semi-submersible model during tests carried out at Laboratorio de Tecnologia Oceanica - LabOceano/COPPE/UFRJ, Brazil. Numerical calculations and tests with regular and irregular waves are used for validation and comparison.Copyright

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.

DOWNTIME ANALYSIS FOR OFFLOADING OPERATION: DP X NON-DP SHUTTLE TANKER

The Brazilian oil company PETROBRAS will install the first FPSO ever in the Gulf of Mexico (GoM). This unit will be installed in a water depth of 2.500m, and in addition to several challenges to develop such a project, one has received special attention: the offloading to be performed by a shuttle tanker in tandem with the FPSO. In a general offloading analysis, the motions of the shuttle tanker and the hawser tension are evaluated only in the maximum operational environmental conditions. This approach has limitations, the most important being the fact that the worst results do not always happen with the most severe environmental conditions and that it does not provide an indication of the operational downtime. In this work, an analysis is performed to evaluate the downtime of a shuttle tanker, with and without dynamic positioning (DP) assistance, under the scatter environmental data taken from the GoM METOCEAN technical specification. Due to the large amount of possible environmental combinations of wave, wind and current, a reduced selection of 60 conditions has been chosen based on statistical procedures. The offloading analysis is performed for a turret moored FPSO, connected with two types of shuttle tankers: a non-DP shuttle tanker (ST) or a DP shuttle tanker (DPST). The DPST uses 2 tunnel thrusters in the bow, 1 in the aft and the main propeller, with a total power of 12,500kW. The ST is assisted by one tug in tandem, which applies at least 10t of force at the ST stern. The calculations are performed with the in-house PETROBRAS software DYNASIM, a fully coupled time domain simulator. In the analysis the position of the moving shuttle tanker is monitored within the green zone, defined as ±45° from the FPSO bow-stern axis, and the mean and maximum hawser tensions, for all defined environmental conditions. The downtime for each loading condition is obtained by the summation of the occurrence probabilities of the environmental conditions under which the ST or DPST results did not stay inside the defined limits of position in the green zone and by the hawser tensions, i.e. the offloading cannot be performed. Machinery failure probability is not considered for the evaluation. As a result a downtime smaller than 3%, or 11 days per year, was obtained for the ST and smaller than 2.4%, or 9 days per year, for the DPST, with the hawser tension limit exceedence being the main cause.Copyright