Sergio H. Sphaier

Laboratory Facilities for Dynamic Positioning System

Abstract Dynamic positioning systems are widely installed in vessels specially those related to the offshore oil industry. Most of these ships require tests in ocean basin to predict their dynamic performance properly since the mathematical models are quite complex and some hydrodynamic phenomena have not been modeled yet. In this paper some aspects of an experimental facility set-up for tests with scale models with dynamic positioning system are described. It includes the topology of the experiment and details of components of the system. Preliminary experimental results are presented and compared with those obtained through digital simulation.

A Two-Dimensional Numerical Investigation of the Hysteresis Effect on Vortex Induced Vibration on an Elastically Mounted Rigid Cylinder

The hysteresis effect on the vortex induced vibration (VIV) on a circular cylinder is investigated by the numerical solution of the two-dimensional Reynolds averaged Navier-Stokes equations. An upwind and total variation diminishing (TVD) conservative scheme is used to solve the governing equations written in curvilinear coordinates and the k-e turbulence model is used to simulate the turbulent flow in the wake of the body. The cylinder is supported by a spring and a damper and free to vibrate in the transverse direction. In previous work, numerical results for the amplitude of oscillation and vortex shedding frequency were compared to experimental data obtained from the literature to validate the code for VIV simulations. In the present work, results of practical interest are presented for the power absorbed by the system, phase angle, amplitude, frequency, and lift coefficient. The numerical results indicate that the hysteresis effect is observed only when the frequency of vortex shedding gets closer to the natural frequency of the structure in air.

On the Occurrence of Mathieu Instabilities of Vertical Cylinders

Large offshore platforms with vertical circular cylindrical hull shapes have been designed and employed in recent times. Classical spar platforms and mono-column concepts are just two examples of designs with such simple configuration, supposed to perform limited motion responses in waves. Reports on investigations on the occurrence of parametric resonance of spar platforms have been published recently in which the relevance of Mathieu amplifications have been assessed making use of different mathematical models. However, some uncertainties still remain on the influence of crucial design parameters as, for instance, metacentric height, draft/diameter ratio, associated damping and mooring system. In an attempt to clarify some of these aspects, in this paper the dynamic stability of a vertical cylinder in regular waves is investigated theoretically and experimentally. A coupled non-linear mathematical model is employed to model and simulate the coupled heave, roll and pitch motions. Theoretical aspects related to the development of resonant motions are discussed. In addition to the numerical and theoretical investigations, an extensive series of experiments with a model of a typical mono-column have been recently conducted at LabOceano. The findings of these investigations are compared and summarized.Copyright

A Numerical Investigation of Vortex Induced Vibration on an Elastically Mounted Rigid Cylinder

The Vortex-Induced Vibration on an elastically mounted circular cylinder is investigated by the numerical solution of the two-dimensional Reynolds Averaged Navier-Stokes equations and results are compared with experimental data. The upwind TVD scheme of Roe – Sweby is used to solve the governing equations and the k-e turbulence model is used to simulate the turbulent flow in the wake of the cylinder. The cylinder is laterally supported by a spring and a damper and is free to oscillate in the transverse direction. Results for the lift coefficient amplitude, displacement amplitude, frequency, phase angle, and power absorbed by the system are presented and compared to experimental data. The code was tested for the fixed cylinder case, and for the moving cylinder. The comparison with experimental data obtained from the literature showed the good quality of the numerical results and validated the code for simulations of vortex-induced vibration.Copyright

Development of a concept of mono-column platform: MONOBR

Over the past few years oil exploitation evolved to deep waters extraction, locations in which oil transport through pipeline systems becomes an unreasonable solution. Solutions based on ships or barges, which allow the storage of oil and its transfer to shuttle tankers, were considered as an advantageous alternative and were widely used. However, these platforms present excessive and undesirable motion for a rigid or even a flexible riser. Furthermore, the recent series of hurricanes in the Gulf of Mexico resulted in strong damages to many installed platforms. These occurrences and climate changes in several areas, all over the world, are a strong motivation for the development of new concepts and new criteria for offshore platforms. New platform concepts, with hulls similar to cylinder trunks and/or cones with shallow drafts, were proposed to the technical community, the goals of which were to respond to small motion in waves. This paper focuses on the study of the motions in waves of the Brazilian conception, MONOBR and its use exposed to severe environmental conditions. Experimental results obtained on the LabOceano (COPPE/UFRJ, Rio de Janeiro) developed in the past few years are presented.

On the safety of offloading operation in the oil industry

A crucial operation for the offshore oil industry is the offloading from a F(P)SO to a shuttle tanker. PETROBRAS, the Brazilian oil company, has developed a technological research programme along the past 10 years to define the shuttle tankers basic requirements for Brazilian waters and the related operation procedures. This programme consists of a wide research work with research centres, universities, etc. This paper presents part of the development programme carried out by PETROBRAS with COPPE/UFRJ and Principia DNV-Brazil. It involves the development of mathematical models and experimental work in ship hydrodynamics to create a database of the F(P)SOs and the shuttle tanker to be implemented in a full mission simulator, a fully dedicated simulator specialised on the offloading operation to work as a preview tool, an adaptable simulation code to be used in new scenarios and a risk analysis to study the use of dynamically positioned shuttle tankers.

Behavior of a Mono-Column Structure (MonoBr) in Waves

MonoBr is the name of a concept of a mono-column structure with a moon-pool developed by PETROBRAS to operate in deep water. A set of tests has been carried out at LabOceano / COPPE / UFRJ to analyze its behavior in waves. Different configurations of the moon-pool entrance have been tested. The main objective of the measurements carried out is to determine the influence of different restrictions on the behavior of the vertical motion of the structure. Results of these measurements are presented and discussed in the paper.Copyright

Asymmetric FPSO Roll Response due to the Influence of Lines Arrangement

In the development of the mooring design of FPSOs in spread mooring system (SMS) configuration, it was observed that the utilization of asymmetric riser arrangement in deep waters might lead to an asymmetrical roll response of the FPSO. In particular, concentrating all riser connections on the portside, it could be observed that roll and heave coupling under the influence of the riser dynamics might lead to a much lower roll response associated with waves coming from portside than from the starboard direction. Simulations were carried using an in-house time domain simulator, where the ship hydrodynamic behavior was represented through the use of impulse response functions and the lines dynamic through the use of non-linear finite element method, using an explicit integration scheme and a lumped mass approach. Non-linear viscous effects could be easily associated to the ship and line velocities. Measured motion responses of an actual FPSO in operation in Campos Basin are compared with the computations.Copyright

Experimental and Numerical Analysis of the Behavior of a Monocolumn With a Moonpool

Although in most vessels the usual function of a moonpool is a passage for production lines, equipments and divers, the moonpool may be used as a device to reduce the vertical motion of floating structures as it can be observed in published studies. From these published works one may say that changes of the moonpool’s internal geometry modify the behavior of vertical oscillation of the water inside the moonpool. Looking for a “hydrodynamic design of the monocolumn-moonpool”, Petrobras has carried out a set of numerical and experimental tests with monocolumn hulls provided with moonpool. Some of these results have been published by the authors in the literature. To increase the comprehension of the dynamics of the joint motion, fluid and structure, numerical results were obtained using a commercial code based on the potential theory and a commercial code based on the solution of Navier-Stokes equations. The paper presents results of these experiments and analyzes the behavior of the coupled motions of mono-columns and the vertical motion of the water inside the moonpool, comparing the experimental and the numerical results.Copyright

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