Experimental and Numerical Evaluation of the Installation of Sub-Sea Equipments for Risers Support

The installations of sub-sea equipments are very complex operations, requiring previous analysis in order to define the correct procedure and the environmental “window” for a safe operation. This paper addresses the installations of a Mid Water Arch (MWA) that consists of a structure to provide risers support. Connecting the risers to the MWA largely eliminates the dynamic forces that would otherwise cause friction and fatigue. Such structure is composed by the riser guides and several buoyancy tanks. It is kept in the water by means of tethers connected to an anchor. The MWA is to be installed 42m from the seabed. The installation procedure has several steps evolving the launching of each component of the MWA (anchor, main structure and the tethers). A tug boat with an A-frame is used during the whole launching, and an assisting vessel is required to keep the buoy away from the tether and the launching cable. The presence of exciting waves induces oscillatory motions in the whole system, and may cause large dynamic forces in the cables and tethers. Due to the complexity of the multi-body system, a comprehensive numerical and experimental analysis was then carried out in order to dimensioning the launching cables and to define the limit environmental condition. The numerical analysis was carried out in the Numerical Offshore Tank (TPN), a multi-processor offshore system simulator that considers the 6 DOF of each body and all environmental forces acting in them. The lines are modeled by finite-element method. Furthermore, a full set of small-scale experiments were carried out at the State of Sa˜o Paulo Technological Research Institute (IPT) towing tank, considering the system excited by a sinusoidal motion at the top, emulating the wave excitation. Comparisons between numerical and experimental results were performed, with good adherence between them. The validated numerical simulator was then used to make predictions of the behavior of the systems during the installation, considering several environmental conditions and configurations.

Seismic Analysis of a Double-Hinged Articulated Offshore Tower

The response of offshore structures under seismic excitation in deep water conditions is an extremely complex phenomenon. Under such harsh environmental conditions, special offshore structures called articulated structures are feasible owing to reduced structural weight. Whereas, conventional offshore structure requires huge physical dimensions to meet the desired strength and stability criteria, therefore, are uneconomical. Articulated offshore towers are among the compliant offshore structures. These structures consist of a ballast chamber near the bottom hinge and a buoyancy chamber just below the mean sea level, imparting controlled movement against the environmental loads (wave, currents, and wind/earthquake). The present study deals with the seismic compliance of a double-hinged articulated offshore tower to three real earthquakes by solving the governing equations of motion in time domain using Newmark’s-β technique. For this purpose Elcentro 1940, Taft 1952 and Northridge 1994 earthquake time histories are considered. The tower is modeled as an upright flexible pendulum supported to the sea-bed by a mass-less rotational spring of zero stiffness while the top of it rigidly supports a deck in the air (a concentrated mass above water level). The computation of seismic and hydrodynamic loads are performed by dividing the tower into finite elements with masses lumped at the nodes. The earthquake response is carried out by random vibration analysis, in which, seismic excitations are assumed to be a broadband stationary process. Effects of horizontal ground motions are considered in the present study. Monte Carlo simulation technique is used to model long crested random wave forces. Effect of sea-bed shaking on hydrodynamic modeling is considered. The dynamic equation of motion is formulated using Lagrangian approach, which is based on energy principle. Nonlinearities due to variable submergence and buoyancy, added mass associated with the geometrical non-linearities of the system are considered. The results are expressed in the form of time-histories and PSDFs of deck displacement, rotational angle, base and hinge shear, and the bending moment. The outcome of the response establishes that seismic sea environment is an important design consideration for successful performance of hinges, particularly, if these structures are situated in seismically active zones of the world’s ocean.

Research on the Pressure Field and Production Ability of Compound Hydrocyclone

Compound hydrocyclone is an important separation equipment in oilfield waste water treatment. In order to grasp the equipment separation characteristics, its pressure field and production ability research is becoming more and more important. In the process of pressure or pressure drop deduced, the hydrocyclone’s vortex field is divided into two parts: semi-free vortex area and compulsive vortex area. In the free vortex area, the pressure and the pressure drop are all deduced by the tangential equation, the pressure gradient equation and the relationship equation of tangential velocity in the hydrocyclone body and the velocity of the rotary crib. In the compulsive vortex area, the pressure and the pressure drop are deduced by the velocity equation and the hydrocyclone’s separation equation. As to the respect of the production ability, it is fixed on the relationship of the inlet flow-rate, overflow pipe diameter, the main diameter of the compound hydrocyclone, pressure drop and the inlet liquid density. The research indicates that the pressure or the pressure drop are all connected with compound hydrocyclone’s diameter, rotary crib’s running velocity and diameter of the maximum tangential velocity track face. As the results of the research, the ascertained key operators, pressure and the pressure drop, the hydrocyclone’s production ability can provide designing consult for the hydrocyclone designers.

Structural Analysis of an Offshore Jacket Model Under Seismic Excitation

In the present work the action of earthquakes upon offshore jacket structures is analysed by means of ADINA software. Our case-study refers to an existing model structure, previously constructed at the Laboratory of Fluid Mechanics of UBI, which has been analysed from the hydrodynamic point of view — Mendes et al. [1, 2]. The seismic excitation will be imposed at the base of this model structure, with frequencies and amplitudes corresponding to actual earthquake conditions transposed to the model scale of 1:45. The FEM software is utilised to calculate the natural frequencies of the model and to obtain stresses at selected members, as well as their nodal displacements. Our purpose is to quantify maximum stresses occurring in critical structural members and to verify the survivability criterion. The predictions of the numerical model, in terms of the reaction forces at the base and acceleration at the top of the structure, are then correlated with the experimental measurements performed when the model structure is excited in an especially designed shaking table (Correia [3]), revealing a good agreement between both results.

Experimental Research on the Vibration Reduction and Impact Resistance Performances of Offshore Structure Based on Magnetorheological Damper

This paper presents a new kind of vibration reduction and impact resistance isolator system based on magnetorheological technique, and its experiment results. The vibration and impact experiments were designed using MTS hydraulic loading system. There were many load cases being applied in the experiment with different mass of the model, exciting forces, and controllable electricity of MR damper (Magnetorheological Damper). The experiment results indicate that this isolator system can control the vibration response very well, especially near the natural frequency of the system; and the isolator system has a good performance in the impact experiment too, the response acceleration was evidently reduced, but the characteristic of MR damper was different form its performance in vibration experiment.

Experimental Study of Sloshing Effects in Roll Motion of Floating Units

Usually, the hydrodynamic loads due to sloshing are considered in the design of liquid cargo ship or floating units concerning the structural. Owing to the increasing size of these structures, resonant sloshing motions may occur and result in the amplification or attenuation of motion of the vessel. In order to assess the effect of sloshing, traditionally the motion of the vessel is calculated at first without considering the dynamic of the liquid inside the tank. After that, this motion of the vessel is inputted as excitation motion acting on the tank and, finally, the sloshing effect is evaluated. In the other words, the coupling effects of sloshing and sea wave in the vessel’s motion are ignored. A bibliographical survey shows that there are few studies that consider the effect of sloshing on the ship motion, acting as a passive device of absorption of the movements. The main goal of this research is to investigate experimentally the roll motion amplification and reduction due to sloshing. The coupling effects of sloshing and sea wave in the vessel’s motion are taking into account by recording the motions, in regular waves, of a free floating model with a partially filled liquid tank. For this purpose, a two-dimensional model is designed to carry out measurements with fixed cargo and partially filled liquid cargo. The experimental results are evaluated by comparing the measured motion of the free-floating model with fixed cargo against the results obtained by traditional approach. Then, the effects of sloshing on floating units are shown by comparison of the measurements from free-floating model with fixed cargo and liquid cargo. The results shown herein provide data for the validation of new numerical approaches for the study of the coupled motions of the floating units and sloshing.

Dynamic Response Analysis of a Spar Platform Subjected to Wind and Wave Forces

Results from a study on dynamic response analysis of a floating production unit (FPSO) excited by wave and wind forces are presented. The FPSO is examplified by a Spar platform considering the motion in surge and pitch. The wind gust is modelled with the Harris [4] and Ochi and Shin [7] wind gust spectra. The effect of the wave age on the wind gust spectrum is included by adopting the Volkov wave age dependent sea surface roughness parameter [10]; the wave age independent Charnock roughness parameter [2] is also used as a reference. Examples of results demonstrate clear effects of wave age on the dynamic response. Moreover, for high mean wind speeds the total wind response is much smaller than the wave response, but for low mean wind speeds the wind appears to be more important.

Computing Large Amplitude Motions of a Floating Offshore Structure in the Time Domain

Based on the panel-free method, large-amplitude motions of floating offshore structures have been computed by solving the body-exact problem in the time domain using the exact geometry. The body boundary condition is imposed on the instantaneous wetted surface exactly at each time step. The free surface boundary is assumed linear so that the time-domain Green function can be applied. The instantaneous wetted surface is obtained by trimming the entire NURBS surfaces of a floating structure. At each time step, Gaussian points are automatically distributed on the instantaneous wetted surface. The velocity potentials and velocities are computed accurately on the body surface by solving the desingularized integral equations. Nonlinear Froude-Krylov forces are computed on the instantaneous wetted surface under the incident wave profile. Validation studies have been carried out for a Floating Production Storage and Offloading (FPSO) vessel. Computed results were compared with experimental results and solutions by the panel method.

An Experimental Investigation of Roll Motions of an FPSO

FPSO roll motions can be major contributor to riser fatigue. This is especially true in regions where wind, waves and currents are non-collinear. Roll motions as high as 23 degrees have been reported in the Campos Basin. The most common roll mitigation strategy consists of adding bilge keels to the FPSO. Motivation for this work came from a need to develop a better understanding of roll motions as a function of bilge keel width. In addition to roll motions, the hydrodynamic forces on the bilge keels were measured. A series of tests were conducted at the LabOceano offshore basin. This new facility has a length of 4 0 m, a width of 30 m, a depth of 15 m and is equipped with a multi-flap wave generator on one side. A ship-shaped FPSO design with sponsons for a deepwater offshore development in Brazil was tested. It has a length of 316 m, a breadth of 57.2 m and a draft of 28.3 m. A 1:70 scale model was constructed. A horizontal soft mooring system consisting of four lines with springs was used. Regular waves of different amplitudes as well as random waves were generated in the basin. Two different loading conditions, ballast (draft = 6.7 m) and loaded (draft = 21.7 m), as well as three wave headings, beam seas (90°), and quartering seas (22.5°, 45°) were considered. Tests were undertaken for four bilge keel configurations, corresponding to a case without bilge keels, as well as bilge keels of 3 different widths (1 m, 2 m and 3 m). In all cases, the bilge keels had a length of 200 m. An optical system was used to measure ship motions in all six degrees of freedom. The hydrodynamic loads on the bilge keels were measured using strain gages.

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.