Arthur Curty Saad

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.

DP Pipe-Laying and Crane Barge: Procedure for Defining Operational Window and Capability Plots Using Dynamic Simulations

Special station-keeping requirements must be defined for a safe operation of a DP pipe-laying and crane barge. When a pipe is being laid in shallow waters, small displacements of the launching ramp may induce large forces on the pipe or even to deviate it from the defined route. Offshore crane operations are performed in close proximity with other vessel or platform, and large loads are transported in a pendulum configuration. Again, a precise positioning of the barge is required, in order to avoid unsafe relative motions, as well as keep the load being transported on a stable position. Due to these special DP requirements, it has been shown in the present work that a simple static analysis of the DP System is not adequate in this case. Dynamic effects related to wind gusts, slow drift forces, propellers response, DP filtering and time-delay must be considered since the initial stages of DP specification. The common approach of considering an amount of 20% extra power to compensate for dynamic effects may underestimate the necessary power. A fully non-linear dynamic simulator was then used to carry out a complete analysis of the barge. Thrust utilization capability plots were obtained for the DP design environmental condition, considering the occurrence of a single failure. After that, an analysis of the environmental conditions in the Brazilian waters was carried out, and a comprehensive set of more than 1700 conditions were obtained. Dynamic simulations were then used to define the operational window of the barge as well as the estimated downtime for each operation. The barge is also able to operate in a DP-mooring assisted mode. The simulations were used to define under which operational and environmental conditions that such mode must be used.Copyright

Numerical and experimental analysis of a typical DP shuttle tanker operating in Brazilian waters

Abstract Brazilian academic and offshore industry communities are developing numerical and experimental tools for the analysis of Dynamic Positioned (DP) vessels. One of the main reasons is the construction of several DP shuttle tankers (DP-ST) for operating in Campos Basin and in the new oilfields of Santos Basin. The DP-ST specifications include the limiting environmental condition and heading angle in which the DP system must be able to keep position. Proper numerical and experimental verification must be performed in order to assure that the vessel will satisfy the specifications. In the present paper, the complete dynamic and DP-holding analysis of a ST is detailed. Initially, experimental model-scale tests were carried out, considering the limiting environmental conditions. A dynamic-simulation tool was then used, and extensive comparisons between numerical and experimental results were done. Good agreement between them was verified for several cases. Some simplifications of the numerical model related to thrusters’ behavior may explain the differences between simulation and experiments for same extreme cases, when saturation of several thrusts are verified. A critical analysis of the validity of simulation tools for DP analysis is done.

OFFLOADING OPERATION WITH A DP SHUTTLE TANKER: COMPARISON BETWEEN FULL SCALE MEASURENMENTS AND NUMERICAL SIMULATION RESULTS

Abstract In the present paper, full scale measurements of a DP offloading operation is presented, considering all stages of the operation: approach to the FPSO, connection, oil transfer and disconnection. Vessel motions, thruster forces and environmental agents were measured. A time-domain offshore system simulator was then used to reproduce the same conditions of that operation, and the results from the numerical code are compared to the full-scale measurements. Good agreement between numerical results and full-scale measurements was observed.

Consequences of a Non-Linear Asymmetric Restoring Force on the Offshore Barge Rolling Motion

A certain barge widely used for offshore operations in Brazil has a ramp to allow riser and pipeline installations. This ramp crosses the free surface, leading to a non-linear restoring force. The restoring force is also asymmetric, since the ramp is placed in starboard side. Because of the irregular geometry, the predicted hydrodynamic behavior obtained considering potential theory is affected. It is usual consider using duffing equation (inclusion of a cubic term in restoring force) to reproduce GZ curve for analysis at large angles. Although non-linear, the behavior of Duffing model (limits of stability) has been stressed by scientific community and can be considered well-known. Unfortunately, for this analysis, duffing model is not the most appropriated to be used, because of the asymmetry. A quadratic model of restoring force may present better results describing roll evolution and should be investigated. Several studies have been done to improve the dynamic behavior of the barge. As a part of this effort, model tests have been conducted to allow the installation of bilge keels to control the rolling behavior. This was a good opportunity to access the non-linear behavior what will permit comparison with theoretical analysis.© 2008 ASME

Subsea Pipelaying Simulation by the “Situa-Petropipe” Software: A User Friendly Alternative

Currently, Petrobras (the Brazilian state oil company) performs numerical simulations of pipelaying operations employing commercial software, such as OffPipe [1]. However, such tools presents restrictions/limitations related to the user interface, model generation and analysis formulations. These limitations hinder its efficient use for analyses of installation procedures for the scenarios considered by Petrobras, using the BGL-1 barge (owned by Petrobras) or other vessels, considering for instance particular types of stingers depending on depth and pipeline, with different lengths and geometries adapted to certain laying conditions in S-Lay procedures. Therefore, the objective of this work is to present the development and application of a tailored, in-house non-commercial computational tool in which the modules follow Petrobras users’ specifications, in order to overcome the limitations for specific needs and particular scenarios in the simulation of several types of pipeline procedures. Such tool, called SITUA-PetroPipe, presents a friendly interface with the user, for instance allowing the complete customization of the configuration of laybarge and stinger rollers. It also includes novel analysis methods and formulations, including the ability of coupling the structural behavior of the pipe with the hydrodynamic behavior of the vessel motions under environmental conditions.Copyright

Analyzing the Stabilizing Tank for the Control of Rolling Motion by Model Testing and the Dynamic Absorber Theory

The FPSOs are a type of offshore platforms that are directional in nature. Since a large ship (presently the use of a VLCC - Very Large Crude Carrier - is very common) is used, there is a great importance the direction the waves hit the hull. Differently from a real ship, the FPSO cannot resort to maneuvering to avoid waves. It has been found that there is a possibility, in certain cases, that a rolling resonant wave may reach some stationary FPSOs and consequently, a very high response may be obtained. Sometimes, it is not possible to use large bilge keel and the alternative is to consider the use of other devices such as stabilizing tanks and U-tubes. Faced with this problem, the present work performed model tests in a deepwater ocean basin showing the effectiveness of the stabilizing tank. On the other hand, it is clear that due to the presence of mass, restoring motion and damping that the design problem may be tackled by the use of classical dynamic absorber theory. For this reason, a simplified problem was formulated by replacing the stabilizing tank by a passive concentrated mass on board. The fully nine dimensional and non-linear model are then recovered. Six degrees of freedom are to describe the ship motion and the renaming three are for the mass on board. Based on these preliminary studies, the work describes the use of tests with reduced models showing the usefulness of the theory in practice. The test results together with 2 and 4 degrees of freedom system addresses the importance of the roll-sway coupling. Subsequently, a careful linearization is made for the purpose of identifying the commanding variables such as the mass, the position above the keel, the damping, the dynamic absorber natural frequencies, etc. After that, several parametric studies have been performed, identifying the range of applicability of the variables. Finally, this theoretical-experimental exercise addresses back the use of the applicability of the stabilizing tank.Copyright