Carlos H. Fucatu

Wind Shielding Effects on DP System of a Shuttle Tanker

The effects of hydrodynamic and aerodynamic interactions on a typical shuttle tanker (ST), when offloading a FPSO moored in Spread Mooring System (SMS) configuration are being studied in an extensive research project conducted at the University of Sao Paulo, in collaboration with Petrobras. The numerical models will be incorporated in the simulators in order to evaluate the impact of such effects on the dynamics of the ST and on its DP System power requirement. Present paper focuses the wind shielding effect, since current and wave wake effects were already treated in previous works (Illuminatti et al., 2009; Queiroz Filho et al., 2009). A detailed CFD model of the FPSO and ST were used to evaluate the horizontal forces and yaw moment induced in the ST by the wind when inside the shadow zone defined by the FPSO. The CFD model was calibrated using wind tunnel measurements of a similar system. Typical tandem configurations were considered for the connection and oil-transfer stages of the operation. The power requirements for each DP thruster were then calculated, considering a thrust allocation algorithm. The comparisons with the stand-alone ST configuration indicated that the wind shielding effect is important concerning DP power. Besides, it is proposed a simplified model for wind forces and moment in the ST, considering the wind velocity field in the wake of the FPSO (CFD calculation). The procedure is based on report [7] where the wind force is evaluated through de summation of forces and moment in the stern, middle and bow parts of the ST. Such procedure has the advantage of requiring only one CFD calculation (for obtaining the velocity field downstream FPSO).© 2010 ASME

Modular Mathematical Model for a Low-Speed Maneuvering Simulator

CNPq (process 302544/2010-0 and FAPESP (process 2012/20531-8 for the grant and financial support to researches related maneuvering

DP Cooperative Control for Subsea Installation with Multiple Vessels: An Illustrative Case Study Based on Numerical Simulations

This paper presents the development of cooperative control technique applied to vessels equipped with dynamic positioning (DP) system. An illustrative case study is suggested: the launching of subsea equipment using two DP vessels. In this example, the cooperative system controls the relative distance between the DP vessels. One of the advantages of this method is the increase of operation’s safety and operational window, since, among other factors, the tension in the launching cable is reduced by half.Initially, it was proposed the control of vessels relative positions, trying to keep the movements at the top of the A-frames in counter-phase. This avoids the slackening of the launching cable. For this, an algorithm based on phase estimator (Hilbert transform) associated with a PD control was implemented. The results showed that for regular waves this strategy was effective.A dynamic mapping was then obtained using simplified 2D simulator, previously validated by comparison with experimental tests. In these maps, two regions are defined — occurrence or non-occurrence of cable slackening — as a function of the distance of the vessels and the depth of the subsea equipment. This map defines the proper set-point for the DP systems for each depth of the subsea equipment. This map is used to define the best relative position for the vessels.In addition, the hoisting control receives the measurements of the vertical motion at the top of the A-frame, and compensates its motion, trying to maintain a constant lowering velocity. This control was implemented considering errors of 10% and delay of 0.5s in the measurements. The results confirmed that the control is able to eliminate the tension peaks and the occurrence of slackening in the launching cable.The conclusion is that the appropriate control strategy, considering regular waves, is to combine the control of both position of the vessels and hoisting of the cable. Therefore the position control, coupled with dynamic mapping, defines the “optimal path” to be followed during the line hoisting, trying to keep the vessels as close as possible to the “no slackening” region.Copyright

Numerical Analysis of a Novel Method for Subsea Equipment Installation

This paper and the companion paper OMAE2010-49946 (de Mello et al., 2011) present the experimental and numerical results of the analyses of a novel method for subsea equipment installation. The conventional procedure, which is usually used, may lead to dynamic amplifications, depending on the launching cable stiffness and length. In the proposed method the equipment is lowered using two tug boats in a Y configuration. Trough the control of their relative position and also of the cable length it is possible to diminish the tension amplification.Copyright

Development of an Innovative Real-Time Simulator for DP-Shuttle Tanker / FPSO Offshore Connection Operation

This paper discusses the prerequisites and the technical aspects of an innovative real-time simulator for DP-shuttle tanker and FPSO offshore connection operation. This simulator was developed by means of a partnership between the University of Sao Paulo and Petrobras. The software is based on the TPN (Numerical Offshore Tank) numerical code which had several modifications, in order perform real-time simulations. The mathematical models considered in the simulator and the computer architecture (hardware and software) will be presented.Copyright

Seasonal Downtime Analysis of DP and Non-DP Offloading Operation in Brazilian Waters

Oil offloading from Spread Mooring System (SMS) FPSO is usually done by means of a shuttle tanker (ST) in tandem configuration. The ST receives the oil pumped by the FPSO from a bow or stern offloading station, and the operation may take up to 3 days. In order to minimize the risks associated with the operation, the shuttle tanker (ST) should be kept within a safety zone with respect to the FPSO, which is usually given as a minimum distance between the two ships and an aperture angle from the FPSO centerline. In order to guarantee the tanker position during the whole operation, the operation is usually assisted by tug boats or performed with tankers provided with DP (dynamic positioning) systems. Since SMS FPSOs may be not aligned to the environmental forces, keeping the shuttle tanker in position may be a hard task either for the DP system or for the tug boats, depending on the environmental conditions. There are non-rare situations in which the ST must be disconnected and the operation interrupted. The present paper presents a methodology based on fully-nonlinear dynamic simulations for evaluating the downtime of such offloading operations, considering both DP and conventional non-DP tankers, in the later case assisted by a tugboat. Typical Brazilian waters environmental conditions (current, wind and waves) are considered for each seasonal period of the year. The main objective is to provide a quantitative comparison between both cases in each season. The results may also be used for strategic definition concerning logistics of oil production. The procedure is based on exhaustive dynamic simulations, and a statistical analysis of the oceanographic data was used to generate a representative set of environmental conditions together with the corresponding probability of occurrence for each season. A full time-domain dynamic simulation is then carried out for each case, considering detailed models for the dynamics of each floating structure, mooring lines and DP system. The vessel excursion, hawser tension and other operational parameters are then evaluated for each case, and the occurrence of unsafe conditions may be properly evaluated. A statistical procedure is finally applied to compute the associated downtime. Illustrative results for two different classes of ST (Aframax and Suezmax) are presented and discussed, and the advantages of employing DP assisted vessels become clear.Copyright

Evaluation of the FPSO Polvo Offloading Process

Abstract This paper presents the results of simulation analysis executed by the TPN – Tanque de Provas Numerico (Numerical Offshore Tank), about the POLVO FPSO (Floating, Production, Storage Offloading) offloading operation, located at Campos basin in the state of Rio de Janeiro, Brazil. Moreover, it is shown the viability to use numerical codes, as TPN simulator, as auxiliary tools instead of carry out expensive and time spending procedures like reduced model tests.

An Empirical Model of Current Shadow Effect

The ship based Floating Production Storage and Offloading system (FPSO) has been largely used in the recent offshore oil exploration. In most of the cases the oil stored in FPSO is offloaded to a shuttle ship that is connected by a hawser in tandem configuration. Although the tandem mooring is a common procedure in the offshore oil industry, there are few publications related to the theme. Among these, the papers concerned with the current flow interference due to the presence of FPSO on upstream of shuttle ship, here called current shadow effect, are even more uncommon. Neither among the commercial simulators that analyze tandem systems there is none that considers the shadow effect of environmental conditions, providing an analysis different from the real world. In previous studies, the authors presented some experimental results showing that the dynamic behaviors of shuttle ships subject to current were strongly influenced by the presence of a FPSO in an upstream position. This phenomenon was expected since the FPSO disturbs the downstream current field acting on the shuttle ship. Observing this fact, a tentative empirical model based on the disturbed current field was developed. The disturbed current field was obtained through a 3D CFD program and the hydrodynamic forces acting on the shuttle ship were computed using the local cross flow method. At that time, a particular experimental case was analyzed and the proposed model showed to be effective. However, during the model evaluation, it was realized that the proposed model wasn’t able to reproduce some experimental cases. This paper presents the last developments on the shadow effect model and a more comprehensive comparison between experimental and numerical results.Copyright

THE EFFECTS OF MOORING LINE DAMPING ANDWAVE DRIFT DAMPING ON MOORED TANKERDYNAMICS

This paper is concerned with the analysis of the dynamic behavior of a tanker, acting as a FPSO, moored through a differentiated compliance anchoring system (DICAS). It is presented, initially, the mathematical models of the moored tanker dynamics, with special consideration on the description of the interaction between waves and current, and the mooring line damping The mathematical formulation is used to simulate the performance of a 130kDWT tanker in the DICAS configuration. The simulation results are compared with model test data in order to evaluate the accuracy of the models proposed to represent the wave drift and mooring line damping.