Isaias Quaresma Masetti

Towards the Integration of Analysis and Design of Mooring Systems and Risers: Part I — Studies on a Semisubmersible Platform

The objective of this paper is to study different analysis methodologies for the design of floating production systems. The main issues are the use of uncoupled and coupled analysis methods, and the integration in the analysis and design of the mooring system and the risers. This paper is a companion to another paper also presented in the OMAE2002 Conference [1] The present paper begins describing a “basic” classic, uncoupled methodology, and proceeds with comments on some refinements in the representation of the behavior of the lines in the motion analysis of the vessel. Comments regarding the introduction of some level of integration between mooring line and riser behavior are also presented. These issues are illustrated with studies applying some of the considered design methodologies to the P-18 semi-submersible platform in Campos basin. The companion paper [1] proceeds describing a fully coupled methodology, and some hybrid methodologies that combine coupled and uncoupled analysis tools, and illustrates their application to a DICAS system for deepwater applications in Campos basin.Copyright

Towards the Integration of Analysis and Design of Mooring Systems and Risers: Part II — Studies on a DICAS System

The objective of this paper is to study different analysis methodologies for the design of floating production systems. The main issues are the use of uncoupled and coupled analysis methods, and the integration in the analysis and design of the mooring system and the risers. This paper is a companion to another paper also presented in the OMAE2002 Conference [1]. That paper describes a “basic” classic, uncoupled methodology, and comments on some refinements in the representation of the behavior of the lines in the motion analysis of the vessel. Comments regarding the introduction of some level of integration between mooring line and riser behavior are also presented in the companion paper [1], and these issues are illustrated with studies applying some of the considered design methodologies to the P-18 semi-submersible platform in Campos basin. The present paper proceeds describing some hybrid methodologies that combine coupled and uncoupled analysis tools, and illustrates their application to a DICAS system for deepwater applications in Campos basin.© 2002 ASME

Numerical Simulation of the Mooring Procedures of the BGL-1 Pipeline Launching Barge

The objective of this paper is to present the application of a computational tool intended to help the crew of the BGL-1 pipeline launching barge to develop safe mooring procedures. This tool is able to calculate the deformed catenary configuration of all mooring lines, regarding the subsea layout and the local environmental conditions, and taking into account one or more buoys attached to the mooring lines in order to avoid interference and accidents with subsea obstacles. One of the main characteristics of this computational tool is the fact that it is able to incorporate the correct definition of the seabed from bathymetric curves, and to automatically consider the position of the subsea obstacles, and possible interferences between the mooring lines and the obstacles. This is performed through a specialized interface with the SGO (Obstacles Management System) database system. This system, developed by Petrobras, contains frequently updated information about the bathymetry and position of subsea obstacles, gathered by a special vessel equipped with a ROV (Remote Operated Vehicle). Case studies will be presented, in order to illustrate the application of the system to the design of actual mooring procedures.Copyright

The Influence at Vertical First Order Motions Using Appendages in a Monocolumn Platform

Oil discovery in ultra-deep-waters is carrying out the ocean engineering to develop new conceptions of offshore exploration and production systems. A promising alternative is a monocolumn platform with moonpool. Concept design has shown its good stability make feasible the use as offshore solution. In order to solve the problem brings by increase on water deep, the use of steel catenary risers and wellhead dry completion has been shown a good solution because it decreases the maintenance cost. However, this solution needs a very low vertical motion. In general, platform like the monocolumn present resonance on vertical motion (15–20 sec to heave motion, 20–25 sec to roll/pitch motion) close to typical wave energy. This fact is so that designers introduce mechanisms to leave the natural periods out of wave energy. The moonpool as passive absorber motion was a solution found by designers. The water inside moonpool acts out-of-phase with platform motion. This difference on phases can attenuate the platform dynamic response. Other solution is water-line-area (AWL) reduction that reduced the restoring forces and consequently increased the value of vertical motions natural periods. These two alternatives showed very efficient to improve the monocolumn performance. This paper presents the gain, in terms of motions, achieved as the appendages introduction on the platform. These appendages consist of two plates, one in form of a disk and other around it. It can be located on the bottom at platform and inserted inside at the moonpool. Model test conduced on the scaled platform shows that heave and pitch natural period increased approximately 4 seconds when this appendages is allocated on the bottom. Practically any change is noted when it is allocated only inside of the moonpool.Copyright

Numerical Moonpool Modeling

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to protect drilling equipment from environmental forces, and its design aims the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies have been carried out to predict the water dynamics inside the moonpool. At most, analytical tools have been used with experimental results, to obtain a good evaluation of viscous effects. Another line of development uses the moonpools as a device to reduce motions of ships or oil platforms. In his context, the use of moonpools in monocolumn type platforms was studied during the development of the concept, through the partnership between PETROBRAS and University of Sao Paulo–USP. An alternative that became viable in the last years is the use of numerical methods to evaluate potencial parameters, being only necessary simple experiments to obtains viscous data to complete the model. This work, that is a continuation of articles about the issue written before, intends to consolidate the calculation method of moonpool to monocolumn units.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.

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 Sao 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.© 2008 ASME

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

Evaluation of the Dynamic Behavior of the P50 FPSO System Using Dynasim: Comparison With Experimental Results

The P50 system is a Floating Production Storage and Offloading System under construction for future operation at Brazil’s Campos Basin, in a water depth of approximately 1200 m. The system is based on a VLCC vessel, moored in DICAS (Differential Compliance Anchoring) system and presents a reasonably large riser porch on the portside for 77 lines. In this paper the dynamic behavior of the offshore system is evaluated using Dynasim, a time-domain simulation code for moored offshore systems, developed by the University of Sao Paulo and Petrobras. Simulations are compared with experimental results. Two kinds of tests were performed: “Calibration” tests were carried out in order to obtain static coefficients of the hull under isolated current and wind loads. “Validation” tests were conducted to evaluate the dynamic behavior under extreme environmental conditions combining current and wave excitation. First and second-order motions were measured as well as mooring line tensions for three different drafts of the ship. A generally good agreement was observed between numerical simulations and experimental results, reassuring the reliability of the numerical code.Copyright

Preliminary Design of Differentiated Compliance Anchoring Systems

The preliminary design of a differentiated compliance anchoring system (DICAS) is assessed based on stability of its slow-motion nonlinear dynamics using bifurcation theory. The system is to be installed in the Campos Basin, Brazil, for a fixed water depth under predominant current directions. Catastrophe sets are constructed in a two-dimensional parametric design space, separating regions of qualitatively different dynamics. Stability analyses define the morphogeneses occurring across bifurcation boundaries to find stable and limit cycle dynamical behavior. These tools allow the designer to select appropriate values for the mooring parameters without resorting to trial and error, or extensive nonlinear time simulations. The vessel equilibrium and orientation, which are functions of the environmental excitation and their motion stability, define the location of the top of the production riser. This enables the designer to verify that the allowable limits of riser offset are satisfied. The mathematical model consists of the nonlinear, horizontal plane fifth-order large-drift, low-speed maneuvering equations. Mooring lines are modeled by open-water catenary chains with touchdown effects and include nonlinear drag. External excitation consists of time-independent current, wind, and mean wave drift.