Abstract

In the last years, the use of FPSOs is becoming a widespread technology because it is a practical and cheap system for the exploitation of small oilfields or deep water oil wells, i.e. oilfields for which the installation of pipelines is too expensive. Recently, growing attention has been devoted to the advanced design of vessels able to operate effectively even when the system is subject to abrupt movements. The study of the behavior of process fluids (gases and liquids) and their mutual system is subject to abrupt movements. The study of the behavior of process fluids (gases and liquids) and their mutual Fluid Dynamics (CFD) techniques.

In this paper, a fluid dynamic model of a FPSO separator is developed using a CFD methodology. This model is able to predict the behavior of the fluid and of their interfaces within the separators to be installed on FPSOs. Several simulations were carried out to study the behavior of the fluids and the efficiency of two vessel types (reboiler and surge vessel). The vessels are subjected to the forcing conditions typical of an ocean storm, i.e. surge, sway, heave accelerations and roll, pitch and yaw rotations, gathered from real historical data. Transient simulations were carried out applying time-varying accelerations to the vessel. The methodology is described in order to give general guidelines for the FPSO simulations.

The main technical contributions of this study are:

  • the understanding of the behavior of the liquid in the separator mounted on FPSOs in different storm conditions;

  • the validations of FPSO separator design to assure their functionality even during heavy storms;

  • the A development of a numerical methodology to implement complex CFD simulations in the presence of time-varying boundary conditions which models the effect of sea movements on on-board ship devices.

Introduction

The ever increasing research regarding new technologies for the exploitations of off-shore oil wells has led to the creation of FPSO in the 1970s. This technology allows the exploitation of oil wells for which the installation of an oil platform and pipeline is too expensive, is not technically advantageous or, in the case where hydrocarbons must be stored, until it can be uploaded onto a tanker or transported through a pipeline. The FPSOs can be either decommissioned tankers or purpose-built ships used for receiving, processing and storing the hydrocarbons produced by nearby platforms or subsea wells.

Several separators and oil treatment systems are installed on an FPSO. These systems are intended to separate water from oil and gas extracted and, subsequently, to process them. Once processed, the fluids are stored awaiting to be transferred. The most common technique to separate oil, water and gas is the use of large gravity-based vessels (separators).

The FPSOs are ships that perform their functions by being moored permanently so the FPSOs and all the systems installed on-board are subjected to wave motion. These movements, that can be very violent i.e. during an ocean thunderstorm, cause sloshing within the vessels. The slosh motion can reduce the separator's efficiency and cause great damage to the structure. It is therefore necessary to study the behavior of the fluids within the separators.

The design verification of the separators to be installed on FPSO can be carried out by numerical simulations. The numerical simulations are a cheap instrument to verify the performance of the separators and the design choices without the need to create a prototype.

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