Optimizing a Large-Vessel Separator Design for FPSO Operation in Bohai Bay
Authors: Durham Karen D. , Sams Gary W. 
Published: October 2008

Designing separation equipment to be placed on an FPSO in Bohai Bay generates a unique set of obstacles that must be overcome. The large capacity of vessels designed for this area, producing low API oils, and the motion of the FPSO due to the local sea states can generate significant process fluid motion inside a separator. Bohai Bay has short wave motion periods which make interface control in a separator challenging. Any fluid motion within the separator can damage the process or the vessel structure if not appropriately contained. Several key separator components, when properly arranged, can control the fluid motions in a large Bohai Bay separator.

These key items will be analyzed and compared to yield a process vessel design with a reasonably quiet fluid interface. Wave amplitude and frequency must be determined for any interface within the vessel. Optimizing the vessel components that are used to control interface waves for the appropriate fill levels will manage the amplitude and frequency.

Wave amplitude can be reduced by dampening and compartmentalizing the fluid within the vessel. This is accomplished by balancing the placement of baffles for flow distribution with baffles for compartmentalization, which adjust the natural frequency.

The amplitude of interface waves generated inside a bare separator will be compared to the relatively quiet interface in a separator that has been designed using the above criteria. A properly designed separator will have an operating range extended into more severe sea states than a bare vessel can handle.

Introduction

The design of oil/water separators intended for installation on FPSO facilities has been the subject of many papers and conferences since the first FPSO was installed. Prior to the design of any FPSO production vessel, the designer must develop a thorough understanding of the vessel purpose, operating philosophy and survival conditions. The needs of the design can be summarized in three broad categories: performance, survival and installed cost.

Separators pose a particularly difficult challenge. They function with a liquid/gas interface that must be properly quieted to sustain performance without introducing too many internal components, which can reduce performance by remixing phases.

While the primary function of the separator is to achieve adequate phase separation during mild to moderate sea conditions, it is not and cannot be the only goal. The secondary function of a design must be to ensure vessel and component survival during severe sea states. Rice points out that FPSO separator designs must ensure process quality specifications and maximize on-stream time. As FPSO vessels move into deeper waters and more severe seas, separator designs must be optimized to maintain process performance while protecting the internals from damage.

Costs

Capital expenses (CAPEX) and operating expenses (OPEX) are certainly considerations that must be properly weighed when making equipment selections. FPSO separators will undoubtedly have high CAPEX resulting from more engineering involvement, robust structural requirements, sophisticated level controls and significant vessel internals. Cost reduction measures such as standard vessel designs with minimum fluid-control internals can mitigate some of these increased costs.