Dynamic Simulation of a Gas Compression System for Relief and Controlled Blowdown Cases
Authors: Moon Kiho , Lee Donghun , Chang Kwangpil , Min Joonho , Kim Junhong , Lee Sejoong , Nam Kiil , Chang Daejun 
Published: October 2008

Abstract
This study presented the dynamic simulation of a gas compression system, proving the viability of operational philosophy and emergency shutdown logic with quantitative process responses in various situations. To avoid unnecessarily high peak in an initial stage of blowdown, this study employed the controlled blowdown and investigated its safety level.

Introduction
This study concentrated on the dynamic simulation of a gas compression system of a topside module on offshore facilities. The gas produced from the wells was separated in the Inlet Separator and routed to the Gas Compression Module. Such various process specifications and operation logics as dew-point control, turbine speed, and compressor surge were taken into account in the dynamic simulation. Revealing the time-varying behavior provided the viability of operational philosophy and emergency shutdown logic, illustrating the detail and quantitative process responses to various disturbances. For example, it showed the transient behavior of the compressor control system consisting of the recycle valve opening at 15% above the surge flow and the surge control valve opening at 10%.
Event scenarios included the possible cases of emergency and relief operation. The history of such process variables as pressure, temperature, liquid level, and flow rate were demonstrated to the virtual event scenarios.
Flare system should be designed to the peak flow rate. The conventional blowdown systems is easy to install and simple to operate. But, it suffers an unnecessarily high peak flow rate in an initial stage of blowdown. In offshore processes, this high peak rate means costly flare facilities. Especially, the flare stack location and weight distribution demand careful consideration for floating units like FPU, FPSO, and LNG FPSO. Therefore, it is important to mitigate the peak flow rate for environmental protection and optimization of the flare system design. In addition to rigorous dynamic simulation, this study also examined the feasibility of the controlled blowdown application to the floating units, to decrease the peak flow rate. The blowdown system was compared with the normal practice in terms of safety level as well as of flare load.

Process description
The gas compression and treatment facilities are rated for gas export capacity of 60 MMscfd and fuel gas requirement of 4 MMscfd at 108 F ambient temperature and 100 % relative humidity. The overall compression requirement is satisfied by two process stages. The Low Pressure (LP) and the Intermediate Pressure (IP) stages are in the form of a complete 100% train, comprising a single driver with two compressors in a tandem arrangement. The gas turbine for LP and IP Compressors is capable of running both on fuel gas and diesel. Diesel is to be used only for field 'black start' when fuel gas is not available. The discharge pressure of the IP compressor is fixed at 638.5 psia (43 barg) to fulfill the required hydrocarbon dew point specification.
The compression module receives the gas from the Inlet Separator, the gas produced from the wells is separated into two phases of gas and liquid. Compression is performed in two stages increasing the pressure from the Inlet Separator operating pressure to the required export discharge pressure.