The discovery of small fields often leads to the installation of low cost facilities. However, further nearby discoveries result in a requirement to handle increased duties. Unreliability of such facilities from off-design operation is common.
The case of an oil/gas producing platform is presented. Process gas is compressed in three-stages. Excessive recycling of the stages caused unreliability. The system was also unable to handle planned tie-backs.
The extent of recycling was analysed and reasons understood. The inflexibility of the compression stages due to running at a common speed was the main cause of unreliability. Off-design operation also resulted in a sub-optimal match of the stages. Control of each stage was achieved by means of recycle valves, leading to excessive recycling and power wastage. A redesign of the LP stage was recommended. This was demonstrated to cost-effectively and significantly reduce recycling and release sufficient capacity for the planned tie-back.