As part of the modification of the Floating Production Storage and Offloading Unit (FPSO) Glas Dowr for operation on the Sable Field (offshore South Africa), a new gas compression system was installed. Associated gas is compressed for use as lift gas and re-injection back into the reservoir for pressure maintenance and optimizing liquid recovery.

Concept selection of the compression facilities was based on technical and commercial criteria in combination with the specific requirements of the Sable field. Large variations in gas molecular weight, compressor efficiency and overall system availability favored the use of reciprocating compressors, despite space constraints and potential vibration problems. A configuration based on 4 × 33% MP/HP reciprocating compressors which includes a spare compressor, was selected.

During the design, specific attention was given to pulsation and vibration control as this was considered crucial for the success of the project. A dynamic analysis of the compression system according API 618 Design Approach 3 was included in the scope of the compressor Original Equipment Manufacturer (OEM). In addition an independent verification study was awarded to TNO, acting as 3rd party.

The required modifications to the piping system resulting from the analysis were implemented as far as practically possible. An extensive field measurement program during the operational phase was conducted which showed vibration levels well within the allowable range.

The application of multiple large reciprocating compressors on the FPSO Glas Dowr showed to be a success. Compressor and piping system vibrations were controlled successfully which resulted in vibration levels well within allowable limits. The stringent space restrictions and high system availability requirements were met in full.

1. Introduction

In 2001 a project was initiated to develop a marginal field offshore South-Africa. With a relative short field life, the development scenarios showed that the application of a leased FPSO was the most suitable option. At that time the existing FPSO Glas Dowr, owned and operated by Bluewater was available for a new contract and showed to have a good basic match with the project requirements. The main modifications to the existing topsides facilities consisted of new gas compression facilities, additional power generation and some modifications to the crude separation system.

To meet the challenging technical and commercial constraints a challenging gas compression system based on reciprocating compressors was selected. Due to the relative small area available for the compressors and the pulsations and vibrations that can be expected with the application of reciprocating compressors, a thorough investigation of the pulsation and vibration behavior of the compressors and the piping system was considered crucial. High pulsations and vibrations may lead to reduced compressor efficiency, pipe failures and possibly unsafe operating conditions. Recognizing this risk, in addition to the analysis that was conducted by the OEM, a similar analysis study was awarded to TNO for the purpose of verification by an independent party.

This paper presents the process of concept development which led to the selection of the optimum compression solution.

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