Rapid Gas Decompression Issues in Subsea Boosting Systems
Authors: Ritchie Norman W. , Rivera Robert J. 
Published: May 2011

The paper describes the Rapid Gas Decompression (RGD) process and how it occurs in Subsea Boosting & Pumping Systems (SBPS). It describes potential subsea equipment damage and how it can be prevented, or reduced, by elements of equipment design, seal choice and equipment operational philosophies. While no SBPS is completely immune to RGD effects, there are known and industry proven steps that can be taken towards mitigating this phenomenon.

Introduction

With the growing number of designed, installed and operated Subsea Boosting and Pumping Systems (SBPS), the Rapid Gas Decompression (RGD) phenomenon should be thoroughly studied and closely addressed in an effort to prevent system damage and potential shutdown or even severe equipment failure.

RGD is the process where gas is initially absorbed into soft goods, (elastomers, epoxy, rubbers, etc) at high pressures and then allowed to quickly expand due to a fast reduction in pressure. This fast pressure reduction causes the gas volume to rapidly increase while still inside the aforementioned soft-goods. The potential end result is the larger gas bubble tearing apart the soft goods in the process. This can dramatically damage the soft goods with detrimental effects on the subsea equipment, with potentially catastrophic effects, including shutdown or eventual system failure.

With subsea valve equipment limitations and communication delays inherent in subsea control systems, RGD can be difficult to control in a subsea installed boosting system. Implementing known RGD solutions into SBPS can dramatically reduce the effect of explosive decompression events, if not eliminate them entirely. Implementing these solutions will greatly increase the reliability of the pumping system from a recognized subsea and downhole decompression phenomenon.

RGD Definition

Rapid gas decompression can be classified as an extreme depressurization event from high pressure (thousands of psi) to low pressure (hundreds of psi down to atmospheric) in the order of seconds, or instantaneously. Ultimately, the composition of the soft goods, the environment and the mechanical design will dictate the tolerable decompression rate. RGD is also known as explosive decompression, for obvious reasons.

For the purposes of this document, a subsea boosting, or pumping, system is a self-contained pump system installed on the seabed aiding with artificially lifting hydrocarbons to the surface. Typically these systems are operated at high pressures and can sometimes observe full wellhead shut-in pressure, with a large reduction in pressure during later start-up activities. It is these high pressure transient events that can cause RGD episodes which in turn can potentially damage the subsea boosting and pumping system.

For RGD to occur there must be the presence of gas in the production system; an entirely liquid packed production system with little GVF should not experience severe RGD issues. However, for the purposes of this discussion, it is assumed that an elevated level of GVF is present in the subsea production system.

Why does the RGD Process affect Subsea Boosting Systems?

So what is so special about SBPS that may cause RGD? Typically it is the introduction of high operational pressures close to the pumping system with a potential for a rapid lower pressure equalization event that will introduce unfortunate explosive decompression opportunities. These opportunities can be devastating to typical soft goods found in SBPS.