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31.102.132371/15 SUBSEA COMPRESSION PLANT Background Subsea compression plants are systems used to pressurise a hydrocarbon stream in order to permit long-distance pipeline transport of the hydrocarbon stream from a subsea location. These subsea locations are generally linked to hydrocarbon production host platforms that receive hydrocarbons from a well(s). Hydrocarbon streams typically comprise a multiphase fluid comprising an intermixed flow of liquids and gases. The gaseous phase of the multiphase fluid may comprise hydrocarbon gases, and the liquid phase of the multiphase fluid may comprise hydrocarbon liquids (e.g. oil) and water. The ratio of fluid in the gaseous phase to fluid in the liquid phase can vary during extraction. Sudden increases in the liquid content of the multiphase fluid cause ‘slugs’ or surge waves. A slug is a liquid plug completely bridging the cross-section of the flowline, whilst a surge wave is a liquid wave that only partially bridges the cross-section of the flowline. Pressurising a multiphase fluid for transport poses a number of challenges, particularly when subsea. Liquids are essentially incompressible and much denser than gases. Conventional compressors are designed to handle single-phase gas. This introduces a need for the gas being completely separated from the liquid before being compressed. Traditionally, this is achieved by using a scrubber upstream of the compressor to remove liquids from the well stream. The liquid phase is re-introduced into the main pipeline downstream of the compressor, such that it mixes back into the compressed gas phase. In order to achieve this functionality, a liquid pump is used to pump the liquids from the scrubber liquid outlet towards the gas-liquid mixing point downstream the compressor. These systems can be large and expensive due to the various components they require. More recently, there has been a focus on using multiphase compressors and flow conditioning units in place of conventional scrubbers and gas phase compressors. Multi- phase compressors compress a multiphase fluid wherein the liquid content of the mixture is within a certain range. However, if the compressor ingests a “slug” of liquid (so that the liquid content is well above that range), the load imposed on the compressor, i.e. the torque required to drive the compressor, can increase dramatically (i.e. abruptly, or rapidly in a short period of time) and lead to damage of the compressor. Accordingly, whilst it is no longer necessary to remove all liquid from gas provided to the compressor, it remains necessary to control the liquid content of the fluid being compressed. The present concept As noted above, the use of multiphase compressors in subsea compression plants is not itself new (although relatively recent). However, the present concept seeks to provide a number of improvements and optimisations to such systems. For example, the Flow Conditioning Unit (FCU) which may be used in the present concept aims to provide a simpler solution for flow conditioning of a hydrocarbon stream. ‘Flow conditioning’ as described herein represents the processing or pre-treatment of a multiphase fluid upstream of a subsea compressor, whereby liquid slugs and surge wave transients are smeared out to a level tolerable by the downstream compressor. Various aspects of the subsea compression plant will be discussed in turn, before the overall subsea compression plant comprising each of the individual various aspects in combination is discussed. It will be appreciated that the present disclosure hence extends to 31.102.13...