SUBSEA COMPRESSION VERSUS TOPSIDE

SUBSEA BENEFITS INCLUDE LOWER POWER CONSUMPTION, FLOW ASSURANCE, HIGHER PRODUCTION AND THE FULFILLMENT OF HEALTH, SAFETY AND ENVIRONMENTAL RESPONSIBILITIES

By George Kleynhans

Figure 1: Subsea compressor

Installers of subsea compression systems are working hard to improve the cost benefits of subsea compression systems. Subsea has been shown to reduce offshore production infrastructure costs (Turbomachinery International, May/June 2016). Further benefits include flow assurance (to increase production) and the fulfillment of health, safety, and environmental (HSE) responsibilities in a manner superior to conventional topside compression.

The wellstream flow from offshore gas wells is multiphase where the dominant phase is natural gas, but it also includes liquid hydrocarbon condensate and water. A combination of pressurized natural gas and water can cause hydrate formation which has the potential to block pipelines and arrest production flow. To mitigate this risk, upstream production typically injects a hydrate inhibitor (monoethylene glycol, for example).

In order to recycle the hydrate inhibitor and transport liquids, it is important to maintain flow assurance and stable production along multiphase pipelines. Flow assurance is provided by maintaining a certain flow velocity for a given gas pressure (or density) so that the liquids (hydrocarbon condensate, produced water and the hydrate inhibitor) can be successfully transported through the pipelines and the riser section to the topside and onshore receiving installation.

Flow assurance

When applied correctly, flow assurance transports liquids stably and avoids liquid slugging at receiving facilities. By applying subsea push compression, higher pipeline flow momentum (a combination of density and velocity) can be maintained to ensure sufficiently high interfacial friction between the gas and liquid phases. Thus stable production flow assurance is obtained efficiently along with increased continuous production.

Locating the compressor close to the well, by placing it on the seabed, increases total recovery by levels of 20% or more. The currently operating non-redundant 2x50% capacity subsea compression solution implemented for the Åsgard field in the North Sea provides flow assurance and increased continuous (21 million standard cubic meters per day) and overall production (increased recovery of 306 million barrels of oil equivalent).

In terms of HSE, subsea compression installation requires human presence only on the installation vessel during installation or retrieval activities. Therefore, no health and safety danger is present related to human presence for operation monitoring and control or airborne transportation to and from offshore installations. To operate in a submersed environment, the compressor must be hermetically sealed so no local emission of environmental pollutants occurs.

The initial subsea compression system, as implemented for the Åsgard license, experienced high costs due to a multitude of non-reoccurring technology development and qualification activities and a conservative production layout that increased system complexity.

However, many current development and qualification activities are addressing subsea system complexity, footprint and cost issues.

Wellstream, or wet gas, compression can reduce complexity by removing the need for large suction scrubbers and a subsea pump with its associated power system and umbilical (Figure 1). Full-scale simulated wellstream operation tests with existing subsea compressors showed that only 4% additional power is needed for 19% additional liquid mass flow rate.

Figure 2: Single (blue), overhung (green) and tandem (red) HOFIM modularity options for a range of production requirements

Additional wet gas tests showed that higher pressure ratios can be achieved compared to dry gas compression, thereby enhancing compressor performance. This multiphase performance gain combined with its compact modular compression configuration options (Figure 2) offer a cost and performance benefit over the wide range of production requirements an offshore field can experience over its lifetime.

The currently operating subsea compressor units have accumulated more than one year of operation with close to 100% availability and give confidence to realize further benefits the next generation subsea compression system can provide.

George Kleynhans is a Technical Advisor in the engineering department of MAN Diesel & Turbo Schweiz AG in Zurich, Switzerland. For more information, visit www.turbomachinery.man.eu