Twister subsea gas dew-pointing
Offshore gas production is faced with increasingly deeper water developments and longer tie-back distances. This trend is leading to a variety of flow assurance challenges, such as hydrate formation, wax deposition and degraded hydraulic performance issues.
Twister offers an unrivalled subsea processing tool which simultaneously dehydrates the gas and removes heavy hydrocarbon components in a single compact modular device. The technology manages the above-stated flow assurance challenges and, as such, has been identified as a key enabler for future subsea developments. Twister BV and FMC completed a feasibility study in 2002 to investigate the feasibility of a subsea Twister system. No show-stoppers were identified, although the study did highlight that some – non Twister – components require further development and qualification, prior to subsea use.
Twister BV, FMC and various other partners subsequently secured EU funding for a four year development programme, which resulted in further development of Twister subsea technology. A joint technology development co-operation agreement was signed with Petrobras in 2006, to test, qualify and develop a subsea dewpoint system using Twister. Under this agreement, Twister BV has since supplied a Twister module to Petrobras which has been installed at an existing onshore gas plant in Brazil, to allow Petrobras to further evaluate the technology. The unit was started up in November 2009 and has operated successfully to date. The second phase of the agreement is expected to kick off during 2010 and will comprise an engineering study for the subsea test on the Petrobras Canapu field. This field is located 20.5 kilometers from a nearby FPSO, in 1600 metres water depth.
Benefits of Twister subsea gas dew-pointing
Twister dehydration and dew-pointing enables dry, single-phase export, eliminating flow assurance risks and limitations associated with wet, multiphase export systems. Benefits of subsea gas conditioning include:
- Hydrate prevention:
As Twister dehydrates the gas, the continuous use of MEG (or other hydrate inhibitors) can be eliminated or lowered substantially. Dedicated chemical pipelines are no longer required, but can be incorporated in the Twister control umbilical. In turn, the problems associated with MEG regeneration equipment (i.e. incompatibility with other chemicals, salt deposition) are eliminated or dramatically reduced.
- Improved hydraulic performance of pipeline :
Multiphase export limits both pipelline turndown flexibility and maximum pipeline size. This usually necessitates multiple smaller diameter pipelines and often eliminates the flexibility to tie-in future developments. By splitting the flow into separate single-phase gas and condensate streams, multiphase flow is avoided and future tie-ins facilitated. Terrain or severe riser slugging is prevented, and costly slug-catchers can be avoided. Routine pigging liquid hold-up mitigation, which is a costly and risky operation, can also be avoided. The elimination of pigging loops also halves the amount of installed subsea pipelines.
- Corrosion prevention:
Twister subsea dew-pointing is expected to be a particularly attractive option for corrosive services. Due to the process Twister enables most corrosive elements to be removed from the gas, and hence may avoid the need to install expensive corrosion resistant alloy (CRA) pipelines. Effective corrosion management for wet gas carbon steel (CS) pipelines normally requires routine pigging to maintain a protective chemical film on the inside wall of the pipeline. This requirement is costly and cumbersome for subsea developments. Twister subsea dew-pointing minimises the need for extensive subsea pigging.
- Field enabling technology by cost saving:
Using Twister subsea gas treatment, an otherwise un-economical or marginal field can be developed without the need to install an offshore surface facility. Twister enables direct gas and liquid export lines from the subsea wellhead to the onshore facility, or to a platform in shallower waters. This enables the economic development of stranded gas fields. The ability to hot tap new fields directly into existing export lines allows asset life extension, further reducing costs for field development, as no new pipelines are required. By combining Twister subsea gas dew-pointing with currently under-development subsea gas compression, offshore surface facilities may in the future be totally eliminated with the associated substantial commercial benefits.