As they drill deeper under the ocean floor, oil producers are asking for even more pressure to sustain the strong, steady crude flows needed to justify their huge development costs and royalties. Every bar of additional pressure counts.
As a leading global manufacturer of mechanical seals, EagleBurgmann continues to raise the bar by setting a new standard for dry gas seals (DGS) used in high pressure gas reinjection systems.
For GE compressors at the Tupi 4 floating production site in the Lula field in the Atlantic Ocean southeast of Rio de Janeiro, new EagleBurgmann mechanical seals are being installed with the highest static design pressure rating of any DGS certified for deep water gas reinjection compressors. The 428 barg (6,206 PSIG) rating is not just a bench test; it is the actual operating point required for the Tupi 4 reinjection compressors’ system startup and anytime the compressor is tripped as the suction and discharge pressures equalize (settle out pressure). That 428 barg (6,206 PSIG) is several bar more than seals employed by petroleum producers in comparable ultra high pressure reinjection systems worldwide.
Gas, whether natural gas or supercritical carbon dioxide (CO2), is displacing water as the most economical reinjection medium. It is an abundant byproduct of offshore oil production at the Tupi 4 site that is useless with the addition of an environmental cost, as it cannot be vented to the atmosphere. Reinjecting CO2, in effect, relegates it below ground.
Discovered in 2006, the Lula field contains pre-salt oil and gas. The ocean depth averages about 2,000 meters (1.2 miles) and the hydrocarbon zones are 4,000-5,000 meters (2.5 – 3.1 miles) below that, holding estimated recoverable reserves of 5-8 billion barrels of oil equivalent. Lula is being brought into full production using Floating Production Storage and Offloading (FPSO) vessel platforms. The Tupi 4 partners, led by Brazil’s state-controlled oil company, need the highest pressure possible from the compressor and mechanical seal within the parameters of safe and reliable operations to create an effective miscible zone to flow the crude to the production well.
The seals developed for the Tupi 4 FPSO are installed on GE Oil & Gas BCL306/D vertical split compressors and represent the leading edge of reinjection sealing. They are designed for a maximum shaft speed of 13,844 min-1. The higher 428 barg (6,206 PSIG) raises the level at which the compressor can remain pressurized in the event that it is tripped. Avoiding depressurization saves process gas and considerable time by dispensing with the lengthy shutdown and repressurization protocols.
For Tupi as it did in the Caspian fields, EagleBurgmann is deploying a tandem DGS with an intermediate labyrinth. Tandem DGS layouts–comprised of a primary and secondary seal–are used widely in petroleum production and pipeline operations and are considered the best choice for ultra high pressure reinjection. The Tupi 4 seal reflects several technical considerations in the EagleBurgmann tandem seal design for ultra high pressure operations to achieve the optimal compromise between leakage reduction and torque at startup. These three are of particular note:
Functional Gap at Sealing Elements
The functional gap of a tandem DGS is the gap between the balancing sleeve and support ring of the dynamic secondary seal. To prevent the sealing element material from extruding, the functional gap is designed as small as possible. Free movement must be ensured under all operating conditions. The functional gap’s design must be no more than a few hundredths of a millimeter. This is a tough challenge to manufacture because the variation of the gap height comes under the influence of temperature and pressure and has to be minimized. To achieve this, EagleBurgmann carried out extensive FE-calculations prior to design finalization.
Stability Under High Forces
At ultra high pressure levels, there are tremendous forces caused by the pneumatic load acting on the seal not only in the radial direction, but also in the axial direction. To ensure maximum stability of the seal at such high loads, the cross-sections of the metal sleeves in the seal cartridge have to be larger than those operating at lower pressure.
In the case of the latter, a single sleeve is used, which is assembled above the shaft sleeve. If only one sleeve is used for an ultra high-pressure DGS, the relatively small cross-sections would be too weak to handle the high axial load. Instead, the sleeves were split to ensure maximum stability of the DGS under ultra high pressure.
Material Selection for Ultra High Pressures
Due to the extreme mechanical loads at ultra high pressure, like the great torque at start-up when the seal faces are still in contact, special emphasis has been given to the selection of materials, including the mechanical properties of the seal faces. EagleBurgmann’s extensive experience with hard-to-hard material combinations of seal faces played a significant role in achieving the optimal compromise between gas leakage reduction and torque at start-up. A special fluid-phase sintered silicon carbide material was chosen to ensure maximum strength of the seal faces while maintaining optimum thermal conductivity. Also, at full load, the power produced mainly in the seal gap is in the range of 25kW due to the sharing action of the high density gas. The seal design and choice of materials is such that this tremendous amount of power is easily dissipated into the surrounding gas and metal parts.
There have been notable advances in re-injection technology since 2000 as new fields have been brought into production in Oman, in the Caspian Sea, and now off the shores of Brazil. This has spurred EagleBurgmann to develop ultra high pressure derivatives of its tried and true DGS, already used widely in petroleum operations. It has been able to progressively raise the DGS design pressure with refinements to existing DGS design and materials without compromising operational reliability or integrity. For example, the sour gas content of the Caspian oil fields (sour gas is natural gas with high levels of hydrogen sulfide, an aggressive corrosive) presented a more challenging environment for compressor and seal integrity and had to be compensated for in the design process and with materials. The Tengiz and Kashagan fields have 23 percent and 17 percent H2S content respectively.
In 2005, EagleBurgmann undertook research at the invitation of GE Oil & Gas to develop a new ultra high pressure DGS for gas re-injection. The result was a seal with a static design pressure of 425 bar (6,163 PSI) and maximum shaft speed of 12,373 min-1 for the Caspian projects, milestones surpassed by the Tupi 4 seal.
Any high-performance seal design must balance multiple objectives to achieve the best possible overall outcome. In the case of the Tupi 4 seal, extensive testing by both EagleBurgmann at its premises and at GE has demonstrated that the seal delivers high reliability in common startup/shutdown scenarios, as well as continuous operations at full load, assuring compressor integrity with minimal controlled leakage despite the great pressure exerted.
Development is not stopping there: Research by EagleBurgmann continues to focus on enhancements for reinjection operations at even higher pressure – up to 550 bar (7,975 PSI) – while ensuring optimum safety and reliability.
For more information on the high pressure PDGS seal, contact us at email@example.com.