High pressure compressor seal for Tupi 4

Mechanial seal for compressor re-injection applications

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Sales locations

Situation

  • GE compressors used for gas re-injection on the Tupi 4 platform in the Lula oil field off the coast of Brazil
Product portfolio

Challenge

  • Extreme mechanical load of the compressor seal due to process pressures of up to 428 bar (6,206 PSI)
Advantages (big)

Solution

  • EagleBurgmann tandem PDGS seal with intermediate labyrinth

The Tupi 4 FPSO (Floating Production Storage and Offloading) is stationed in the Lula oil field, which was discovered in 2006 and is located in the Atlantic Ocean, south east of Rio de Janeiro. It contains pre-salt oil and gas, so called because hydrocarbon-bearing zones are situated under layers of rock and salt. The ocean depth averages about 2,000 meters, the oil- and gas strata, holding 5-8 billion barrels of oil equivalent, are situated 4,000-5,000 meters below that. Lula is brought into full production using FPSO platforms, like Tupi 4.

As they drill deeper into the ocean floor, oil producers demand higher process pressures to sustain the steady crude oil flow and therefore justify the large investments and costs. Every PSI of additional pressure counts.

EagleBurgmann – one of the internationally leading producers of sealing technology – sets a new standard for dry gas seals (DGS) used in ultra-high pressure gas re-injection systems. The EagleBurgmann mechanical seals installed in GE compressors have the highest static design pressure rating of any DGS certified for deep water gas re-injection compressors.

The design pressure of 428 bar(g) (6,206 PSI(g)) is not just a test-bench achievement: it is the actual operating point for the Tupi 4 re-injection compressors. The pressure is required for system startup and when the compressor is tripped as the suction and discharge pressures equalize (settle out pressure). Those 428 bar(g) (6,206 PSI(g)) exceed the design pressure of any seal employed in comparable ultra-high pressure re-injection systems worldwide, by several PSI.

Natural gas or supercritical carbon dioxide (CO2) are more and more replacing water as the more economical re-injection medium. At the Tupi 4 offshore oil production site, it is an abundant byproduct, which is valueless, even an environmental cost as it cannot be vented to the atmosphere. Re-injecting CO2 sequesters it below ground.

The Tupi 4 partners, led by Brazil’s state-controlled oil company, require the highest pressure possible from compressors and mechanical seals, within the parameters of safe and reliable operations to create an effective miscible zone to flow the crude to the production well.

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The seals developed for the Tupi 4 FPSO and installed in GE Oil & Gas BCL306/D vertical split compressors represent the leading edge of re-injection sealing. They are designed for a maximum shaft speed of 13,844 min-1. The high design pressure of 428 bar(g) (6,206 PSI(g)) raises the level at which the compressor is tripped. Avoiding depressurization saves process gas and considerable time by dispensing with the lengthy shutdown and re-pressurization protocols.

Previously proven in the oil fields of the Caspian Sea, EagleBurgmann deployed a tandem DGS with an intermediate labyrinth on Tupi 4. Tandem DGS layouts – comprised of primary and secondary seal – are widely used in petroleum production and pipeline operations. They are considered to be the best choice for ultra-high pressure re-injection.

 

Illustr.: Gas injection compressor at the Tupi 4 FPSO

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The Tupi 4 seal combines all technical aspects that are required to achieve the optimal compromise between leakage reduction and torque at startup. Three considerations are of particular note:

Functional gap between sealing elements

The functional gap of a tandem DGS is the gap between the balancing sleeve and the 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 has to 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, extensive FE-calculations were carried out by EagleBurgmann 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, 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 with a single sleeve assembled above the seal shaft. For an ultra-high pressure DGS the relatively small cross-sections of this solution would be too weak to handle the high axial load. Instead, the sleeves were split.

Material selection for ultra-high pressures

At ultra-high pressure, extreme mechanical loads occur – such as the high torque at start-up when the seal faces are still in contact. A special emphasis must be put on the selection of materials and the mechanical properties of the seal faces.

EagleBurgmann’s extensive experience with hard/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, and at the same time maintain optimum thermal conductivity. Also, at full load, the power produced mainly in the seal gap is in the range of 25 kW due to the sharing action of the high density gas. The seal design and choice of materials make sure this tremendous amount of power is easily dissipated into the surrounding gas and metal parts.

 

Illustr. 1: EagleBurgmann PDGS compressor seal (cross-section)
Illustr. 2: Cross-section of a Tupi 4 compressor seal

As a result of the exploitation of new fields in Oman, in the Caspian Sea and off the coast of Brazil notable advances in re-injection technology have taken place since 2000. This has led EagleBurgmann to develop ultra-high pressure variants of its proven DGS, which is used in petroleum operations worldwide.

By optimizing the existing DGS design, it was possible to progressively raise the DGS design pressure, without compromising operational reliability and integrity.

The sour gas content (a natural gas with high levels of hydrogen sulfide, an aggressive corrosive) of the Caspian oil fields presented a more challenging task. The Tengiz and Kashagan fields have 23 % and 17 % H2S content respectively. This had to be taken into account during design and the material selection. In 2005, at the invitation of GE Oil & Gas, EagleBurgmann undertook research 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 a maximum shaft speed of 12,373 m-1 for the Caspian projects – milestones that were surpassed by the Tupi 4 seal.

Any high-performance seal design must balance multiple objectives to achieve the best possible overall result. In the case of the Tupi 4 seal, extensive testing at the EagleBurgmann R&D center 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.

The potential for development is not yet fulfilled: Research at EagleBurgmann continues to focus on enhancements for re-injection operations at even higher pressure up to 550 bar (7,975 PSI) – while ensuring optimum safety and reliability.

Details about the PDGS
Details about the PDGS

Additional information about the PDGS can be found here...