Off the coast of Brazil, the Lula field (formerly Tupi field) was discovered in 2006 and contains pre-salt oil and gas, so called because the hydrocarbon-bearing zones are situated beneath layers of rock and salt. The ocean depth averages about 2,000 meters, and the hydrocarbon zones are 4,000 to 5,000 meters below that, holding estimated recoverable reserves of 5 to 8 billion barrels of oil equivalent. As operators drill deeper under the ocean floor, they require even more pressure to sustain the strong, steady flows needed to justify huge development costs and royalties. Every bar of additional pressure counts.
Dry-gas seals (DGS) used in ultrahigh pressure gas-reinjection systems are setting a new standard in offshore, deepwater applications. These mechanical seals are being installed on compressors at the Tupi 4 floating production site in the Lula field in the Atlantic Ocean, southeast of Rio de Janeiro. They have the highest static design pressure rating of any DGS certified for deepwater gas re-injection compressors. The 428 barg (6,206 psig) rating is not just a test-bench achievement. It is the actual operating point for the Tupi 4 re-injection compressors, which are required during system startup and when the compressor is tripped for any reason, as the suction and discharge pressures equalize (settle out pressure). That 428 barg (6,206 psig) is several bars more than seals employed by producers in comparable ultrahigh-pressure re-injection systems worldwide. Gas—natural gas or supercritical carbon dioxide (CO2)—is surpassing water as the most economical re-injection medium. It is an abundant byproduct of offshore oil production at the Tupi 4 site that is valueless. It is actually an environmental risk because it cannot be vented to the atmosphere. Re-injecting CO2, in effect, sequesters it below ground. 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, needed the highest pressure possible from the 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. The seals developed for the Tupi 4 FPSO and installed on the vertical split compressors represent the leading edge of re-injection sealing. They are designed for a maximum shaft speed of 13,844 rpm. The higher 428 barg (6,206 psig) raises the level at which the compressor can remain pressurized if it is tripped. Avoiding depressurization saves process gas and considerable time by dispensing with the lengthy shutdown and repressurization protocols. A tandem DGS with an intermediate labyrinth was the choice for this service. 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 ultrahigh pressure re-injection. The Tupi 4 seal reflects several technical considerations in the tandem seal design for ultrahigh pressure operations that help it achieve the optimal compromise between leakage reduction and torque at startup. The major factors are discussed in the remainder of this article.
Functional Gap at 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 to be 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 challenge to manufacture because the variation of the gap height may be influenced by the temperature and pressure. This influence must be minimized. To achieve this, extensive finite element calculations were carried out by the seal manufacturer before finalizing the design.
Stability Under High Forces
At ultrahigh pressure levels, tremendous forces caused by the pneumatic load act on the seal in the radial and axial directions. To ensure maximum seal stability 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. At the lower pressure, a single sleeve is used, which is assembled above the shaft sleeve. If only one sleeve was used for an ultrahigh-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 ultrahigh pressure.
Material Selection for Ultrahigh Pressures
These extreme mechanical loads at ultrahigh pressure—such as the intense torque at startup when the seal faces are still in contact—require that special emphasis be given to the selection of the materials of construction, including the mechanical properties of the seal faces. The seal manufacturer’s 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 startup. A special fluid-phase sintered silicon carbide material was chosen to ensure maximum the seal face strength 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 kilowatts because of the sharing action of the high-density gas.
The seal design and material choice are such that this tremendous amount of power is easily dissipated into the surrounding gas and metal parts. Notable advances in re-injection technology have occurred since 2000 as new fields have been brought into production in Oman; in the Caspian Sea; and most recently, in Brazil’s offshore, pre-salt fields. This activity has spurred the development of ultrahigh-pressure derivatives of DGS used widely in petroleum operations. Progressively, these DGS’s design pressures have been increased with refinements to existing DGS design and materials without compromising operational reliability and integrity. For example, the sour gas (natural gas with high levels of hydrogen sulfide—H2S) content of the Caspian oil fields presented a more challenging environment for compressor and seal integrity. The design and material of construction had to compensate for this aggressive and corrosive component. The Tengiz and Kashagan fields have 23 percent and 17 percent H2S content respectively. In 2005, a seal manufacturer undertook research at the invitation of GE Oil & Gas to develop a new ultrahigh-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 rpm for the Caspian projects, milestones surpassed by the seal. Any high-performance seal design must balance multiple objectives to achieve the best overall outcome. In the case of the seal, extensive testing by the seal and compressor manufacturer demonstrated that the seal delivered high reliability in common startup/shutdown scenarios and during continuous operations at full load, assuring compressor integrity with minimal controlled leakage despite the great pressure experienced by the equipment. Development will not stop there. Research 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.