A successful offloading operation has been carried out with a mono-column type Floating Production, Storage and Offloading (FPSO) vessel-

FPSO Sevan Piranema

The FPSO Sevan Piranema is moored in 1000 m of water, (left) where it operates under lease to Brazilian national oil company Petrobras S.A.The FPSO is shown here during its tow-out to the Piranema field. (Right) The hull of the FPSO was built at the Yantai Raffles Shipyard in Yantai, Shandong Province, China, and then moved by a heavy lift transporter vessel to the Keppel Verolme Shipyard in Rotterdam, Netherlands, where the outfitting and assembly of the processing plant took place.

believed to be the first ever in the world using such a unit. Piranema Serviços de Petróleo, a subsidiary of Sevan Marine do Brasil, reported the maneuver which involved the FPSO Sevan Piranema and the shuttle tanker Navion Bergen and took place recently in the Piranema Field, offshore Brazil. The movement was a simulated offloading operation in partnership with Petrobras in which all the systems and procedures related to the offloading were tested and approved prior to the first oil transfer from the FPSO Sevan Piranema, after the start of production.

The FPSO Sevan Marine is moored in 1000 m of water, where it operates under lease to Brazilian national oil company Petrobras S.A. The hull of the FPSO was built at the Yantai Raffles Shipyard in Yantai, Shandong Province, China, and then moved by a heavy lift transporter vessel to the Keppel Verolme Shipyard in Rotterdam, Netherlands, where the outfitting and assembly of the processing plant, power generation and crew quarters took place.

With an oil storage capacity of 300 000 barrels, an oil processing capacity of 30 000 barrels per day and a gas injection capacity of 3.6 million m3/d, the FPSO also has a processing plant for hydrocarbons installed on the deck, and depending on the characteristics of the field the processing plant has different modules for processing oil, gas and water.

OPRA Turbines supplied three of its OP16-3 dual-fuel gas turbine generator sets for the Sevan Piranema rated at 1.84 MW each. Combined with two smaller emergency diesel generators offering 1 MW each, the power module can deliver up to 7.7 MW of electrical power under ISO ratings to a split 690 volt bus for powering all ofthe FPSO’s operating, utility, hotel and emergency needs.

From tow-out at Rotterdam harbour, under tow across the Atlantic to Brazil, and until oil and gas production began at the Piranema Field, the OP16 turbines — operating as liquid fuel engines — provided electrical power for the FPSO and its onboard crew. When field production began and associated gas became available, the units switched to gas fuel. As long as natural gas is available on the FPSO it will be the fuel of choice. When natural gas is not available due to field shut-in or separation plant downtime, for example, the units will again be operated as diesel-fueled turbines.

Each of the OP16-3 turbines is installed in its own weatherproof, sound-attenuating enclosure incorporating a control room at one end of the turbine generator package. The air intake filter house with demisters, filters and inlet silencer is mounted on top of the enclosure. The filter house has a separate integral section to provide ventilation air to the enclosure.

The OP16 is an all-radial 2 MW gas turbine for midsized power generation applications in the on- and offshore petroleum industry and in combined heat and power and combined cooling heating and power installations in a variety of industries, the company said.

OPRA said its main design criteria when developing the OP16 included a nominal 2 MW gas turbine generator unit, robust and simple design, simplecycle thermal efficiency and compactness. The company said that some of these design aims could have been met by using a conventional axial compressor and axial turbine but said that such a configuration does not allow the above benefits. An axial configuration requires multiple stages of internally cooled turbine discs, blades and guide vanes, which all add length and complexity to an axial turbine.

The turbine

The turbine has 13 blades and 29 radial flow guide vanes, as contrasted with the many blades and guide vanes of an axial turbine.

OPRA added that its solution was to develop an all-radial gas turbine and achieve compactness through the use of a single-stage, one-piece centrifugal compressorand a single-stage radial turbine resulting in a compact configuration.

The company said the OP16 incorporated a range of other design features, which is helping OPRA to secure significant orders worldwide. These features include four reverse-flow combustion cans with a manifold and radial inlet guides, which allow the compressor and turbine to be mounted back-to-back with no added space requirements for an annular or can-annular combustion system — with the OP16 rotor providing a length reduction of 40%.

The turbine produces more than 4 MW of power, most of which is employed todrive the direct-coupled compressor with the net power of 2 MW delivered through the reduction gear to the generator. To achieve the 6.7:1 compressionratio that contributes significantly to good fuel (thermal) efficiency, OPRA said the compressor must turn at 26 000 r/min.

OPRA said a unique design aspect of the OP16 is its overhung rotor configuration with all bearings located in the turbine’s cold end where they are not exposed to high temperatures that can reduce oil life and increase operating costs.

OPRA reported that the oil system absorbs little heat from the rotor and only collected a small amount of heat from both the rotor and anti-friction reduction gear bearings. According to the company, oil cooling requirement are minimal, with a heat rejection rate of just 7000 cal/hr and the oil inlet temperature maintained at 50°C.

The transmission of power is achieved by a quill shaft attached to the rotor shaft with an epicyclic gear system that reduces rotor speed from 26 000 r/min to 1500 or 1800 r/min depending on whether generator output of 50 or 60 Hz is required. The company decided to opt for epicyclic reduction gear as part of its aim to select turbine components with reduced size and weight.

By using four combustors in the gas turbine, fuel flexibility was also achieved along with the ability to configure fuel nozzles and mixing swirlers for a variety of liquid and gaseous fuels, with the company adding that the engine operates on low heat value fuel as low as 30% of natural gas.

In an effort to reach the low exhaust emission capability with gaseous fuels, the company decided to adopt two different combustor versions for its two versions of gas turbines the OP16-3A and the OP16-3B. The formermodel has conventional fuel nozzles and film-cooled flame tubes.

To reach the even “leaner burn” demandedfor reduced NOx and CO levels, a different combustion flame tube, head and combustion air flow path was needed to permit increased air flow through the 3B combustor. The 3B model uses an impingement cooling flame tube design so that a higher percentage of the air flow passes through the flame tube’s burn zone.

With the single-shaft rotor configuration where the rotor operates a constant speed to maintain the correct electrical frequency of the generator — overall air flow through the turbine remains constant. Under part-load conditions — where less fuel is introduced into the combustors — the 3B model diverts part of the compressor discharge air through a bleed valve to the engine exhaust diffuser.

An added benefit of a radial inflow turbine is the need for fewer blades and radial flow guide vanes, both with increased cross-section flow path for the combustion gases. The turbine has 13 blades and 29 radial flow guide vanes as contrasted with the many blades and guide vanes of an axial turbine. This configuration allows the turbine to operate on a wide range of fuels with less effect of combustion deposit build-up that constricts the flow path of an axial turbine.

Bob Kietzman, president OPRA Turbines Inc., based in Houston, Texas, U.S.A., said he is optimistic of future sales for the OP16. He added, “We do recognize that the single-shaft radial gas turbine does have some application and growth constraints, but I believe that the future prospects are promising for the engine with its particular power range and engine characteristics.

dual-fuel gas turbine

OPRA Turbines supplied three of its OP16-3 dual-fuel gas turbine generator sets for the Sevan Piranema rated at 1.84 MW each.

“There are numerous applications which will use the OP16 in petroleum production and transportation, as well as marine auxiliary power generation and in industrial and commercial CHP installations.

“With current technology, the power limit of an all-radial gas turbine is around 2.5 MW. The OP16 is, however, expected to grow to power ratings approaching that level, while continuing to offer all of the performance and operational benefits which it provides.

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