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Gas pipeline recompression is changing as global demand for long distance gas transportation skyrockets

19th May 2014

Emerging markets are crying out for natural gas to fuel their booming economies, whilst production levels have soared due to new concepts increasing output and targeting new reservoirs, but connecting the two requires robust new technology

Natural gas pipeline compressor units: Pipeline compression units need to push harder and work longer
Pipeline transportation remains the chief method of natural gas supply

Gazprom and CNPC have just signed an historic 30-year trade agreement worth USD 400bn to supply China with 38 billion cubic metres a year of natural gas via a new pipeline. The world needs gas, and pipelines remain the most effective means of transportation. If the human world was one organism compressor units would be the thousand mechanical hearts that keep it alive.

Pressure pushes the gas through the pipeline, but as the miles fly past the gas starts to lose momentum and stagnate. This is why the gas needs to be intermittently recompressed and pumped back into the pipeline at intervals of around 100 kilometres depending on temperature, pipeline diameter and gradient.

This has always been the case, but new challenges in pipeline construction are appearing as operators try to find ways of supplying growth markets with fuel. China alone saw natural gas imports from overseas jump 25 per cent in 2013. Liquefied natural gas (LNG) is of course an attractive new area of technological pioneering but it is nowhere near as efficient at transporting very large volumes of gas as through a pipeline.

Around 25,000 kilometres of natural gas pipeline are laid every year – which therefore amounts to around 250 new compression stations being required each year too.

So, as greater volumes of gas are pumped through pipelines, the compression stations along the way need to become ever more reliable and efficient, as well as their being a strong focus on reducing their footprint and level of emissions. There are traditionally three basic types of engine that power gas compressor units. A turbine/centrifugal compressor uses a small amount of gas from the pipeline to fuel the turbine, which rotates the large centrifugal fan that pumps the gas. An electric motor centrifugal compressor uses a high-voltage motor to power the centrifugal fan, which creates little on-site emissions but requires a very reliable source of electrical power. Thirdly there are reciprocating engines/centrifugal compressors, which use gas from the pipeline to power an internal combustion engine. The pipeline gas is fed inside the engine’s reciprocating pistons and pumped back into the network.

The Russians are leading the way in compression station size and power. The Portovaya Compression Station is the world's most powerful recompression facility. Its aggregate capacity is 366 MW; it boosts the pipeline to a working pressure of 220 bar, and provides a gas transmission distance above 1,200 kilometres. It is located in the Portovaya Bay near St Petersburg and secures gas transmission for the Nord Stream pipeline. But Portovaya is soon to be eclipsed by its southern cousin – the Russkaya Compression Station for South Stream Transport. This station has the unenviable task of pumping natural gas the entire length of the pipeline’s subsea section, which equates to nearly 1000 kilometres of subsea pipeline, until it touches land again in Bulgaria.

“Each of the four offshore pipelines will be 931 km long, with an maximum operational pressure of 284 bar at the gas inlet in Russia, dropping to well below 100 bar at the outlet in Bulgaria,” explained South Stream Transport spokesperson Jasper Jensen. “On the Bulgarian shore, the project will connect to a compressor station of the joint venture company South Stream Bulgaria, which is responsible for the construction and operation of the South Stream Onshore Pipeline through Bulgaria.”

For both projects, South and North Stream, Gazprom has used its compression unit technology, the Ladoga Gas Compression Unit (GCU). Russkaya will be equipped with fourteen of these GCUs, resulting in 448 MW of aggregate power use.

The units are manufactured by REP Holdings (REPH), a St Petersburg-based firm closely linked to Gazprom, and are based on the GE Oil & Gas’ MS 5002E gas turbine design. REPH bought the contract for the design back in 2012, believing the ME 5002E design ideally suited to the needs of ultra-long-distance natural gas pipeline transportation.

The key benefit of the MS 5002E/Ladoga 32 turbine is that it is adaptable and can be customised onsite for specific regions and specifications, which is especially important considering the harsh weather conditions and remoteness of areas in Russia where transporting equipment is costly and difficult.

In Russia’s case, compressor unit technology isn’t really about introducing new concepts but tackling old problems with more cost effective solutions. So they have taken a relatively simple design that is proven to perform consistently and to require little maintenance, and then simply used lots of them to reach output requirements. The alternative being that they invest in very heavy duty machines that may promise incredible levels of productivity, but which can’t cope with the extremes of the region’s weather and will cost more in downtime and maintenance than originally budgeted.

Pipeline compressors see an extremely large range of operation, due to the cyclical nature of both daily demands (night versus day) and seasonal demands (winter heating versus summer cooling) of natural gas. Most pipeline compressors are characterised with a low pressure rise and a high volume flow.

Traditionally, clients want compressor stations to be reliable and cost effective to increase their profitability and enhance their reputation of reliable transportation of natural gas.

Many western pipeline companies are experiencing a similar demand from operators as those in Russia, but do not believe that having many compressors at one station is an alternative to fewer better compressors.

“The power required by the compressor is a function of the mass flow rate of the gas being handled and the pressure ratio to which the gas is being compressed,” says Harry Miller, director of emerging technologies at Dresser Rand. “We have seen a trend toward larger power, especially in electric motor driven stations. Due to the economies of scale, larger units always provide a lower installed “cost per brake horsepower (bhp).” Also, since larger compressors are more efficient, they also provide the transmission company with a lower power cost. This trend is aided by advances in remote monitoring and the ability to “optimise” the operation of the overall pipeline. The pressures and flows between individual stations can be balanced – remotely – in real time to maximise overall efficiency. This approach offers less capital cost and better efficiency compared to having multiple small units per station.”

Flexibility and modularization are also key characteristics that operators are looking for because many compressor stations are in very remote locations and the equipment will go through varying operating conditions in its lifetime.  The ability to minimise footprint and field construction time are also important factors because the total installed cost of a compressor station is often three to four times the price of the compressor. Achieving this without losing efficiency or power is the key driver of compressor technology development.

“New designs are being launched to provide enhanced capability and more effective solutions. Many pipeline companies are investigating the use of non-traditional centrifugal compressor technology, such as magnetic bearings, hermetically-sealed compressors and high-speed motors for applications in the future.

“For example, the Datum C compressor is a hermetically sealed, integral high-speed motor-driven compact compressor with magnetic bearings designed for natural gas pipeline and process gas applications. Its small footprint and modular compressor bundle make installation and maintenance easier than for conventional units, while minimising the requirements for auxiliary systems and buildings.

“New designs are also being created to meet new market needs, and as emissions regulations on gas-powered compression become stricter, more technology is needed on the pre-combustion and post-combustion treatment in order to reach increasingly low levels.

“As of today, none of the basic principles of centrifugal compression are being replaced by newer concepts. However, newer concepts are being investigated, such as supersonic compression which uses shock waves to accomplish the compression function. Although not ready for commercial application, Dresser-Rand is engaged in continued research and development in this area.”

Much of the new technology being developed is targeted towards North American shale plays, mainly because this is an area which garners a lot of funding. But as other regions of the world start to unlock their own unconventional reservoirs, the same technology can be applied and adapted to each area’s unique requirements.

“We see the demand for compression coming from all three primary sectors of the oil and gas marketplace (up-, mid- and downstream) which encompasses oil and gas production, gas transmission, and refining and petrochemical, respectively. The continued demand for energy and the need to replenish dwindling reserves continue to spur the upstream, whereas, the recent emergence of shale gas is driving the midstream and downstream segments.

 “Unlike traditional gas reservoirs, shale gas cannot be “locked” into the well without risking the life and viability of the well. Therefore, the shale gas is either sold or flared. This inelastic “boom” in supply is driving a large amount of pipeline infrastructure changes.

“Internationally, energy demand is growing exponentially as developing nations advance, so new pipelines must be built to bring cost effective energy to areas that need it.”

There will remain a continued need to develop new gas compression technology as rapidly developing nations like China and India gobble gas as fast as possible to sustain economic growth and avoid international criticism over excessive coal-sourced emissions.

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