How conservative is Oil & Gas industry?
Oil & Gas industry is known for its conservative attitude. Such statement is principally correct. However, it depends what you understand under being conservative. Moreover, are there any exceptions?
While being generally conservative, oil & gas sector is also implementing very sophisticated and complex technologies, often operated in demanding service conditions. Such applications can be considered as novel. In this post we show few examples that may change your view on this industry a bit.
Conservative approach
You may associate oil and gas industry mainly with the conservative approach. There are quite clear reasons behind it. The liquidated damages in case of unavailability of the system are so huge that it is worth to spend more money when it can make the system more robust. The price tag of a trip and subsequent downtime of few hours may be in similar order of magnitude as the purchase price of the (drive) equipment. Thus, a measure eliminating such trips has a very quick payback time.
Design of drive systems employed in oil and gas is known to have design margins (i.e. power overdimensioning) and many accessories for protection or condition monitoring that are less usual in other industries.
The concept typically involves following points:
- Design margins
- Dimensioning for the worst case
- Reduced temperature rise
- Redundancy at different levels
- Rigorous testing with strict acceptance limits
- Condition monitoring (offline and online)
- Only proven solutions accepted
Examples of such design “features” may be:
- Margin stacking: The dimensioning power of the electric motor is easily 10-20% higher than the real load demand. The inverter may again have 10% margin compared to the motor rating. Thus, the inverter will likely never be operated at its full rating.
- Temperature rise: The allowed temperature rise is sometimes reduced compared to a common practice in other industries. Operation at reduced temperature shall provide more margin and extend the lifetime of the equipment.
- Online condition monitoring: Online partial discharge monitoring of transformer or motor, vibration monitoring.
High-tech applied in oil and gas
Despite the generally conservative approach, the oil and gas industry has been adopting many high-tech solutions and even played a pioneering role in some of them. Knowing these cases gives you a different view at oil and gas.
Power from shore
Many offshore platform currently use local power generation based on gas turbines or diesel engines. That is the conventional solution. However, the aim to reduce local emissions puts power from shore into the game. There are several principal solutions. The power transmission can be done using AC or DC. And may involve a static frequency converter.
Mentioned technologies are pure high-tech. They apply power electronic systems in a way that was not used before. The environment is extremely challenging, the reliability expectations are high and top performance is required.
A front runner in the field of power from shore is Norway. As the country has an excess of renewable hydroelectric power, it makes sense to power the offshore consumers with power from shore. Newly built platforms mostly have power from shore concept as integral part of the design. Existing platforms may eventually be retrofitted: decommission local gas turbines or diesel generators and supply the power from onshore grid.
General challenges of power from shore:
- Long distance from shore
- Medium to high power demand
- Limited weight and space offshore
- Low short circuit power and larger voltage variation
- Large surplus of reactive power in case of AC cable transmission
- Need for reactive power compensation (static and dynamic)
- Demanding transients, such as full load rejection
- Rough ambient conditions offshore (corrosive environment, strong wind, hazardous area)
a) AC transmission
Power from shore can be transferred by using a submarine AC cable laid on the seabed. Current record is 161 km of cable length (project in operation) and will be surpassed with the new case reaching 250 km of cable (commissioning planned in 2027).
b) HVDC Link
Some oil and gas platforms use HVDC link to bring the power from shore to the consumers.
Long step-out systems
These systems are used in configurations where the VFD is on the platform (topside) while the motor and compressor are installed at the seabed. The cable length between the inverter and the motor ranges from several kilometres to tens of kilometres. Moreover, the motor is often a high-speed design (direct drive) to eliminate the mechanical gear. To minimize the transmission losses and voltage drop in the system, a step-up transformer at the inverter output and step-down transformer at the end of the cable (before entering motor terminal box) is used. The high-tech solution is then mainly the motor-compressor operated subsea. Special hermetically sealed casing is used.
Subsea development
Next step in the subsea development is to place also the VFD and input transformer at the seabed. This solution is still in the development phase although some tests in shallow waters have already been successfully performed in shallow waters.
Subsea comes with extremely demanding requirements similar to space exploration. The high availability, generally required in oil & gas industry, becomes even more crucial here.
Subsea drives significantly differ from the traditional industrial drives. While normal drives are either air-cooled (at low and medium power) or water-cooled (in medium and high power range), the subsea drive is submersed inside oil vessel. The principle is well-known from transformers but integrating power electronics into an oil tank is a bit different.Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.
Pioneering installations
HVAC for Power from Shore
Martin Linge employs the world’s longest subsea cable for power from shore with astonishing 161 km of length. The power from shore was connected in 2018 and the platform is on stream since June 2021.
Technical data [3]
Cable length: 161 km
Cable voltage: 145 kV
Transmission: AC
Cross section: 3 x 1 x 300 mm2
Max. power demand: approx. 55 MW
Load profile: approx. 35 MW
Martin Linge project pushes the boundaries of cable AC transmission system. According to the Figure 5 in [3], for the given power and cable length the project is almost at the borderline between AC and DC transmission. The choice for AC, despite several drawbacks, was mainly the reduced number of components offshore (DC transmission would require DC/AC converter terminal offshore plus possibly a harmonic filter).
The currently executed project NOAKA [4] will surpass Martin Linge in terms of AC cable length.
Technical data
Cable length: 250 km
Transmission: AC
Power capacity: approx. 120 MW
NOAKA will combine two power electronics based power quality solutions: STATCOM and Thyristor controlled series capacitor (TCSC). The series capacitor enables further extension of the AC cable length while STATCOM helps to balance the reactive power. Commissioning of the project is planned for 2027.
HVDC for Power from Shore
Troll A platform is the tallest structure ever moved by people over the surface of Earth [1]. It was the first platform in the Norwegian continental shelf to receive power from shore. Two HVDC links supply HV motors on the platform (see motorformer in the next section). Thus, it is a special customized type of HVDC where the DC/AC offshore converter generates variable AC voltage with frequency range 42 to 63 Hz [2]. It is basically an HVDC link and a variable frequency drive combined in one.
Motorformer
Did you know that offshore oil & gas industry sector has one of the first installations of a motorformer?
Troll A platform has one more world record. It is the first installation of a motorformerTM. Motorformer is a large-power high voltage motor that can be connected directly to HV grid eliminating the need for a step-down transformer [5]. The stator winding is made of XLPE cables which makes the main difference to the conventional machines with VPI insulation system.
On Troll A, two motorformers are installed. Each is rated 40 MW with nominal stator voltage of 56 kV. These motors are supplied directly from the HVDC Light links. This is a pioneering installation of variable speed HV motor supplied directly from HVDC inverter without any step-down transformer. Such solution is very attractive for offshore installations where weight and space is always critical.
When introduced in late 1990s, Motorformer was ahead of his time. Consequently, only few projects had been realized to date. The more interesting is to know that the pilot installation was in oil and gas offshore application.
Long step-out
Åsgard subsea is the world’s first subsea compression system. Two 11.5 MW motor-compressors are in operation at the seabed (first unit since September 2015, second unit since January 2016).
One of the challenges in Åsgard gas field is that there are no local operators. The system needs to be designed and manufactured in modules that are then submersed and put together on the seabed close to the wellheads some 300 meters below the sea level.
This technology is a milestone for the subsea industry opening up new possibilities for future development.
Summary
Oil and gas industry is generally a conservative segment when it comes to dimensioning, design rules or implementation of new technology. However, there are companies and areas that implement high-tech solutions and set the new standards. This is especially the case in demanding operations like offshore and subsea. This article provides several examples of high-tech innovative technology implemented in oil and gas: power from shore (HVDC, long AC transmission, high power STATCOM, TCSC), HV motor (motorformerTM), long step-out drive systems and subsea drive systems.
References
[1] Troll A gas platform, Wikipedia – the free encyclopedia, available online,
https://en.wikipedia.org/wiki/Troll_A_platform
[2] Troll A provides a platform for HVDC Light and Motorformer, ModernPowerSystems, November 2005, available online, https://www.modernpowersystems.com/features/featuretroll-a-provides-a-platform-for-hvdc-light-and-motorformer-/
[3] Thibaut, B. Leforgeais, Selection of power from shore for an offshore Oil and Gas development, PCIC Europe 2013
[4] Hitachi Energy and Aker BP on schedule to a world record in the North Sea, available online, https://www.hitachienergy.com/news/features/2022/12/hitachi-energy-and-aker-bp-on-schedule-to-a-world-record-in-the-north-sea
[5] Eriksson, MotorformerTM – A new motor for direct HV connection, ABB Review 1/2001, available online, https://library.e.abb.com/public/96556b75eef8ad35c1256ddd00346e8e/22-25%20M697.pdf
[6] Leijon et al., “Breaking conventions in electrical power plants”, CIGRE, Sept. 1998
[7] Åsgard – Solving one of the subsea’s biggest challenges, Aker Solutions, available online, https://www.akersolutions.com/what-we-do/projects/asgard-solving-subseas-biggest-challenge/
[8] Monsen, K.S. Rongve, T. Lagreid and C. Gutscher, Asgard Subsea Gas Compression – Technology qualification testing with high-speed VSD and very long step-out cable, IEEE Trans. on Industry Applications, Vol. 50, No. 2, March/April 2014
