David Bond, Benning UK, looks at the challenging environmental conditions faced by offshore oil platforms, and investigates power protection strategies.
The first submerged oil wells were drilled from platforms in the fresh waters of a lake in Ohio around 1891; by today’s standards, a reasonably benign situation. Since then, offshore oil extraction activities have extended to all parts of the world – creating widely varied oilfield settings and locations, all presenting their own particular challenges to the people and equipment working on the platforms and vessels involved.
Figure 1. Diagonal braces used to increase cabinet strength.
One challenge arises through the density, complexity, range and often sensitivity of the equipment found in oilfield environments. Much of this equipment must have a clean, uninterrupted electrical supply, with demand possibilities including three phase, single phase and DC inputs and outputs, sometimes at very high power levels. These complex, large scale requirements can rarely, if ever, be met by off-the-shelf uninterruptible power supply (UPS) systems, especially when the hardware also has to survive and operate reliably in extreme weather conditions. Instead, a more tailored approach that caters for the mix of electrical, physical and environmental demands specific to the target platform or vessel must be used.
Below are some solutions that can now be facilitated through the use of modern power processing technology. This article starts by reviewing the types of electrical and electronic equipment that typically need quality power, together with the issues associated with delivering it effectively within offshore installations. It then moves on to discuss how UPSs and related components can be designed into an efficient, resilient and reliable solution – the ‘tailored approach’ as mentioned above.
Oil platform equipment and its requirements
Sensitive equipment includes instrumentation and process control systems directly related to the platform’s extraction or exploration activities. Behind this there is data processing equipment for analysis, administration functions and communications, located in office and accommodation areas as well as in the production sections. Emergency lighting is a critical function that requires secure AC power, especially as there are strict regulations for oil platforms, demanding that emergency lighting remains fully operational for 72 hours after a call for evacuation.
Figure 2. Single power protection system providing both AC and DC outputs.
24 VDC supplies for switchgear and circuit breakers must also be secure to ensure that switching and tripping functions can continue with battery-backed support if the mains power fails. Auxiliary systems with protected AC and DC power requirements include fire and gas, navigation support and signalling equipment. Lube oil pumps create another demand for secure power.
These varied requirements create a considerable population of electrical and electronic modules that must be contained within a limited area; this makes space a major issue in oilfield and particularly offshore environments. Power conditioning equipment, like every other function, must occupy a minimal footprint with high power density. Power supply and protection equipment for the various supported systems is usually provided in separate cabinets, although AC and DC functions can be provided within the same enclosure. The mix of AC and DC power very much depends on individual customer requirements.
Ease of installation is also important. Traditional, more monolithic systems are not readily transportable and are often permanently welded to the platform floor. By contrast, more modern modular systems offer better manoeuvrability; cabinets can be carried by four people if their internal modules are removed. The modules are simple to re-install once the cabinet has been securely sited – a cost-effective solution, particularly for offshore platforms where exchanging equipment is associated with high costs.
A long operational life and very high reliability are essential, particularly because of the cost of any lost production time and the risk to safety and the environment resulting from a power failure. However, failures create further costs in these environments and these costs can be minimised by the rapid swap out of any faulty module.
Power system providers today have a number of strategies that can be flexibly combined to overcome these challenges effectively. These strategies relate to product quality, reliability and ruggedisation, modularity, design flexibility, efficiency and support.
Irrespective of its topology, product quality and longevity is fundamental for power equipment intended for offshore environments. Benning’s approach, for example, is to design products for a working life of 15 - 20 years to match that of the oil platform and other equipment. This is in contrast to commercial IT applications where the expectation is to replace equipment more frequently to keep pace with rapid technology advances. Such durability is achieved by paying attention to every aspect of the hardware, from the subassemblies and components to the external cabinets. Quality is also maintained by manufacturing and assembling the products in-house.
Printed circuit boards (PCBs) are populated with over specified components, carefully laid out to help prevent ambient contamination such as dust and moisture from creating short circuits across tracks and other problems. In more hostile ambient environments, these PCBs can also be coated to provide additional contamination protection. Power modules including rectifiers, inverters and DC-DC converters using these PCBs can then be integrated into enclosures entirely designed and built by the provider.
To complement the PCB design, the enclosures are IP-rated in accordance with EN 60529, preventing ingress of water, moisture, dust and vermin to customer-specified protection levels. They can also be designed to resist shock, vibration or even seismic activity using diagonal braces (Figure 1) or cruciform shapes. Cabinet packages with welded designs can be used for more extreme conditions. The resulting systems are not only reliable and long-lived, but also ruggedised appropriately for their environment.
Modularity in two forms
Modular topology can further enhance system reliability and availability while also tackling another offshore challenge – space efficiency. Within the context of these power systems, modularity can be used in two different ways, within one installation if appropriate.
The first is an innovative approach in which the UPS is not necessarily considered as an indivisible unit. Some applications simply require incoming AC mains to be conditioned and supported with battery backup before being passed on to the critical load. For these, a standard UPS configuration remains appropriate. Other situations, however could call for a range of conditioned outputs – AC and a set of DC voltages at different levels for example, to be derived from an AC input. An ability to operate from a DC input as well as the AC supply may also be necessary.
Under these circumstances, it makes more sense to think of the UPS in terms of its major components – the rectifier for converting incoming AC into DC for battery charging, the battery system, the DC-DC converter for providing multiple voltage levels and the inverter for converting DC back into AC. If components at this level are made available as modular building blocks, they can be assembled into efficient power solutions. Any combination and number of three phase and single phase AC and DC inputs and outputs can be accommodated, with DC-DC converters used to provide multiple DC outputs. The assembly could achieve considerable space-saving through implementation within a single enclosure, rather than dedicating one enclosure to a basic UPS and others to additional inverters and DC-DC converters respectively (Figure 2).
Modularity’s second form is well established within the commercial UPS sector. Now it is being supplied into oilfield and other ruggedised applications by manufacturers such as Benning. Modular UPSs comprise modules with a capacity down to 10 kVA each rather than a single monolithic solution. Each module is an entirely self-contained complete UPS. A number can be paralleled within an enclosure to achieve the target UPS capacity. This is efficient in terms of both cost and space because modules can be incrementally added to closely match any critical load requirement. A 50 kVA load, for example, could be exactly satisfied by five 10 kVA modules. By contrast, if only 40 kVA monolithic UPS were available, two would be needed to support the 50 kVA load, leaving a 30 kVA overcapacity.
This modular UPS topology also facilitates efficient implementation of redundant configurations. UPS reliability and availability can be greatly improved with minimal excess capacity. The 50 kVA load, for example, could be supported by six 10 kVA modules. If one fails, the others can fully support the load until the failed unit is repaired or replaced. This arrangement is known as N+1 redundancy.
‘Hot swap’ capability
Systems like Benning’s further improve availability through their ‘hot swap’ capability. A failed module can be removed and replaced without need to interrupt power to the load, so minimising repair time – which, together with extending mean time between failures, is essential in maximising UPS availability. Increases in load demand, as more equipment is installed, can also easily be accommodated, simply by plugging further modules into the power protection enclosures as required. All of the company’s power products – Enertronic UPSs, Tebechop rectifiers, Invertronic inverters and Tebechop DC-DC converters are available in modular, hot swap form, ready for 19 in., rack mounting, and a single rack can contain any combination of products. This allows configuration of easily maintained, high availability, high power density, minimal footprint systems that benefit from modularity in both its forms.
High efficiency design
While high power density, which is essential in oil platform installations, is boosted by the space efficiency of a modular design, it also depends on the design of the modules themselves. Benning’s UPS, rectifier, inverter and DC-DC modules employ Switch Mode Power Supply (SMPS) technology and are intended for industrial applications. These modules are four to five times lower in weight and volume and up to 20% higher in efficiency than traditional solutions.
Control and monitoringPower protection systems exist to provide clean, stable and uninterrupted power to their critical loads. To ensure they perform their role reliably, effective monitoring and control of their own condition is essential. Communication with local operators, and remote operators via Ethernet and the web, must be provided. External alarms and battery conditions should be monitored, with programmable alarm limits and set points. Data logging should record and time-stamp measured system values and events including alarms, device switching, faults and changes. Exchange of data with central control systems such as SCADA packages should be supported.
Project lifetime support
An offshore platform environment is often highly specific as well as highly demanding in its power requirements. Most power protection projects are designed to customer specifications and require unique engineering or design work with significant customisation effort. This makes it essential for operators and integrators to partner with power component providers that do not simply offer off-the-shelf products. Benning works with many of the world’s biggest engineering contractors, providing technical and commercial liaison from the start of a project through to maintenance and servicing of operational systems.
The company provides dedicated management teams that engage with the customer through all stages of a project, including specification review, project management, detailed documentation and engineering calculations.
Once the project moves from design to the implementation phase, further engineering capability together with a local presence become important. Benning has manufacturing plants in European locations, with wholly-owned subsidiaries in 25 countries and a network of agents across Europe, the Americas and Asia. Local assembly and testing facilities are often provided, and are capable of assembling customised power solutions and performing customer-defined system tests. On request, third party agencies or certification bodies such as DNV, ABS or TÜV may witness such tests. Factory acceptance tests (FAT) can often also be accommodated.
Oilfield platforms need power protection systems to meet specifications that are not only demanding but also highly specific to each installation. Nowadays it is possible to obtain power protection components and systems that can handle these requirements while being rugged enough to survive in oilfield environments. However, oilfield power systems typically require significant custom design and engineering effort as well as the building block components. Accordingly it is important to partner with a power protection supplier that can also supply the necessary engineering effort and international support.
Edited from an article by David Bond, Benning UK.
Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/24112016/offshore-power-protection-challenges/