Need a lift?
Published: 15 October, 2014
ODEE spoke with Martin Üre Villoria, engineering director at Sulzer Pumps (UK) Ltd., about the different applications requiring the lifting of seawater on offshore oil & gas applications, and the critical role pumps play in this process.
In fixed platforms, wellhead platforms or floating production, storage and offloading (FPSO) units, seawater is required for a large number of on-board applications. These include cooling, reinjection into the well, and firewater (firefighting applications). In the case of an injection loop, for example, seawater is pumped up to the platform, where it is then filtered and cleaned. The treated seawater can then be injected into the well to increase the pressure.
There are many explosive gases and liquids on offshore oil & gas platforms, so fire protection is a critical issue. In offshore applications seawater is a plentiful liquid source that can ensure firewater systems are as effective as possible. Firewater applications require independent loops and need to be available 24 hours a day in case of a fire incident, and so the technology must be in place to ensure that seawater can be lifted as and when required.
Martin Üre Villoria, engineering director at Sulzer Pumps (UK) Ltd., explains that a typical lifting arrangement could comprise a diesel driver with an independent diesel reservoir. “The diesel driver usually drives the lift pump, although the pump can also be driven by a hydraulic system,” he said. “In the case of a hydraulic system the power is transferred from the diesel shaft to a hydraulic power unit where the rotation and mechanical energy is converted into hydraulic energy. This means the oil is raised to a certain pressure and flow. This oil can then drive a submerged hydraulic motor, which in turn drives the pump.”
Üre Villoria explained that seawater lifting concepts are usually initiated by the large oil and gas companies such as BP Shell and Chevron, and are undertaken by the contractors and the designers of these platforms. “As our clients, these contractors and designers come to us with a datasheet and the required technology to lift seawater,” he said. “Although the decision to lift sea water for a specific purpose has usually been made before it lands on our desk, there are a number of ways to fulfil the task, and we can discuss all these with our clients.”
Specifically for the task of lifting seawater for various tasks, Sulzer has designed a centrifugal submersible pump, the SJS. The pump design is based on Sulzer’s own vertical turbine bowl assemblies, which, states Üre Villoria, are recognised for providing reliable and efficient performance. “The SJS pumps utilise a submersible motor to drive the Sulzer vertical pump stages,” he continued. “The main benefit of the new design is that the normal vertical lineshaft and bearings are eliminated. This allows the submersible motor and bowl assembly to operate at higher speeds, possibly reducing pump and caisson size. In the standard design, the submersible motor is assembled below the bowl.”
Üre Villoria added that, for applications that require a low net positive suction head (NPSH) and accordingly low submergence, Sulzer can also deliver an inverted design, which has the motor mounted above the bowl. The motor is then constructed with a double-wall motor case to guide the flow from the bowl assembly along the motor and into the column pipe.
Üre Villoria reflected that finding the best motor for the SJS pump was an important step in the development process. “When we talk about lifting seawater for applications such as cooling, injection or firewater, there are a number of proven methods to transfer power mechanically; for example, a hydraulic-driven system, or a line shaft system that normally comprises a 20 or 30m long shaft,” he said. A third possibility is electro submersible motors, and this was Sulzer’s preferred option for its SJS pumps. “The use of water-filled submersible motors with hydrodynamic bearings is a highly effective means of lifting seawater,” said Üre Villoria. “However, the biggest challenge is ensuring that the submerged electric motor doesn’t incur any seawater ingress. With this and other challenges in mind, we entered into a collaboration agreement with submerged electrical motor supplier, Indar, based in Northern Spain, in order to optimise the design of the motors for offshore applications.”
Indar’s motors are often employed for the purpose of lifting groundwater from municipal wells where relatively clean water is used. Therefore Sulzer and Indar set out to adapt the technology by applying materials that were more compatible with seawater, as well as improve some internal design aspects of the motor, including the winding procedures.
“For example, there are stator windings in the motor,” said Üre Villoria. “Sulzer has considerable experience of various types of windings because we have a division called Rotating Equipment Services and regularly undertake service and maintenance work on submersible motors. In the case of the electro submersible pump motor our intention was to develop the product in collaboration with Indar to offer new products for new platforms.”
For the water-filled motors, a waterglycol mixture was chosen. “The waterglycol mixture is much more environmentally friendly than oil, which is often used in motor designs for offshore applications,” explained Üre Villoria. “Another benefit of the waterglycol mixture is that cleanliness requirements are less stringent than with dielectric oil. This makes maintenance and refilling a more straightforward process.”
For the standard configuration (pump above the motor), Üre Villoria pointed out that the motor is connected through hoses to a simple topside expansion tank. “This tank provides an overpressure of the water inside the submerged motor, allows thermal expansion of the water during operation, and adds the possibility to control any leakage of the system with a simple sight glass and a level switch,” he said.
For the inverted design, a pressure accumulator is applied to provide the required overpressure. Üre Villoria also explained that, unlike an oil-filled motor, this waterglycol-filled motor tolerates some sea-water ingress. “It can continue to run for a certain time because the motor stator windings are insulated and do not short circuit as typical oil-filled motor designs would,” he said.
Another benefit is that the modular design of the Sulzer SJS with the motor coupled to the pump, rather than impeller mounted on the motor shaft, allows access to motor bearings and motor mechanical seal without the need to dismantle the pump bowl assembly. Üre Villoria explains that this facilitates easier maintenance. Also, temperature readings to the windings and the thrust bearings are standard features that provide sufficient operational machine protection. A variety of materials are available for the SJS pump, depending on the customer’s preference and operating conditions: cast iron or carbon steel for freshwater; 316, Duplex, or Super Duplex stainless steels for seawater; or coated wear part and bushing materials can be selected to meet specified requirements.
Simple but effective design
In conclusion, Üre Villoria commented: “Our customers like the simple design of water-filled submersible motors. They like the topside expansion tank in combination with a well-balanced cooling, water-lubricated, hydrodynamic bearing design inside the motor and the possibility of accessing both main modules – pump and motor – separately.”