FPSO vessels are used for processing oil and transferring it to tankers or pipelines. Positioned close to an oilfield, they remain there until the field is exhausted. Originally designed with a service life of over 20 years, FPSOs represent a huge investment for international oil companies, and to maximize their potential, they are refitted and relocated once a project is completed.
The three packages included 8-stage pumps designed to deliver 3’500 m head with a flow rate of 350 m3/hr using a 4.5 MW motor.
Before relocation, the vessel must undergo a refit. For example, an additional 20 m of height will be added to the bow to handle waves in the North Sea. All work will be carried out in a dry dock.
Retrofits are said to offer benefits over pump replacement. Financial savings equate to approximately 10% of the cost of a new pump. However, time is often the priority. With its engineering facilities, Sulzer delivers a pump retrofit in around six weeks.
Equally important are pump efficiency and reliability. To attain optimum application performance, pumps need fine-tuning. In this case, two duty points were required – flows between 330 m3/hr and 387 m3/hr with a differential head of 1750 m as well as a low flow, high pressure point of 104 m3/hr at 1960 m head.
To achieve the best solution, Sulzer proposed a pump de-staging, removing four of the eight stages. Super Duplex stainless steel de-staging tubes will guide the flow between the remaining impellers, preventing turbulence and maintaining efficiency.
The reduction in output also equates to lower power input. Here, the original fixed-speed, 11 kV, 5.5 MW motor could be replaced with a 3 kV, 3.5 MW unit controlled via a variable speed drive. Power will be reduced from 4.5 MW to 1.8 – 2.0 MW, depending on operating conditions, while motor efficiency will increase from 96.4% to 96.9%, providing considerable energy savings. This reduces carbon footprint, which is important when governments are imposing environmental taxes based on carbon emissions.
The reduction in motor power and discharge head enabled many original components to be retained, but a new lateral analysis of the de-staged unit was conducted, confirming the pump rotor stability throughout the operating speed range.
Due to the reduced speed of the new electric motor, the lube oil system output had to increase. A modification of the lube oil pump increased output by 30%, satisfying the demand for the pump, gearbox, and motor.
Pipework also needs to be approached carefully to avoid costly reworks on site. This can be achieved cost-effectively by using pipework with ‘green ends’, allowing it to be cut and welded in position on site, achieving a perfect fit. An alternative is to deliver the pipe and fittings to all the pipework to be fabricated on site.