Meeting a power demand

Producing the world?s most powerful FPSO turbine .

The on-board electrical power demands of floating production, storage and offloading vessels (FPSOs) have risen substantially over recent years as bigger ships are built to facilitate higher volumes of production. Specialist engineering company Peter Brotherhood Ltd has manufactured a power generation system that is believed to be the most powerful used on an FPSO anywhere in the world.

There are a number of ways in which power can be generated on a ship. The most cost-effective and efficient way to do this on an FPSO is with a steam turbine driven alternator set. By burning gas extracted from the drilling process, water is heated and steam raised. The steam is then used to drive a turbine which is connected to an alternator that generates electricity.

UK-based Peter Brotherhood has designed and manufactured two condensing turbo-alternator sets for a vessel owned by Single Bouy Moorings. Each set produces 12MW of power, which is understood to be 50% more than any other in operation on an FPSO anywhere in the world. They are being installed on board the FPSO Marlim Sul, which will operate in the Marlim Sul field off the coast of Brazil.

“There are many technical factors to be considered when designing a turbo-alternator set for an FPSO,” said Ray Bowell general manager for Peter Brotherhood?s turbine division. “For example, the pitch and roll of a ship would have a fatal effect on the system if it were not taken into account during design.

“If this was not compensated for pumps used to circulate oil for the lubrication system would run dry as the vessel undulates. We allow for this and the degree to which this is done will depend on where in the world the vessel will be located and the severity of the weather conditions usually encountered in the region.

“A turbo-alternator set can be installed internally or externally on a vessel. We are often constricted by the space that is available and have to negotiate and work around fixed obstructions such as pipe work.

“We use computer aided design (CAD) to create virtual models of every component and the complete turbo-alternator set. We then integrate this with a software model of the installation area. This allows us to identify potential problems of pipe work clashes and/or difficult or impossible maintenance access areas which may have otherwise only become apparent during installation.”

This has helped to achieve reductions in the time between receiving an order and delivering the product. It also reduces the potential for human error and the need for ?fixes? to be found during installation.

“Turbo-alternator sets used on FPSOs typically operate at a very high temperature and pressure. The system we have developed for Single Bouy Moorings will reach temperatures of around 490°C and the steam at the inlet will be at 60 bar ABS,” says Bowell. “With the system operating in this condition and in a confined area, safety is paramount. We install numerous sensors to continuously monitor the status of the system. Should a problem be detected an emergency stop valve is able to shut down the turbine in 0.6 seconds.

“To help design steam turbines for FPSOs, we use a number of specialist techniques,” continued Bowell. “For example, computational fluid dynamics (CFD) allows us to model and understand the behaviour of the steam at every stage in its passage through the turbine. The ability to optimise aerodynamics in real time allows us to tailor each individual machine to its intended application.

“Finite element analysis allows us to calculate precisely stresses in critical areas and therefore to build in safety factors that are more accurate than methods used in the past. This has led to more economic use of materials and reductions in overall machine weight and cost.

“Piping stress analysis and acoustic modelling ensure that pipework and ancillary equipment which forms part of the turbine package meets the same high standard as the machine itself and permits us to minimise noise and vibration.

“We are also looking at the precision casting of nozzle segments and nozzle banks using moulds created using ‘rapid prototyping’ technology direct from 3D computer design models.

“Other recent changes in our turbine design have included adoption of fabricated casings (ten years ago most mid range turbines had cast casings) and laser-cut and welded (rather than vacuum brazed) diaphragms. Brush seals, common on larger machines for many years, are replacing labyrinth seals on mid range machines, allowing us to secure the same pressure drop over a shorter length of shaft and therefore contributing to reductions in the overall size and weight of the turbine.

“Twisted and tapered blades, designed to maximise efficiency, are now common on even relatively small machines. More design enhancements to improve efficiency further are being introduced ? such as flared rather than parallel diaphragms.”

Reliability is crucial

“When designing steam turbines for application other than for FPSOs there is always a trade off between the total cost of the system and its operational efficiency,” says Bowell. “FPSO applications are different. Because the fuel used is in effect free (the gas would otherwise be flared) efficient conversion of fuel to energy is not an issue. What is an issue is reliability. Once an FPSO is in operation offshore it has limited access to spare components and maintenance teams.

“Properly maintained it is not unreasonable to expect a steam turbo-alternator set to operate reliably and efficiently for a minimum of 25 years ? and this is why they are the most economic way for FPSOs to generate electricity,” continued Bowell.

“Two key aspects to long operational life are the cleanliness of the water and the oil used for lubrication. If the water contains impurities, these will build up on the turbine rotor blades and will ultimately cause the system to fail. Impurities in the oil will have a similar effect, leading to degradation of bearings and other moving parts.”

“Until recently turbine maintenance has been schedule-based. After a defined number of hours running, certain maintenance tasks were carried out. They were done whether they were necessary or not – because there was no way of determining whether or not they were needed without disassembling the turbine to take a look.

“Modern monitoring systems – continuously checking such things as temperatures, pressures, vibration and pressure drop ? allow operators to determine when maintenance is actually required. Because old-fashioned maintenance intervals were based on precautionary and conservative principles, the advent of requirement-based maintenance has led to less frequent servicing when things are operating normally ? and swifter intervention when things are starting to go wrong.

“Taken together, these two effects have not only dramatically reduced downtime but also the number of skilled maintenance personnel which an operator requires to optimise performance. This in turn has reduced operating costs, justifying the increased capital cost of installing the monitoring systems in the first place.

“Alongside this has come a considerable extension of remote operation and monitoring. Starting a steam turbine from cold used to be a highly skilled manual operation. Now it can be completely automated and carried out remotely from a control room.”

Peter Brotherhood?s expertise in designing and manufacturing steam turbines for FPSOs has led the company to adopt ways of working which can provide substantial financial benefits for FPSO operators. The most significant is the company?s design and manufacturing process, which has been ?tuned? so that it is able to build systems much faster than the industry average.

“There are many challenges when developing turbo-alternator sets for FPSOs,” said Ray Bowell. “Always the most critical challenge is delivery. Because our systems need to be installed shore side, precise timing is essential so that we do not hold up the launch of the vessel.”