As an offshore processing plant and storage facility, space is a premium commodity on an FPSO. Usually, production modules on FPSOs are installed closely to each other, creating tight working environments. Because FPSOs bear refinery-grade heavy equipment on a marine vessel, managing weight translates directly into savings in capital expenditure investment evaluations. Lowering weight by just one tonne (2200 lb) can result in construction savings of $30 000 to $50 000.
On FPSOs, hydraulic actuators are preferred to pneumatics due to their smaller power cylinders. In the case of one enormous valve specified on this FPSO, however, conventional hydraulic actuators were too large for the available space.
Situation: Size and Weight of Conventional Actuators — For this FPSO, the builders specified a DN 50 (NPS 20), PN 420 (Class 2500) ESD valve from Argus™. The valve required fail-safe, spring-return automation with a spring end torque of 250 000 Nm (184 000 ft-lb). When the actuator was sized, the dimensional footprint turned out to be almost 2 meters (6.6 ft) longer than for what the vessel layout plan had called. In such cases where space is a constraint, the conventional response is to turn to helical spline actuators. However, helical actuators are much heavier in comparison to Scotch yoke configurations.
A Custom-Designed Alternative — Flowserve Limitorque proposed a different approach: a custom-engineered layout. Limitorque R&D engineers designed an innovative quad-yoke system consisting of two dual-yoke mechanisms. This solution permitted a significant torque multiplying factor in a very limited horizontal displacement. The final design was code named “Margherita” (the Italian word for “daisy”) based on the shape of the quad-yoke mechanism.
Design Challenges — The design challenges involved ensuring the mechanical efficiency of the system and the precise synchronicity of the quad-yoke mechanism. To further reduce the critical horizontal dimension of the actuator, the hydraulic power cylinders were installed within the coil of the fail-action springs. The quad-yoke configuration was extensively modeled and validated through FEA before being released for construction, and the completed actuator was accurately tested for torque and performance to ensure compliance to expected and required operational criteria.
The overall design allowed construction of the FPSO to proceed according to the originally planned layouts, with no need for costly and invasive redesign. The Limitorque quad-yoke actuator solution also ended up weighing approximately 2000 kg (4400 lb) less than competing helical mechanism actuators.
Considering the significant cost-to-weight relationship of installed topsides on offshore facilities, this innovative design from Limitorque not only satisfied the customer’s performance requirements, but also helped in realizing significant construction cost savings.
The effort to custom design an actuator to work within tight space and weight restrictions demonstrates the capability of Limitorque to respond to critical customer needs, with unique and efficient solutions that combine technical excellence with economic practicality. The custom-engineered Margherita solution revealed itself to be sufficiently innovative for immediate patenting, providing a new turnkey actuator solution where size and weight are critical considerations.