Moog has developed an innovative hydraulic 'flow fuse' for protecting plant and personnel on offshore drilling rigs.
Performing drilling operations in rough seas is fraught with hazards. As the sea swells relative to the drilling platform, the drill pipe experiences heavy alternate compressive and tensile loads.
However, Moog has developed an ingenious servo hydraulic protection system that provides an intelligent 'flow fuse' that not only automatically resets, but also regulates the platform speed in the event of an exceptional wave - or brings the whole platform to a controlled stop in the event of a drill break.
A 68-tonne drilling platform such as that fitted to a semi-submersible floating crane and pipe-laying vessel can move up to 4m at 1m/s in these conditions, and therefore needs to be supported by hydraulic actuators to compensate for this heave.
A break in the drilling pipe on a heave-compensated offshore platform can be lethal, so protection systems are required. Traditionally these have been based on a mechanical system that shuts and locks if the flow across it exceeds a predefined level. When activated the system remains locked until manually reset, which can take some time. While this reset time is a minor setback if the system has activated as a result of a drill stream break, nuisance tripping can occur if a larger-than-normal wave is encountered.
Accumulator and actuator
Moog's system works by pre-charging an accumulator to a pressure, which is sufficient to hold the drill weight. As the platform moves up and down with the heave of the sea surface, hydraulic oil passes to and fro between the actuator and accumulator, keeping the platform stationary relative to the seabed.
Each actuator is fitted with a position sensor, which enables the Moog MSC Controller (M3000) to monitor the position and velocity of the platform. Two Moog DSHRE 80 servo cartridge valves are used to regulate the flow of the hydraulic oil with capability to flow 6100 litres/minute and to open or close in just 18ms.
Simon Furnell, Senior Applications Engineer at Moog, explains: "You have a huge counter-balance force to compensate for the drill weight. If there should be a break in the drill, the energy force released makes the platform shoot upwards and crash against the end stops. This would destroy equipment and endanger life. Although the current mechanical system protects from this potentially fatal event, it takes time to recharge and the customer wanted an active system that would monitor and adjust, depending on conditions, so there is no delay or no nuisance tripping.”
One additional benefit of the active system is that the customer was able to replace expensive hydraulic buffers with a simple rubber block at the top end of stroke because the system was able to come to a controlled stop before the buffers were reached.
Furnell states: "After we initially installed the system on the platform for testing, it was fitted with instrumentation to monitor acceleration forces. The operators of the monitoring equipment asked us to alert them to when we would start the drill break test; in fact the test had already happened and the results recorded. The results logged on the MSC show that the system worked exactly as predicted by the dynamic simulation Moog carried out at the initiation of the project.”
Moog works with users to understand their complex needs and deliver innovative systems that yield greater efficiency, less noise, easier control, tighter tolerances and exceptional reliability. The application of new ideas to improve safety and eliminate downtime as shown in this application reinforces Moog’s pre-eminent position as one of the hydraulic industry’s leading integrators of precision control components and systems.