System reduces the dangers of drilling at sea

A Moog product story

Edited by the Engineeringtalk editorial team Jan 29, 2008

As the platform moves up and down with the heave of the sea surface, hydraulic oil passes backwards and forwards from actuator to accumulator, keeping the platform stationary relative to the seabed.

Drilling in choppy seas is fraught with hazards.

As the sea swells relative to the drilling platform, the stresses and strains induced on a drill pipe will alternately take the weight of the ship or try to pull itself out of the seabed.

A servo hydraulic protection system from Moog provides a intelligent "flow fuse" that not only automatically resets, but regulates the platform speed in the event of an exceptional wave and 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 semisubmersible floating crane and pipe-laying vessel can move up to 4m at 1m/s in rough seas conditions and 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.

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 backwards and forwards from actuator to 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 the ability to flow at 6100 litres per minute and to open, or close, in 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".

"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", Furnell said.

"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".