Drilling rigs stand up to the power of the ocean

Manmade fluid power systems work to counteract Mother Nature's own fluid power -- ocean waves and currents.

Land-based drilling has been used for more than a century in the search for oil and gas. The first "offshore" wells weren't drilled until the early 1900s, and even these were drilled from piers in swamps and lakes -- generally in very shallow water. It wasn't until the mid 1900s that drilling in 100 ft or more of water became viable.

Modern offshore wells were first drilled from mobile platforms that were fixed to the ocean floor over the oil reservoir. These jack-up rigs, which are still in wide use today, are self-elevating. This enables operators to lift the drilling platform out of the reach of waves and tides. Today, jack-ups are generally limited to water depths of a few hundred feet.

For reservoirs in deeper waters, it was not practical to build a support structure to keep the drilling platform on location. Deeper wells required drilling to be accomplished from a floating structure.

The 1960s saw the predecessors to modern day floating drilling vessels. These vessels, unlike jack-up rigs, must have control systems to combat the effects of the ocean: waves, tides, currents, and winds. The motion compensator system is one such system, which uses hydraulics to compensate for wave-induced vertical motion of the drilling structure.

Motion compensator system

Today, drill ships and semi-submersible drilling platforms use motion compensator systems to allow drilling in water depths to 10,000 ft., while experiencing waves as high as 40 ft. A simplified offshore drilling system is represented in Fig. 2. The motion compensator is made up of:

* compensator cylinders,
* a main accumulator,
* multiple air pressure vessels (APVs),
* an isolation valve, and
* a pilot accumulator.

All of the compensator components are rigidly fixed to the drilling vessel, except for the cylinder pistons that are positioned at mid-stroke and are attached to the drill string (a term used for the entire length of joined sections; also called drill pipe). As long as the MCV is open, fluid flows from the cylinders into the main accumulator when the vessel heaves upward and flows in the opposite direction when the vessel falls into a wave trough. Thus, the drill string maintains its vertical position while wave action moves the vessel.

Pressure in the system is set so that the net weight of the drill string (weight on bit) is at the desired level. For example, assume the desired weight on bit is 5,000 lb. If the weight associated with the drill pipe and other components supported by the compensator cylinders is 1,000,000 lb, and the compensator cylinder control areas equate to 500 in.2, the hydraulic pressure would be set at 1,990 psi, resulting in the desired weight on bit. Pressure of the system is adjusted as drill pipe is added or the desired drilling force is changed.

Any variation in the fluid pressure is seen in the system as weight-on-bit variation. Rig operators want to minimize this variation because it reduces drilling efficiency and component life. With day rates as high as $250,000, the last thing an operator wants to do is waste time pulling a 30,000-ft or longer drill string from the hole to change a worn out bit.

Pressure in the system will vary as the gas volume changes during a typical wave heave. The pressure will be highest when the rig is on a wave crest, because this is when the gas volume is minimal. In a wave trough, the gas volume will have expanded and the pressure will be at its minimum. These pressure extremes can be somewhat controlled by adding a greater volume of gas.

Another pressure variation is due to fluid pressure losses in the system due to fluid flow. The majority of this variation takes place at the wave mid-point and is minimized by properly sizing the pipes and by utilizing a flow efficient control valve.

The control valve, in addition to being very flow efficient for the weight on bit issue, needs to perform multiple functions for everyday operations. Also, it must automatically isolate the cylinders from the main accumulator should the drill string break. The isolation valve is a system in itself and the heart of the motion compensator system.