Thursday, April 24, 2008

Keeping a Lid On Blowouts

by: OilOnline.com
Tuesday, April 15, 2008

Author Information
Eric Milne is electrical and software chief engineer at Hydril. Reach him at emilne@hydril.com.

As industrial accidents go, a blowout in offshore natural gas or oil exploration surely ranks among the worst. When not contained, a blowout can release high pressure gasses that are frequently toxic and flammable, resulting in environmental damage and possible fatalities. Keeping a lid on these events requires a combination of the right hardware and control strategy.

Offshore drilling is a complex process. A drill pipe or drill string extends from the drilling rig down thousands of feet to the wellhead and into the wellbore on the sea floor directly beneath. This drill string is contained within a riser, or solid casing, to create an enclosed space. Inside the riser is drilling mud, a fluid mixture whose sheer weight is intended to contain any upsurge from the highly pressurized formations that are the ultimate target.

But sometimes the "kick" from the newly released hydrocarbons can literally push the drilling mud up the drill string and riser. If not blocked by heavier mud or stopped by a pressure control system, the oil or gas can rush up the pipe and emerge like a geyser on top of the rig. This is a blowout, and on something as small as a drilling platform, it is a disaster.

Containing blowouts

Thankfully, blowouts are rare today. This stems in large part from the emergence of sophisticated and robust pressure control devices called blowout preventers (BOPs), controlled by real-time computing networks. Hydril LLC invented the first hydraulically operated BOP as well as the annular BOP, which features an opening lined with high-strength rubber that narrows or widens to control oil or gas flow.

Hydril's reputation for such inventions goes back into the 1930's, and still today the company is a leader in the design and manufacture of BOPs and drilling control systems. Hydril products are used worldwide, at drilling sites with the most extreme environments in terms of depth and wellbore pressure and temperature.

One major development goal has been to provide multiple levels of backup protection. Current designs achieve this by integrating three separate software disciplines - real-time operating systems (RTOSs), embedded high-availability database systems, and enterprise-style SQL server database management - into a single control system.

The enterprise database, Microsoft's SQL Server, lives on the rig and is used for archiving, reporting and troubleshooting functions. The embedded database, McObject's eXtremeDB High Availability, runs on QNX's Neutrino RTOS within the individual controllers, both on the rig and subsea, and provides duplicate "working copies" of data to support real-time processes.

Hydril's objective is to produce the safe and reliable BOPs and drilling control systems for the industry. The real risk takers are the specialist contractors who do exploratory drilling to determine whether or not there is a resource worth extracting. The initial drilling is critical since, much less is known about what's underground, and surprises can be greatest. These contractors are the most concerned about having failsafe solutions should the oil or gas within the wellbore deliver an unanticipated kick.

A pressure control system's key hardware is the BOP, which is essentially a large, heavy valve to contain pressure. A system will use multiple, specialized BOPs, configured in a vertical stack that sits on top of the well head. Above the stack are redundant BOP control pods, each consisting of a lower unit containing hydraulics to control the BOPs, and an upper electronics housing (EH). The EH is contained in a three-inch thick steel domed container to protect sensitive electronics gear from the surrounding water pressure. The pod also controls solenoids that operate hydraulic valves on the BOP.

Drilling a complex process

During operation, each redundant Pod continuously gathers data from remote sensors, including:

Temperature and pressure in the wellbore - Obviously it is very cold at depths of 10,000 feet, but the energy used in drilling can generate temperatures as high as 82-176 °C (180 - 350°F) or higher. Sensors in the wellbore capable of measuring both high temperatures and high pressures monitor pressure spikes to help predict and control kicks.

Positioning - The stack should remain vertical, with the riser extending straight to the surface, but environmental conditions and positioning of the drilling vessel can make this a challenge. A KVH fiber optic gyro within the pod's electronics housing monitors stack rotation, while inclinometers measure tilt. If the riser is in an unacceptable position with regards to either the stack or the surface vessel, a dynamic positioning system on the rig corrects the problem.

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