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Straight in the Water PUBLIC ACCESS

Oil Rig Operators Draw on Automated Help to Hold their Own Against the Elements.

[+] Author Notes

Associate Editor.

Mechanical Engineering 122(05), 52-54 (May 01, 2000) (3 pages) doi:10.1115/1.2000-MAY-2

This article reviews that deepwater oil platforms are kept afloat and the oil they produce is kept pumping with the help of a host of advanced technologies. Not the least among them is the technologies that play a particularly strong role in calculating the critical operations of large oil rigs, like that of the Chevron Genesis project. The Genesis platform is located in water where weather is powerful and changeable, and is therefore always a concern. The crew must be ready to deal with weather at a moment’s notice, and also must deal with the day-to-day events of life aboard an oil drilling plat form. Software also plays a role in positioning the risers. They must be located in such a way as to allow them to remain standing against the force of the waves in deep waters. Piping systems are often designed and laid out in a computer-assisted drawing or modeling program. The plan for the pipes is downloaded from the design system to the analysis program in order for engineers to study the structure and to calculate stresses and loads.

Deepwater oil platforms are kept afloat and the oil they produce is kept pumping with the help of a host of advanced technologies. Not the least among them are the technologies that play a particularly strong role in calculating the critical operations of large oil rigs, like that of the Chevron Genesis project.

The Genesis platform is located in water where weather is powerful and changeable, and is therefore always a concern. The crew must be ready to deal with weather at a moment’s notice, and also must deal with the day-to-day events of life aboard an oil-drilling platform. For instance, the oil drill on board is sometimes moved about the platform. Operators need to make sure that moving this heavy piece of equipment won’t imbalance the rig and leave it listing.

The weather and the platform list concerns are both addressed at the Chevron project with software from Computerized Processes Unlimited of New Orleans.

“The vessel is out there floating, tied to mooring lines. Obviously, they don’t want it leaning,” said Charles Robert, marine systems business unit manager at CPU. One problem that platform operators have is how to keep this thing from leaning.

To move the deepwater oil platform from well to well, operators aboard the Chevron Genesis rig consult graphical software showing the rig's position within a circle of chains that moor it, to determine the best way to go.

Grahic Jump LocationTo move the deepwater oil platform from well to well, operators aboard the Chevron Genesis rig consult graphical software showing the rig's position within a circle of chains that moor it, to determine the best way to go.

The platform is leveled on the gulf by the use of ballast tanks that maintain precisely metered water levels and by properly maintained tension applied to the rig’s mooring lines. The mooring lines are chains that have been plunged into the gulf floor in a 140-foot circular pattern that radiates from the vessel, deep below its waterline, to anchor the rig. The CPU software constantly monitors and measures the water levels in the ballast tanks and tension in the mooring lines, Robert said.

A three-dimensional model of a planned piping structure can graphically show the elevation of each pipe and where it is to be placed within the system.

Grahic Jump LocationA three-dimensional model of a planned piping structure can graphically show the elevation of each pipe and where it is to be placed within the system.

Software includes mooring advisory, ballast advisory, load management, and environment and position monitoring capabilities. These areas form what’s called the distributed control system used on the platform, Robert said. Marine operators on the Genesis drilling spar refer to the statistics generated by the software for guidance on how much water to let out or take into the ballast tanks as conditions change, such as a movement of heavy equipment aboard the spar. They also adjust mooring chain tension by following the software readings.

The spar might need to adjust the mooring chains to change its position. Operators can move the platform in order to drill at another well, or riser, within the radius of mooring lines.

“The only way they have to adjust the vessel’s position is to adjust these mooring lines,” Robert said. “You can let more line out or keep more line back.”

Robert detailed a typical spar move, which usually begins with the request that the spar be moved over a particular well to stab what’s called a drill riser. The riser is a structure of pipe rising from the gulf floor over a well. When stabbed, or tapped, the riser connects the well with the rig for oil production.

Before the move, an operator uses the software to define a path that shows the route for moving the spar from its present location to the requested location. The operator graphically maps the path, avoiding obstacles like the risers. After this is done, the software determines how the tension in each mooring line will have to change in order to keep the platform steady as it moves on its way. The operator then programs those changes into a chain jack system, which executes them.

The software also calculates how to keep the platform steady when the drill is moved. It determines the ballast adjustments that are required to offset the effect of the moving weight.

“The only thing they can do to offset that drill rig position change is to redistribute water from the tank,” Robert said. “But how much water they need to redistribute won’t be intuitively obvious to them. The software offers advice on how much water to let out to offset that load change.”

The technology also helps operators prepare for severe weather—always a consideration in Gulf waters.

“Wind, waves, currents—any of these weather forces can exert a force on the platform and get it to try to change its position from where it is now,” Robert said. Operators need to offset this by changing tension in the mooring fines.

“Say they want to stay in the same place, but a big storm is coming,” he added. “The software simulates the storm and shows operators how the platform will be repositioned if nothing changes. Operators use that information to adjust the mooring lines so the storm doesn’t severely affect the position.”

Pipe Positioning

Software also plays a role in positioning the risers. They must be located in such a way as to allow them to remain standing against the force of the waves in deep waters. For this, platform designers use a pipe stress analysis program that helps them decide how to best locate pipes.

“Ever try to hold a beach ball underwater?” asked Richard Ay, vice president of Coade in Houston, which provides pipe stress analysis software. “That’s called buoyancy, which also needs to be considered when deciding how to position pipes underwater.”

Pipe stress analysis software is used in any industry where pipe positioning is not only key to operation but is critical to protect life. For instance, if strong waves should overwhelm the pipes that carry oil and natural gas and cause them to rupture, a poisonous substance is released into the atmosphere, Ay said.

But other systems, such as a steam system, aren’t dangerous to the environment, he said. “Still, a steam system is considered a critical high-pressure piping system because if it failed, it could take out a building.”

Pipe stress analysis programs are also used in breweries and in the food processing industry, where substances are transported from one location to another via piping.

“The software operator has to determine the loads applied to the piping system, and that would include addressing things like wave and current theories. These can be addressed within the software,” Ay said. “Once the loads are determined, the effects can be computed and the decision is then made about whether the system design is okay.”

The wave theory used in undersea pipe stress analysis is similar to force-of-wind theory used in aboveground plants that use pipes, he added.

“It’s a little more difficult with wave theory, because wave theories are based on the height of the wave, the depth of the water, and the current where you are,” he said. “But once you’ve determined the waves and how they correlate with the wind, you can apply that force to each piece of pipe. The pieces of pipes are essentially risers that change in diameter as they go up through the water. You can see what the force would be on each part of the pipe, and you put that together to determine the complete load.”

The analysis software doesn’t tell designers where to locate the pipes or how to lay out the system, but it checks a pipe system design to ensure that it falls within certain tolerances, Ay said. If it doesn’t meet requirements, the designers have to change a part of their design.

Piping systems are often designed and laid out in a computer-assisted drawing or modeling program. The plan for the pipes is downloaded from the design system to the analysis program in order for engineers to study the structure and to calculate stresses and loads.

Pipe systems, ranging from the underwater drill risers used in the oil drilling industry to the pipe systems used in the brewing industry, are often designed and laid out in a computer-aided drawing or modeling program.

Grahic Jump LocationPipe systems, ranging from the underwater drill risers used in the oil drilling industry to the pipe systems used in the brewing industry, are often designed and laid out in a computer-aided drawing or modeling program.

As in other areas of mechanical design, three-dimensional computer modeling is making gains over 2-D design programs. Sharp Design, a Woodbury, N.J., design engineering firm, says it was able to complete design of an off-gas treating unit for a major oil refinery in nine months using a 3-D design program. Mark Gorman, director and project manager at Sharp Design, estimated that the project would have taken 15 months had it been completed using 2-D design techniques.

The project was designed to reduce emissions by blowing off-gas through a caustic solution that reacts with hydrogen sulfide to reduce sulfur emissions believed to cause acid rain. Sharp’s design included two packed towers and a venturi scrubber. Each tower is 24 inches in diameter and 5 feet long.

In using a 3-D model to visualize the project, Sharp used plant design software from Rebis in Walnut Creek, Calif. With the software, the piping engineer begins by defining the number, material, dimensional specifications, and contents of each pipe. The engineer then specifies where he or she would like each piece of equipment to be located. Then, the engineer locates the ends of each piping segment. The 3-D model can graphically show the elevation of each pipe.

Using this third dimension allows the model to be viewed from any view or rotation angle, Gorman said.

An automated feature of the software places a full 3-D pipe in the position the engineer defined. Viewing the design in 3-D made it possible for Sharp Design engineers to find the best placement of pipe nozzles and to evaluate different piping routes at the beginning of the design process, Gorman said. This helped engineers build a model that exists within the extremely tight plant footprint they were to work with, he added.

Copyright © 2000 by ASME
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