April 2007

Drilling advances

Several new gizmos improve offshore work

Vol. 228 No. 4  

Offshore gizmos. Why is Man so fascinated by the sea? It’s the pursuit of the bounty from and made possible by those waters, I suppose. From subsistence fishermen through the Phoenician trade ships to the New England whalers—from the Spanish galleons hauling treasure from the New World, to the supertankers bringing oil to it—each sailor saw the sea as a way to make a living. Even today, we marvel at the unfettered ease of shipping (note the term) across its surface.

There is an arresting strangeness to the sea that no one can overcome. No matter how much time we spend offshore, we simply cannot become totally familiar with it. Perhaps it’s because the human eye cannot penetrate the sea’s surface and gaze into its depths; it’s sort of like drilling a hole into the earth to search for oil and gas. I have never known anyone that can truly see very far down the hole (except for a few disillusioned braggarts that claim “downhole vision”).

Early man got a glimpse below the surface by simply holding his breath and diving. Next came various bells that trapped a small volume of air, allowing underwater breathing. Helmeted divers made salvage and commercial diving an economic reality. Then the military capitalized on our lack of visual acuity by creating the ultimate stealthy weapon, the submarine.

In the early search for oil offshore, geophysicists found ways to bounce sound off rock layers below the mudline by harnessing water’s ability to transmit acoustic energy. The earliest sound source used? Dynamite. That probably didn’t conserve fish stocks. Thankfully, air guns and other acoustic pulsers have far less impact on fish and other marine organisms.

Now, there’s a new kid on the block, seabed logging. Here, the sensors are laid in a pattern on the sea floor. Then, a controlled-source electromagnetic emitter is towed over them. The EM radiation transmits through the water and penetrates the rock. Returned EM signals not only indicate the rock layers’ shapes, but also indicate the fluid resistivity within the rock. That sure beats looking for "bright spots" on old, 2D seismic lines!

In a similar light, we are now seeing low-frequency radio waves used to communicate with subsea instruments and devices. Formerly, controls used acoustical signals transmitted through the water column, if the hydraulic or electric connection to the surface failed. Now, radio waves can penetrate deeply into the water column, allowing communication with downhole sensors and control devices. It’s almost like talking with a satellite in outer space, isn’t it?

This allows us to forego umbilicals, like another new kid on the block, the Autonomous Underwater Vehicle (AUV). Every subsea engineer and offshore driller is familiar with the robots that provide bottom pictures and allow work to be done without diving, the ROV. The remotely operated vehicle is generally dispatched on a cable, linked by electrical connections to a surface operator. The ROV is tethered to the ship; the AUV is not. The AUV is simply dispatched over the side and it swims about, performs its duties and communicates by radio with the operator’s console at the surface. No longer is the robot constrained by the length and weight of the umbilical. It can go as far as radio communication will permit.

In the undersea mining industry, AUVs don’t even have hands-on operators. In that industry, they use the AUVs to scan the seabed for obstructions, map the seafloor and take mineral samples. How? They program the AUV with a search pattern. The robot is then unceremoniously thrown over the side and begins swimming the programmed pattern, storing the data on computer chips.

When its tasks are complete or its batteries are expended, the AUV dumps its ballast and rises to the surface. There a light beacon and radio transmitter are activated, and the little robot merrily bobs up and down on the waves, as it beams its pick-up request to the mother ship. Once aboard, batteries are recharged or replaced, samples are removed, repairs and maintenance are performed, the robot is reprogrammed, and over the side it goes again. The operators go back to more important activities, like watching football beamed from a satellite or playing pinochle. Not a bad way to conduct offshore operations!

Here’s another nifty idea: strap-on vortex shedders for subsea risers. Subsea currents create a vortex (like a whirlpool) on the downstream side of drilling risers. These can excite severe resonant frequency vibrations in the riser that cause all kinds of problems. Think of a high-tension overhead electric line in a windstorm. The lines often hum, then wobble as sine waves develop along their length. As the vibrations increase, lines can separate from the connectors and literally self-destruct. So can a riser in offshore service.

Formerly, vortex shedders were welded directly to the riser. These were spiral-shaped vanes that surrounded the riser and made riser-handling a nightmare. The vanes are often fragile and they sometimes break off, requiring time to reattach and repair them before the riser can be run. Problems, and time required to run risers equipped with these vortex shedders, have been the sources of many swear words on offshore rigs.

Now, a plastic vortex shedder has been perfected that straps to the riser’s outside without welding. These can also rotate about the riser, so they are always oriented in the optimum direction to prevent vibration excitation. They are environmentally safe, don’t rust, don’t bend or break off, and riser running and pulling speeds are dramatically improved. Language on the rig floor has also improved dramatically.

The long-ballyhooed underwater drilling rig is now undergoing development, I understand. This robotic, hydraulically powered rig will sit on the seafloor and be operated by personnel on a surface support vessel. Bits will be attached to bottomhole assemblies, BOPs tested, connections made-up and broken-out, and all other functions normally performed by rig-floor crews will be made without the touch of the human hand (except on a joystick in a control room). There will be no riser, no control lines for BOP functions, no booster lines and nobody on the floor to get fingers and toes mashed.

I suppose the next thing is to somehow get the little AUV to perform maintenance on the underwater rig without ever bringing it to the surface. Then, we would truly have a hands-free offshore drilling operation.

Keep turning to the right. I’ll see you at OTC. WO

Les Skinner, a Houston-based consultant and a chemical engineering graduate from Texas Tech University, has 32 years' of experience in drilling and well control with major and independent operators and well-control companies.

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