Drilling advances

	Vol. 230 No.2
LES SKINNER, PE, CONTRIBUTING EDITOR, LSKINNER@SBCGLOBAL.NET

Lifecycle rig design

Last month, I was privileged to attend a course on materials and corrosion provided by a major oil company. In this course, we were taught about processes by which corrosion occurs and how to combat each by the proper selection of materials in all areas of a well.

Probably the most interesting aspect of the study involved long-term use of the wellbore. Most of us design for current conditions of pressure, temperature, fluid and gas analysis and production. However, when a well is designed, one must consider the complete lifecycle of the well.

It became apparent that the same questions could be asked of a drilling rig design. Will this rig always be drilling shallow Spraberry wells in West Texas? Is there a reason to build it with a taller substructure, so a rotating head can be installed under the rotary beams? Are the pumps and mud tanks good enough to handle oil-based mud?

Some drilling rigs are designed properly with engines sturdy and powerful enough to deal with a variety of conditions demanded by current and future use. Others have engines barely strong enough to pull the pipe off bottom, and floating casing in the hole is not an option-it is a requirement. Stuck pipe almost always involves cutting and leaving some of the drillstring in the hole, since working the pipe is out of the question.

With age, all engines suffer wear. The outcome is poor performance, even when engines are run at higher rotary speeds to compensate for the reduced power. Higher rotary speeds result in an even greater wear rate.

How many of us have seen rigs with powerful engines, good mud systems, great pumps and super drawworks coupled to a derrick that looks like it was welded together by a middle school metal shop class? At other times, we’ll see a good derrick, good pumps, good mud system and an antique drawworks held together by bailing wire, duct tape and chewing gum. Performance and downtime in either situation are predictably bad.

What about wear and allowances? I have seen ancient pumps-many are duplexes, incidentally-that are poor performers despite the pump rating and their performance curves. Pump ratings are good for new pumps only. Once the pump has some wear through use, the pump cannot meet that performance. So pump flowrate and horsepower curves are pretty useless.

When pumps are replaced or overhauled back to new pump specifications, flowrate and pressure improve, often causing leaks in other components of the high-pressure mud system like the standpipe manifold or old, dry-rotted kelly hose. In other cases, a bigger engine is installed to maintain pump rate and pressure with an obvious mismatch between engine and pump.

So where is all this discussion heading? Two places. First, the worldwide fleet is aging. Newbuilds are helping overcome this aging process, but there are a number of rigs out there that are now past their design lives. Good maintenance, upgrades and overhauls have kept many of them viable. Second, we are advancing into increasingly difficult-to-reach hydrocarbon accumulations. These reservoirs are deeper, hotter, farther away from decent drillsites and may contain fluids and gases that would have precluded them from being producible in the past, with high concentrations of corrosives and inerts. Can we expect our aging drilling fleets to continue working safely at the remote edges of their operating envelopes indefinitely?

Sadly, during this downturn in drilling, there will be a good number of drilling contractors who will take the opportunity to refurbish their old rigs with new or like-new components. Some of these will be from other rigs that are stacked or decommissioned with their component parts sold on eBay. In the past, components such as engines, drawworks, derricks, substructures and whole mud plants from one rig were simply pulled off and mounted on an old, worn-out rig to “bring it up to speed.” Then, the newly cobbled-together rig was painted, given a new name or number, and its depth rating revised. Some of these are the rigs of today-collections of parts re-assembled into “new” rigs. Problem is: If all the parts are from 30- to 40-year-old rigs, it’s not really a new rig.

When this fleet of “new” rigs is assembled during the coming downturn, what can be expected from them through their extended lifecycle? Is such a “new” rig more capable of performing than any of its components? Answer: Sometimes yes, sometimes no.

Last year I looked over some rigs that were pieced together from components bought at auctions during previous downturns. A very capable operator had purchased engines from one place, pumps from another, drawworks from yet another and a derrick in a sale. None of the components was ever on the same rig or one even resembling the “new” rig. Surprisingly, the rigs were just great. Care had been taken to purchase only those parts that would perform well together. Pumps and prime movers were carefully skid-mounted with controls that allowed them to function properly. The derrick was mounted on a self-elevating sub in a way that took advantage of the strengths of both without introducing weaknesses in either.

These rigs were assembled to drill only one type of well through their entire useful lives, making the lifecycle issue moot. Notably, the crews that assembled the rig in the yard were the same ones operating it as a finished drilling unit. Not surprisingly, they knew where every nut and bolt was located, and who tightened each one.

If this could be done with a “new-build” assembled from dissimilar parts, it seems likely that using rebuilt parts from one rig to overhaul, rework and repair another rig for a longer serviceable life is feasible. If the process is properly managed, the resulting rig may have an extended life and the same or more capabilities than the original product.

A word of caution: This is not a task left to the inexperienced or ignorant. Fatal flaws can be introduced if everything is not done carefully. Function and safety are still the best drivers for this type of work-not cost.