How Do Drilling Rig Derricks Work?
Social media is cool. I’m able to keep up with people in the industry, all over the world. The different methods and equipment make for some interesting conversations. I got into a discussion the other night about derrick design, the different rigs and the reasons for the designs, so I thought I’d share a little of what I’ve learned over the years.
Derricks in the drilling industry basically come in two types: internally loaded and externally loaded. Examples of externally loaded derricks would include a standard Smeal rig or an oilfield workover rig. This means that the load is dispersed to the foundation by transmitting and dividing it on the block to the load on guy wires, or hydraulic cylinders. This design is very handy, as it allows you to rig up over most any hole and allows plenty of working room. Guy wires can be either to the ground or back to the rig itself. Most smaller rigs use guys to the front of the unit, and any ground wires are just for wind stability. Pulling power is often limited by the weight of the unit. I’m sure you have seen or heard about situations that lifted the front wheels off the ground. That’s pretty much the limit, and it’s hard on equipment. Downsides include flex and overall capacity. They will flex and move as loads are increased. These derricks can be built very strong, but the engineering challenges get larger on the big units.
Very common in all classes of the drilling industry are internally loaded derricks. Everyone is familiar with Mayhew, Gardner-Denver and Failing rigs, as well as many others. This includes the biggest oilfield rigs. The rotary, and the load, are inside the derrick. Forces are transmitted directly from the block to the foundation, without external guys or support. These derricks are typically vertical and are self-stabilizing, and don’t require any headache or ground wires. This makes for a very strong and compact unit, but sacrifices working room on smaller rigs.
Older designs, such as cable tool rigs, are externally loaded, but have such a tried-and-true design that they’re able to handle most of the loads put on them. Eventually, rotary rigs came along and were internally loaded derricks. These are very strong derricks for their size, which is a good thing. On most water well rigs, the drawworks don’t have enough power to damage the derrick. Whether this is a design feature or an economic consideration, it sure is safer. I’ve seen drillers who would ignore the weight indicator (if they had one) and just “give ‘er hell.” With most water well rigs, they run out of clutches or horsepower before they have a train wreck. Oilfield rigs are different. They have the power to do anything the rig will take, and more. That’s why they have weight indicators and crews are trained to use them. When these derricks take a load, several things happen. They will almost always “squat” with a significant load. This can be anything from half an inch to several inches, depending on load, foundation and design. But they usually stay over the hole. In my years as a borehole fisherman, I have been on many stuck pipe jobs. This generally requires that I pull more weight than the rig sees on a day-to-day basis. Sometimes, the company has strict rules but, sometimes, it is up to the people on the rig as to what their iron will stand. It’s not uncommon to pull way over the rated capacity. When I have to do this, I always look very carefully at everything. As I said, when you load a conventional derrick, it will squat when it takes the load. It will also twist a little, but stay over the hole. However, as you increase the load, most derricks will twist a little more in proportion to the load. When the load is released, the derrick will spring back to its original size and shape. This is normal elasticity. At some point, though, as you increase the load, the derrick will not continue to flex or twist. This is the danger zone, and indicates that you have reached the limits of elasticity of the steel. Been there, done that. It’s not pretty. It’s then time to try a different approach.
Usually, a well-designed derrick will last a lifetime of daily use — and occasional abuse. Since they are over our heads, we should know, understand and maintain our derricks. If you crater a derrick — and live through it — you will have a lot of explaining to do.
Newer rigs are built either way, commonly with box tubing and an external load. These are great rigs and very popular. But, while they allow more working room, they do take more maintenance. I guess it’s what you grew up with and what you are used to.
Keep ’em turning to the right and let me know if I can help.
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.