The Hoisting System

The hoisting system is the most visible of the four drilling systems. It consists of a variety of structures such as the mast, winch (drawworks), cables (wire rope), and pulleys (sheaves). It has one major purpose – to raise and lower things into and out of the borehole.

As we mentioned previously, there’s a lot of weight going in and out of that borehole. Add to that the fact that these loads sometimes need to be lowered to the bottom of a well far exceeding ten thousand feet long; it is clear that this system needs to have serious power and flexibility.

The extreme operating requirements of the hoisting system mean that every component must be carefully designed, tested, and maintained to ensure rig safety and effective mechanical operation.

The largest component of the hoisting system is the mast (if mobile) or derrick (if permanent). This is the large tower that most people think of when they picture a drilling rig. The tower, which can reach heights of 200 feet, provides support for all of the weight that will be lifted by the hoisting system as well as support the drill string standing within the mast.

Why not just have the hoisting system supported right at the surface?

A tall mast is required because the hoisting system needs to lift pipe sections over 90 feet long out of the borehole. If there were no mast, it would be far more complicated to actually lift anything up and out of the hole.

The hoisting system is a complex system of cables and pulleys. Typically, the primary cable, known as the drilling line, is a hardened steel multi-strand cable between one and two inches thick.

The drilling line is secured at both ends. The deadline, the end of the drilling line that doesn’t move, is anchored to a point below the rig floor. This end of the line is only reeled in and out if a new length of cable is being added to the main line. This occurs when the segment of the main line passing through the pulleys has reached the end of its safe operating lifetime in a process called “slip and cut.”

From the deadline anchor, the line extends up to the crown block, a system of pulleys fixed to the top of the derrick. It is then reeved (threaded) through the travelling block. By passing the line through multiple pulleys on each block, giving a mechanical advantage meaning, the lifting capacity of the line is multiplied and the draworks torque required to hoist a given amount of weight is reduced.

From the crown block and traveling block, the line extends downward to the drawworks. The drawworks is a large drum attached to a powerful motor. During hoisting operations, the drawworks is used to wind and unwind the drilling line.

As you might imagine if you’ve ever struggled to reel in a big fish, lowering a heavily weighted line into a hole is going to cause the drawworks to want to rotate faster and faster. A powerful braking system is required to keep the drawworks under control. Many drawworks use a band brake, which tightens around a drum mounted on the axle to slow the rotation of the draw works. Usually, this brake isn’t enough, so a second brake is installed. This second (or auxiliary) brake slows the drum using a reverse electric current, water brake, or some other energy dissipation system.

The part inside the derrick that you see traveling up and down on a drill rig is the traveling block. Typically, a large hook hangs from the block on a shock absorbing spring. The hook is the primary connection between the hoisting system and the object being hoisted. On top drive systems, the object being hoisted is screwed directly into the top drive.

Let’s take a closer look at how the block and tackle system used by a rig helps it lift heavy loads.

Transcript

Hoisting Calculations – Alfred William Eustes – Colorado School of Mines

The primary role of the oil derrick is to provide structural support for hoisting operations. A long drill string can weigh a lot. By a lot, I mean hundreds of thousands of pounds. No hoisting cable in existence can hold that much weight by itself.

How do drillers overcome this problem? Well, by using a block and tackle style pulley system, they reduce the amount of weight on a single strand of drill cable.

Here’s how it works: The drilling cable, which riggers call rope, is rolled on and off of a reel in the drawworks. It passes up to the top of the rig, where it’s threaded through the pulleys, called sheaves, of the crown block and through travelling block. After passing through the sheaves, the rope runs to the rig floor, where it’s anchored very firmly.

By passing the rope through multiple sheaves, the force exerted on each part of that rope is reduced by the number of lines strung between the crown and traveling blocks.

In exchange, the amount of rope that needs to be pulled in by the drawworks to lift the traveling block a certain distance is much greater than that distance. You do not get something for nothing.

Drillers refer to the weight of anything being lifted by the block and tackle as the “hook weight”, because everything is hanging on a hook at the bottom of the traveling block.

Hook weight changes based on a lot of factors: how much pipe you are hanging down in the hole is a big one, as is the weight of the casing per foot, which varies depending upon the diameter, wall thickness and alloy composition.

The amount of pipe hanging can be as little as one piece, called a joint, or go up to thousands of feet of pipe based on how deep the hole is.

Whatever that nominal weight is, it can be reduced by several factors too. The load can be reduced by the weight-on-bit (or WOB) or by the segment of the drill-string lying on the borehole wall.

Pumping mud into the hole with great force can actually push the entire string upwards out of the hole, that’s called the ram effect, so sometimes the drill string must be supported from the top, but also chained down to prevent it from being pushed up.

The process when you have to shove the pipe in under pressure when it wants to shoot back out is called “snubbing”. When the weight of the pipe is enough to allow the pipe to overcome the pressure at the surface, then that process is called “stripping”. It depends upon whether you are “pipe light” or “pipe heavy”.

Based on how heavy the hook weight is, and accounting for all these variables, we need to calculate how many times we pass the rope through the sheaves of the travelling block and the crown block. We obviously need to make sure the tension on the rope is less than the maximum amount it can safely hold by some safety factor.

The number of segments of rope running between the crown and traveling blocks divide the mass of the hook load evenly among them.

As an example, if the hook load is 100,000 lbs, and there are four rope segments, each segment will have a tension of 25,000 pounds.

Getting these calculations correct is imperative. Whatever that final segment load is determines the rotational power needed by the winch in our drawworks which is going to wind up that wire rope to lift that traveling block.

So all the weight on the hook is supported by the rope wraps going up and around the crown block.

So what’s supporting the crown block? That’s the derrick or mast as it is called if it is portable, and that structure also has to be able to support the forces involved.

You might think that the total weight of the hook load would be the same as the tension exerted on the derrick, but it’s not quite that simple.

You need to take into account the tension on the deadline (that’s the rope end that is anchored to the derrick) and the fastline, (that’s the rope that goes around the winch).

You also need to take into account the friction in the pulley system. The more lines strung between the blocks, the more friction.

The upshot of all this is that the derrick needs to be able to withstand more tension than just the weight of the hook load. Derrick construction requires serious mechanical engineering, and we won’t get into that other than to recognize that derricks have weight limits, and again, staying within those limits is mandatory.

The drilling plan that we produce before we get started will dictate what kind of drilling rig will be needed to do the job, as well as providing insight about all those other variables I mentioned, and it’s our job to make sure we properly account for all the possibilities we might encounter.

Now that you’ve seen some of the science around how the drill string is raised and lowered, it’s time to learn how it is rotated while hanging from the derrick.

Images: “Hook Shock” by Michael Black ; “Draw Works” by Michael Black