Hook load is the sum total of all the downward forces pulling on the hook. This hook is normally attached to the bottom of the travelling block and has a J-shape to make it easier to hang other equipment like the swivel, kelly, elevator bails or even topdrive units.
In other words, the hook carries the drilling load; it is the actual weight of the drillstring as measured from the surface. The hook load will therefore be at its maximum when all the weight attached to the hook is suspended freely in air with no support. Any form of support for the load suspended on the hook will reduce the hook load.
Maximum Hook Load
For instance during drilling, the drillstring (drillpipe and other bottom hole assembly) will have to be immersed in the drilling fluid in the hole. In so doing, some of this load will be transferred to the drilling fluid, thereby reducing the hook load. This support is as a result of an upward force of buoyancy being exerted on the drillstring from the drilling fluid.
Hook load will also be seen to reduce when the bit touches the bottom of the hole in vertical wells as some of the weight is transferred to bottom. As a matter of fact, in high angle wells (common these days), hook load also reduces due to friction as the drillstring makes contact with or rests on one side of the borehole.
In essence, hook load reduces once there is any form of support for the weights attached to the hook. Maximum hook load is felt when there is no form of support whatsoever for the weight attached on the hook such as a situation where the drillstring is suspended in air.
Importance of Knowing the Hook Load
The driller can easily tell what the hook load is from the weight indicator located on the driller’s console. The knowledge of the hook load helps the driller to control weight on bit. By monitoring the hook load, the driller can decide to increase or decrease the weight imposed on the drillbit.
During a stuck pipe scenario; to free the pipe the driller may need to pull hard on the drillpipe. To do this the driller must know what the overpull is. If the tension (from pulling action) exceeds the overpull then the drillpipe may part at the joint or even break. If the hook load is known, then subtracting the hook load from the pipe’s yield strength will give the exact overpull needed to free the pipe.