Buoyancy & Hookload - Introduction

Buoyancy & Hookload

Introduction

Drillstrings weigh less in weighted fluids than in air due to a fluid property
known as buoyancy. Therefore, what is seen as the hookload is actually the
buoyed weight of the drillstring. Archimedes’s principle states that the
buoy force is equal to the weight of the fluid displaced. Another way of
saying this is that a buoy force is equal to the pressure at the bottom of the
string multiplied by the cross sectional area of the tubular. This is due to
the fact that the force of buoyancy is not a body force such as gravity, but a
surface force.
For example, the buoy force exerted on 7.5-inch x 2-inch drill collars in a
700 ft vertical hole with 12 ppg mud would be 17,925 pounds.
Buoy Force = Pressure x Area
Hydrostatic Pressure = 0.0519 x MW x TVD
= 0.0519 x (12) x (700)
= 436.8 psi
Cross Sectional Area = p/4 x (OD2 - ID2)
= p/4 x (7.52 - 22)
= p/4 x (56.25 - 4)
= 41.04 in2
Buoy Force = 436.8 x 41.04
= 17,924.99 pounds
By looking at the API RP 7G it can be determined that the air weight of
these 7.5-inch drill collars is 139 pounds per foot. If we have 700 feet of
collars, the total air weight would be 97,300 pounds.
Total Air Weight = weight per foot x length
= 139 x 700
= 97,300 pounds
The buoyed weight of the collars, or the Hookload, is equal to the air
weight minus the buoy force.
Hookload = Air Weight - Buoy Force
= 97,300- 17,925
= 79,375 pounds
This method for determining the buoyed weight is not normally used.
Instead, the following formula, which incorporates a buoyancy factor, is
used and recommended by the API.
MW=Mud Density (ppg)
Hookload = Air Weight x Buoyancy Factor
= 97,300 x 0.817
= 79,494 pounds
Buoyancy Factors rounded off to three places can also be found in the API
RP 7G (Table 2.13).
Note: The formula above for hookload does not take into
account axial drag. Hookload, as determined in the
formula above is the approximate static surface hookload
that would be displayed by the weight indicator in a
vertical hole with no drag, excluding the weight of the
traveling block, drill line etc.
In practice, hookload will vary due to motion and hole drag. Pick-Up Load
refers to the hookload when pulling the drillstring upwards. The highest
hookload normally encountered will be when attempting to pick up the
string. Slack-Off Load refers to the hookload when lowering the drillstring.
Drag Load refers to the hookload when drilling in the oriented mode. Other
references to hookload are Rotating Off-Bottom Load and (rotary) Drilling
Load.