Forces lec ( 10 )

Tubular Forces 

Determining the stress levels that the completion string and components will
be subjected to, under the best and worst conditions, is a critical step in
completion design. Properly assessing the length and force changes will avoid
premature failures and costly remedial operations.

•  Temperature
•  Pressure
•  Weight
•  Fluid gradients
•  Friction

Tubing Forces

 Each completion will have a variety of downhole conditions which affect the
total design, choice of downhole tools and the operation of the completion
components once in place. Changes in temperature, pressure, applied weight,
fluid gradients and friction are a few of the variables that must be considered.
The choice of completion equipment not only must meet minimum stress
requirements, but they themselves contribute directly to these stress
calculations. The illustration in Fig 7-1 summarizes the principal variables that
need to be considered in a completion operation.For more detailed discussion
on basic forces refer to the “ Completions Hydraulics Handbook.”

Factors Influencing
Completion String Length
and Force Changes

Length and
Force Changes

The most important aspect when evaluating a packer installation is the
determination of the length and force changes due to varying pressures and
temperatures. When the magnitude and direction of these length and force
changes have been calculated, this information can then be used as shown
below.

•  To aid in the packer selection process
•  To determine if tubing damage will occur
•  To determine the proper spacing-out procedure for the packer and completion components

There are four different effects which create length and force changes. Each of
these effects must be analyzed and combined to assess the total effect for any
packer installation.

•  Piston effect
•  Buckling effect
•  Ballooning effect
•  Temperature effect

This piston effect, bucking effect and ballooning effect result from pressure
changes in the system. The temperature effect is related only to temperature
change and is not affected by pressure changes. While some effects are
related to each other, each must be calculated individually. Each calculated
effect will be a magnitude and direction. Once each effect is known, they are
combined to obtain the total effect. The decision to add or subtract when
combining is based on the direction that each effect (resultant force) acts.
The approach used to evaluate packer installation problems will depend on the
type of tubing-to-packer hookup being considered. If the packer system will
not permit length change in the direction of the total effect, then the packer
installation is evaluated by calculating the force changes. If the packer system
permits length change in the direction of the total effect, then the calculation
would be as a length change.

Piston Effect 

See Completions Hydraulics Handbook