Installation loads

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After the casing string has been designed to withstand the anticipated collapse and burst loads, it should be checked against the loads that will be experienced during the installation, and against the loads experienced during cementation and pressure testing.

Such loads are calculated on the basis that the string is fixed (suspended) at surface but free to move at the shoe.

These loads should include:

  1. Self weight (in air) loads;
  2. Pressure (buoyancy) loads;
  3. Bending loads;
  4. Dynamic drag loads;
  5. Shock loads;
  6. Point loads;
  7. Static drag loads.

Temperature effects do not lead to additional stresses in the installation phase since the casing is free to move at the shoe.

Below a brief description of the origin of these loads is included.

  1. Self weight (in air) loads: The self weight load is the load imposed on the string by gravitational effects (sa). This load depends on the weight per unit length of the string and the suspended vertical length below a point at the pipe axis.
  2. Pressure (buoyancy) loads: The pressure load, which results when casing is submerged in the drilling fluid, mud and/or cement, is generally referred to as the buoyancy load (sa, sr, st). This load is the result of the integration of the hydrostatic pressure over the submerged internal, external and free-end surface of the casing. It will depend on the density of the fluid(s) in which the casing is submerged, the presence of any applied surface pressures, and the vertical depth of the casing. Typical examples are the dynamic pressure loads generated when circulating mud prior to a cement job and during the actual cementation. The hydrostatic pressure load caused by the difference in fluid densities, acting on the sealing casing shoe after the cementation, also falls in this category.
  3. Bending loads: Bending of the pipe through any curved portion of the hole will induce bending stresses in the pipe walls (sa). Such stresses will be tensional in the outer or convex wall and compressional in the inner or concave wall. Bending is induced directly by the well path. The drilled well trajectory may be intentional, as with a build-up or drop-off, but may equally be inadvertent due to changes in formation, dip, drilling assembly, or applied drilling operation
  4. Dynamic drag loads: Dynamic drag loads are the result of sliding resistance between the casing and the borehole wall. The velocity profile at the point of contact results in axial and tangential drag force components. Hence, drag loads may result in torsional (t) and axial stresses (sa). Drag loads can vary considerably as a function of hole conditions, hole and casing geometry, and the mud system in use.
  5. Shock loads: When a casing that is being run into the hole is suddenly obstructed at a point somewhere along the casing, two shock waves will be generated: an upward travelling compression wave above the contact point and a downward travelling tension wave below that point (sa). A similar effect occurs when the casing is being pulled out-of-hole and it is suddenly stopped. Then the tension wave will travel upwards the compression wave downwards. The origin of shock load can be found in for example the spider elevator early closing or the casing string hanging up on a ledge.
  6. Point loads: Point loads, in the installation phase, result usually from operational activities related to pressure testing (sa, sr, st). For example, pressure testing using retrievable packers or directly after the cement displacement.
  7. Static drag loads: These drag loads, referring to the remaining stresses after casing movement, have an influence on the distribution of stresses within the casing after it has stopped moving (s a). Evaluation of these loads requires a knowledge of the movement "history" of the casing. Subsequent behaviour of the casing depends on the magnitude and direction of these "sliding resistance" loads.

The casing design should be checked against the combination of these loads that result when the string is moving, i.e. being run, and against the combination that result when the string is stationary, i.e. landed off. The applicable loads during these dynamic and static phases can be determined from the following table:

Load

Dynamic Phase

Static Phase

Self-weight (in air) load

yes

yes

Pressure (buoyancy) load

yes

yes

Bending load

yes

yes

Dynamic drag load

yes

no

shock Load

yes

no

Point load

no

yes

Static drag

no

yes

 

The following criteria, together with the design parameters, should be used to generate the load conditions against which the capacity of the earlier generated pressure vessel design should be checked.

1. Dynamic loads

The earlier generated casing string should be checked to confirm that it is capable of withstanding the sum of the loads (sa, sr, st) within the pipe wall resulting from self weight, pressure (buoyancy), bending, drag or shock. Any rotational loads (t) which are experienced while running the casing to its setting depth should also be checked. Pipe reciprocation/rotation during cementation is considered part of this phase and is subjected to the same check criteria.

The individual loads should be established applying the following rules:

1.1 Self weight (in air) loads

Self weight loads should be calculated from the product of nominal unit weight and the vertical projection of the well trajectory.

1.2 Pressure (buoyancy) loads

Pressure (buoyancy) loads should be based on the lowest anticipated mud/cement pressure gradient and the vertical projection of the well trajectory.

1.3 Bending loads

Bending loads should be based on the planned rate of curvature for a well trajectory increased with an additional dogleg severity. This additional dogleg severity value should be based on local Opco-specific experience.

In the absence of such knowledge an additional dogleg severity of 2°/100 ft, above the planned value of rate of curvature for any point in the well trajectory, should be used.

1.4 Dynamic drag loads

The incremental axial load, experienced over the self weight load plus the pressure (buoyancy) load and bending load, due to drag while both running and pulling casing should be estimated. Additional rotation of casing strings introduces a torque load, the values of which should be estimated.

In the absence of local specific knowledge on friction coefficients to establish these loads the following empirical values should be used:

Mud type

Open hole

Cased hole

Water based mud with barytes

0.30

0.30

Water based mud with dolomite

0.30

0.25

Oil based mud with barytes

0.20

0.15

Brine or water

0.30

0.50

 

1.5 Shock loads

Shock loads are to be calculated from the peak casing running velocity which is assumed to be one and a half times the average casing running speed.

In the absence of specific information on average running speeds an average running speed of 13 seconds per 40 ft joint, giving a peak velocity of 4.5 ft/second, should be used. If such loads are found to be excessive, the casing running speed should be reduced accordingly, rather than adjusting the casing design.

Potential shock loads during reciprocation are not as severe as those that may be encountered during installation due to the reduced velocities involved. Here a similar approach should be followed.

When a casing that is being run into the hole is suddenly obstructed at a point somewhere between the top end and the bottom end of the casing, two stress waves will be generated: an upward travelling compression wave above the contact point and a downwards travelling tension wave below that point. A similar effect occurs when the casing is being pulled out-of-hole and is suddenly stopped. Then the tension wave will travel upwards and the compression wave downwards. In this section the quantification of these shock loads will be addressed followed by a qualitative evaluation of concurrent shock and drag loads

1.6 Combined loads

While it is possible that shock and drag loads may occur at the same time, they usually act in the opposite direction. For example, while running in, the drag force gives rise to a compressive load, while a surface shock load caused by kicking in the slips will give rise to a tensile shock load. On the other hand a compressive shock load, caused by running into a ridge with the casing shoe, will be damped out rapidly due to the high wall contact forces that exist in those parts of the casing which experience high drag loads. As a result, shock and drag loads can be considered to be mutually exclusive, and the larger of the two should be used in calculating the total dynamic load.

The pressure related radial (sr) and tangential stresses (st) are usually low compared to the axial stress (sa). However, for completeness they may be analysed to derive the Von Mises equivalent stress (sVME).

Static loads

The casing string designed as described above should be checked for ability to withstand the loads (sa, sr , st) within the pipe wall resulting from self weight, pressure (buoyancy), bending and static drag loads. Any point (pressure) loads (sa, sr , st) arising during pressure testing should also be checked, not to exceed the capacity of the casing.

The self-weight, pressure (buoyancy), and bending loads should be analysed. The static drag load is not well known. The design factor takes this unknown into account. The point (pressure) loads should be analysed for:

1.Any pre-cementation pressure test load (against a retrievable packer);

2.Any post-cementation pressure test load (against the sealed float shoe).

In summary the maximum expected axial load (s a) during the static phase of the installation phase is the greater of:

The pressure related radial (s r) and tangential stresses (s t) are mostly low compared to the axial stress (s a). However, for completeness they may be analysed to derive the equivalent stress (s VME). Specially for the larger OD casing strings thepost-cementation collapse load should be evaluated.

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