Surface Protection (BOP)

In this section we describe the load case "Surface Protection (BOP)" available in Oliasoft WellDesign™.

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Surface protection (BOP) is a burst load case, where the unknown is the internal pressure profile of the tubing.

NOTE!
Note: In this documentation we denote any tubular as tubing. All calculations however encompass any tubular, such as tubings, casings, liners, tie-backs etc. //: # (end - note)

Summary

Surface protection load is a less severe load case than the “Displacement to Gas” and is a criterion when designing a blow-out preventer. The pressure at hanger is the wellhead equivalent pressure assumed the whole casing is filled with water. Then the case reflects a casing filled with gas, giving the internal pressure profile of the hanger pressure with gas column from hanger to the casing shoe.

Illustrating Pressure Profile Graph

Printable Version

Oliasoft Technical Documentation - Surface Protection (BOP) //: # (end - printable_version)

Inputs

The following inputs define the Surface Protection (BOP) load case

1. The true vertical depth (TVD) along the wellbore as a function of measured depth. Alternatively, the wellbore described by a set of survey stations, with complete information about measured depth and inclination.
2. The true vertical depth / TVD of
   1. The hanger of the tubing, TVD$_{hanger}$
   2. The shoe of the tubing, TVD$_{shoe}$
3. The temperature profile of the wellbore, $T$.
4. Water density, $\rho_{w}$.
5. The fracture pressure at the shoe, $P_{f@s}$
6. The gas gradient, $\rho_{\text{g}}$, or gas specific gravity, sg$_\text{gas}$.
7. The gravitation constant, $g$.

Calculation

The hanger pressure is given by

$P_{\text{hanger}} = P_{\text{f@s}} - g\, \rho_{\text{w}} \, \left( \text{TVD}_{\text{shoe}} - \text{TVD}_{\text{hanger}}\right). \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad (1)$

The internal pressure of the tubing is given by the hydrostatic pressure from gas, from hanger to shoe. If gas specific gravity is entered, $\text{sg}_\text{gas}$, a gas gradient, $\rho_{\text{g}}$, is calculated using z-factor and Sutton correlations [1], using the temperature and hanger pressure as input. Precisely,

$P_i = P_{\text{hanger}} + g\, \rho_{\text{g}}\, \left(\text{TVD} - \text{TVD}_{\text{hanger}}\right), \quad \text{TVD}\, \in\, [\text{TVD}_{\text{hanger}}\,, \text{TVD}_{\text{shoe}}]. \qquad \qquad \qquad \qquad \qquad (2)$

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References

[1] Curtis H. Whitson and Michael R. Brul$\acute{\text{e}}$. Phase behavior, volume 20 of Henry L. Doherty series. SPE Monograph series, 2000.