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Negative Pressure Test

In this document we describe the load case Negative pressure test available in the Oliasoft application.#

Negative pressure test is a collapse load case, where the unknown is the internal pressure profile of the tubing1^1.

1^1 We denote any tubular by tubing. All calculations encompass both tubings and casings.


The pressure profile consists of the hydrostatic salt water pressure to ll meters below the mud line, plus an additional pressure drop, and the hydrostatic mud pressure below.


The following inputs define the negative pressure test 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, inclination, and azimuth.
  2. The true vertical depth/TVD of
    1. The rig RKB, TVDRKB_{RKB}.
    2. The wellhead/BOP interface, TVDWH/BOP_{WH/BOP}.
    3. The TVD from rotary table to mud line, TVDRT to ML_{RT}\ to\ _{ML}.
    4. The hanger of the tubing, TVDhanger_{hanger}.
    5. The shoe of the tubing, TVDshoe_{shoe}.
  3. Length below mud line to use seawater gradient, lml_m, default is lml_m = 1000ft (\approxeq 300m)
  4. Pressure drop, pnp_n, default to pnp_n = 500 psi (\approxeq 3.4 MPa)
  5. The mud weight/density, ρm\rho_m
  6. The salt water density, ρsw\rho_{sw}


The internal pressure profile, parametrized by TVD, of the tubing is then given by

pi={ρswgTVDpn,TVDTVDRT to ML+lm,pml+lm+ρmg(TVD(TVDRT to ML+lm)),else,p_i = \begin{cases} \rho_{sw}\, g\, \text{TVD} - p_n, \quad &\text{TVD} \leq \text{TVD}_{\text{RT to ML}} + l_m, \\ p_{\text{ml+l}_m} + \rho_m\, g\, \left(\text{TVD} - (\text{TVD}_{\text{RT to ML}} + l_m) \right), \quad &\text{else}, \end{cases}

where gg is the gravitational constant, and pml+lmp_{ml+l_m} is the hydrostatic salt water pressure at the wellhead plus lml_m including the pressure drop, i.e.

pml+lm=ρswg(TVDRT to ML+lm)pnp_{\text{ml+l}_m} = \rho_{sw}\, g\, (\text{TVD}_{\text{RT to ML}} + l_m) - p_n