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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.

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


Summary#


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.


Inputs#


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}

Calculation#


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