Riser Margin

Riser margin is a collapse load, where the unknown is the internal pressure profile of the tubing.

Note: In this documentation we denote any tubular as casing or tubing. All calculations however encompass any tubular, such as tubings, casings, liners, tie-backs etc.

Summary

The pressure profile consists of the hydrostatic salt water pressure to the wellhead, and the hydrostatic pressure from mud below. This load case is similar to the external load case {\em{Fluid gradient for open water}}.

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Inputs

The following inputs define the riser margin 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

a. The rig RKB, $\text{TVD}_{\text{RKB}}$ b. The wellhead/BOP interface, $\text{TVD}_{\text{WH/BOP}}$ c. The hanger of the tubing, $\text{TVD}_{\text{hanger}}$ d. The shoe of the tubing, $\text{TVD}_{\text{shoe}}$

3. The mud weight/density, $\rho_m$

4. The salt water density, $\rho_{sw}$

Scenario Illustration

Calculation

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

$$\begin{equation} p_i = p_{\text{sw,WH}} + \rho_m \,g\,\left( \text{TVD} - \text{TVD}_{\text{WH/BOP}}\right) \end{equation}$$

where $g$ is the gravitational constant, and $p_{\text{sw,WH}}$ is the hydrostatic salt water pressure at the wellhead given by

$$\begin{equation} p_{\text{sw,WH}} = \rho_{sw}\, g\, \left(\text{TVD}_{\text{WH/BOP}} - \text{TVD}_{\text{RKB}}\right) \end{equation}$$