Stove pipe, foundation pile, marine conductor and conductor string

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The load determination for a stove pipe, foundation pile or marine conductor is a complex issue. This determination is usually followed by specialised stress analysis resulting in the involvement of a Structural Engineering department.

The load determination and related stress analysis for a conductor casing is less complex, but still requires a good knowledge of the boundary conditions.

This section will expand on the issues involved and presents, where possible, methods for manual, first order, calculations related to axial load, wellhead settlement and wellhead growth determination.

1. Stove-pipe, foundation pile or marine-conductor design

For land wells, the stove pipe is usually driven or cemented to surface. Without a landing plate, such strings then carry the buoyant weight of the conductor casing until the cement is set and the landing ring removed. The weight of all subsequent inner strings, wellhead, BOP, Xmas-tree and any tubing-to-packer force is taken by the conductor casing. Shear forces between the conductor casing and the cement will transmit loads to the stove pipe.

For subsea wells, the foundation pile is usually cemented to seabed. Therefore these strings might be considered as a rigid foundation, which will permanently carry the total buoyant weight of inner strings, the wellhead, the BOP, the Xmas-tree, and any tubing to packer force. Shear forces between the foundation pile and the cement will transmit these loads into the soil.

For offshore wells with surface wellheads, a marine conductor is installed to protect the other casings from lateral environmental loading. The marine conductor is usually not designed to withstand the axial loads carried by the conductor casing. In order to prevent lateral loads being transferred from the marine conductor to the conductor casing, and axial loads being transferred from the conductor casing to the marine conductor, the conductor casing should not be cemented above seabed or mudline suspension system, if present.

The design of these marine conductors  should be carried out by a Structural Engineering Company.

2. Axial loads on conductor strings, compressive and thermal movement

Apart from the design criteria as laid down in the pressure vessel design criteria, the conductor casing is usually subjected to axial loading, caused by the weight of subsequent casings, BOPs, tubing and Xmas-tree. Also possible future work-over operations like compressive loads generated by snubbing units when in operation, should be considered here.

These surface point loads introduce compressive forces and axial strain as a function of material properties and geometries.

It has to be checked whether these compressive forces cause yielding or buckling. The wellhead settlement calculation is required to check whether interference will occur with the stationary surrounding structures as for example marine conductor or well bay area.

Because of the distinct difference in length of the uncemented sections between land/subsea wellhead systems and platform/mudline wellhead systems these will be addressed separately:

2.1 Land wells or wells with subsea wellheads (no free-standing conductor string)

For land wells, the conductor casing is cemented over its entire length but is not directly supported by the stove pipe. The conductor casing alone will carry the total buoyant weight of the inner strings, wellhead, BOP, Xmas-tree and any tubing-to-packer force.

For subsea wells, the conductor casing is cemented over its entire length and may be considered as a rigid foundation with the foundation pile which will carry the total buoyant weight of the inner strings, the wellhead, and any tubing-to-packer force. Shear forces between the conductor casing and cement will transmit these loads via the foundation pile into the soil.

If a full analysis of the stresses in the conductor casing is required, a Structural Engineering Department should be consulted.

2.2 Offshore wells with surface wellheads (with free-standing conductor string)

If the conductor casing is not entirely cemented, e.g. for an offshore platform well, the uncemented portion will initially compress elastically. Any such compression will transfer some of the total load to the inner strings.

For a mudline suspension system, the casing strings are hung off at seabed. Only the surface to seabed tie-back section of each casing, together with the wellhead load and the tubing load, is supported by the conductor casing. This assumes a correctly designed tie-back system where overpull does not remove load from the mudline suspension system.

The distribution of applied loads between the conductor string and any inner strings must be known in order to check the ability of each string, but particularly the conductor string, to withstand those loads. The resulting wellhead movement must be known to ensure sufficient clearance when designing the wellhead area of the platform.

The load distribution between the strings can be calculated by considering the behaviour of the casings already in place as a set of parallel springs.

 

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