Support Systems For High Wellhead Loads
Authors: Anderson K.A. 
Published: March 2010

This paper outlines the method presently in use by Shell Canada Resources Limited to support high wellhead loads at surface. Wellhead load is the total of all casing and tubing loads hung at surface. This load is normally carried by the surface casing and ultimately transferred to the earth by the surface casing cement. On deep wells the load can exceed the compressive strength of the surface casing. On wells with inadequate surface casing cement, the load can cause surface casing buckling and wellhead subsidence. Either failure can have catastrophic consequences to pressure integrity or production capability. Repair of wellhead support failure can be costly. An example of a subsidence problem is discussed. Guidelines are presented jar the compressive capacity of surface casings. If anticipated wellhead load exceeds the capacity of available surface casing, or if a poor surface casing cement job is anticipated a landing base casing bowl is designed to transfer load to the conductor pipe. The conductor pipe must be adequately sized to support the wellhead and transfer the load to the earth via a cement to formation shear bond. Geotechnical properties of the conductor hole formation must be known. This paper outlines considerations necessary for the design of a wellhead support system.

Introduction

Adequate support of loads imposed on the wellhead has not been a problem in Canadian onshore drilling in the past. Prior to 1978 Shell Canada had drilled many wells in the 4000 m to 5000 m depth range, and had no apparent problems with wellhead support. With the advent of deeper exploration wells and an increasing tendency to pre-tension casing strings as they are hung off it was recognized that wellhead load could exceed the capacity of standard surface casings. A wellhead subsidence problem on an exploration well highlighted the fact that inadequate surface casing cement can also cause wellhead support problems. Methods outlined in this paper are the result of a study undertaken to provide solutions to the problem of wellhead support.

Wellhead Load

Wellhead load includes the as-cemented weights of all casing strings (excepting surface casing and liners), tubing string weight, the weight of the wellhead and BOP equipment, thermal or pressure induced loads and any pre-tensioning loads applied to casing strings when they are hung off. Intermediate and production casing strings are often pre-tensioned (hung off with a load higher than their as-cemented weight) in order to prevent buckling at the cement top. Buckling can occur with changes in fluid densities, thermal expansion or changes in surface pressure. Casing pre-tension loads can be a significant portion of the total wellhead load.

The following example gives a wellhead load breakdown for a typical foothills well:

Calculation (Available In Full Paper)

Load Transfer

Wellhead load is transferred through the casing bowl to the surface casing, and/or from a landing base to the conductor pipe. It is transferred from the pipe to the cement via the casing - cement bond. The load is transmitted through the cement sheath by virtue of its shear strength.