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Abstract

A new wellhead installation technique has been developed and field tested. The new wellhead system employs a cold forging process which eliminates the need for welding the casing head to the initial casing string an operator sets. Advantages of the new system are its speed of installation, elimination of undesirable welding stresses, and the technique's insensitivity to weather. Described in this paper are the features of the new wellhead installation technique and details surrounding the industry's first installation on a 13-3/8" casing string in July of 1987 in the Gulf of Mexico. Also documented is a discussion of field and engineering data to test its metal to metal sealing characteristics.

Introduction

Operators have historically had problems with achieving high quality weldments of starting casing heads to the initial pipe string set on a newly spudded well. It has been the experience of the authors to witness initial pressure tests of the weldment fail or a leak in the weld or heat affected zone occur during the drilling operation, often requiring complete replacement of the wellhead weeks after it was installed. To maintain wellbore pressure integrity and avoid rig downtime for wellhead repairs operators have conducted metallurgical tests on both the casing and wellhead and have implemented various welding procedures. The welding procedures often require extensive post heat treatment that may keep the rig from operating for 24 hours or longer depending on weather conditions or casing metallurgy.

Another concern operators have faced installing weld on equipment is the necessity to "shut-in" other producing wells when performing an installation on offshore platforms. Production loss can be substantial. Also, some wells may require remedial work to be restored if these shut-in periods are extensive.

In offshore wells that set casing across unexpected shallow gas hazards welding can be extremely dangerous, often requiring elaborate safety precautions to install the initial casing head. To address these problems and concerns a wellhead system was developed and tested on an offshore platform located in West Delta, block 35, in 70' of water. The technology employed was transferred from a pipeline repair system that has been successfully used for emergency repairs of subsea pipelines.

Welded Wellhead Installations

The most common method for attaching wellheads to surface casing is by welding. For this, the lower portion of the wellhead housing has a weld preparation. A typical wellhead having a conventional slip-on weld (SOW) preparation is shown in Figure 1. After the conductor or surface casing has been run and cemented, the casing strings are cut and fluid levels drained to provide a dry welding surface. The wellhead is slipped on to the surface or conductor casing, and the base plate landed on the first casing string run (normally drive-pipe). The welding procedure is then performed which typically includes preheating, welding of the head to casing, postheating, and cooling. Detail specifications of this procedure are generally a function of casing and housing material and established by the operator. After cooling, the wellhead casing annulus is hydrostatically tested within limits of the casing collapse strength. Although this process is fairly straightforward, there are inherent aspects of these conventional techniques that have caused field problems.

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