ABSTRACT

Severe environments exert .a variety of extreme loading conditions on subsea wellheads. Cyclic loading during both the drilling and production modes make fatigue a critical consideration. In addition, long casing strings plus high pressures and temperatures create critical situations with regard to support stresses and the effectiveness of annulus seals. This paper describes the analysis and design of wellhead system features that address these problems.

INTRODUCTION

Historically, subsea wellheads have been supported in a 30-inch conductor housing by a single shoulder with additional centralizing fins or gussets providing a coarse "centralizing" effect below the support shoulder (Figure 1). This approach worked satisfactorily until operators drilled in deeper waters and more severe environments. Fatigue failures in the wellhead 20-inch casing transition led investigators 1, 2 to examine the mechanism of load sharing between the wellhead and the 30-inch housing.

Findings indicate that although the severe loads of the drilling environment are critical, situations that call for the use of a production riser may be even more critical even though the load levels are lower. The studies indicate that a two point support between the wellhead and the 30-inch housing is essential to efficiently transfer the riser loads of both the drilling and production cycles into the 30-inch conductor.

In addition to bending moment loads on the wellhead, there are other loads that complicate the design of a severe environment wellhead. Thermal strain in casing strings can cause lift off of the wellhead from the 3D-inch support which can negate any stabilization benefit derived from these supports. Casing weight and pressure loads require adequate hanger support in the wellhead. This paper describes a 15,000 psi wellhead which accommodates these loads and incorporates a true metal-to-metal seal.

STABILIZATION

The loads applied to the top of a wellhead by a drilling or production riser are coupled through the wellhead body into the 30-inch housing and into both the 20-inch and 30-inch pipe below. The other casing strings have little direct effect on these loads except that thermal strain can cause lift off of seating shoulders between the wellhead and 30-inch.

A stabilized wellhead is one in which the wellhead is coupled to the 30-inch conductor such that there exist two radially stiff contacts between the bodies which transfer the bending moment to the 30" (Figure 2). A continuous contact over an appreciable length would also affect such a "moment" coupling, but in practice this is difficult to achieve. Conventional single shoulder support (Figure 1) couples the 30-inch and 20-inch pipe with respect to shear loads but it transfers very little bending moment to the 30-inch; therefore, the 20-inch carries the majority of the bending moment. If the cyclic loading experienced at a particular well location is severe enough, the 20-inch transition can fail in fatigue.

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