ABSTRACT

The subsea wellhead for a tension leg platform application may be subjected to large external loads when the drilling riser remains connected during the 100 year storm. In addition, because the subsea wellhead is tied back to the platform for production, it must withstand a large number of smaller loads induced by the production riser. Therefore, the design of the wellhead requires the consideration of both large external and cyclic fatigue loadings. This paper will discuss the mechanism of load transfer, analytical techniques, and component design considerations.

INTRODUCTION

Until recently, the basic structural design of subsea wellheads has remained essentially unchanged. The reason is that operating companies have gradually drilled in deeper, more harsh environments using existing equipment, and by and large, the equipment has performed satisfactorily. However, as new drilling and completion systems are designed, the structural integrity of the subsea wellheads must be verified. This verification requires knowing the design loads and an understanding of how riser loads are transmitted through the wellhead into the soil. In designing a subsea wellhead for a tension leg platform, one observes that the design loads can vary substantially from "existing" applications. Subsea wellheads for TLP's may be required to withstand large external loads from drilling operations, including 100 year storm conditions. In addition, if the subsea wellhead is tied back to the vessel with a production riser; the wellhead must withstand a large number of smaller cycle fatigue loadings. Therefore, both drilling and production riser loads must be considered for TLP applications.

This paper provides an explanation of how loads re transferred through the subsea wellhead to the 20 inch and 30 inch casing strings. In addition, variables which affect the load transfer mechanism are discussed and experienced gained using 2-D and 3-D finite element analysis techniques is presented. Design considerations for the major structural components of a subsea wellhead are summarized

MECHANISM OF LOAD TRANSFER THROUGH THE WELLHEAD TO THE 30 INCH AND 20 INCH CASINGS

The shear, moment, and tension loads applied to the top of the wellhead body by the drilling or production riser are reacted through the wellhead body and into the 20 inch and 30 inch casing below (Figure 1). It was found that the other casing strings (13 3/8 inch, 9 5/8 inch, etc.) react little or no applied loads.

The load transfer mechanism can be complex and depends on a number of factors as follows:

  1. Reactive shear at template - A template which is relatively rigid laterally (as in a TLP) can exert a significant reactive shear on the subsea wellhead assembly. This shear reduces the effective moments on the assembly below the template as illustrated in Figure 1. A single well completion or a non-rigid template will not have this shear.

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