Deepwater High-Capacity Collet Connector
Authors: Hsu Jacqueline C. 
Published: October 2008

New generation deepwater drilling rigs have increased the specifications for mechanical capacity of drilling riser, subsea BOP stack, wellhead connectors and wellheads. This paper describes an innovative approach to the development of a wellhead connector system that meets the challenges of these increased design capacities while maintaining compatibility to existing wellhead profile designs. Backward compatibility to existing wellhead designs is critical to rig owners to offer maximum flexibility to their customers and their drilling needs. The 18 ¾" wellhead connector described in this paper is rated for 15,000 psi service while maintaining enough reserve capacity to withstand maximum external bending and tension loads even under full pressure load.

Introduction

The new generation rig design criteria for wellhead connector load capacities were considerably higher than existing designs could withstand. The bending capacity under pressure was more than double the capacity of existing designs. It was immediately obvious that existing wellhead profiles were inadequate to meet this challenge. A new wellhead profile became inevitable. To increase the connector systems load capacity, the new wellhead profile would have to be larger and bigger in diameter. The obvious pitfall in this approach would be the proliferation of yet another wellhead profile thrust upon the industry and the inability of the new generation connectors being used on existing wells currently used offshore.

The Deep water, High Capacity (DWHC) was conceived to provide the increased design load capacity while maintaining backward compatibility to existing 18 ¾"-15,000 psi Cameron hub type wellhead profile. This was accomplished by adding a second load shoulder below the standard wellhead hub load shoulder. The added load shoulder provides the increased load bearing area required for the increased capacity while maintaining the original standard hub profile. Not only can the new DWHC connector mate to the existing hub type wellhead profile, the existing hub type wellhead connectors would compatibly mate to the new DWHC profile. This backward and forward compatibility of the new connector system to the existing systems in service offers the best and least impact solution to the offshore developers and rig owners while fully meeting the design criteria demanded by the new generation rig owners.

Design Development

The design for an ultra strength connector was driven by the oil exploration industry for use in water depth up to 10,000 feet. The original design challenge was to meet a combined loading condition of 3.0M lb. of tension, 5.5M ft-lb. of bending and 15,000 psi bore pressure in 8,000 ft of water for normal operating conditions. The final design achieved a loading condition of 8.0M lb. of tension in combination with the 5.5M ft-lb. of bending and 15,000 psi bore pressure.

The ABAQUS analysis program by Hibbitt Karlsson Sorensen, Inc was employed to complete the axisymmetric finite element analysis (FEA). The model consisted of twodimensional axisymmetric elements, which are augmented with additional incompatible displacement modes (CAX41). This eliminated the parasitic shear stresses and the artificial stiffening due to Poisson's effect that a standard 4-noded element may experience in bending. The contact pair approach with finite sliding contact interactions was used to model the contact between the components of the DWHC. Material properties were isotropic throughout the model except for the collet segments. The twenty-two collet segments are not a continuous ring, which result in zero hoop strength. Therefore, orthotropic material properties were used to model the segmented natu