An improved connector design to produce and positively maintain preload to oppose hub separation forces is presented. The basic design features and the resulting benefits are explained. Finite element analysis is used to determine the stress distribution, develop the connector/hub stiffness ratio, and to optimize the design configuration and simulate operating conditions. Test results from two independent sources are-presented and shown to confirm the design calculations. Preload development is consistent and positively maintained by the flat-to-flat lock. Forces required to cause hub separation confirm the design's stiffness ratio to be similar to that of a bolted flange.
In subsea drilling there are many critical operations that must be performed remotely. One of the most critical is the connection between the sub sea wellhead and blowout prevented stack. Hydraulic connectors are used to attach the BOP to the wellhead hub or mandrel profile and energize a metal seal gasket. The effective compressive clamping force (preload) of these connectors should one equal to or greater than the combined separation forces due to well pressure, riser tension, and bending moments to maximize the integrity of the connection.
The preload of these connectors is produced mechanically by a hydraulic piston or pistons forcing locking members inward. The outer member(s) or locking segments that makeup the tensile pre-stress load path are stretched and compressive force is generated across the top face of the wellhead. Should the separation forces overcome the preload, the seal gasket may eventually fail due to relative motion causing fretting. The amount of separation causing seal failure or length of time the seal may wear before failing is indeterminable due to the variable load conditions. Our industry has therefore accepted hub separation as the best detention of failure at this interface.
In addition to generating preload sufficient to counteract the separation forces, the wellhead connector must securely maintain a positive preload during the entire drilling operation. One common design method is to maintain locking pressure on a tapered locking mechanism throughout this time period. This approach has two significant disadvantages. First, drilling vibrations can cause a taper-locked mechanism to wedge tighter. The connector could become traction-locked such that the required releasing pressure is increased to the point that the connector cannot be unlocked. Second, should the desired locking pressure bleed down, drilling vibrations, separation forces, and temperature fluctuations could cause a significant preload loss. This possible problem is especially critical on a completion or permanent installation.
The following paper describes a second design method, for a flat-to-flat locking mechanism that maintains constant and fixed preload without the continual application of locking pressure. The first section explains the mechanics of the preload generation and maintenance. The second section describes the finite element analysis procedure for generating the pre-stress loop and determining the stiffness of the connector. The third and fourth sections describes two full-scale tests which demonstrate the preload capability and maintenance of this connector design.