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The flex joint in the lower marine riser package accommodates up to 10°

deflection from the vertical to allow for any horizontal movement to which the

drilling vessel might be subjected. When using a ball joint, the weight of the riser

and the drilling fluids within it creates an unbalanced force on the socket section,

tending to force it down onto the ball section. This compression force is balanced

by pressurized lubricating oil between the top socket section and the ball section.

The hydrostatic head of drilling fluid in the marine riser and the overpull of the

riser tensioners creates an upward (or tension) force on the socket section, acting

on the lower face of the ball section. These upward forces are variable and are

again compensated by pressurizing the lubricating oil between the ball section and

the lower socket section. A single ball flex joint pressure-balance system is shown

in fig. 6.20. Hydraulic fluid from the BOP stack control system is applied to the

base of the floating piston in another fluid-oil separator. This transfers the pressure
into the lubricating oil, maintaining the required balancing force to compensate for

the overpull and mud weight. The required hydraulic pressure is dependent upon
the tension load on the riser, water depth, and mud weight.

To avoid twisting the flexible choke-and-kill lines around the flex joint, an
antirotation pin is incorporated to prevent the socket from rotating relative to the

ball section. Extensive wear on the inner bore of the ball joint can occur as the

result of drilling while the vessel is displaced from the well bore or is off location.

Inserting a replaceable wear bushing will help avoid irreparable damage to the ball

joint. A tensile load capacity of 1.000.000 lb has proven ample for running the

larger BOP stacks and for the tensioning loads required of the riser.

Because of the pressure requirements anticipated for a ball joint in deep

water (3,000 to 6,000 ft), a nonpressurized flex joint was developed with the high

tensile capability to handle the deep-water subsea equipment. Vetco's Uniflex (fig.

6.21) is an example. Since the Uniflex joint requires no hydraulic balance pressure,

its operation is simplified and service and maintenance requirements are

substantially reduced. The inner surfaces, subject to drill pipe wear, carry

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