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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. 2.12. 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. 2.13) 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 removable bushings. Primary flexing takes place at
each of the two bearing rings in the upper and lower sections (fig.
2.14). The two pieces in the middle, the seal assembly, are
composed of the same flexing material and mainly seal between
the internal mud pressure and the external ambient pressure. The
flex material is laminated layers of steel and rubber. The action is
more like a sliding, compressive loading than the pivotal loading

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