Page 178 - Well Control for Completions and Interventions

170 Well Control for Completions and Interventions

Nitrogen charging valve
Nitrogen charging valve
Gas cap

Accumulator body
Nitrogen charged Accumulator body
bladder

Poppet
Piston
Spring
Hydraulic cap
Figure 4.24 Bladder accumulator (left) and piston accumulator (right).

4.9.2 Accumulator bottles
Accumulator bottles are available in several sizes and types. By far the
most common is the 11 or 34 US gallon cylindrical bottle containing a
flexible rubber bladder. The bladder is attached to the top of the cylinder
using a threaded connection, allowing the bladder to be filled with nitro-
gen. Since nitrogen is inert it is safe for use at the wellsite. The quantity
of nitrogen used depends on the size of the bottle, but in most cases the
bladder will be pressurized to 1000 psi. This is termed the precharge pres-
sure. Hydraulic fluid is pumped into the bottom of the bottle using the
accumulator unit pumps. The nitrogen charged rubbed bladder is com-
pressed as the bottle fills. When the bottle pressure reaches the operating
pressure for the system, the pump stops, leaving the hydraulic fluid stored
under pressure. When released, the pressurized fluid is forced out of the
bottle by the nitrogen in the bladder expanding.
Other cylindrical bottles use a buoyant float instead of a bladder. In
these bottles, the nitrogen is pumped into the top of bottle above the
float. Fluid pumped into the bottom of the bottle below the float forces
the float up, compressing the nitrogen.
Accumulator bottles should be protected by a pressure relief valve,
normally set to 3300 psi (Fig. 4.24).

4.9.3 Accumulator system pumps
The pumping system supplies hydraulic fluid to the accumulator bottles.
Most accumulator units have two independent pumps: a triplex pump