This topic describes how an accumulator (Koomey Unit) works. First of all, I will start with accumulator bottles. The accumulator bottles are used to store hydraulic pressure for closing/opening all blow out preventers. Each bottle, which has a rubber bladder inside, has a storage volume of 10 gallons. The rubber bladder inside is pre-charged to 1,000 psi with Nitrogen (N2). Each bottle will be pressured up 200 psi over the pressure charge pressure. At this stage, 1.7 gal of hydraulic fluid is used in order to compress N2 to minimum system pressure of 1,200 psi (200 psi over pre charge pressure), called “minimum operating pressure”. Hydraulic oil will be pumped into the bottle until pressure in the bladder reach 3,000 psi, called “Operating Pressure”. Volume of hydraulic fluid used to pressure up from 1200 psi to 3000 ps,which is called “Useable Fluid”, is equal to 5 gallons,
Note: I will show you how to use Bolye’s Law to figure out usable fluid later.
Accumulator Pressure Charging System
In an accumulator unit as shown in the figure below as an example, there are one electric pump and two pneumatic pumps, which will be automatically pumped hydraulic fluid, which is stored in a reservoir tank when pressure in bottles is below set pressure in order to maintain bottle pressure at 3,000 psi. The electric pump is mainly used to maintain pressure. Pneumatic pumps will be use in case of 1) the electric pump failure and 2) helping the electric pump to increase bottle pressure quicker.
Note: you may see different set up depending on the rig but the concept is the same.
Hydraulic fluid under pressured is divided into two following ways:
1. The first way is used for controlling an annular preventer. Hydraulic under pressure of 3,000 psi in bottles passes a pressure regulator (in the figure is a regulator “A”) in order to regulate pressure from 3,000 to 500-1500 psi for operating annular preventer. Pressure used to operate the annular preventer must be less than pressure used to operate rams preventer because high pressure will damage a rubber seal inside of the annular preventer.
2. Another way is used to control rams preventers (VBR’s rams, blind/shear rams, pipe rams) and HCR valves. Hydraulic under pressure of 3,000 psi in bottles passes a pressure regulator (in the figure is a regulator “B”) in order to reduce pressure from 3,000 to 1,500 psi for operating rams preventers and HCR valves. The regulated hydraulic fluid under pressure of 1,500 goes to a manifold which is used to divide hydraulic pressure in many ways to operate equipment. This is normally called “Manifold Pressure”.
All pressure activated equipment as annular preventer, VBR’s and HCR valves have two hydraulic fluid ports. One is for opening equipment and another one is for closing equipment. You can operate the BOP equipment by pushing remote switches or handles at the accumulator unit. In case of opening equipment, pressured hydraulic fluid will be transmitted to the open port and hydraulic power will drive an internal mechanism inside equipment in order to open equipment. Fluid in closing side will be pushed out and go back to a reservoir tank. On the contrary, in case of closing equipment, the process is quite as same as opening process but only fluid flow direction is opposite.
Reference book: Well Control Books