The accumulator unit is one of the critical well control equipment and its main aim is to supply the pumps with atmospheric fluid while also storing high pressure operating fluid for operating BOP stack. In this article, we will learn about requirements of critical components of an accumulator unit including accumulators, reservoir, pneumatic pump, electric motor driven pumps and hydraulic control manifold/valve & fitting.
Surface BOP Control Systems Equipment
For storing high pressure fluid, accumulators are pressure vessels (ASME coded). Depending on requirements, the accumulators can be found in all sorts of types, sizes, pressure ratings, and capacities. Most commonly, ‘float’ and ‘bladder’ accumulators are used which come in ball or cylindrical shapes. Furthermore, they can be top or bottom loading.
Figure 1 – Accumulator Bottles
If bottom loading, servicing will require them to be removed from the accumulator unit. If top loading, both float and bladder can be removed while mounted on the accumulator unit. Without destroying their stamp of approval, both types of accumulators can actually be repaired in the field whenever necessary. Continue reading
4-way valves in the accumulator (Koomey) unit are used to control the position of Blow Out Preventer (BOP). Today we will go into the detail of 3 positions of 4-way vales in order to see how each position affects to the BOP.
Read more details about Koomey Unit here =>mechanism of Koomey unit.
Four-Way Vale in Open Position
When the valve is turned into the open position, it directs hydraulic pressure from the manifold into the BOP openning port therefore the BOP is in the open position. The hydraulic fluid in the ram closing chamber will return back to the reservoir tank. Figure 1 illustrates how the hydraulic pressure is lined up to open the BOP.
Figure 1 – Open position of the 4-way valve
This topic will demonstrate you how to determine accumulator bottles required for Koomey Unit (Accumulator Unit) in order to close the surface BOP stack.
This is a specification of Accumulator (Koomey) Unit.
Accumulator 3,000 psi system
Volume each bottle is 10 gallon.
Pre charge pressure is 1,000 psi.
Minimum operating pressure is 1,200 psi (200 psi over pre charge pressure)
Usable fluid per gallon is 5 gallon.
Safety factor require is 50 %. This safety factor may be different depending on each company’s requirement.
BOP Stack and HCR’s as listed below:
|| Gallon to open
|| Gallon to close
| Annular Preventor
| Upper Pipe Rams
| Blind Shear Rams
| Lower Pipe Rams
| Choke HCR
| Kill HCR
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
An accumulator or Koomey unit is a unit used to hydraulically operate Rams BOP, Annular BOP, HCR and some hydraulic equipment. There are several of high pressure cylinders that store gas (in bladders) and hydraulic fluid or water under pressure for hydraulic activated systems. The primary purpose of this unit is to supply hydraulic power to the BOP stack in order to close/open BOP stack for both normal operational and emergency situation. Stored hydraulic in the system can provide hydraulic power to close BOP’s in well control operation, therefore, kick volume will be minimized. The accumulator should have sufficient volume to close/open all preventers and accumulator pressure must be maintained all time. According to API RP53, your reservoir tank should have a total volume at least 2 times of usable volume to close all BOP equipment.