New Control System for Blowout Preventers
Authors: Vujasinovic Ado , Bates John 
Published: December 2007

In a conventional blowout preventer (BOP) control system, a substantial amount of stored high-pressure fluid is depleted for each closing operation. Large annular preventers may require closing fluid volumes greater than 60 gal [0.227 m3]. The new BOP control system takes advantage of the differential area between closing and opening sides of BOP hydraulic operators. When pressurized fluid is directed simultaneously to the closing and opening side of the piston, the preventer will start the closing process. The BOP is driven by the force that results from pressure acting on the piston area differential. The fluid supplied equals the volume differential between closing and opening sides because the fluid from the opening side is short circuited to the closing chamber inlet.

Introduction

In a conventional BOP control system (Fig. 1) the motion of opening and closing a BOP is controlled by a single selector valve. When shifted, this valve directs the pressurized fluid from the accumulators to either the pressurized fluid from the accumulators to either the closing or opening chamber of the BOP (Fig. 2). Because the entire chamber has to be filled before the desired function is accomplished, this system uses a large amount of high-pressure fluid. Consequently, a substantial amount of stored high-pressure energy is depleted for each operation.

The need for more efficient closing systems has become quite apparent as BOP's have grown in size over the years. This is particularly true with annular preventers that require closing fluid volumes of 60 gal preventers that require closing fluid volumes of 60 gal [0.227 m3] or more. Analyses of actual operating requirements reveal that the force needed to close an annular preventer is relatively low during approximately the first 75 % of the stroke and increases rapidly toward the end. The stroke/force relationship, as well as the closing fluid flow requirements in a conventional system as function of the stroke, are shown in Fig. 3. It is obvious that the large volume of high-pressure fluid acting on the entire area of the closing side of the piston can generate a force far greater than the actual requirements over the initial three-fourths of the piston stroke. Moreover, since the flow of pressurized fluid into the closing chamber is accompanied by a corresponding pressure drop, it is clear that this system must satisfy the pressure drop, it is clear that this system must satisfy the maximum force requirements (at the end of the stroke) when its pressure is at the minimum level. The result is a large accumulator system and a high-capacity pumping unit.

The new BOP system takes advantage of the differential area between closing and opening chambers and of the closing force requirement described previously. When pressurized fluid is directed simultaneously to the closing and opening side of the piston, the preventer will start the closing process driven by the force resulting from the pressure acting on the piston area differential. Fluid volume supplied by the stored energy equals the volume differential between the closing and opening chamber because the fluid from the opening side is short-circuited to join with the supply fluid at the closing chamber inlet, Fig. 4.

Energy Saving Valve (ESV)

The heart of the new system is the ESV shown schematically in Figs. 5 through 7. When the sequence valve is shifted from open to close, pressurized fluid is introduced initially into both the closing and opening chambers of the BOP (Fig. 5). This condition will prevail for as long as the piston (A) is, as indicated, on prevail for as long as the piston (A) is, as indicated, on the left side of its chamber. After the annular sealing element is partially closed, resistance builds up as the element deforms. At approximately 75% of the stroke, pressure increases to the setting level of the relief valve pressure increases to the setting level of the relief valve in the control circuit (C) and allows pressurized fluid to enter the chamber on the left side of the piston (B). Because piston B is larger than Piston A, both Pistons B and A will shift to the right (Fig. 6). In this ESV position, the opening chamber is vented to tank and the position, the opening chamber is vented to tank and the high-pressure fluid acts on the entire area of the closing chamber, allowing full closing force to be applied to the annular BOP (Fig. 6). Reversing the selector valve applies pressurized fluid to the opening chamber of the BOP and connects the closing chamber to tank (Fig. 7). As the BOP opens, the ESV resets for the next cycle.

Pressurized fluid saving is represented by the cross-sectional area (Fig. 3), which amounts to the volume of the opening chamber up to 75% of the stroke. Beyond that, the opening side is connected back to tank. From the force viewpoint, 75% of the stroke is accomplished with minimum effort and the last 25 % with the full force available on the closing side of the piston.

A built-in backup function of the ESV system is the override mode shown in Fig. 8. Activation of the over-ride applies pressure to the back of Piston B, which results in an instantaneous shift corresponding to the conventional closing mode.

JPT