September 20th, 2024
Critical Calculations
Ensuring Accumulator Performance When Safety Is on the Line
When it comes to off-the-cuff accumulator calculations, most personnel are already aware the equations below will put you in the ballpark.
These equations simply mean that, in a closed system (such as an accumulator bottle), the pressure times volume at a particular condition will equal the pressure times the volume at another condition.
In other words, if you increase the pressure, the gas volume will decrease because gasses are compressible. Similarly, a decrease in pressure will produce an increase in volume. For safety critical rig systems, however, being in the ballpark is not even close to good enough. This is why the process for calculating accumulator capacity is standardized in API 16D.
Reading through a 16D accumulator calculation can be frustrating and at times confusing, so what really matters when it comes to these assessments?
Step 1: Define What You Need the Accumulator System to Be Able to Do
In a Dedman/Autoshear (DMAS) system, ensuring the ability to shear and seal the well's worst-case tubular is critical. But here’s the catch—the "worst case" isn’t always the biggest pipe in the well plan! While it's easy to assume that a 6-5/8" 40# V-150 pipe would be harder to shear than a 6-5/8" 34# V-150, that's not always true. If the lighter pipe is in a section with significantly higher wellhead pressure, it might require more hydraulic force to shear than the larger pipe at lower pressure.
To make sure nothing is left to chance, Aquila Engineering kicks off the process with a detailed Shear Verification Analysis. We consider well-specific parameters, along with the tubulars, wirelines, and assemblies the rig will encounter. By performing these analyses for each installed shear type, we deliver a complete picture of the required pressures for a safe operation.
Once we’ve determined the theoretical pressure requirements to shear or shear and seal the worst-case tubular with a Casing Shear Ram (CSR) and Blind Shear Ram (BSR), we move forward confidently to the next critical step.
Step 2: Define What You Already Have
We know what pressure we need, but what about volume? Now it’s time to identify what the rig has installed. What is the hydraulic fluid volume required for closing a CSR or BSR? Does this include the locks, time delay circuit, and any other functions? Does the BOP use standard piston type bottles, or depth compensated bottles (DCBs)? How many bottles and how large are they? What is the gas volume of each bottle? Does the rig use Nitrogen as a pre-charge gas, or does the rig use Helium?
This is also the time to define seawater and control fluid pressure gradients, or weight per unit volume (typically pounds per gallon in the US). Other constants that can be gathered which impact the accumulator’s ability to perform are the riser and control fluid air gap, and the HPU output pressure. Next, we must identify our well-specific variables.
Step 3: Define the Well-Specific Parameters
When it comes to accumulator efficiency, well-specific variables play a huge role. Factors like mud weight, water depth, maximum anticipated wellhead pressure (MAWHP), and even the temperature at both the surface and subsea wellhead can dramatically impact performance. These variables determine how much fluid is available at a given bottle pressure for a specific pre-charge.
In the high-stakes environment of subsea operations, the bottle pressure we care about most is the one that will deliver the shear force needed to handle our ‘worst-case’ tubulars. Each detail matters in ensuring the system can meet these critical demands.
Step 4: Finally Putting It All Together
Now it’s time to crunch some numbers. Parameters like depth, fluid pressure gradients, and shear pressures all come together to calculate the theoretical pressure required to shear the well’s worst-case tubular. This calculation factors in environmental variables, including wellbore pressure, which adds to the overall force needed to shear.
With this data in hand, we now know the exact pressure the Casing Shear Ram (CSR) and Blind Shear Ram (BSR) need to get the job done at the wellhead. The next step? Determining whether the system can deliver that pressure when it matters most.
To tackle this, we dive into some advanced physics- using a sophisticated version of PV/T calculations that incorporate factors like density and entropy. As gas expands, it cools, and colder gas exerts lower pressure than the same volume of warmer gas. This is a key consideration when calculating the volume needed to close a set of Casing Shear Rams (CSRs).
We model how the accumulators respond when gas is rapidly withdrawn, leading to a sharp drop in temperature. This cooling effect reduces the pressure inside the bottles, so we carefully compare the remaining pressure with what's needed for the Casing Shear Ram (CSR) to function. If the available pressure is still greater than what's required, we’re in the clear! We then repeat this process for the Blind Shear Rams (BSR) to ensure both systems are ready to perform effectively in any emergency scenario, giving us confidence that the entire setup is good to go.
Next, we calculate a range of pre-charge pressures, establishing both a minimum and maximum boundary. Within this range, the BOP will maintain enough fluid volume to exceed the required shear pressure, ensuring the DMAS sequence performs flawlessly in an emergency.
Finally, once these calculations are set, we catalog and verify the actual pre-charge pressure and temperature. This final check guarantees that the accumulator system is fully capable of delivering when it counts, ensuring the safety and functionality of the operation.
Precision Under Pressure: Ensuring Safety with Accurate Accumulator Calculations
In the high-stakes world of subsea operations, getting accumulator calculations right is non-negotiable. A miscalculation could mean the difference between a flawless emergency response and a critical failure.
That’s why at Aquila Engineering, we leave nothing to chance. From pinpointing the worst-case tubulars to diving deep into well-specific variables, we ensure every detail is accounted for.
Our process doesn’t stop at just meeting requirements—we go further with advanced PV/T calculations, ensuring that your Casing Shear Rams (CSR) and Blind Shear Rams (BSR) are primed for action. By meticulously calculating pre-charge pressures and verifying system performance, we make certain that your equipment will perform under pressure, literally. When you’re relying on precision and safety, trust in a system that’s built to deliver when it matters most.