The Role of Pressure Relief Systems (PRS)
Central to maintaining this delicate balance are Pressure Relief Systems (PRS). These are not merely components; they are critical safety barriers ensuring that high-pressure operations remain within safe boundaries, protecting personnel, multi-million-dollar assets, and the sensitive marine environment.
At its core, a PRS is designed to automatically prevent the pressure within any part of an offshore facility’s equipment—from pipelines and vessels to wellbore components—from exceeding its designated Maximum Allowable Working Pressure (MAWP).
Typical Overpressure Scenarios Offshore
Overpressure can result from equipment malfunctions (e.g. pump failures or stuck chokes), unexpected formation responses during well treatments, or process upsets.
One critical event in well stimulation is a “screenout,” where proppant (typically sand) blocks the flow path and causes rapid pressure escalation. In such cases, a PRS provides a vital safeguard as it is engineered to detect such an incipient overpressure condition and automatically open a path for the excess pressure to be safely vented..
Pressure Relief System Options
Spring-Loaded Pressure Relief Valves (PRVs) click to readmore
These are perhaps the most traditional type. A spring holds a disc closed until upstream pressure overcomes the spring force, lifting the disc and relieving pressure.
While conventional PRVs are simple and reliable in clean service, their set pressure can be significantly affected by back pressure at the valve outlet—potentially causing premature opening, inconsistent performance, or failure to open at the correct pressure in high back pressure scenarios. Balanced designs help minimize—but do not always eliminate—this risk.
Balanced PRVs (using bellows or a piston) are designed to counteract the effects of back pressure, ensuring more consistent operation in systems with variable downstream pressures. However, their effectiveness depends heavily on system conditions and maintenance. In dynamic or harsh environments, they may still suffer from performance drift, component fatigue, or sealing issues, which can compromise overpressure protection if not properly managed.
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Pilot-Operated Relief Valves (PORVs) click to readmore
These are more sophisticated systems where a smaller pilot valve controls the operation of a larger main valve. System pressure is typically routed to the top of the main valve’s piston (the “dome”), creating a net downward force that keeps it tightly sealed, often allowing the system to operate closer to its set pressure without leakage. When system pressure reaches the pilot’s set point, the pilot actuates, venting the dome pressure. This allows the main valve to open and relieve the system overpressure.
PORVs can use the process fluid itself to actuate the pilot. However, this makes them vulnerable to clogging if the fluid contains particulates, a major concern in well stimulation.
Some designs utilize an external, clean medium (like nitrogen) for pilot actuation, isolating the sensitive pilot mechanism from potentially erosive or clogging process fluids. Nitrogen can also be used for pre-charging domes in certain PORV designs or for testing valve set points.
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Burst Discs (Rupture Discs) click to readmore
These are thin membranes engineered to rupture at a precise pressure. Upon rupture, they open instantly, providing full-bore relief with no moving parts exposed to the process fluid.
Forward-acting (tension-type) discs have the process pressure acting on their concave side, causing them to stretch and burst under tension.
Reverse-acting (compression-loaded) discs have pressure on their convex side, causing the dome to buckle and reverse at the set pressure, often opening along pre-scored lines. These generally offer better fatigue resistance and tighter burst tolerance under cycling conditions.
Burst discs are highly valued for their speed, simplicity, and reliability—especially in severe service environments involving corrosive, abrasive, or dirty fluids. Unlike valves, they are immune to clogging or mechanical failure before activation. Due to these properties, burst disc technology is also widely used in nuclear power plants and aerospace applications, where failure is not an option and safety margins are extremely stringent.
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Modern Innovations in PRS click to readmore
Innovative systems like dual-line burst disc modules mitigate the traditional drawback of non-reclosing discs. These allow rapid switchover to a secondary line, minimizing non-productive time (NPT) after disc rupture.
One example is the Mark & Wedell Annulus Pressure Relief System (APRS), which uses parallel burst disc lines with rapid switchover capability. This design maintains the benefits of burst discs (speed, simplicity, reliability) while minimizing downtime.
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Other Solutions click to readmore
While less common as primary overpressure protection for entire systems, shear-pin relief valves are sometimes used, particularly for protecting positive displacement pumps. These use a pin of calibrated strength that shears when pressure exceeds a certain limit, allowing the valve to open. They are manually reset and generally less precise than PRVs or burst discs.
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Why PRS are Crucial in Offshore Oil and Gas
The importance of robust PRS in the offshore oil and gas sector cannot be overstated. Operations frequently push the boundaries of pressure and temperature, often dealing with flammable, toxic, and environmentally sensitive fluids.
Safety and Environmental Stewardship
The primary driver is safety—protecting personnel from injury or fatality, preventing catastrophic equipment failure, and safeguarding the marine environment from uncontrolled hydrocarbon releases or chemical spills. Failure in pressure containment can have devastating and far-reaching consequences.
Asset Integrity and Economic Viability
PRS protect high-value assets, from downhole completions and wellheads to surface facilities and pipelines. Preventing overpressure damage avoids costly repairs, replacements, and significant non-productive time (NPT). In an industry where daily operational costs, especially offshore, are immense, reliability is paramount.
Managing Extreme Operational Conditions
A key factor in managing these risks is understanding the pressure regimes typically encountered in offshore stimulation operations. These jobs generally fall into three main pressure categories, based on industry field experience:
- Low to moderate pressure (1,500–3,500 psi / 100–240 bar): ~15–20% of stimulation jobs, typically matrix acidizing or lighter treatments
- Moderate to high pressure (3,500–6,000 psi / 240–415 bar): ~60–70% of jobs, including most sand-acid and conventional proppant fracturing operations
- Extra high pressure (6,000–10,000+ psi / 415–690+ bar): ~15–25% of jobs, often associated with deep, high-pressure reservoirs or HP/HT stimulation applications
Common examples include:
- Sand-acid jobs: typically 3,500–5,000 psi
- Crosslinked gel fracturing: up to 7,500 psi
- Deep carbonate acidizing: 5,000–8,000 psi
- HP/HT fracking (less common offshore): 10,000–15,000+ psi
These pressure levels significantly influence equipment selection. While spring-loaded PRVs may suffice for lower pressures, they often struggle at higher ratings due to bulk, back pressure sensitivity, and erosion risk. PORVs can handle mid- to high-pressure but may be compromised in contaminated or abrasive fluid conditions. Burst discs, by contrast, remain compact and effective across the full pressure spectrum—offering fast and reliable protection even under extreme conditions.
Well stimulation—activities like hydraulic fracturing and acidizing—involve pumping complex, often abrasive slurries (containing sand, proppants, and chemicals) at very high pressures and flow rates to enhance reservoir permeability. The risk of “screenouts,” where proppant bridges off and causes a sudden, dramatic pressure surge, is a significant concern.
Annulus Protection:
A critical aspect is protecting the well annuli (the spaces between concentric casing strings or between casing and tubing). While dedicated annulus pressure management systems exist for issues like thermal expansion, the PRS on surface treating lines play a vital indirect role during stimulation. By preventing the treating pressure from exceeding the limits of the tubing, casing, or downhole packers, these surface PRS help avert breaches that could lead to high-pressure fluids entering and damaging the annulus.
Regulatory Imperative:
Stringent international and national regulations, often guided by industry standards from organizations like Det Norske Veritas (DNV), the American Petroleum Institute (API), the International Organization for Standardization (ISO), and the American Society of Mechanical Engineers (ASME), mandate the use, proper design, installation, and maintenance of PRS. These standards ensure systems meet rigorous performance and safety criteria for offshore operations.
Comparison of Pressure Relief Systems
Selecting the optimal PRS involves a careful trade-off based on specific operational needs, fluid characteristics, and risk tolerance.
| Feature | Spring-Loaded PRV | Pilot-Operated PRV (PORV) | Burst Disc (Rupture Disc) |
| Response Time | Fast (2–10 milliseconds/ ms) | Slower (~100 ms) | Extremely Fast (1–3 ms) immediate full-bore opening |
| Pressure Range Suitability | Suitable for moderate pressures up to ~3,500–4,000 psig (240–275 bar); can become bulky or unstable above this range | Suitable for pressures up to ~6,000 psig (415 bar); performance may degrade in dirty or cycling service | Covers full offshore stimulation range: 3,500–10,000+ psig (240–690+ bar); compact even at ultra-high pressure |
| Performance in Erosive/Slurry Service | Fair to poor; erosion common in the majority of offshore stimulation jobs | Poor (system fluid pilots clog easily); improved with external actuation | Excellent; no moving parts exposed before actuation |
| Leak Tightness (Pre-Actuation) | May simmer/leak near set pressure | Excellent; can operate very close to set pressure | Excellent; hermetically sealed until rupture |
| Reclosing | Yes (automatic) | Yes (automatic) | No; but is often installed in parallel configurations for rapid switchover |
| Back Pressure Sensitivity | Sensitive (conventional) | Many designs are inherently balanced/less affected | Not sensitive, fully passive design |
| Maintenance | Frequent; most offshore stimulation jobs involve abrasive or contaminated fluids | Complex; requires pilot cleaning and specialist knowledge | Minimal: inspection after burst, simple replacement |
| Cost (Typical) | ~USD 50/day rental; 2–4 units per job | ~USD 150–300/day rental; 1–2 units per job; requires N₂ system, hoses, test setup, and specialist operation | USD 1,000–4,000 per disc (purchase); typically 2 installed per job in parallel with switchover configuration; unruptured discs can often be reused for future jobs; requires integration with a pressure-retaining mounting system (typically rented) |
| Primary Advantage | Simple, cost-effective for clean systems | Precise control and high capacity | Fastest response, highest reliability in harsher and safety critical environments |
| Primary Disadvantage | Leak risk and erosion in harsh conditions | Susceptible to clogging and complexity | One-time activation (unless rigged up in parallel for quick switchover), requires replacement |
Due to their simplicity, speed, and robustness, burst discs are often the preferred solution in environments where reliability is non-negotiable—such as nuclear facilities, aerospace systems, and high-risk offshore stimulation operations.
Key Considerations for Offshore Operations
For Abrasive Slurries (e.g., during hydraulic fracturing)
Burst discs are often favored due to their superior resistance to erosion and clogging before actuation. Externally actuated PORVs, where the pilot is protected from the process fluid, offer a reclosing alternative but with increased complexity. Standard spring-loaded PRVs and system-fluid PORVs face significant reliability challenges in these conditions.
Rapid Pressure Spikes (e.g., screenouts)
The millisecond response time of burst discs is a critical advantage. Direct-acting spring PRVs are also very fast.
Minimizing Non-Productive Time (NPT)
Dual-line burst disc systems like APRS from Mark & Wedell address the main drawback of traditional rupture discs (i.e. their non-closing nature).