By Josh Cosford, Contributing Editor
Environmentally friendly fluids, corrosion resistance, and fire- and explosion-proof technologies are just some of the design options necessary in offshore use.
Offshore oil and gas applications are one of the most demanding environments for a machine of any name, let alone hydraulic equipment. Offshore describes the open water, of course, and those waters are nearly always salty oceans. If you’re reading this from the Salt Belt, you understand the destruction left in the wake of sodium chloride crystals spread across roads and highways to melt snow. Salt is very corrosive, especially in the presence of water, making the ocean a prime source of oxidative stress on metal components and structures.
Not all oil rigs are in bodies of water with names ending in Gulf, as it turns out. You’d be surprised with the number of oil reserves found in the Arctic. Take the effects of saltwater and add to it extreme cold, and you’ve got an extreme operating environment by any standard. So, you’d guess correctly that fluid power operates in all corners of the earth, and offshore oil and gas are not exempt.
Hydraulic power finds itself the primary motivation for many machines and functions on offshore oil rigs, both above and below sea level. For example, motion compensators, blowout preventers and snubbers and various material handling applications take advantage of the power and control of hydraulics. In fact, some drill rigs employ fifteen or more hydraulic cylinders to activate various functions.
A common thread to all hydraulic components in offshore oil & gas describes both the construction and finishing techniques. To stomp with the big offshore dogs, hydraulics must withstand the threats of its environment while also operating successfully in its sensitive environment. As it turns out, crude oil is quite flammable, so the control systems must take care to avoid being an ignition source.
The right fluids for the job
The hydraulic fluid used in offshore applications must exhibit unique qualities suitable to the environment. First and foremost, that environment is the environment. As you’d suspect, the opportunity to harm our ocean’s flora and fauna with a toxic hydraulic oil leak should be mitigated without prejudice. Off-the-shelf mineral-based hydraulic oil need not apply for this role. Environmentally friendly fluids mitigate the damage to fish, coral and plant life should a leak or blowout occur.
Should it leak into the ocean, subsea environmental fluid is acceptable to discharge without causing any harm to its sensitive surroundings directly. Manufacturers offer both oil and water-based fluids for subsea operation, and regardless of the base compound, both must meet the criteria for these extreme conditions. Extremely low viscosity fluid created from a synthetic base offers an advantage over conventional hydraulic fluid.
A little-known property of hydraulic fluid is its propensity to become more viscous as pressure increases. In fact, hydraulic fluid may double dynamic viscosity as pressure increases from 1 to 400 bar. 400 bar (5,800 psi) sounds like extreme pressure, and in absolute terms, it is. However, you must consider how undersea operating depths may offset the design pressure of hydraulic components. At 10,000 feet deep, ambient pressure is 4,330 psi, so as far as the hydraulic components are concerned, they’re only withstanding 1,470 psi of relative pressure. However, in absolute terms, that 5,800 psi still increases viscosity.
Because industrial hydraulics usually perform within stable thermal conditions, temperature changes are not extreme enough to change viscosity much. Conversely, undersea temperatures vary widely, sometimes in the exact location, so undersea hydraulic fluid should be manufactured with high viscosity index (its ability to maintain viscosity under a wide temperature range). Seals should be suitable for low-temperature operation because of near-freezing underwater temperatures and because not all offshore hydraulic components are underwater during operation. Design hydraulic machines for their environment, whether for an arctic SPAR or semi-submersible in the Gulf of Mexico.
Because high-pressure crude oil sprays with such vigor during a blowout, it atomizes like it was sprayed from a giant fuel injector, making a highly flammable combination of fuel and air. Hydraulic fluids for offshore oil & gas should not support flame, so high water-based fluids make an ideal candidate, especially for above-sea applications. Any oil-based hydraulic fluid should be relegated for use only in subsea applications, where it cannot mix with air to create a flame thrower.
Corrosion-resistance tops the needs
Whenever possible, hydraulic components such as cylinders should be constructed with corrosion-resistant materials. Reliability in the face of extreme conditions trumps every other consideration since something like a blowout preventer must absolutely work the first time, even if it hadn’t been engaged in years. The alloy of choice for cylinders, motors, valves and major components subject to rush is 316 stainless steel.
Stainless can be expensive, especially for huge cylinders (and very many are large in oil & gas). So, using other methods of corrosion protection may be acceptable. For example, electroless nickel plating offers excellent correction protection, wear resistance, and surface hardness. The laborious plating process isn’t inexpensive but offers a solution a fraction of the investment in something like a 12-in. bore cylinder with some 20-ft stroke or more.
Valves constructed using corrosion-resistant technology are more important than you think. Sure, you can paint the outside of any valve with marine-grade epoxy, but that does nothing to help the internals. Yes, corrosion and rust can occur inside steel components, even in the presence of oil. For undersea hydraulic applications, it’s nearly impossible to keep seawater out permanently. In fact, specialty hydraulic fluid manufacturers often publish data on their fluid’s resistance to seawater. Seawater must not be left entrained in hydraulic fluid, as it will degrade the fluid over time should it be left alone.
With the potential for internal corrosion, stainless steel valves make sense for offshore oil & gas applications. Aluminum valve bodies and manifolds are ideally suited for internal exposure to seawater, but epoxy paint must be applied to external surfaces to prevent a galvanic reaction that corrodes aluminum. Also, aluminum should never be mixed with stainless steel, where this reaction is most extreme.
Materials are key in fire-resistance
Epoxy paint for offshore applications must offer myriad performance benefits, such as flexibility, fire resistance and corrosion protection, all the while formulated with consideration to the environment. In addition, any paint used in offshore applications must be biodegradable to prevent damaging the surrounding animal and plant life. The rig manufacturer will provide the specs to the manufacturer of the cylinder, motor, valves or power units, which is likely the same type and brand used on the other critical rig components.
As previously mentioned, conditions conducive to creating and supporting flame must be avoided. Not only is fire dangerous because of the quantity of fuel available, but it can also be tough to extinguish. Furthermore, once started, a fire can lead to explosions, which result in severe injury, death and loss of property.
Passive fire protection is essential, like flame-resistant epoxy paint, insulated power unit enclosures, or all-metal construction, where possible. However, just as resisting fire is important, so too is avoiding igniting any leaking oil. A technology called explosion-proof refers to the valve coil, controller or other components unable to send a flame or spark outside of itself, avoiding the possibility to ignite any fuel source.
Explosion-proof technology is part of the industry specification and requires rigorous certification through independent bodies such as the Underwriter Laboratories. Unfortunately, the process is also expensive, so few manufacturers offer explosion-proof valves and components. For that matter, manufacturing anything for offshore oil & gas is expensive because reliability and safety are number one. But because hydraulics is just so good at what it does, the effort put into specialty components to serve the offshore oil industry is worth it.
Filed Under: Engineering Basics, Fluids, Trending, Valves & Manifolds