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Picture that it’s 10,000 feet subsea, and a remotely operated vehicle (ROV) is navigating its way through the black, frigid waters of the ocean’s aphotic midnight zone, far beyond the reach of sunlight, guided only by lights and cameras to its destination. The conditions are such where no human diver could survive. Then a white structure comes into the camera’s view, with the paddle handles of subsea valves and black faces of subsea gauges becoming clearer as the ROV nears its target.
It slows to hover in front of the panel sitting on the sea floor. No obstructions are visible, and all gauges read zero. A human operator topside watches closely as the robotic arm of the ROV extends forward to pull out a dummy plug from the panel and then sets it into a holder on the side. A live hot stab is inserted in its place. Swiftly, gauges begin to move. Oil enters the hydraulic tubing, powered from an umbilical connected to a hydraulic power unit stationed topside. They are “go” for operation.
Most hydraulic equipment operates far away from the corrosive environment and extreme ambient pressures of ocean waters—perhaps mining, transportation, or even wind energy. Yet, the oil and gas industry has made incredible advances in subsea technologies, bringing the capabilities of hydraulic equipment to the most extreme places on the earth. Hot stabs make this possible.
A hot stab is a subsea hydraulic coupling device that transmits hydraulic fluid from a topside hydraulic power unit to energize subsea equipment. Essentially, it is a hydraulic quick-acting coupling designed for the deep-sea application. Two major types of stabs are used: live stabs and plug stabs. Both mate to a receptacle—the live stab creates a path for hydraulic fluid to flow, while the plug stab typically acts as a placeholder on subsea equipment to prevent water ingression into the hydraulic system.
An ROV will remove the plug stab and set it into a designated place holder, then place the live hot stab into the female receptacle. Once the connection is sealed, fluid from the topside will run through an umbilical, through the live hot stab, and into the subsea equipment being activated. The configuration of the hot stab can vary greatly depending on its application, namely the porting, material composition, flow paths, and special testing requirements.
Hot-Stab Design
When specifying a hot stab, one of the first considerations concerns the flow path. Is high pressure at low flow needed, or low pressure at high? When all operating scenarios are evaluated, a standard hot-stab product offering may work for a majority of applications. The American Petroleum Institute’s API 17H recommended practice for subsea production systems has several standard types of hot stabs and equipment with flow paths ranging from ¼ to 3½ in. Even at the bottom of the ocean, though, equipment must obey the laws of physics, and burst pressures and collapsing loads must be evaluated prior to being put into service.
Porting on the hot stabs and receptacles is what integrates the stab into the hydraulic system. The connection type and size need to be carefully selected to ensure that flow rates and designed working pressures are maintained. A wide variety of options is available to meet these requirements, from flanges and hubs to threaded connections and unions.
Depending on the application, single- or multi-port stabs may be used. For applications that cannot tolerate ingression of sea water, integration of check valves into the hot stab help to prevent water from entering the hot stab.
Material Selection
As is the case with most subsea equipment, special materials are often necessary to withstand the corrosive combination of salt water and oxygen. In addition to corrosion, subsea equipment can be subject to embrittlement, making the metal vulnerable to cracking.
Depending on the composition of the metal, temperature, galvanic reaction, and other factors, exposing the metal to hydrogen sulfide may cause catastrophic, premature equipment failure. Many corrosion-resistant alloys have a proven track record in subsea environments. Understanding when to use each one is critical to the success of a project.
The table shows several factors that may play into which material is selected for a hot stab. Strength and corrosion resistance are essential to the long-term survival of subsea hot stabs, but they can come with a significant price tag. Note that the table is based on my experience; different grades, forms, production methods, and level of annealing may result in variations. Always check with your supplier for mechanical properties.
Testing Requirements
Hot stabs and receptacles are often placed thousands of feet subsea, and it can take hours or even days for equipment to reach its final destination on the sea floor. Removing faulty equipment can be intensely time-consuming and expensive. Thus, most hot-stab equipment must go through an extensive testing process, including nondestructive examination (NDE), nondestructive testing (NDT), and destructive testing, depending on the application and materials selections.
NDE procedures may include both ultrasonic and dye-penetrant inspections. First, ultrasonic inspections are conducted on raw materials. Sound waves that are audible to humans typically fall in the 20- to 20,000-Hz range. Ultrasonic testing equipment produces ultrasonic waves (100,000 to 2,500,000 Hz) far beyond the range of human recognition, though, and can be used to detect imperfections. Technicians are able to scan raw materials and interpret the results to determine if items being inspected are of good quality. Imperfections could be cold shuts, porosity, or inclusions, all of which can affect the integrity and fatigue life of these components. Such discontinuities may cause the material to be rejected.
Once a raw material passes ultrasonic testing, it is sent back to the hot-stab manufacturer for machining before being shipped out for dye-penetrant inspection. During dye-penetrant inspection, sub-components are coated with a fluorescent dye. Capillary action then draws the dye into any imperfection. After a specified time period, excess dye is removed and a developing agent is applied. Technicians inspect each part for defects, ensuring that the machining process did not damage the integrity of the hot-stab materials. If a small line or other flaw is revealed by the penetrant dye, it could indicate a crack developed during a machining process, and may be cause for material rejection.
NDT procedures are a bit different, and include proof testing and hardness checks. Proof testing, or pressure testing, is usually conducted by applying hydraulic pressure to the hot stab’s ports and taking them either up to the designed working pressure or beyond for a predetermined amount of time, depending on test requirements. Hardness checks are also critical for subsea equipment, because materials with a higher hardness may be more susceptible to cracking.
A Rockwell hardness test is the most common for hydraulic components. It indicates hardness by measuring the depth of penetration of an indenter under a high load and comparing it to the penetration made by a preload. Different scales are used depending on the material. Steels are usually measured with the C scale, which applies a force of 120 kgf using a 120-deg. speroconical diamond indenter.
As in most industries, the world of subsea equipment has no one-size-fits-all solution. Every application is different, and each hot stab must be tailored to meet the rigid demands of an unforgiving environment. Each location and operation presents its own set of challenges, and engineers are always on the lookout for features that can modify a product for optimal performance.
Chauntelle Baughman Subsea & Offshore Product Specialist at Hydraquip Inc., Houston, Texas, an exclusive distributor for DMIC hot-stab products. For more information, call her at (713) 680-1951, or email [email protected].
