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This offshore handling system is built to weigh less than 50 kg, but it still must produce 50 tonnes of pulling force with a linear stroke of 650 mm — highly unlikely without a carefully designed sealing system. |
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With 800 bar internal pressure and guides designed to prevent equipment seizure, wider gaps are a must-have. To prevent extrusion, though, the seal systems (to the left is the piston version; to the right, the rod version) have to operate below a certain working temperature. |
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This T23 hydraulic rod seal is made from 95 AU V142 Novathane, a material that exhibits a high elastic modulus and resists abrasion. The asymmetrical sealing lip geometry optimizes the sealing action. The backup ring, made from POM PO202, prevents extrusion at 800 bar of internal pressure as long as the operating temperature does not exceed 50° C. |
Offshore oil and gas production, underwater construction, deep-sea exploration, salvaging, and underwater mining all push man and machine to the limit. And when subsea conditions are too extreme for human divers to perform assembly and maintenance work, the tasks are done by remotely operated vehicles (ROVs). However, the ROVs used by Kvaerner Oilfield Products did not have the hydraulic guidance, positioning, and pulling attachments needed to connect and assemble such parts such as pipes, anchors, and braces. So Kvaerner Oilfield Products spelled out its requirements for such a system in a specification sheet containing some pretty hefty demands.
To make the handling system easy to transport, mount on the ROVs, and use above water on drilling rigs, it had to weigh 50 kg or less. Plus, it needed a stroke of 650 mm and deliver 50 tonnes of pulling force — a tall order for this weight class.
The solution, Figure 1, came from Norwegian specialist Malm Orstad. The only way to generate 50 tonnes of pulling force in a system weighing 50 kg or less was to manufacture high-strength hydraulic cylinders from titanium. Then, to actually produce the 50 tonnes of force, the entire system was designed for an operating pressure of at least 700 bar. To provide a margin of safety, the system was designed for 800 bar.
Dealing with large gaps
To develop a sealing system for this fascinating new product, Malm Orstad's engineers turned to Merkel Freudenberg Fluidtechnic. Being a system primarily for subsea ROVs, it had to be extremely reliable and virtually maintenancefree. Reliability was a major concern because a handling system failure could result in the total halt of oil production. To attain the highest possible degree of reliability, Malm Orstad wanted the rod and piston sealing systems to use guides. The guides, which prevent the individual components in the hydraulic cylinder from seizing, required larger gap dimensions that were absolutely necessary to seal the high pressures.
The main concern with wide gaps is that they can allow the extrusion of polymer seals. Even a requisite backup ring would not keep the primary seal in its groove at 800 bar. After gap dimensions, the Merkel-Freudenberg specialists soon discovered a possible solution: a Merkel primary seal T23 AU V142 combined with a backup ring made of polyoymethylne (POM), Figure 2.
The main concern with wide gaps is that they can allow the extrusion of polymer seals. Even a requisite backup ring would not keep the primary seal in its groove at 800 bar. After gap dimensions, the Merkel-Freudenberg specialists soon discovered a possible solution: a Merkel primary seal T23 AU V142 combined with a backup ring made of polyoymethylne (POM), Figure 2.
However, because of its temperature-sensitive characteristics,the seal would only work if it was protected from the regular peak temperatures for hydraulic systems of 110° C. At these high temperatures, a T23 AU V142 primary seal with a POM backup ring would only provide adequate service up to 400 bar.
Larger gaps sealed at 800 bar
As it turned out, the maximum acceptable operating temperature was the key to the solution. Merkel Freudenberg specialists calculated that, as long as operating temperatures did not exceed 50° C, this sealing system with the primary seal and a POM backup ring, Figure 3, would not exhibit extrusion, even at internal pressure to 800 bar. The seawater side is sealed by a similar Merkel seal that easily protects the hydraulic cylinder against the environment with the same gap and fit dimensions at pressure to 300 bar.
The seal system was to be rated for rod and piston seals with a wide range of diameters at an operating pressure of 800 bar and an operating temperature of 50° C. So Merkel-Freudenberg engineers designed the acceptable gap and fit dimensions for the various diameters. They then used these data to determine how large gap dimensions could get before the seal and backup ring began to extrude.
Their calculations set the maximum gap at 0.5 mm and the minimum gap at 0.2 mm. These gap dimensions, which are rather large in comparison to the pressure conditions, prevent even the smallest risk of any seizure between the cylinder and the piston or rod. The guides that handle this task in the seal systems are Merkel Guivex guides. They feature a patented convex profile on the static installation space side that reduces dangerous peak stress loads that occur around the dynamically loaded edge. These guides evenly distribute loads, no matter how high, effectively preventing disastrous edge fractures.
Any Malm Orstad handling system outfitted with these seals can also be used on shore as required. Because temperatures in the hydraulic system can easily exceed 50° C without the cooling action of the seawater, the operators have to ensure that the internal pressure of the hydraulic system does not exceed 400 bar.
Malm Orstad tested the hydraulic seal system extensively, as did the end user, Kvaerner Oilfield Products, before giving it the final seal of approval. This example shows that it is possible to develop costeffective seal solutions — even for critical operating parameters that far exceed standard requirements.
Asger Rasmussen, Merkel Freudenberg Fluidtechnic GmbH, Hamburg, Germany, provided this report. For more information, e-mail gerd.woelk@freudenberg.de or visit www.simrit.com.
