Pipeline &
Gas Journal
December issue 1999:
Light Hyperbaric Pigs Bring New Levels Of Safety To Subsea Pipeline Servicing
Arevolutionary new generation of lightweight, composite-bodied hyperbaric pigs made by International Pipeline Products Ltd (iNPIPE) of Catterick, Yorkshire, UK, has just been used with great success in a complex diver-based servicing program on a 40-inch crude oil import line from a subsea PLAM (pipeline-and-manifold), about two miles offshore Greece. Contracted by Hellenic Petroleum to carry out essential maintenance, Stolt-Comex divers worked from its Seaway Discovery vessel. They soon found their activities in poor visibility conditions much easier, quicker and, most importantly, much safer with the new hyperbaric pigs, which are fast-inflating and simple to handle and use. The cost-effectiveness of the operations was outstanding.
Design Criteria For Hyperbarics
Historically, hyperbarics have been subject to lengthy inflation times and caused costly vessel hire and downtime for the operator. More important, diving operations have their own intrinsic safety and cost criteria. So the principal design parameters that iNPIPE faced were ensuring 100 percent safety and reliability, ease and speed of operation in a difficult environment, stability and sealing ability during the welding process and, of course, its ability to successfully pig the line.
Standard hyperbarics have always been a highly stressed tool. However, the new tools incorporate patented seamless tires which remove the need for filling or venting operations and reduce the number of critical, stressed components by three-quarters. The inflation time of the new pigs is dramatically reduced - to less than one-tenth that of a standard hyperbaric. In addition, a buoyant core allows significant weight reduction in water, which greatly enhances subsea handling characteristics. For example, a new 16-inch hyperbaric weighs less than seven pounds in water and a 42-inch version is predicted as weighing just 33 pounds against 640 pounds for a traditional type. These two design developments alone significantly enhanced the integrity of the system and its fast, safe operation, along with greatly reducing the cost of a difficult subsea maintenance job.
New Valves & Seals
In order to complement these improvements, a new design of quarter-turn hyperbaric valve was built into the new pigs. Benefiting from CNC-manufactured components, all of which are interchangeable, the operation of the new valve could not be simpler. The inflation tool is located into a tapered valve housing at the front of the hyperbaric, pushed forward and turned through 90 degrees. This opens the valve and engages the locking mechanism. The pressure-balanced valve design prevents inadvertent opening due to external pressure, thereby removing the risk of over-inflation.
The system employs a positive spring-loaded valve which provides positive location of the tool during inflation, thus preventing inadvertent dislodging. This is especially important in conditions of zero visibility. Finally, the new design employs low-friction, long-life PTFE-based seals, all contributing to a remarkable advance in safer subsea pipeline servicing operations. P&GJ