You’re offshore on one of the largest rigs in the world. It was put together with all the latest equipment with all the “bells and whistles.” The rig was built, erected and went into production in record time. As with any new unit, it takes a while to get all the “bugs” out, tune things and get lined out. While observing the rig, you see something you don’t like — process fluid dropping out of a flanged joint. The flange connection is a new quick connect type joint (QCJ). The maintenance crew chief is called and he sends someone out there to tighten the bolts on the QCJ, and the small leak stops. You’re relieved and in subsequent observations, you see no leak. A few weeks later, you walk by and you see “drip, drip, drip …” You call maintenance again, they tighten the clamp, and the leak stops. Once again, you don’t see it at first, but it pops up again in several weeks. You ask, “What’s going on?”

Your company has approved the QCJ for this service; it has been tested, and it is used widely all over the company. The QCJ comes from a well-respected manufacturer, and your company has had a long relationship with them. You ask yourself, “What is the problem” and call for help.

The situation described above is not necessarily an isolated case in the business. In cases where we have a line in a service that is cycling “hot to cold” due to the inherent design of the process, the QCJ is subjected to cyclic load and pipe stress loading. Depending on the QCJ design, typically a high seating stress or seal contact stress is required to prevent leakage. It is important to torque the QCJ properly in accordance with the manufacturers’ specifications.

With all that said, in most cases the external pipe stress loading will derate the allowable pressure for the QCJ. Pipe stress loading must be included as part of the design. The combination of pressure and external loadings is usually too complicated to calculate. Most applicable codes and standards handle the pressure-only case quite well, and even the manufacturers own design tools can do a good job. But when high external loadings are imparted onto the QCJ, the ball game becomes more challenging. Fortunately, the finite element tool can be used to look at these QCJs. The methodology is as follows:

1. Lay out the pipe design.
2. Perform a code pipe stress analysis.
3. Define the loads for the QCJ.
4. Specify the type of QCJ you want.
5. Develop an FE model of the QCJ.
6. Analyze the loads.
7. Specify a large QCJ or move the clamp to a strategic location.

The problem of external loads on QCJ is challenging in some applications, but it is also a problem with standard flange joints as well. Many times our leaks in the field are misdiagnosed as a gasket problem or whatever, when it is simply the external pipe stress loading and the cycling thermal load causing the problem. It is recommended that a registered professional engineer competent in this field reviews any designs. As with any piece of hardware implemented in industry, the complete ball game has to be defined and analyzed prior to it being put into service.

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