Many operators have a register of flanged connections identified as having fitted stud bolts below acceptable standards and, on many occasions, in critical condition requiring urgent attention. Corroded stud bolts seriously undermine an asset’s integrity and unscheduled pressure releases can have a devastating impact on offshore operations.
In 2009, the situation prompted several operators in the United Kingdom Continental Shelf to contact Stork Technical Services regarding safety concerns related to incidents of uncontrolled pressure releases and future potential releases, as a result of severe stud bolt corrosion/failure on four-bolt flanged connections. Many of the corroded stud bolts were proving difficult to remove and replace during planned shutdown activities, because of their location and the presence of existing systems.
Stork was asked to develop a more sophisticated method of removing and replacing corroded stud bolts that would mitigate risk and improve the safety of operations and the wider asset, while simultaneously reducing operating costs and optimizing productivity.
Comprehensive R&D
Stork embarked on a rigorous research, design, and testing program and worked with a specialized manufacturing company to develop the hot bolt clamp (HBC) technology. The patented HBC system enables the safe removal and replacement of corroded stud bolts on live four-bolt flanged connections, with no disruption to ongoing production.
Since its introduction and development in 2012, the technology has successfully reworked thousands of four-bolt flange connections. Since developing the initial standard HBC, which covers Class 150 rating flanges from 2 in. to 3 in., Stork now produces a range of HBCs to cover all four-bolt flanges, including those designed for tight, restricted access applications.
Hot bolting (the practice of removing, replacing, or freeing and retightening stud bolts on live piping and equipment) of flanged connections with eight bolts or more is a well-established practice. Traditionally, any stud bolt activity on four-bolt flanges required the system to be shut down, isolated, and purged prior to breaking the flange integrity to enable rework. This is a lengthy process requiring the system to be shut down (typically, with lost production) as well as associated alternative services to be in place to cover system purging and subsequent system testing to prove the integrity of the disturbed flanged connections.
The HBC system allows hot bolting to be carried out on four-bolt flanges without a shutdown. Corroded stud bolts can be removed and replaced on live connections while maintaining flange integrity, which delivers cost, efficiency, and time benefits.
The system hydraulically clamps pressurized bolted pipeline flanges together in a controlled manner so that the corroded stud bolts can be safely removed. Once all the bolts have been replaced, the hot bolt clamps are depressurized and removed. Change-out of the bolts is achieved without taking the flanges out of operation, disrupting standard line pressure, or the danger of a medium release.
Using the HBC, a four-bolt flange can be reworked in only 40 minutes and, as the system can be used on live assets, typically removes approximately 1.5 days of activity from a planned 1-month shutdown, which would otherwise equate to hundreds of thousands of dollars lost in deferred production. This also reduces the number of personnel required on board during the shutdown period at a time when bed space is at a premium.
Converting the initial clamp concept into a safe, practical, and efficient tool posed a number of significant challenges. The clamp dimensions had to be compact enough to fit around the flanged dimensions, as well as having suitable rigidity to deliver sufficient load so as to start transferring the retained stress from the studs to the clamps. At the same time, the clamp also had to be adjustable to cater to a wide range of flanges.
The initial conceptual design was carried out with 3D computer-aided design to check for limiting access/geometry constraints and using finite element analysis modeling. Rigorous testing was then carried out on manufactured clamps, with hydraulic cylinder, full-cyclic pressure tests to simulate real environment use. The cylinders were pressurized beyond maximum working pressure for 10,000 pressure cycles with realistic dwell times between each pressurization.
Once the manufacture of the clamps was complete, the process of developing the site service was initiated, which saw the clamps being operated in conjunction with ultrasonic bolt load monitoring equipment to ensure that minimal load transfer could be detected while preventing excessive load transfer through the assembly to the energized sealing gasket.
Case Study
Stork was contracted by Shell UK to carry out four-bolt flange replacement on the Nelson North Sea platform. It was also asked to carry out a prejob survey on the platform to determine a fixed scope of work. This allowed work to be planned efficiently in line with ongoing work scopes. All flanges worked were under live working conditions, which enabled the HBC team to work on them without the need to depressurize and purge any lines, thus removing the requirement for a costly shutdown.
Following the initial trip and successful bolt change on 20 flanges, the company continued using the HBC system to operate a 5-month program in which more than 500 four-bolt flanges were successfully hot bolted. The full work scope was carried out in time and within budget, and the work was completed safely.
After receiving requests from a number of operators and technical authorities, the next evolution of the clamp design is currently under way. Further innovation and new applications will be developed and covered by the technology.