Well well - detecting fatigue in subsea structures

Materials World magazine
8 Oct 2012
Oil rig

Are you feeling fatigued? Sick of being monitored all the time? When Eoin Redahan went to a recent subsea engineering event in London, UK, he found out exactly how the equipment felt. Grainy footage shows a 300-tonne blowout preventer (BOP) swinging above the seabed like a sapling in the breeze. If neglected for long enough, all oil will break loose.     

Fixing the problem is tricky. The fact that the BOP rests more than 1,000m below sea level is awkward enough, but identifying the root cause of the fatigue loading is even more difficult. Engineers need to find out what mechanism is causing the response. Is it wave loading, current effects or internal flow vibration? In the end, John McGrail, Director of 2H Offshore Engineering Ltd, in London, UK, revealed that the high velocity current running past the blowout preventer caused the vortex-induced vibration that swayed the BOP.    

McGrail’s video at Subsea 2012: Designing for Integrity and Operation, hosted by the Institution of Mechanical Engineers, illustrated several key aspects of subsea engineering – once equipment is laid, it stays where it is; an intimate knowledge of the local environment is important; and the correct interpretation of collected data is crucial.    

Of course, nature doesn’t make any of this easy. Each reservoir is as distinctive as a dental record. One facility could be situated 1,000m below the mudline off Angola, while another will be 9,000m deep in the Gulf of Mexico. Similarly, how can you compare the waves, current, rock types, topography and climate of the North Sea to rigs in the Pacific Ocean?    

Once the equipment is in place, there are other factors to consider, such as well and flow control, impact loads, installation loads, corrosion, chemistry and hyperbaric effects. All of this takes a heavy toll on the materials used. McGrail explained, ‘No material is perfect when it comes to a microscopic level. Over a period of time, these flaws will grow. [So] what is the allowable flaw growth we can tolerate?’    

Subsea systems are monitored using data loggers, sensors, mechanical interfaces and software packages, such as finite element analysis. McGrail continued, ‘If we understand the equipment within these systems, we can actually determine a high accuracy of the stress range. [But] when we start seeing deepwater systems with complex responses, the greatest difficulty we have is understanding the loading mechanism. If we knew what it was, we could put it into the software. The challenge is understanding the complexity of the offshore environment.’    

One of the main problems associated with reliability data collection is time. ‘We’re talking years,’ said Stephen May, Reliability Engineering Manager at Cameron in Houston, Texas. ‘Reliability data is not a short-term solution to anything. You need to monitor [equipment] for years or even decades afterwards.’    

Furthermore, fatigue tests cannot simulate a piece of equipment’s full lifespan, and performance data can be slow to filter back to manufacturers. May highlighted the disparity between the operators’ and manufacturers’ mean time between fail statistics to highlight the data collection divide. With not all equipment failures reported, manufacturers can sometimes assume a false sense of confidence in their products. ‘Some things are too big to test, such as multi-test flowmeters,’ added Professor John Strutt, Director at Astrimar Ltd, based in Milton Keynes, UK. ‘You can’t test it before it gets used. There’s a lot of equipment that falls into that situation.’    

Closer industry collaboration should improve the information flow. Joint industry projects and guidelines, such as API RP17N (Recommended Practice for Subsea Production System Reliability and Technical Risk Management), give operators information on qualifying new equipment. 

The Energy Institute Joint Industry Project (whose steering committee includes BP, Chevron, Nexen, Shell, Total, HSE and LR) is also publishing guidance for the avoidance of vibration-induced fatigue failure. According to Rob Swindell, Principal Consultant of Xodus group, based in Southampton, UK, instrumentation equipment that is subjected to multiple cycles per second can be susceptible to rapid fatigue accumulation. ‘We’ve had problems on instrumentation, [including] cables unscrewed due to vibration loading,’ he said. ‘[Fatigue failures] happen fairly rarely, but when they do, the consequences can be severe.’    

Swindell added that simple design tweaks can offset the problem. ‘The piping layout can have a big impact on how a pipe responds to excitation. We have to make sure the structure is robust – avoid multiple bends in series, use two plane welded braces for small bore connections, and aim for a minimal piping structural natural frequency.’    

Gordon McCulloch, Principle AIM Technologist at INTECSEA, based in Woking, UK, explained that the availability of standards is gradually aligning industry practice. ‘International standards are beginning to copy each other’s language,’ he said. ‘And that’s the natural process that is occurring from companies taking documents such as API RP17N and implementing them.’    

For all the ills of the Gulf of Mexico oil spill in 2010, it has brought the subsea community together, and will improve the reliability and integrity of operations. In the wake of the accident, the UK Oil Spill Prevention and Response Advisory Group [OSPRAG] was formed to create a capping device that could be deployed in the case of a catastrophic event. With a slew of companies involved, including Exxon Mobil, Total and BP, a device was manufactured that could stem the flow of a well within 30 days and handle 75,000bbls of fluids per day.    

On the far side of the Atlantic, contingency caps have also been made. However, Paul Dymond, Operations Director at Oil and Gas UK, noted that the cap is a form of tertiary control, and that it is more important to focus exerts on aspects of primary control, such as designing and maintaining well integrity. He added that the capping device might not even be the appropriate response to the next disaster.    

‘We could end up having shelves upon shelves of potential equipment. That’s why we need primary and secondary responses,’ Dymond said. ‘[But], the minister needs to have something to respond to the public [with].’    

Until then, subsea engineers will battle fatigue and stress to battle fatigue and stress. ‘It’s very hard to predict exactly where the next major problem will be,’ event Chairman Bil Loth concluded. ‘There’s no such thing as a typical catastrophe. That’s why our discipline is so important.’