Evaluation of a Riser After 10 Years of GOM Service Life
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As part of an effort to evaluate existing riser systems, an operator launched an inspection and testing program to investigate risers retrieved following well abandonment after a service life of nearly 10 years in the Gulf of Mexico (GOM). This represented a rare opportunity to evaluate the state of the threaded and coupled risers after such a long service life and assess the remaining longevity of the riser system. Specifically, it was a chance to observe and measure the effects of actual service life as opposed to calculated service conditions on the pipe and connections. The findings provided valuable insights into the future viability of similar riser systems.
The Constitution truss spar was first placed in service in 2005. The spar featured six dry tree production risers with two subsea tiebacks. It operates in the Green Canyon region of the GOM in almost 5,000 ft of water. The production risers were run in 2006. In 2015, the decision was made to abandon several of the production dry tree wells. There was a high degree of interest in investigating the risers to assess remaining life. Special attention was paid to evidence of corrosion, cracking, or wear on the outer diameter (OD) or inner diameter (ID) of the pipe, along with water ingress into the connection causing any corrosion from seawater on the bare metal of the connection.
The complete paper includes a detailed discussion of the original design, testing, and first running; this synopsis will concentrate on post-service testing and assessment.
Riser Pulling, Evaluation, and Testing
Pulling and Review. In late 2015, on the basis of internal evaluation, the operator pulled several of the production risers. In order to validate the remaining life of the risers and to help inform future decisions on the longevity of threaded and coupled risers, the operator then initiated a detailed review of the risers. The stress joints, tension joints, and first connectors after those joints were kept, along with several of the risers from the middle of the string. During the pulling operations, little evaluation of the state of the risers was possible; however, photographs of several of the connections were taken as they were pulled. Most notable was the lack of corrosion on the connection.
Evaluation and Inspection of the Riser. Connection Observations. The first detailed evaluation of the connections came after they were pulled and had been in a yard for several weeks. The results were not promising; rust was observed throughout the connections along with galling on the internal seal of the pin and threads. This troubling find led to an investigation to determine if connections might not be suitable for continued use. Wet magnetic particle inspection was also performed on the connections to search for indications beyond the rust and galling, but no further indications were seen on the connection.
Connection Investigation. Rust. The first observation was the lack of protection on the connections. Very little storage compound was present, especially on the pins (which were supplied with “as-machined” end finish), and the little that had been applied was mostly gone because of the lack of thread protectors. This pointed toward storage after pulling as being the primary cause of the rust.
In addition to review of the photos taken during pulling, the “mill ends,” or those ends with the coupling still attached that were not broken out or removed during pulling, were inspected. The couplings were taken off as part of the retesting process. The complete connections showed no damage or signs of rust. These connections would have been subject to the same loading conditions as those broken out on the spar as part of the pulling, but without exposure to storage or handling issues. Taking into account both the photos from the rigsite as well as those broken out on shore, it was concluded that the rust observed was a result of storage after breakout and was likely not present on the in-service riser.
Galling. Inspection of the pin-end connections pulled on the spar showed galling on the internal seal as well as heavy galling on the threads. This was a point of high concern for threaded and coupled risers still in service. Galling on metal-to-metal seals usually leads to leaking. The first step was to inspect the corresponding coupling connections from which the galled pins were taken. This could help determine if the galling was a result of in-service damage or was introduced as a result of handling while being pulled. Review of the box connections revealed no galling or damage on the internal seal (Fig. 1). The internal seal and shoulder is the first area to disengage during breakout of the connection. Because no galling was observed on the box internal seal, the conclusion was made that the galling was not present on the connection while in service. Galling was most likely introduced by rotating the connection while pulling against the threads with the pin seal rubbing against the box threads.
Pipe-Body Investigation. Along with evaluation of the connection, a detailed evaluation of the riser pipe was required. Review of the pipe involved chemical analysis of corrosion, visual inspection by camera along the full pipe ID, ultrasonic inspection, and thickness evaluation of the OD coating. As with the connections, these investigations were vital to understanding the remaining life of similar in-service risers.
Corrosion Analysis and Coating Thickness. The OD of the pipe exhibited white spots. These seemed to follow a helical pattern along the OD and were more severe in some locations. Chemical analysis of the spots showed they were aluminum oxide, the product of seawater corrosion of the thermal sprayed aluminum (TSA) coating. This led to an evaluation of the remaining coating thickness. Review of the thickness in several locations indicated, on average, 15 mils of remaining TSA coating—a positive indication for coating life.
Evaluation of the ID. Initial visual inspection of the pipe ID revealed a new layer of iron oxide. This was most likely the result of open-air exposure during pulling and transport before inspection. However, it was important to evaluate if any corrosion or pitting could be attributed to in-service conditions. A power wash was performed along the full pipe-body length to remove any recent superficial corrosion and to check for cracks or pitting. A camera system was then used to visually inspect all surfaces of the pipe. Full review of the videos showed no visual evidence of pitting or cracks along the pipe-body length.
Riser Post-Pulling Testing
Static Load Tests. The pulled pin ends of the connections were in a state that did not allow for further testing. To overcome this challenge, one connection was made from two of the pulled riser joints. This was achieved by removing the coupling from half of the pulled risers. The broken-out pin was then made back up into one of the other pipes and couplings. This not only allowed further investigation of the pin and box, but allowed the tested connection to involve both pins with a field service life.
Following makeup of the connection, a single sealability sample was run. Testing was successful, with no leaking observed in the tests with internal and external pressure, internal tests with bending, or mechanical and thermal cycle tests. This was a positive indication of the viability of threaded and coupled risers for continued in-service use.
Fatigue Tests. Another important step was further fatigue testing to indicate remaining expected life. The specimens taken were from the middle of the riser, but known factors can be used to estimate the life at the top and bottom of the riser. For that purpose, two fatigue specimens were taken from the pulled risers by breaking out some of the connections and remaking them, as described previously. Those two specimens were then subjected to fatigue cycles at low, medium, and high stress. The fatigue cycles, described in detail in the complete paper, yielded a very positive indication with regard to the continued service life of threaded and coupled risers.
An extensive review of the pipe body and connections was performed following pulling of the riser. The inspection methods included visual inspection, wet magnetic particle, and ultrasonic inspection of the pipe and connections. The nondestructive-evaluation (NDE) inspections showed the pipe and connection had performed very well in service, with few signs of wear, excessive corrosion, or cracking in the deepwater environment. All of these evaluations were positive, with strong evidence of any such issues having been introduced during pulling and storage.
In addition to the positive results of the NDE inspections, the connection evaluation for internal and external sealability performance, as well as fatigue testing, provided positive results. The tested samples passed sealability testing with no issues, and the fatigue samples had a high number of cycles before failure. In both cases, the results were consistent with testing performed on similar pipe and connections before service.
This level of review following a 10‑year service life was the first for a threaded and coupled riser. The results provide positive data toward continued long-term use of similar risers in offshore applications.
Evaluation of a Riser After 10 Years of GOM Service Life
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04 October 2019
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