The previous article Rigless Origins (OE: March 2014) detailed the Zakum project, which was carried out in 1969 offshore of Abu Dhabi, where all methods of running wireline from a vessel—in open water—were executed and assessed(1) (2). More importantly, the article captured the evolution of rigless methods and techniques in regards to the subsea lubricator, soft riser, and submersible winch method, while employing a now versus then approach to show where the aforementioned technologies presently stand. A similar approach will equally be used in this follow up article to demonstrate—in greater detail—how today’s multi-service vessels (MSV), and subsea and rigless intervention equipment/methods have evolved to meet the demands of contemporary rigless light well interventions (RLWI).
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Contemporary MSV used for rigless projects. |
Size disconnecting: vessel comforts
One of the key findings made at Zakum was the need for “larger and better equipped [vessels to conduct] operations in deeper water and more exposed areas”(2).
Comparatively speaking, today’s MSVs are better suited for such deepwater environments as exemplified by their use at depths in excess of 3900ft when operating in a rigless fashion(3). Conversely, the forward bridge dive vessels—Sharifah and Ajax—used at Zakum operated at very shallow water depths (1) (2). In spite of these advances, today’s MSVs must contend with having to keep station during deepwater operations (Station keeping was also a concern at Zakum). Furthermore, a glitch on an MSV’s station keeping system can cause it to lose station, leading to a drive off, resulting in brash and uncontrolled tugging of topside to subsea conduits that tether a vessel to a fixed subsea point.
Protective measures to prevent tugging is key, which is due to the fact that these conduits make it possible to intervene and communicate with a well, without a marine riser. For this reason, it is of the utmost importance that conduits have the ability to disengage—in an emergency, independent of a remotely operated vehicle (ROV), or an intervention workover control system (IWOCS)—while facilitating the following: Parting and sealing on wireline (slickline and E-line) via a blow out preventers’ (BOP) rams when employing downhole tools; ejection of an umbilical via a hydro-mechanical “kick off” junction plate to separate the couplings which provide hydraulics and chemicals to an intervention kit; and the disengagement of rigless through bore connectors that make it possible to communicate with a well via a quasi-flexible medium such as coiled tubing.
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Left: A topside acoustic system used to engage subsea accumulators. Right: Coiled tubing running through a vessel’s moonpool. |
Contingent disconnections
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Subsea accumulators, which are mounted on rigless kits. |
It is important to note that failing to disengage any topside to subsea conduits during anomalous conditions is highly detrimental, which can cause them to part, while adversely affecting both topside and subsea equipment. Because of this, accumulators and mechanical weak links are paramount for disengaging said conduits in a safe and controlled fashion (All conduits must be able to isolate/seal off at their disengament point).
Functionality-wise, weak links work by being tugged or pulled via an external wire or apparatus that attaches to a topside to subsea conduit—such as a wire strapped to coiled tubing—to release a through bore connector, for example; conversely, subsea accumulators operate by storing and quarantining the needed pressure and flow, to close the rams on a BOP via a set pre-charge, which ensures that the rams’ close at a fixed and set time. This is driven by the American Petroleum Institute.
The use of weak links and accumulators is key in not only mitigating anomalous vessel conditions, but to equally allay adverse weather, and downhole conditions. Furthermore, the release of an accumulator’s stored energy requires setting off an external trigger, such as a topside acoustic system. It must be noted that due to Zakum’s depth, the use of accumulators and mechanical weak links for contingent disconnections were not addressed.
Post disconnecting
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A four-point anchor vessel on standby during a RLWI project due to weather conditions. |
The authors also recognized the need for vessels to be of “sufficient size and comfort to remain on location during severe weather” after the closing of operations(2). In spite of the advances made over a 40-year span, it can be difficult for MSV’s to remain on location during volatile weather, resulting in vessels having to return to dock while conditions subside.
It is imperative to highlight that once a vessel has disconnected during harsh weather—and operations are closed—that all topside to subsea conduits be recovered in a controlled and methodical manner. In addition, all conduits and associated equipment must be able—by design—to withstand volatile recovery conditions until they can be safely brought above the water line, or to a predetermined safe zone. Failing to execute the aforementioned can cause the disconnected conduits to crash or become entangled. A more severe scenario is one where conduits become intertwined with subsea assets. For this reason, having a safety procedure/protocol in place for the immediate spooling up/recovery of conduits, during anomalous weather, is of high benefit.
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(Left) A cut through view of a through bore connector. (Right) A connector prior to being fully mated Photo from SECC. |
Resume work
The need to immediately resume work after severe weather passes was an additional recommendation for the future(2). At present, this is achieved by remating the previously mentioned kick off junction plates and through bore connectors. This section, will focus on the latter, as they enable the execution of more than just wireline work, and facilitate communicating with a well in the absence of well control equipment, such as a BOP, when acidizing wells: the authors correctly inferred that this type of work would be conducted in the future from a vessel.
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17H hot stab mounted on an ROVs porch. Photo from Reaching Ultra. |
From a mating standpoint, through bore connectors work similar to 17H hot stabs, in that they require mating a male (This end attaches to a coiled tubing line) to a female mating end that is located on a pump in port on a rigless kit. In addition, through bore connectors and 17H hot stabs greatly differ, since fluid pumped through the former is measured in barrels per minute, while the latter’s flow rate is measured in gallons per minute. Additionally, said connectors are specifically engineered to mechanically lock the male (stab) within the female (bore), and require an external trigger (weak link) to unlock the male end.
Application-wise, through bore connectors are conducive for acidizing wells where large volumes of chemicals/cocktails need to be introduced in to a well, to reverse their decline in production. However, through bore connectors can also facilitate dewatering, flushing, abandoning pipelines, placing cement plugs, and pumping of kill fluids to plug and abandon (P&A) a well – without the use of a rig.
Conclusion
The future of rigless intervention technology, post-Zakum, will continue to have inherited challenges and obstacles. Notwithstanding, the rigless community and ancillary participants, have not only technically addressed such challenges and obstacles, but have equally, as demonstrated above, developed innovative techniques and unconventional technologies to safely and properly execute in an open water setting. The result: tripling the depth of projects in a span of five years, while furthering the proof of concept of rigless technology (3).
Works cited
1. Goodfellow, Ron. Underwater Engineering. Tulsa : The Petroleum Publishing Company, 1977. 0-87814-065-4.
2. Subsea Experienced Gained at Zakum. Ron Goodfellow, Alan Webb. Dallas, Texas : Society of Petroleum Engineers, 1974.
3. Myths And Misperceptions About RLWI. Bevan Morrison, Ole Eddie Karlsen. Houston : Atlantic Communications, 2013.
Fernando Hernandez is the subsea technical advisor at Reaching Ultra. Hernandez speakings three languages and has extensive field experience in the ROV tooling, automated controls, subsea and well intervention sectors.
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