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| Running line in open water |
Rigless intervention hit the scene in 1969 with Abu Dhabi's Zakum pilot program. Reaching Ultra's Fernando Hernandez explains how the technology has evolved to meet new challenges.
The Zakum pilot program was launched in 1969, and with it(1) (2), it introduced technology that is only now becoming viable. These ideas were employed and assessed over a three-year span in shallow waters near Abu Dhabi, including the development and execution of processing and separation at depth, with subsea power and controls, and distribution via the world’s first–subsea specific–electrical wet mate connector(2).
Furthering the program’s significance was the fact that all facets of rigless intervention via wireline, as they stand today, were executed from a vessel(1)(2).
Subsea lubricator: Wireline compensation
Of the three methods employed at Zakum, the use of a subsea lubricator to deploy wireline in open water is the most common method used today.
This method works by landing, or mating, a blowout preventer (BOP) with well interfacing components onto a subsea tree. A lubricator, which houses the wireline tool string, is then placed on top of the BOP. Lastly, a sealing element, such as a stuffing box for slickline or a grease injection system for electric line, is mated above the lubricator (installation varies per location and project).
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| Recovery of perforating guns after use in open water |
With this completed, thorough engagement of the well via wireline follows. However, the sea’s conditions—at present— and at Zakum, can hamper such runs.
To mitigate the sea states at Zakum, a swell-compensating system was developed to better control the wireline’s movement(2). Today’s wireline-specific compensators counter both swell and heave. Offsetting unfavorable sea states continues to be imperative when using electric line, for example.
The erratic movement of the wire makes it difficult to accurately perforate at the required depths, which can cause operations to halt until the sea subsides. Rough seas equally affect running slickline, as it can cause the wire to go from being static to being rapidly jerked upward, which can cause slickline tools to engage prematurely. Deepwater further complicates running wireline, since the sea’s currents can act on the wire at different depths and opposing directions. Currents can also drag the wireline and disrupt deployment, operability, and general positioning.
Zakum showed that running wireline in open water, including the wireline’s extended exposure from surface to depth, did not greatly hamper/affect wireline operations(2). However, deepwater operations have shown that the sea’s swells, heave, and currents are formidable obstacles to mitigate and overcome with exposed wireline.
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| Braided line | Slickline |
Subsea lubricator: Grease delivery
Another obstacle experienced with subsea lubricator systems today is the delivery of grease when running braided line for electric line functions: The deeper the well, the more difficult it is to seal on braided line via a topside umbilical.
Because of Zakum’s depth, this was not the case. However, if it was deepwater, the authors would have observed the technical challenges associated with sealing on E-Line.
Such difficulty is driven by the fact that topside pumps must overcome hydrostatic pressure, which affects the flow and pressure delivered to create a seal on braided line. The grease’s viscosity can be equally hampering. Localized methods are used to circumvent this, which include subsea pumps, and accumulators.
Understanding such delivery at Zakum would have been a key insight for today’s operations, and in the process, streamlined the learning curve of sealing via grease.
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| Tensioners in operation (colored in blue) |
Authors deemed the lubricator method to be “simple,” with a forecasted depth usage of 328ft to 492ft(2)(1). Because the forecasted depths have now been exceeded, new barriers, as illustrated above, have arisen, adding layers of complexity to what was once viewed as a “simple” method.
Subsea wireline winch
Also explored was the use of a marinized wireline unit with surface controls that operate directly above the well. The winch also had electronic and power capabilities and utilized a flexible conduit for electrical power and controls( 1). For this reason, it was deemed to be complex and necessitating new methodologies.
Presently, running wireline subsea continues to garner interest, as demonstrated via the “riserless subsea intervention [research and development] program” that was initiated in 2003(3), resulting in a fully-submersible wireline system(3).
The working system that came to fruition from this research and development program improved upon the design used at Zakum, as it was designed for housing and swapping an array of wireline tools at deepwater depths. The need to trip to surface to swap tools was not necessary, giving the subsea wireline unit an advantage when compared to the subsea lubricator approach. Per the findings at Zakum this method was believed to have the highest degree of success and depth applicability for future deepwater operations: depth limitation was not assigned. Currently, the marinized wireline unit, originating in 2003, has been surpassed by the subsea lubricator method, since after 2013 the marinized approach was no longer being pursued or developed.
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| Moonpool on an MSV |
Lubricator as a riser
The third method employed was the use of lubricator to function as a soft riser. This approach pressed the question why does “rigless have to be riserless”? It also provided a viable answer(2)(4). Soft risers run lubricator from the vessel to operational depth to establish a ridged through-bore connection(2).
The authors saw the through-bore option as a key feature when compared to the subsea lubricator method, as it allowed for performing more than just wireline work.
Presently, outfitting multi-service vessels (MSV) with ridged conduits continues to be explored. This is exemplified by a heave/swell compensation system that is under development to run riser from a vessel’s moonpool(4). The intent is to retain stability while performing rigbased functions by way of two specialized through-bore, self-compensating, cylinders—“that work in tandem to exert force against each other” via nitrogen charged accumulators(4).
Installation-wise, the cylinders are secured above the riser to be run while remaining directly below the moonpool. Furthermore, the cylinders’ ID is designed to match the riser of choice, which facilitates, for example, running coiled tubing through the middle of said cylinders. Comparatively, the cylinders’ self-compensation design is key, as it foregoes the need to have a mechanical link, as with rig tensioners. The goal of this system is to unlock and redefine the work that can be conducted from a vessel’s moonpool in deepwater. The authors believed the soft riser method to be simple(1), with a forecasted depth applicability of 984ft to 2952ft(1), a depth that the heave compensated cylinder is being configured to exceed.
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| MSV utilized to conduct rigless work |
Further vessel considerations
The commonality shared by the aforementioned methods is the use of a vessel, which gave the authors insights into the future applicability and roles of vessels. The authors opined that future operations that worked “without a central platform” would require larger vessels than the 70ft and 165ft forward bridge vessels used at Zakum(1) (2), in order to carry out “flow testing, acidizing, and remedial work” from future vessels(1).
Today’s MSVs are in excess of 400ft in length. They complete the above mentioned functions, fortifying the authors’ foresight. Presently, they are also used to perform the following:
There is an ongoing trend with the build of new vessels. On one hand, they are increasingly being tailored for rigless activities. On the other hand, however, vessels continue to enter the market with general and non-specific intervention templates: Both have their inherited advantage. However, the latter gives vessels greater flexibility as it does not commit them to a set of specific functions.
Conclusion
Rigless technology has yet to become fully accepted and adopted technology. However, the last five years “have seen advances in vessel and deployment technologies,” tripling the depth of rigless wireline projects(5), further validating rigless technology. In addition, the lack of available rigs, when in high demand, is also contributing to an uptick in rigless interventions. When this is coupled with the substantially lower day rates associated with vessels, it furthers the attractiveness of rigless interventions.
Works Cited
1. Subsea Experienced Gained at Zakum, 1974. Dallas, Texas: SPE. 2. Underwater Engineering, 1977. Tulsa: Petroleum Publishing Company. 3. Advances in Subsea Intervention, 2009. Houston: Gulf Publishing. 4. Running Rigless With A Riser, 2012. Houston: Atlantic Communications. 5. Myths And Misperceptions About RLWI, 2013. Houston: Atlantic Communications.
Fernando Hernandez is the subsea technical advisor at Reaching Ultra. Hernandez speaks three languages and has extensive field experience in the ROV tooling, automated controls, subsea and well intervention sectors.
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