Part II: Integral connections on large-bore pipe

The Gulf of Mexico deepwater arena has several areas with one or even two shallow water flow zones. These zones create major well design challenges since each zone must be cased off with one of the precious few casing strings available to the well designer.

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By Rich DeLange, Jim Light, Marvin Odum, Curtis Parker
Grant Prideco

PART II: This is the second of a two-part series. Part I focused on the search for cost-effective technologies.

A simple geometry problem

The Gulf of Mexico deepwater arena has several areas with one or even two shallow water flow zones. These zones create major well design challenges since each zone must be cased off with one of the precious few casing strings available to the well designer. The challenge to the designer becomes a simple geometry problem: How to fit the desired number of casing strings inside the 33-in. inside diameter (ID) of the typical deepwater conductor, while allowing adequate annular clearances for drilling tool passage and cementing, and still achieve the desired production casing or tubing size at the target depth. Many deepwater operators are discovering that Grant Prideco's XL Systemstrademark large-diameter integral connections with profiles that are flush at the ID and flush at the outside diameter (OD) at the connection are a cost-effective solution for challenging deep-water well casing designs.

Case studies

A deepwater Gulf of Mexico well had to be drilled through two shallow water flow zones, each requiring a casing string be set. Including the 36-in. conductor, six large-bore strings were required to cement the 16-in. string: 36-in., 30-in., 26-in., 22-in., 18-in., and 16-in. Connections on the first four strings were conventional weld-on connectors with ODs larger than the pipe. This created very small annular clearances for these strings, similar to the values for typical weld-on connectors. Casing running was slow to prevent hydraulic fracture of the formation, and cement-circulating rates were well below the optimum. Running casing slowly for the 30-in., 26-in., and 22-in. strings added at least three unplanned days to the drilling program. At $350,000 per day, the well was $1 million over budget after drilling and casing only 1,700 ft of the wellbore.

The next well in this field, designed in late 2001 and spud in early 2002, used the XLFtrademark flush ID/OD integral connections in place of conventional weld-on connectors for the 30-in., 26-in., and 22-in. casing strings.

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Including the 36-in. conductor, six large bore strings were required to cement the 16-in. string: 36-, 30-, 26-, 22-, 18-, and 16-in.
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The XLF integral connection is a specialized wedge thread machined directly into the wall of the pipe. Grant Prideco's Atlas Bradfordtrademark flush joint connections were used on the 18-in. and 16-in. strings. The annular clearances for the large-diameter strings were increased significantly. The slow casing running problem was solved, the cementing pump rate was optimized, and the drilling program progressed on schedule and on budget through the upper section of the well.

Solutions

Casing designers have found many solutions to the challenge of drilling difficult deepwater wells inside the physical limitations of a 36-in. conductor. Using conventional weld-on connectors and managing the small annular clearances is a common solution. Tight annular clearances can cause the upset connectors to act as pistons inside the hole as the casing is run, increasing fluid pressure downhole. The risk of hydraulic fracture is high, and casing must be run slowly.

Increasing the size of the conductor and other large diameter strings to accommodate typical weld-on connections is a simple solution that increases material costs for each string up-sized, increases the size of the hole that must be drilled, and increases mud and cement volumes.

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The annular clearances for the large diameter strings increased significantly.
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Using expandable liner strings downhole is relatively new technology, but extremely effective for increasing the number of casing strings that can be installed in a wellbore. Expandable casing strings are often more expensive than conventional casing.

Decreasing the production tubing or casing size is never a desirable decision at the casing design stage. It can reduce the peak production volume from the well.

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The casing program for these wells included a 36-in. conductor, a 26-in. surface casing string, and 2,100 ft of 20-in. casing with build angles ranging up to 3 degrees per 100 ft.
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Eliminating the option for contingency casing strings and liners increases the risk of having to abandon the well before reaching total depth if problems arise while drilling. The high cost of most deepwater wells makes this risk undesirable.

The use of the large-diameter integral connection is a simple, cost-effective solution that uses standard products. The flush ID/OD connections occupy no more of the valuable wellbore real estate than that required for the pipe. Increased annular clearances eliminate operational delays associated with running casing in close-clearance situations. The flush ID/OD connections are compatible with standard wellhead systems and casing hangers.

Deviated strings

Deepwater production wells tied-back to a floating surface production facility often target shallow reservoirs at large horizontal offset distances. It is becoming increasingly common to see deviation start in the 20-in. casing string for these wells. It is necessary to start the deviation at shallow depths (in the larger-diameter casing strings) to reach the distant production zone target at a reasonable inclination angle.

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To date, 24 full-scale connections have been tested to failure under rotating bending fatigue loading.
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As straight pipe is run in a deviated hole, the pipe bends to follow the path of the drilled hole. Bending pipe creates two phenomena that must be considered by the well designer: bending stresses in the pipe and friction between the pipe and the wall of the drilled hole as the pipe is being run.

The bending stress for pipe run at a constant build angle or constant curvature, typically expressed in degrees per 100 ft, is a linear function of the pipe diameter. For the same build angle, bending stress in 20-in. OD pipe is twice that in a 10-in. diameter pipe because the diameter is doubled. This means that bending stresses in a large diameter casing string, such as 20-in. diameter, are significantly high even for angular build rates considered "moderate" for smaller diameter strings. The large-diameter pipe and connections must be designed to resist these high bending forces.

The force that causes the straight pipe to bend and follow the path of the drilled hole is a bearing force against the wall of the drilled hole. The pipe bearing against the hole wall creates a frictional force, which must be overcome to either run or pull the pipe through the deviated hole section. As pipe diameter increases, the pipe becomes dramatically stiffer (pipe bending stiffness is roughly a function of diameter squared), and the bearing forces required to force the pipe into the curved shape also increase. In turn, the frictional force increases, and it becomes increasingly difficult to run or pull the pipe through the curved section. The pipe and connectors must be designed for high compression forces required to push the pipe through the curved hole section and, if the pipe gets stuck, the same connectors must withstand a high tension or pulling force that might be used to free the stuck pipe. Ideally the connectors should also provide high resistance to over-torque in case the pipe is rotated through the deviated section.

In late 2001 and early 2002, large-diameter pipe and connections were supplied for a multi-well deepwater Gulf of Mexico drilling program. The casing program for these wells included a 36-in. conductor, a 26-in. surface casing string, and 2,100 ft of 20-in. casing with build angles ranging up to 3 degrees per 100 ft. The high bending stresses created by this curvature plus the expected high compression and tension forces while running casing through the deviated section required 20-in. connections with full pipe body strength.

A weld-on version of the XLF connection, named XLF-RBtrademark (reduced bore), was supplied for the 20-in. casing strings. This connection provides mechanical strength equal to or greater than the pipe body in tension, compression, or bending. The connection OD remains flush, but the ID is slightly reduced. The mechanical characteristics of the wedge thread technology are perfect for this application because all mechanical loads are transferred across the engaged dovetail, locking threads. Once fully made-up, the connection pin and box react to all applied loads as a solid body. No torque shoulders, which can be damaged or neutralized under heavy applied loads, are used to transfer compression and bending loads. The dovetail thread flanks offer equal strength ratings in tension and compression. The wedge threadform provides high torsional capacity and excellent resistance to anti-rotation.

Connection fatigue

The severe environments in which deepwater wells are drilled today are putting greater demands on the performance of large-diameter well conductor and casing pipe and connections. Unlike conventional wells drilled in shallow water depths, assessing the fatigue performance of the large diameter connectors for a deepwater well is an important part of the casing design and connector specification process. This trend was recognized many years ago, resulting in a full-scale fatigue testing program of the XLCtrademark threadform. To date, 24 full-scale connections have been tested to failure under rotating, bending fatigue loading.

An often-overlooked aspect of threaded connection fatigue performance is the influence of corrosion on the fatigue life of the threads. Exposing the connection threads to a corrosive environment, such as seawater, can severely reduce the fatigue life of the connection. The XL Systems XLC-Strademark connection incorporates an external, metal-to-metal seal to prevent seawater intrusion into the threads.

Cyclic fatigue loading on the large-bore pipe and connections in a deepwater well comes from several sources. One source is the drilling or production riser. For development wells on a floating production structure such as a tension leg platform (TLP) or Spar, the conductor and surface casing connections must have adequate fatigue life to withstand 10 to 20 years or more of severe service.

A second and often overlooked source of fatigue loading for deep-water well casing and connectors is vortex-induced vibrations during make-up and running of the casing string. Rapid fatigue damage of pipe and connections can result.

A fatigue prediction model has been developed and used numerous times to calculate the fatigue life of 36-in. diameter connectors typically used on subsea well conductors. In each case, the fatigue life of the threaded connection can be predicted with a high degree of certainty since the fatigue prediction model is based on full-scale fatigue test data. The nature of the fatigue prediction model also allows for direct calculation of the probability of connection fatigue failure using risk-based design methods.

Conclusions

Drilling deepwater wells from expensive floating rigs presents a cost minimization challenge unequalled in the history of drilling hydrocarbon wells. With rig costs exceeding $300,000 per day, eliminating rig-days and even rig-hours can have a dramatic and positive effect on the cost of any deepwater well. Large-diameter, wedge-thread connections have geometry, strength, and fatigue characteristics that can be leveraged for significant time and cost savings in ambitious, deepwater wells. The savings can exceed $1 million per well. The connections dramatically increase annular clearances with the use of flush ID/OD connections so that pipe installation speed is optimized and compatibility is maintained with standard wellhead and casing hanger components. The connections minimize drag with the use of flush OD connections and maximize the probability that deviated, large-diameter strings get down without delays. The connections use high-strength casing with integral connections to improve annular clearances and still satisfy demanding performance requirements. They also minimize fatigue-related risks by applying fatigue test verified XL Systems connection technology.

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