As high resistance to fatigue loading together with high overtorque and compression capacities are at the top of a list of required features for connections to be used in new drilling and completion techniques such as casing drilling, then special connections with enhanced performance need to be developed. On the other hand, a cost-effective solution should be chosen to balance the performance when considering low-demanding shallow wells for which sophisticated premium connections could be uneconomical.
This paper describes the development and evaluation of premium and semi-premium connections for tubing/casing which were developed to stand cyclic loads and reach an extended number of cycles under such conditions. The development process of an integral connection for casing sizes targetting very demanding applications comprised Finite Element Analysis (FEA) and Full Scale Fatigue Test (FSFT) showing Stress Concentration Factors (SCF) lower than 2. In addition, during the development of the semi-premium connection to cover less demanding applications, some of these techniques were used to optimize the results until getting SCF lower than 3, good enough when low doglegs are present. The results of the tests are plotted in S-N (Alternating Stress vs Number of cycles to failure) curve with a standard curve as a reference.
As per the results, both types of connections achieved the objective set at the beginning of the development process now being suitable alternatives for low and high demanding drilling/completion operations.
The use of OCTG connections for drilling appeared some time ago as a possibility to reduce costs by eliminating tripping in and out of drill pipes, and moreover to solve some instability problems [1,2] that normally occur while drilling conventionally. However, this new technology brought the necessity of improving the fatigue resistanc of OCTG connections. Traditionally, due to the standard drilling and completion techniques, all OCTG connections (API and proprietary) were designed to stand static loads while they only needed to be rotated for complex operations such as directional, horizontal and extended reach wells.
Casing Drilling, so far, has been mainly applied on low demanding vertical wells for which in almost all the cases API connections were used sometimes with some modifications to improve torque capacity. For these applications where the application is pushed by an important reduction in cost, a semi-premium connection with high fatigue resistance, low sealability requirements, extra torque and compression has been developed. The tighter tolerances of a semi-premium connections allows for a more stable, fatigue-wise behavior.
Nevertheless, as the current horizon of this drilling technique is being moved towards more demanding and critical applications [3,4] such as horizontal/ deviated oil/gas wells, and also offshore oil/gas wells where most of the wells are drilled directionally, new types of connections need to fill the vacuum left by the standard premium connections. The driver requirements for these connections are reliability, sealabity, high overtorque capacity and ultra-high fatigue resistance as the stresses produced by high bending and vibrations while rotating could reduce the life of the connections.