Premium-Connection Design
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 17221, "Premium & Semi-Premium Drilling-With-Casing Applications," by N.J. Santi and G.E. Carcagno, SPE, Tenaris, and R. Toscano, Centro Investigation Industrial, prepared for the 2005 Offshore Technology Conference, Houston, 2-5 May.
High resistance to fatigue loading together with high torque and compression capacities is necessary for connections used for drilling with casing. On the other hand, cost-effective solutions must be found to provide connections for shallow wells for which sophisticated premium connections are not economical. The full-length paper describes the development and evaluation of premium and semipremium connections for tubing/casing.

Introduction
Drilling with casing originally was used for nondemanding vertical wells where, in almost all cases, American Petroleum Inst. (API) connections (sometimes with modifications to improve torque capacity) were used. As drilling with casing was used in more demanding and critical applications such as horizontal, deviated, and offshore wells where most wells are drilled directionally, new connection types were needed. Requirements for these connections include reliability, sealability, high overtorque capacity, and ultrahigh fatigue resistance because the stresses produced by bending and vibrations while rotating can reduce connection life.

Considerations
Use of oil-country-tubular-goods (OCTG) connections for drilling began as an attempt to reduce costs. Standard OCTG casing connections are designed to withstand static loads of tension and compression, internal and external pressure, and bending. Torque requirements for these connections are low. However, for drilling with casing, some critical issues that become important for drilling directional and deviated wells include the following.Wear protection.High torque and compression resistance.High fatigue resistance.Cost effectiveness.
Wear protection is provided by means of centralizers or other hardened surfaces located close to the pipe end to avoid damage to the connection. Higher torque and compression requirements are met by designs that include robust torque shoulders, thread interferences, and reduced gaps between mating threads. Fatigue resistance is a very important issue for drilling with casing and perhaps the most difficult challenge to address.
Fatigue Response of OCTG Connections. Fatigue is a complex process that affects mechanical components subjected to cyclic loads. Fatigue failure is not a sudden crack, instead it involves different failure phases that occur until component failure becomes evident. This failure occurs at stress levels much less than the elastic limit of the material. When subjected to cyclic loading, ordinary OCTG connections are weaker than drillpipe connectors that are designed to work with alternating stresses. The fatigue-failure location of threaded connections is in the area with the highest stress-concentration factor (SCF). Location of the SCF can be estimated with finite-element analysis (FEA), which also can yield a numerical value of the SCF. For standard premium OCTG connections with loads expected in drilling with casing applications, the SCF normally is greater than 2. However, from a fatigue perspective, it is desirable that the SCF be as low as possible. Threaded-connection response to fatigue loads can be plotted as a stress vs. number-of-cycles-to-failure curve.