Connections Selection and ordering

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When selecting a casing connection the actual load on the connection should be compared to the connection's capacity. This is applicable for all types of wells: exploration, appraisal and development wells.

·Design load: The load can be derived from the same design loads as laid down for the complete casing string.With respect to the strength of the connection two aspects should be considered:

  1. Structural load bearing capacity: With the manufacturer's information (see for example Figure L18) or the tables in API Bull. 5C2 [318] the structural load bearing capacity of a connection can be obtained. Specific qualification testing is strongly recommended to obtain these values as a function of size, weight, grade/alloy composition and surface treatment.
  2. Sealing capacity: The sealing capacity of a connection is a function of many factors as discussed in the previous sections. These include geometrical as well as running thread compound related aspects. The sealing capacity of Premium connections should result from qualification testing. Specific qualification testing is strongly recommended to obtain these values as a function of size, weight, grade/alloy composition and surface treatment. For API connections it should be noted that the tables in API Bull. 5C2 only quote the structural capacity and not the sealing capacity. Based on an in-house study, the following limits with respect to the field of application of an API connection:
  • If the connection could be exposed to an appreciable gas pressure loading for a prolonged period of time then a Premium connection with a metal-to-metal seal should be selected. This applies to all hydrocarbon gasses with or without traces of H2S/CO2.
  • If the differential burst/collapse pressure is in excess of 4,000 psi (27,580 kPa) then a Premium connection with a metal-to-metal seal should be selected.
  • If, the temperature is in excess of 250°F (120°C) then a Premium connection with a metal-to-metal seal should be selected.

As explained above, these limits do not so much reflect the structural limitations of API connections, which can be far in excess of these values, but the limitations to the application of the compound as the sealing mechanism.

It should be pointed out that these limits reflect field experience and are widely used throughout the industry. Although the working range defines a sharp cut off to the use of API connections, special applications, based on thorough testing and qualification, form an exception to this general rule. It is worth mentioning that recent research on the sealability of API round connections under special make up conditions has upgraded this connection to application for maximum 6,000 psi (41,370 kPa).

 

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