Strip-Out Behavior of NPT Threads under High Pressure

Florian Dietz

Research output: ThesisMaster's Thesis


In 2008 IADC reported an incident on a drill rig in which one worker threaded a 2in. NPT nipple into the stand pipe. This nipple then failed due to a pressure spike. It was blown out of the receptacle of the standpipe and injured personnel present on the rig floor. During an investigation it was determined that the cause of the accident was most likely that the length of engagement for the nipple was too short. For a 2in. NPT thread there is a minimum length of engagement as well as a maximum allowable pressure, both specified in industry standards. OMV E&P Austria GmbH applies even stricter rules to prevent any such event from happening. This thesis analyzes the possible cause for such incidents and makes a recommendation in respect to the necessity for OMV to be even stricter than industry standards. This analysis is done by using a FEM Simulation to examine the internal processes within a 2in. NPT connection when pressure is applied. The simulation is also used to determine if there is a difference in stress if the connection is strained by internal pressure compared to external tension. Furthermore, the possibility of crack propagation within the thread is examined. All of the results are then used to design practical tests in which a specifically designed 2in. NPT connection is stressed by internal pressure in a region well above the maximum allowable pressure defined by API. These tests are conducted for different lengths of engagement to recreate the circumstances which led to the incident reported by IADC. After evaluating the results of the FEM Simulation as well as the practical test results it is now possible to tell what is happening within a thread and how well a thread performs under high pressure with different lengths of engagement.
Translated title of the contributionAbstreifverhalten von NPT Gewinden unter Hochdruck
Original languageEnglish
  • Thonhauser, Gerhard, Supervisor (internal)
Award date14 Dec 2012
Publication statusPublished - 2012

Bibliographical note

embargoed until 14-05-2017


  • IADC
  • API
  • NPT
  • thread
  • pressure
  • OMV
  • incident
  • FEM
  • simulation
  • crack
  • connection
  • engagement

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