Snag Loading and Conductor Assessments
Snag Loading Subsea wells are often located in areas which offshore fishing trawlers operate.
The trawl gear of fishing vessels can be dragged along the seabed and may snag on the subsea tree, imparting significant loads on the wellhead and conductor system. ISO 13628-1 provides guidance on the trawlboard snag loading to be considered if an over-trawlable structure is not installed on production trees which could be subject to trawl snag loading.
The structural integrity of the Tree, Wellhead and Conductor system based on the expected snag loading is assessed during a snag load assessment. This analysis is best performed at the planning stage of well design, where improvements can be made to the planned system if the snag loading results in loads which exceed the structural integrity of the system.
Conductor Length Assessments
Offshore wells utilise a conductor which is axially supported by seabed soil. This provides the foundation for subsequent casing strings to be installed and supported by the conductor. It is therefore important during the well planning stage that a suitable conductor specification is selected to support the maximum expected casing weight of the well. The conductor can be installed via Jetting, Driving or Drilling and Cementing. The axial load support from the soil is dependant on the shear strength of the soil and also the installation method selected for the conductor.
It is important that the cumulative load at each stage of well construction is understood and compared to the axial resistance offered by the soil to conductor interface. An example of the load stages of well construction are listed below:
- Running the conductor
- Cementing the conductor
- Running the surface casing
- Cementing the surface casing
- Landing the BOP
- Running the intermediate casing
- Cementing the intermediate casing
- Drill String Suspended from BOP after shearing during potential emergency situation
The axial resistance of the soil is calculated in accordance with API RP2GEO and the governing code for the assessment is API RP2A.
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