Snag Loading and Conductor Assessments
Subsea wells are often located in areas where offshore fishing trawlers operate.
The trawl gear of fishing vessels sometimes drags along the seabed, causing it to snag on a subsea tree. This can impart significant snag loads on the wellhead and conductor system.
ISO 13628-1 provides guidance on the trawlboard snag loading. This is important if an over-trawlable structure is not installed on at risk wells.
Snag load assessments verify the structural integrity of Trees, Wellheads and Conductor system. Expected snag loads are applied and checks to ensure the well will remain safe are carried out. The analysis is best performed at the planning stage of well design, where improvements are then made to the system, if necessary.
Conductor Length Assessments
Offshore wells utilise a conductor axially supported by seabed soil. This provides the foundation for casing strings to be installed and supported by the conductor. During well design, a suitable conductor specification should be selected to support the maximum expected casing weight. The conductor is installed via Jetting, Driving or Drilling and Cementing. The axial load support from the soil is dependent on the shear strength of the soil and also the installation method selected for the conductor.
It is important to understand the cumulative load at each stage of well construction. It is then 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. The governing code for the assessment is API RP2A.
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