Need:

As part of the design and qualification of 18-3/4"-15K Wellhead System, Pythagoras Engineering Pvt Ltd were tasked with structural integrity and seal assessment of 18-3/4" - 15K subsea packoff. The FEA analysis incorporated assessments of packoff during seal energization, annulus pressure of 10K and topside pressure of 15K. Upon FEA investigation, it was found that the structural integrity of seal element and contact pressure generated to prevent leak are within the allowable limits and the design is acceptable.

Scope:

The objective of this FEA is to evaluate structural integrity and seal capacity of 18-3/4"-15K Packoff seal element as per ASME section VIII division 3. The following conditions are considered for the analysis.

Annulus pressure of 10 ksi

Top side pressure of 15 ksi

Methodology:

A FE mesh was generated with PLANE182 element - planar element with axi-symmetric option. The model was meshed so as to have a minimum of four elements through any wall thickness. The mesh was also been further refined at critical locations such as seal interface, lock ring interface and other critical locations.

Elastic-plastic FEA analysis was carried out based on the actual strain-hardening of the materials and large strain theory. The FE analysis was based on ASME section VIII division 3, KD-230 Elastic Plastic analysis methodology. The leakage of the seal will be evaluated by taking a safety factor 2 for minimum allowable seal contact pressure.

Results Outcome:

Load Case 1 - Seal Energization

The load to energize the seal is 80,500 lbf.

The 18-3/4"-15K Packoff design can withstand the load during seal energization and the stress/strain are acceptable as per ASME VIII D3.

Load Case 2 - Topside pressure of 15,000 psi

The 18-3/4"-15K Packoff design can withstand the load during seal energization and topside pressure of 15,000 psi. The stress/strain are within the limit as per ASME VIII D3.

The contact pressure beyond 30,000 psi was established at the seal-casing interface and seal-HPH interface with a minimum length of 0.3 inch.

The stress and strain of casing, HPH, energizer and lockring are acceptable as per ASME VIII D3.

Load Case 3 - Annulus pressure of 10,000 psi

The 18-3/4"-15K Packoff design can withstand the load during seal energization and annulus pressure of 10,000 psi. The stress/strain are within the limit as per ASME VIII D3.

The contact pressure beyond 20,000 psi was established at the seal-casing interface and seal-HPH interface with a minimum length of 1 inch.

The stress and strain of casing, HPH, energizer and lockring are acceptable as per ASME VIII D3.