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In
current trend, the oil and gas industry is targeting operations in much deeper
waters and well depths resulting in high pressure and high temperature (HPHT)
applications, creating a need for development of new technology. API 17
Technical Report 8 (API 17TR8) provides the High Pressure High Temperature
(HPHT) design guidelines. Design Verification of any HPHT equipment in
accordance with API/ASME using Finite Element Analysis is a mandatory process
to undergo external certification like ABS, DNV etc

A
variable bore RAM (VBR) enables a ram-type Blowout Preventer (BOP) to close and
seal on more than one size pipe. VBR is a pressure controlling member placed in
a BOP cavity intended to control the movement of well bore fluid. The rated
working pressure of the VBR design is 20,000 psi at temperature of 250°F.

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The
objective of this work is to verify the design of the VBR closed on 7-5/8″
diameter drill pipe for the failure modes like protection against plastic collapse:
Global criteria 1 (Load Factor of 2.4) & Global criteria 2 (Load Factor 2.1),
protection against local failure (Load Factor 1.7), ratcheting criteria
(loading and unloading of minimum three cycles) and hydrotest condition (Load
Factor 1.5) using elastic plastic materials in the model.

Initially,
a sensitivity study was performed for the full assembly model versus a
simplified FEA model consisting of the critical components in the load path to
understand the behavior. The study demonstrated that the results were similar
in both the methods and the simplified model proceeded further for the design
by analysis which reduces the complexity and a faster turnaround time.

Analysis
was performed for the factored load cases required by ASME using 20 ksi bore
pressure, its associated end loads and rubber pressure. Convergence is achieved
which shows the FEA model is stable and satisfies the global criteria 1 & 2
and hydrotest criteria. Accumulated strain limit damage value is less the 1.0
which satisfies the requirement of protection against local criteria. In
ratcheting criteria, the growth of plastic strain is zero, results there is a
development of elastic core in primary components which satisfies the
requirement.

 

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