The Zeta Model software is one of the two primary components
that comprise the Zeta Safety System for well intervention
operations. The Zeta safety system utilizes data and modeling
to eliminate the guesswork commonly associated with well
intervention design and operations. The system was designed to
avoid catastrophic buckling and bending failures of well
intervention stacks. These failures are costly, and could
involve human injury and unplanned release of wellbore pressure
to the atmosphere.
Benefits of the Zeta Model Application - Cost-effective equipment selection (Gimble tables,
load frames, lubricator size/material)
- Adequate safety margin for the specific
intervention operation
- Optimized rig-up configuration
- Avoid catastrophic buckling and bending failures of
the intervention stack
- Eliminate guesswork and unproductive
rig-time
Answers Provided by the Zeta Model - Stress levels for the entire well intervention
stack
- Accurate modeling of stacks with moving wellheads
and or floating platforms (dynamic analysis)
- Stress changes resulting from stack loading
changes
- Need for additional stack supports and their
location
- Weight required to buckle the stack
- Maximum stress location
- Stack vibration frequency
Typical Zeta Model Applications - Tall well intervention stacks
- Moving wellhead and/or floating structure (TLP,
SPAR) intervention stacks
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Description of the Zeta Model
The Zeta model is based on a non-linear finite-element
(FEA) approach, purpose-written for analyzing intervention
structures. The FEA analysis is run repeatedly at very small
time increments to perform a dynamic analysis. A finite
difference, forward difference scheme is used to determine
the acceleration of each component in the stack, and the
associated dynamic forces. The model accurately calculates
the buckling instability of the stack. It also calculates the
Von Mises stress throughout the structure, and compares the
guy-wire loads to user-defined limits. Thus, it examines the
structure for both modes of failure: failure due to buckling
instability and failure due to yielding of a component in the
structure.
Validation of the Zeta Model
Extensive work has been performed to validate the results
of the Zeta model. The model predictions vs. analytical
results were excellent.
- Buckling load model comparison were within 1% of each
other under the same conditions (zero gravity situations - as
typically stipulated in the analytical solution).
- Critical frequency model comparisons were within 2% of
each other under the same conditions.