Page 34 - Pipelines and Risers

8 Chapter I

Better quality control in pipeline production allows more accurate modeling of material while
FEM analysis tools allow engineers to simulate the through-life behavior of the entire pipeline
system and identify the most loaded sections or components. These are integrated into a
detailed FEM model to determine the governing failure mode and limit criteria, which is
compared to the design codes to determine where there is room for optimization. The
uncertainties in the input data and responses can be modeled with the help of statistics to
determine the probability distributions for a range of loads and effects. The reliability
approach to design decisions can then be applied to optimize and document the fitness for
purpose of the final product.

Engineers have long struggled with analytical methods, which only consider parts of the
structural systems they are designing. How the different parts affect each other and, above all,
how the structural system will respond to loading near its limiting capacity requires a non-
linear model which accurately represents the loads, material and structure. The sophisticated
non-linear FEM programs and high-speed computers available today allow the engineers to
achieve numerical results, which agree well with observed behavior and laboratory tests.

The simulation of global response together with local strength is often necessary because
design parameters and local environment are project-specific. A sub-sea pipeline is subject to
loading conditions related to installation, seabed features, intervention works, testing, various
operating conditions and shut-downs which prescribe a load path essential to the accurate
modeling of non-linear systems involving plastic deformation and hysteresis effects. For
example, simulation can verify that a pipeline system undergoing cyclic loading and
displacement is self-stabilizing in a satisfactory way (shakedown) or becomes unstable
needing further restraint. The simulation of pipeline behavior in a realistic environment
obtained by measurement allows the engineers to identify the strength and weakness of their
design to obtain safe and cost-effective solutions. Traditionally, pipeline engineers compute
loads and load effects in two dimensions and either ignore or combine results to account for
three-dimensional effects. This approach could lead to an overly conservative or, not so safe
design. DTA has demonstrated the importance of three-dimensional (3D) FE analysis for
highly loaded pipelines undergoing large thermal expansion.

Design Through Analysis (DTA) involves the following activities:

1. Perform initial design according to guidelines and codes
2. Determine global behavior by modeling complete system
3. Simulate through-life load conditions
4. Identify potential problem areas
5. Check structural failure modes and capacity by detailed FE modeling
6. Develop strategies for minimizing cost while maintaining uniform safety level
7. PerForm design optimization cycles
8. Document the validity and benefits of the design
9. Provide operation and maintenance support.