Page 140 - Pipelines and Risers

On-bottom Stabiiity 113

Numerical method:
finite element formulation with straight beam elements with two degree of freedom at
each node (rotation and transverse displacement)
small deflection theory (small rotations) for the beam elements with linear material
behavior (no updating of nodal co-ordinates)
geometric stiffness is included
solution in time domain using the Newmark and incremental formulation
Rayleigh damping may be specified for the pipe
damping in the linear range of the soil may be specified
concentrated mass formulation
constant time step (user specified) with automatic subdivision in smaller steps in highly
non-linear interval (if required)
simple trapezoidal integration for the distributed loading along the beam elements (nodal
forces only, nom moments)

8.4.3 PIPE
PIPE is based on the use of non-dimensional parameters, which allow scaling of the
environmental load effects, the soil resistance and the pipeline response (lateral pipe
displacement).

Three options are available for the description of the long-term wave environment:

1) scatter diagram of significant wave height, Hs and the peak period, Tp
2) analytical model for the long term distribution of Hs and Tp
3) Weibull distribution based on the definition of Hs and Tp for two return periods

Wave directionality and shortcrestedness can be specified for all options.

The long-term wave elevation data are transformed to water particle velocity data. Together
with the current data, these velocities form the basis for the description of the long-term
hydrodynamic loading process and are used by the program for the pipeline stability design
according to the specified design criteria.

Two principally different design checks are made for the stability control of the pipeline:

1) The first check is relevant for an as laid on-bottom section (not artificially trenched or
buried). For a pipeline on sand soil, the design control is based on a specified permissible
pipeline displacement for a given design load condition (return period). The basis for the
design process is a generalized response database generated through series of pipeline
response simulations with PONDUS. For the on-bottom design check on clay, a critical
weight is calculated to fulfil the ‘‘no breakout criteria”. The critical pipe weight has been