Model Options

Infinite:

No stiffness influence of straight pipe on adjacent bend. The bend is considered to have a 'infinite' bend angle from a stiffness point of view as in the case of a pure ring-shaped pipeline structure. This is the standard built-in situation.

Limited:

Stiffness influence of straight pipe on adjacent bend considered. The bend is considered to become stiffer due to the adjacent straight pipe section dependent on the magnitude of the bend angle. The smaller the bend angle, the greater the influence. This option gives a better approach of the bend behaviour.

This option is only available if the Non-Linear Limited Bend Angle (Q) module and the Ovalisation Redistribution (R) module (required for module Q) are present.

This option is only selectable when analysis type General is selected

Linear:

The geometrical linear method is the commonly used 'small displacements' theory. The pipeline stiffness matrix is based on the 'undeformed' structure and thus the equilibrium system of the loads is also based on the 'undeformed' structure.

The linear option includes, as is the case for all other options as well, the non-linear soil support model.

Non-Linear:

In case of the geometrical non-linear option, the external loads make equilibrium on the 'deformed' structure.

The deformation of the structure takes into account the 'large displacement large rotations' theory, which means that the end of an element due to a lateral force does not only move in the direction of that load but also in axial direction, due to circular instead of perpendicular displacement.

The calculation method is based on disequilibrium relaxation.

After a 'linear' iteration the loads are placed on the deformed structure, where the 'rotational deformations' are translated into additional forces.

The disequilibrium is calculated as the difference between the original load vector and the newly constituted load vector.

The disequilibrium load vector is placed on the deformed structure found so far and the results of the new 'linear' iteration are added to the already found deformations. This is done until the remaining disequilibrium vector becomes small enough in comparison to the 'modified' original load vector or if this vector is a 'zero' vector (e.g. a free elastic bend) the disequilibrium vector is compared with a standard vector, deduced from the pipeline cross-sectional properties.

During the combined soil/geo-nln calculation process geometric iteration results are shown on the control line to enable the designer to follow the calculation process:

DISEQ: smaller integer value of {ratio of calculated and specified relative disequilibrium} and {ratio of calculated and specified absolute disequilibrium}, if ratio<1 then 'NEAR STABILITY', if GEOCTL criteria satisfied then 'STABILITY'

DISPMAX : largest node displacement (translations only!) due to total load

DISPADD : largest additional displacement due to correction load

It is recommended to apply this option even when it is not prescribed due to the choice of the other Model options.

This option is only available if the Geometric Non-Linear (N) module is present.

Non-Ovalising:

This is the common standard bend stiffness and stress intensification procedure: the bend stiffness is independent of the bending curvature of the bend section. This method is a first order approach.

Ovalising:

In reality the bend stiffness is non-linear with the curvature due to the ovalisation of the cross-section. This second order effect is taken into account if this option is selected. It also implies the geometrically non-linear and limited bend angle options.

As a result of the ovalisation in the bend the adjacent straight pipe sections will become ovalised as well, resulting in an ovalisation redistribution over the bend and adjacent pipe sections. This redistribution of the ovalisation and inherent stressing is taken into account, as well as the interaction between soil loads at bends and the resulting cross-sectional ovalisation and related longitudinal pipe bending.

The influence of the 'Ovalising'-option increases when the bend radius decreases.

When straight pipeline parts bend (which will almost always be the case) the Ovalising option also affects the results (see also table CSGEN, column RADIUS).

Even when the pipeline stays straight but does ovalise (resulting in a decreased moment of inertia) this option influences the results.

It is recommended to apply the Ovalising option for buried pipelines, especially if mitre bends are involved.

This option is available only if the following modules are present:

•Geometric Non-Linear (N) module

•Non-Linear Limited Bend Angle (Q) module

•Ovalisation Redistribution (R) module (required for module Q)

Standard:

The standard soil behaviour is applicable for completely buried pipelines as described sub DF3.2. The parameter values from Design Function 3.2 are kept constant during the iteration process.

Upheaval Buckling:

The soil parameters are adapted after each iteration to meet the values valid at the newly calculated depth location. This option is useful for so-called upheaval buckling calculations where the pipeline may even move to (above) the ground level, which has to be specified in table G-LEVEL.

This functionality is only applicable on pipeline parts where the RVT > 0.

This option is only available if the Upheaval Buckling (Z) module and the Geometric Non-Linear (N) module are present.

Pipe on Seabed:

The soil parameters have to be specified for a pipe with the axis at ground level, so half the diameter is below the ground surface, which has to be specified in table G-LEVEL. The soil parameters are interpolated dependent on the calculated imprint in the ground surface. This option is used in offshore pipe-lay calculations to improve iteration stability.

This functionality is only applicable on pipeline parts where the RVT = 0.

This option is only available if the Back tension offshore pipe-laying (B) module and the Geometric Non-Linear (N) module are present.

Model options

Analysis types

General

NEN 3650-2

Belgian Law

ASME B31.4

ASME B31.8

Bend Angle

  1

Geometry Model

Section Model

  1

Material Model

Soil Ring-Stiffening

Soil Model

Ovalisation Redistribution

Loading Redistribution