Model Options

Initiate pipeline design

 

The various options to influence the way calculations are done can be selected here.

Design Function Model

Click to expand/collapseModel Options: Bend Angle

This option lays down the influence of the stiffness of the straight pipe on the adjacent bend.

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.

Info 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.

Info This option is only selectable when analysis type General is selected

Warning This options is not available in the free Educational version.

Click to expand/collapseModel Options: Geometry Model

This option allows switching between geometrically linear or non-linear analysis.

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

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

Click to expand/collapseModel Options: Section Model

This option allows switching between 1st (Non-Ovalising) and 2nd order (Ovalising) ovalisation effect.

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.

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

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

 

Warning This options is not available in the free Educational version.

Click to expand/collapseModel Options: Material Model

This option allows switching between material elastic (Linear) or elasto-plastic (Non-Linear) behaviour.

Info This option is only available in the Non-Linear / Elasto-Plastic Material (M) module. However the following modules are also required (for module M):

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

 

Warning This options is not available in the free Educational version.

Click to expand/collapseModel Options: Soil Ring-Stiffening

This option allows taking into account the cross-sectional lateral soil support (Apply) or disregard it (Ignore).

If Soil Ring-Stiffening is applied, horizontal soil support springs are applied at the points of the cross-section which move outward. To the soil springs the value of KLH or (KLT+KLS)/2 is assigned whichever has the lowest value. The soil resistance is maximised at 50 % of the lowest bearing capacity (horizontal RH or vertical (RVS+RVT)/2). Average values of the soil parameters involved are applied, so without uncertainty factors.

Info This option is only available if the Soil Ring-Stiffening (U) module is present. However the following modules are also required:

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

 

Warning This options is not available in the free Educational version.

Click to expand/collapseModel Options: Soil Model

This option allows switching between "Standard", "Upheaval Buckling" or "Pipe on Seabed".

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.

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

Info 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.

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

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

Warning This options is not available in the free Educational version.

Click to expand/collapseModel Options: Ovalisation Redistribution

In case the Ovalisation Redistribution (R) module is available, redistribution of cross-sectional deformations can be enabled. If disabled (None), the successive cross-sections deform independently.

If enabled (Allowed), the redistribution of deformations due to the shell behaviour of coupled adjoining cross-sections is taken into account.

Info This module requires the Stress (S) module.

Warning This options is not available in the free Educational version.

Click to expand/collapseModel Options: Loading Redistribution

If this option is enabled (Apply), when settlements are specified and the pipeline does not follow due to its stiffness or when - without settlements - the pipeline moves upwards, so that a gap results beneath the pipeline, and the existing top (soil) load exceeds the downward soil reaction from output table SOILREA, additional loadings, bending moments, soil reactions and displacements resulting from the loading difference are collected in output table ADDCROS.

If this option is disabled (Ignore), output table ADDCROS remains empty.

Click to expand/collapseModel Options  for Analysis Types

Not every model option is available for all analysis types. Only for the General analysis type all options are available. For the other analysis types some options are not available because they are not supported by the associated codes.

Model options

Analysis types

 

General

NEN 3650

Belgian Law

ASME B31.4

ASME B31.8

Bend Angle

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  DeleteRed 1

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Geometry Model

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Section Model

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Material Model

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Soil Ring-Stiffening

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Soil Model

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Ovalisation Redistribution

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Loading Redistribution

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Info1 The NEN 3650 does not support the Bend Angle and Section Model options. However, when Soil Ring-Stiffening is applied the Bend Angle option is set to Limited and the Section Model Option is set to Ovalising.


H1003, last changed: 14/09/2016

See also:

Design function 1