D-Geo Pipeline known issues and FAQ

Known issues

For HDD and Direct Pipe, the default value of the allowable deflection for PE pipe (10% * S * Do) is not always correct (MDR-2322)

The default value of the allowable deflection for PE pipe in the Factors dialog was changed into 10% * S * Do in version 23.2.1 (where Do is the outer diameter of the pipe and S the factor of importance) instead of 8% * S * Do in previous versions. According to article 11.2.1.6 of the Dutch standard NEN 3651 (Additional requirements for pipelines in or nearby important public works), the allowable deflection is 10% * S * Do and must also meet article 8.5.4.4.2 of the Dutch standard NEN 3650-3 (Additional specifications for plastic pipelines) where a value of 8% * Do is given. The value of the allowable deflection should be therefore the minimum value between 10% * S * Do and 8% * Do.

Solved in 22.2.1. Access violation when vertical is near the entry-or exit point

When a vertical is placed close to the entry or exit point, the top of the pipeline may be above ground level. This is not allowed and a message about an ‘access violation’ will be displayed during the calculation. The verticals where the pipeline is fully below ground level will be calculated and displayed correctly in the report. Workaround: By removing the relevant vertical in the Calculation Verticals screen (under GeoObjects) the ‘access violation’ does not occur.

Solved in 18.2.2. For Micro tunnelling and HDD models, the determination of the modulus of subgrade reaction and horizontal bearing capacity can lead to unrealistic high values

• Issue 1 (Micro tunnelling and HDD models) For the determination of the modulus of subgrade reaction and horizontal bearing capacity, a distinction is made between shallow and deep situation. The boundary depends on the value of the passive stress: if the maximum passive stress is reached, the pipe is considered as deep. However it seems that this criterion is not always relevant and that a deep pipe can be considered as shallow pipe, which leads to unrealistic high values for the modulus of subgrade reaction.
• Issue 2 (HDD) As a consequence of problem 1, the maximum value of the modulus of subgrade reaction used during the Stress Analysis is too high and leads therefore to unrealistic values of the pulling forces.

As a workaround for these issues, please use the previous release, 16.1, or wait for version 18.2.2.

Solved in 18.2.2. Some contingency factors are not applied during the calculation of the pulling forces

For the calculation of the pulling forces, the contingency factors on borehole radius and on modulus of subgrade reaction are not included leading to under-estimated values. As a workaround for these issues, please use the previous release, 16.1, or wait for version 18.2.2.

Solved in 18.2.1. The overall factor on bending moment (f_k) is not correctly calculated

For the calculation of the axial bending stress Sigma_b (Strength calculation), an incorrect formula is used for the overall factor f_k: The correct formula is f_k = f_M * f_install * f_R but the program uses f_k = f_M * f_install / f_R where :f_M is the contingency factor on the bending moment

f_install is the load factor on installation

f_R is the contingency factor on the bending radius

Frequently Asked Questions (FAQ)

What is the meaning of the 'boundary undrained/drained'?

The boundary separates drained reacting soil (below the boundary) and undrained reacting soil (above the boundary) when loaded by pressure from the drilling fluid. This is important for the calculation of the mud pressures.

What is the meaning of the 'boundary compressible/uncompressible?

The boundary separates compressible soil (above the boundary) and incompressible soil (below the boundary). This is important for the calculation of the reduced soil loading and therefore for the strength calculation.

Where should I place the 'boundary compressible/uncompressible' when I have an intermediate sand layer present in the compressible layer group?

When an intermediate sand layer is present, e.g. a Pleistocene sand base with an intermediate sand layer in the compressible stack above and the pipe is below this intermediate layer, it is preferred to lay the boundary on the Pleistocene sand and define a fictive Su for the intermediate sand layer.

How should the directions ‘left to right' and 'right to left' be interpreted in the mud plots?

The mud plots indicate the course of the minimum required mud pressures for both possible operating directions in the relevant drilling phase.

How do the mud pressures develop during the 3 different phases of the drilling?

A certain minimum mud pressure is necessary in a borehole. This minimum required pressure at the bore front is determined by two factors:

• The difference in height between the bore front and the exit point of the return flow of bore liquid to the surface.
• The minimum pressure required to move the bore liquid (including soil that was drilled free) in the borehole over a certain distance.

During the pilot drilling, the drilling mud can only flow back to the surface through the borehole that was just drilled. The pilot drilling is therefore the normative phase in the drilling process. During the pre-reaming, reaming and pull back operation, the drilling mud can flow towards both sides (entrance and exit point, which are the RIG and pipe sides). The drilling mud chooses the path of least resistance. During the pre-reaming and the reaming and pull back operation, the direction of flow of the return flow reverses. In practice, this amounts to a return flow at the start of the phase in the direction of the exit point. At the end of the pre-reaming or pull back stage, the direction of the return flow is towards the entrance point. The location of this transition point depends on the geometry, borehole and pipe diameters of the relevant and preceding phase and the characteristics of the drilling mud.

Is the pipe filled on the roller conveyor during pull back?

The calculation in D-Geo Pipeline is based on a filled pipe (also on the roller conveyor); When the pipe on the roller conveyor is not filled, the tensile forces in the first part of the drilling will be lower than those calculated. The calculated tensile force at the end of the pull back operation remains the same because here the pipe is no longer on the roller conveyor.

N.B. The tensile force at the start of the pull back operation of a filled pipe (but not filled on the roller conveyor) is equal to the tensile force at the start of the pull back operation of an unfilled pipe.

Why do I have problems reading DXF exported files in AutoCad?

The option to export pictures in DXF format was developed for, and tested with, AutoCad 14. In more recent releases of AutoCad something has changed in relation to the import of DXF files. This means that these versions do not read our DXF files properly (detected by us in versions 2005 and 2006). Most viewers, for instance “Bentley view”, do not experience problems with our exported DXF files.