Usually, this type of arrangement will require a concrete stop block to limit pipeline thermal expansion. Only stress analysis conducted on underground pipeline can determine requirement for the stop block. In the case that stop block is used, it must be sized in such way that resisting soil pressure does not exceed allowable soil pressure. Please see the figure below.

Normally, comprehensive stress analysis using Caesar II is required, since high stresses are expected. Pig launcher stress can be obtained by Caesar II and it can be easily modelled, since the pig launcher is just big pipe having reducer.

Modeling SWAY BRACE in Caesar II:

The steps involved in modeling a sway brace in Caesar are as follows:

Select the sway brace from catalogue depending on given pipe nominal diameter or depending on the force calculated to restrain the pipe work. (SB45 as per C&P catalogue, reproduced in Fig. 1)

Fig.1: Sway Brace selection Table from C&P Catalogue.

Mark a node (Node 10-Fig.2) at the location in the piping system where sway brace will be installed. Run Caesar analysis and note down the displacement of the point in specified direction from cold to operating condition. For the sake of example, let’s assume that CAESAR II calculated displacement from cold to operating position is 0.5 inch in +X direction.

Now in CAESAR II input spread sheet (See Fig. 2) check the restraints box and define bi-linear restraint (X2 for the assumed case) at Node 10 with CNode at 101. Here, K1 is the initial stiffness of a bi-linear restraint. Do not enter anything on this cell as the restraint is assumed to be rigid. The value of K2and Fy to be obtained from catalogue. Where, K2= Post yield stiffness of a bilinear restraint. When the load on the support restraint exceeds Fy then the stiffness on the support restraint changes from K1 to K2. Fy = Yield Load. If the load on the support restraint is less than “Fy” then the initial stiffness K1 is used. If the load on the support restraint is greater than “Fy” then the second stiffness ” K2″ is used.

Define restraint X at node 10 with CNode at 101. Provide a gap of 3 inch (=distance the sway brace is able to move in both positive and negative direction before it gets locked/ become fully rigid depending on manufacturer= 3 inch as per C&P catalogue)

Check the displacement box and define the displacement for Node 101. It is the displacement for node 10 as noted earlier (0.5 inch in X direction, leave other cells i.e., DY, DZ, RX, RY, RZ blank.).

Add D2 in sustained and operating load cases. Now run the analysis to obtain results.

Fig. 2: Caesar II Spread sheet for Sway brace modeling.

Underground or buried piping

Underground or buried piping are all piping which runs below grade. In every process industry there will be few lines (Sewer or drainage system, Sanitary and Storm Water lines, Fire water or drinking water lines etc), part of which normally runs underground. However the term buried piping or underground piping, in true sense, appears for pipeline industry as miles of long pipe run carrying fluids will be there.
Analyzing an underground pipe line is quite different from analyzing plant piping. Special problems are involved because of the unique characteristics of a pipeline, code requirements and techniques required in analysis. Elements of analysis include pipe movements, anchorage force, soil friction, lateral soil force and soil pipe interaction.

To appreciate pipe code requirements and visualize problems involved in pipe line stress analysis, it is necessary to first distinguish a pipe line from plant piping. Unique characteristics of a pipe line include:

• High allowable stress: A pipe line has a rather simple shape. It is circular and very often runs several miles before making a turn. Therefore, the stresses calculated are all based on simple static equilibrium formulas which are very reliable. Since stresses produced are predictable, allowable stress used is considerable higher than that used in plant piping.
• High yield strength pipe: To raise the allowable, the first obstacle is yield strength. Although a pipe line operating beyond yield strength may not create structural integrity problems, it may cause undesirable excessive deformation and possibility of strain follow up. Therefore, high test line with a very high yield to ultimate strength ratio is normally used in pipe line construction. Yield strength in some pipe can be as high as 80 percent of ultimate strength. All allowable stresses are based only on yield strength.
• High pressure elongation: Movement of pipe line is normally due to expansion of a very long line at low temperature difference. Pressure elongation, negligible in plant piping, contributes much of the total movement and must be included in the analysis.
• Soil- pipe interaction: The main portion of a pipe line is buried underground. Any pipe movement has to overcome soil force, which can be divided into two categories: Friction force created from sliding and pressure force resulting from pushing. The major task of pipe line analysis is to investigate soil- pipe interaction which has never been a subject in plant piping analysis.

Normally this line does not have high design temperatures (of the order of 60 to 80 degree centigrade) and only thermal stress checking is sufficient for underground part. Common materials used for underground piping are Carbon Steel, Ductile iron, cast Iron, Stainless Steel and FRP/GRP.