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# Finding a Variable through iterating. This one is killing me!

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## Finding a Variable through iterating. This one is killing me!

(OP)
Hey guys, I'm relatively new to mathcad and have hit a wall.  All the data needed is attached to this message.

The formulae are lifted straight out of DNV-OS-F101 for the Local Buckling - Collapse of off shore Pipelines.

The Problem is that the data provided must be used to find a value of t_nom by setting the value of P_c = P_coll.

In Excel, the program runs with an estimated value of t_nom and iterates until the value of P_c > P_coll, therefore giving a suitable value of t_nom.

Is there any way of doing this in mathcad?

Thanks guys, I'm really stuck here.

### RE: Finding a Variable through iterating. This one is killing me!

Generally, this type of problem can be solved using a solve block.  Please save the workbook in version 11 or 13 and attach.

### RE: Finding a Variable through iterating. This one is killing me!

(OP)
To IRstuff:  I haven't altered the equation at all though, it's pulled straight out of a DNV document, so you would have thought they'd balance?!?!

In the document you've presented, t_nom is still undefined earlier in the equation, and Find(t_nom) is also undefined, how do I solve this?

I really am a complete newbie with mathcad, I'm talking baby steps...

### RE: Finding a Variable through iterating. This one is killing me!

#### Quote:

To IRstuff:  I haven't altered the equation at all though, it's pulled straight out of a DNV document, so you would have thought they'd balance?!?!
Often sources show equations, particularly empirically derived ones, with constants that require particular units.  You either need to use those particular units and not explicitly show them in MathCAD or adjust the equations for the units used.

### RE: Finding a Variable through iterating. This one is killing me!

The OP has two variables that he uses that ostensibly should have units, E and sigma.y;  assuming both have pascals as units allows the equations to be balanced.  However, the RHS expression is extremely flat, while the other side is quite steep, making it difficult for the solver to find a solution.

As a general rule, one should graph the expressions to see whether they behave as expected, and whether a solution is plausibly achievable.  The equations, as written have graphical solution ~0.5183226, which seems absurdly small.  There also is an anomalous D.0/t.nom expression, where all the other instances are t.nom/D.0.  However, changing that one instance makes little difference to the problem as a whole.

http://files.engineering.com/getfile.aspx?folder=0dec261c-5751-42be-8ed0-c874bcf5ac47&amp;file=Collapse2.mcd

### RE: Finding a Variable through iterating. This one is killing me!

0.518m doesn't seem absurdly small (it is meters, not mm) if this is pipeline wall thickness.  If D.o = 350mm is the pipeline outside diameter, a 518mm wall thickness is impossibly large.  If you use Minerr instead of Find, you get a solution 0.5183226*m.

### RE: Finding a Variable through iterating. This one is killing me!

You're right, I'd forgotten that the quickplot defaulted to meters, and yes, Minerr gets around the issues with Find.

### RE: Finding a Variable through iterating. This one is killing me!

The problem might be the units.  The modulus of elasticity of steel is about 2·10^11 Pa, not 207000Pa.  If you use 2.07·10^11Pa, you get a t.nom = 5.16mm

### RE: Finding a Variable through iterating. This one is killing me!

The OP seems to have lost interest, but thanks for the new info; it's only just so much math for me, since it's well outside of my purview.  Half a millimeter seems a bit thin, to me, but it feels much more plausible than half a meter.

### RE: Finding a Variable through iterating. This one is killing me!

Just for the record, as IRstuff said, the OP has lost interest a long time ago: If you change the Modulus of Elasticity from whatever it was to steel then you probably have to look at Sigma.y too. If you use Pascal as unit, I think, it refers to soft butter.

### RE: Finding a Variable through iterating. This one is killing me!

(OP)
Hey guys, sorry, I didn't lose interest in this post, but I've been away for the bank holiday weekend.  I'm just looking through everything now and will reply properly shortly.

### RE: Finding a Variable through iterating. This one is killing me!

(OP)
I didn't realise I'd missed the units for E and sigma.y, my bad.  The units are MPa.

5.16mm is extremely close to the desired answer, a hell of a lot closer than I had been getting.

Just to clarify, is Minerr a pre-built in function to Mathcad?

Thanks for all the help guys

### RE: Finding a Variable through iterating. This one is killing me!

Yes, Minerr is build in, but I did not use it. With the new units I came to 5.5mm - check it out.

### RE: Finding a Variable through iterating. This one is killing me!

(OP)
Occupant,

Do you have the file you have used?  Could you post it please?

### RE: Finding a Variable through iterating. This one is killing me!

I think if you'll blow the thing up even more closer to zero, and include a little negative thickness, you'll see the other solutions.  The workbook I attached to my post yesterday was your workbook with the new E and sigma values and using Minerr.  Your graph showed the two sides of the equation separately and the way I blew it up, they definitely cross in three places.

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