lines2ss, changing coords of other mooring lines

[TheMercer]

Description

When you have Three lines. Two being purely chain and third being semi-taut (chain-nylon).

When using line2ss it changes the fairlead position of the other two lines.

Steps to reproduce issue

1. Two mooring of Chain
2. One line being semi-taut
3. Run line2ss on ms

Current behavior

Coords change

Expected behavior

Coords of other lines stay the same after change.

Fix i did so far (however i don't know how good it is)

I moved the initialisation and equilibrium in lines2ss to lines2subsytem

Which fixed this behaviour

Moorpy.helper:line 1121 & 1122 into 1120 function call

[vamseeachanta]

@TheMercer , please provide example input file to understand the behavior and the expected behavior?

[TheMercer]

I will get something in the week.

Make a subsystem which is made of two lines (semi taut, like chain or nylon or even just chain and chain) just something where one of the main mooring is subsystem.

Have the other two lines as just full chain and non subsystem.

When you apply lines2ss it will affect the other two lines in the system when doing the equilibrium part. Which is why I moved that part of the code

[lsirkis]

Hi @TheMercer,

I have re-created an example of your described system (2 single-section catenary lines and 1 double section semi-taut line) and checked the fairlead point locations. In my example (see provided code at the bottom) I am not able to re-create your issue though. I am using the dev branch commit f7e3ac5

Please respond with a working example if you find that the following explanation does not resolve your concerns

In the absence of an example by you showing this problem, I believe that there are 2 possible reasons you may have come to the conclusion the point locations are changing:

1. Subsystem internal pointList locations are relative (so the fairlead point is always shown to be at [0, 0, z_fair]). If you were looking at the subsystem pointList locations rather than the moorpy system pointList locations for that semi-taut line, it would seem that the line location has changed, but its global location is still correct (and can be seen in [point.r for point in ms.pointList if point.type==-1]. The reason the other 2 (catenary) lines do not show this, is lines2ss will not create a subsystem for lines that are made of a single section type, so they remain moorpy line objects which don't have internal pointLists and are always in global coordinates.

2. When printing out the rB locations (fairlead point) from ms.lineList, (i.e. [line.rB for line in ms.lineList] you saw that the point locations are the same but the order is different from that printed out by the pointList. This difference in order is because lines2ss doesn't delete the fairlead point when creating a subsystem, but it does create a new entry at the end of the lineList, so the order of fairlead points printed from [line.rB for line in ms.lineList] is different than the order of fairlead points printed from [point.r for point in ms.pointList if point.type==-1]

My example re-creation of your system is as follows (the file MoordynSemiTautUpd.dat can be found in here), with my output from this example below that:

import moorpy as mp
from copy import deepcopy
from moorpy.helpers import lines2ss

# load in 3-line semitaut system
ms = mp.System(file='MoordynSemiTautUpd.dat',depth=200)
ms.initialize()
ms.solveEquilibrium()

ms.plot()

# delete last semitaut line and associated points
ms2 = deepcopy(ms)
ms2.lineList.pop(5)
ms2.lineList.pop(4)
ms2.pointList.pop(8)
ms2.pointList.pop(7)
ms2.pointList.pop(6)

# delete 2nd to last semi-taut line and associated points
ms2.lineList.pop(3)
ms2.lineList.pop(2)
ms2.pointList.pop(5)
ms2.pointList.pop(4)
ms2.pointList.pop(3)

# add 2 catenary lines with anchor and fairlead at the same places as the old system
ms2.addPoint(1,ms.pointList[3].r)
ms2.addPoint(-1,ms.pointList[5].r) # fairlead point types are -1 here since no body
ms2.addLine(690,'0',pointA=4,pointB=5) # '0' type is chain
ms2.addPoint(1,ms.pointList[6].r)
ms2.addPoint(-1,ms.pointList[8].r)
ms2.addLine(690,'0',pointA=6,pointB=7)

ms2.initialize()
ms2.solveEquilibrium()

# check point locations
print('Before lines2ss')
for point in ms2.pointList:
    if point.type == -1:
        print(point.r)

# convert lines to subsystems (if they have more than 1 connected section)
lines2ss(ms2)

# re-check point locations
print('After lines2ss')
for point in ms2.pointList:
    if point.type == -1:
        print(point.r)
        
# check line rB (fairlead) points
print('Check rB points')      
for line in ms2.lineList:
    print(line.rB)
    
# plot again for sanity check
ms2.plot()

Output:

Before lines2ss
[ 29.    50.23 -14.  ]
[-58.   0. -14.]
[ 29.   -50.23 -14.  ]
After lines2ss
[ 29.    50.23 -14.  ]
[-58.   0. -14.]
[ 29.   -50.23 -14.  ]
Check rB points
[-58.   0. -14.]
[ 29.   -50.23 -14.  ]
[ 29.    50.23 -14.  ]