Finishing touches on static-dynamic stiffness:

- Cleaned up moorprops-default.yaml, removing extra EAs entry and
  cleaning up notes.
- Cleaned up associated functions in helpers, line, and system to
  match and to just give a warning message when the users calls
  activateDynamicStiffness but EAd value isn't provided (only if
  display > 0).
- Added documention of the new feature in the Usage page!!

docs/usage.rst

@@ -14,10 +14,40 @@ MoorPy Usage
 .. role:: stnd
 
 
+Advanced Features
+^^^^^^^^^^^^^^^^^
 
 
 (A list of key functions to be added here)
 
+Variable Rope Stiffness Behavior
+--------------------------------
+
+MoorPy supports separate static and dynamic stiffness coefficients, 
+to approximate the variable stiffness characterisics of synthetic
+fiber ropes. For using this capability, the mooring line type 
+information must include a static stiffness value (EAs), and a 
+dynamic stiffness value (EAd). An additional factor on EAd that
+scales with mean tension (EAd_Lm) can also be included. When using
+a mooring line properties library (i.e., a yaml file), these values
+should be specified by the EA_MBL, EAd_MBL, and EAd_MBL_Lm keywords,
+respectively. See the (moorprops library section to be added) for 
+more information.
+
+Two System methods control switching between static and
+dynamic stiffness properties:
+
+activateDynamicStiffness switches the mooring system model to dynamic 
+line stiffness values. It also adjusts the unstretched line lengths
+to maintain the same tension at the current system state. If EAd has 
+not been set, it will not change anything (call it with display > 0 
+to display a warning when this occurs).
+
+:func:`.system.activateDynamicStiffness`
+
+revertToStaticStiffness resets the mooring lines to use the static
+stiffness values and return to their original unstretched lenths.
+
 
 Additional Parameters in MoorPy
 -------------------------------

moorpy/MoorProps_default.yaml

@@ -17,22 +17,10 @@
 # EA_d : # stiffness per diameter [N/m]
 # EA_d2 : # stiffness per diameter^2 [N/m^2]
 # EA_d3 : # stiffness per diameter^3 [N/m^3]
-# EA_MBL or Kr # stiffness per MBL [N/N]
-# EAs_MBL or Krs # quasi-static stiffness per MBL [N/N]
-# EAd_MBL or Krd_alpha # dynamic stiffness per MBL [N/N]
-# EAd_MBL_Lm or Krd_beta # dynamic stiffness per MBL per fraction of mean load (not %) [N/N]
-
-
-
-NEW:
-# Krs or EAs_MBL : # quasi-static stiffness per MBL [N/N]
-# Krd_alpha or EAd_MBL : # dynamic stiffness per MBL [N/N]
-# Krd_beta or EAd_MBL_PML : # dynamic stiffness per MBL per % mean load [N/N]
-# (% mean load will be input later in the modeling code)
-
-
-
-# 
+# EA_MBL : # (quasi-static) stiffness per MBL [N/N] (aka Kr, Krs) 
+# EAd_MBL : # dynamic stiffness per MBL [N/N] (aka Krd or Krd_alpha)
+# EAd_MBL_Lm: # dynamic stiffness per MBL per fraction of mean load (not %) [N/N] (aka or Krd_beta)
+#
 # dvol_dnom : # volume-equivalent diameter per nominal diameter [-]
 # 
 # cost_0 : # cost offset [$/m]
@@ -70,7 +58,7 @@ lineProps:
  MBL_d2 : 170.5e6 # minimum breaking load per diameter^2 [N/m^2] 
  dvol_dnom : 0.86 # volume-equivalent diameter per nominal diameter [-] 
  cost_MBL : 1.65e-05 # cost per MBL [$/m/N]
- EAs_MBL : 14 # quasi-static stiffness per MBL [N/N]
+ EA_MBL  : 14 # quasi-static stiffness per MBL [N/N]
  EAd_MBL : 14 # dynamic stiffness per MBL [N/N]
  EAd_MBL_Lm : 0.34 # dynamic stiffness per MBL per fraction of mean load (not %) [N/N]
 

moorpy/helpers.py

@@ -688,8 +688,8 @@ def getLineProps(dnommm, material, lineProps=None, source=None, name="", rho=102
  + mat['cost_mass']*mass + mat['cost_EA']*EA + mat['cost_MBL']*MBL)
  w = (mass - np.pi/4*d_vol**2 *rho)*g
 
- # >>> expected inputs for viscoelastic approach <<< need to add some kind of mode switch to handle this throughout MoorPy <<<<
- EAs = mat['EAs_MBL']*MBL # quasi-static stiffness: Krs x MBL [N]
+ # stiffness values for viscoelastic approach 
+ EAs = mat['EA_MBL']*MBL # quasi-static stiffness: Krs x MBL [N]
  EAd = mat['EAd_MBL']*MBL # dynamic stiffness constant: Krd alpha term x MBL [N]
  EAd_Lm = mat['EAd_MBL_Lm'] # dynamic stiffness Lm slope: Krd beta term (to be multiplied by mean load) [-]
 
@@ -761,7 +761,6 @@ def loadLineProps(source):
  output[mat]['EA_d2' ] = getFromDict(props, 'EA_d2' , default=0.0)
  output[mat]['EA_d3' ] = getFromDict(props, 'EA_d3' , default=0.0)
  output[mat]['EA_MBL' ] = getFromDict(props, 'EA_MBL' , default=0.0)
- output[mat]['EAs_MBL' ] = getFromDict(props, 'EAs_MBL' , default=0.0)
  output[mat]['EAd_MBL' ] = getFromDict(props, 'EAd_MBL' , default=0.0)
  output[mat]['EAd_MBL_Lm']= getFromDict(props, 'EAd_MBL_Lm',default=0.0)
 

moorpy/line.py

@@ -1051,20 +1051,23 @@ def attachLine(self, lineID, endB):
  pass
 
 
- def activateDynamicStiffness(self):
+ def activateDynamicStiffness(self, display=0):
  '''Switch mooring line model to dynamic line stiffness
  value, including potential unstretched line length
  adjustment. This only works when dynamic line properties
  are used.'''
 
- # switch to dynamic stiffness value
- EA_old = self.type['EAs']
- EA_new = self.type['EAd'] + self.type['EAd_Lm']*self.TA # this implements the sloped Krd = alpha + beta*Lm
- self.EA = EA_new
-
- # adjust line length to maintain current tension (approximate)
- self.L = self.L0 * (1 + self.TB/EA_old)/(1+self.TB/EA_new)
-
+ if self.type['EAd'] > 0:
+ # switch to dynamic stiffness value
+ EA_old = self.type['EAs']
+ EA_new = self.type['EAd'] + self.type['EAd_Lm']*self.TA # this implements the sloped Krd = alpha + beta*Lm
+ self.EA = EA_new
+
+ # adjust line length to maintain current tension (approximate)
+ self.L = self.L0 * (1 + self.TB/EA_old)/(1+self.TB/EA_new)
+ else:
+ if display > 0:
+ print(f'Line {self.number} has zero dynamic stiffness coefficient so activateDynamicStiffness does nothing.')
 
 
  def revertToStaticStiffness(self):

moorpy/system.py

@@ -2906,29 +2906,24 @@ def checkTensions(self, N = None):
  return(ratios)
 
 
- def activateDynamicStiffness(self):
+ def activateDynamicStiffness(self, display=0):
  '''Switch mooring system model to dynamic line stiffness
- values, including potential unstretched line length
- adjustment. This only works when dynamic line properties
+ values and adjust the unstretched line lengths to maintain the
+ same tensions. This only has an effect when dynamic line properties
  are used.'''
 
  for line in self.lineList:
- line.activateDynamicStiffness()
-
- #initialize?
+ line.activateDynamicStiffness(display=display)
 
 
  def revertToStaticStiffness(self):
- '''Switch mooring system model to dynamic line stiffness
- values, including potential unstretched line length
- adjustment. This only works when dynamic line properties
- are used.'''
+ '''Revert mooring system model back to the static stiffness
+ values and the original unstretched lenths.'''
 
  for line in self.lineList:
  line.revertToStaticStiffness()
-
- #initialize?
-
+
+
  def getDepthFromBathymetry(self, x, y): #BathymetryGrid, BathGrid_Xs, BathGrid_Ys, LineX, LineY, depth, nvec)
  ''' interpolates local seabed depth and normal vector