New sophisticated stiffness capabilities:

- Proper description will be provided in a later commit that finishes these...

moorpy/MoorProps_default.yaml

@@ -8,11 +8,21 @@
 # mass_d2 : # linear mass density per diameter^2 [kg/m^3]
 # mass_d3 : # linear mass density per diameter^3 [kg/m^4]
 # 
+# MBL_0 : # minimum breaking load offset [N]
+# MBL_d : # minimum breaking load per diameter [N/m]
+# MBL_d2 : # minimum breaking load per diameter^2 [N/m^2]
+# MBL_d3 : # minimum breaking load per diameter^3 [N/m^3]
+# 
 # EA_0 : # stiffness offset [N]
 # 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 : # stiffness per MBL [N/N]
+# 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]
@@ -21,11 +31,7 @@ NEW:
 # (% mean load will be input later in the modeling code)
 
 
-# 
-# MBL_0 : # minimum breaking load offset [N]
-# MBL_d : # minimum breaking load per diameter [N/m]
-# MBL_d2 : # minimum breaking load per diameter^2 [N/m^2]
-# MBL_d3 : # minimum breaking load per diameter^3 [N/m^3]
+
 # 
 # dvol_dnom : # volume-equivalent diameter per nominal diameter [-]
 # 
@@ -60,11 +66,14 @@ lineProps:
 
  polyester : # polyester synthetic rope
  mass_d2 : 797.8 # linear mass density per diameter^2 [kg/m^3] 
- EA_d2 : 1.09e9 # stiffness per diameter^2 [N/m^2] 
+ #EA_d2 : 1.09e9 # stiffness per diameter^2 [N/m^2] 
  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]
+ 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]
+
  nylon : # nylon synthetic rope
  mass_d2 : 647.6 # linear mass density per diameter^2 [kg/m^3]
  EA_d2 : 1.18e8 # stiffness per diameter^2 [N/m^2] 

moorpy/helpers.py

@@ -688,6 +688,11 @@ 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]
+ 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) [-]
+
  # Set up a main identifier for the linetype unless one is provided
  if name=="":
  typestring = f"{type}{dnommm:.0f}"
@@ -697,7 +702,8 @@ def getLineProps(dnommm, material, lineProps=None, source=None, name="", rho=102
  notes = f"made with getLineProps"
 
  lineType = dict(name=typestring, d_vol=d_vol, m=mass, EA=EA, w=w,
- MBL=MBL, cost=cost, notes=notes, input_type=type, input_d=d, material=material)
+ MBL=MBL, EAs=EAs, EAd=EAd, EAd_Lm=EAd_Lm, input_d=d,
+ cost=cost, notes=notes, input_type=type, material=material)
 
  lineType.update(kwargs) # add any custom arguments provided in the call to the lineType's dictionary
 
@@ -755,6 +761,10 @@ 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)
+
  output[mat]['MBL_0' ] = getFromDict(props, 'MBL_0' , default=0.0)
  output[mat]['MBL_d' ] = getFromDict(props, 'MBL_d' , default=0.0)
  output[mat]['MBL_d2' ] = getFromDict(props, 'MBL_d2' , default=0.0)

moorpy/line.py

@@ -46,9 +46,12 @@ def __init__(self, mooringSys, num, L, lineType, nSegs=100, cb=0, isRod=0, attac
  self.number = num
  self.isRod = isRod
 
- self.L = L # line unstretched length
+ self.L = L # line unstretched length (may be modified if using nonlinear elasticity) [m]
+ self.L0 = L # line reference unstretched length [m]
  self.type = lineType # dictionary of a System.lineTypes entry
 
+ self.EA = self.type['EA'] # use the default stiffness value for now (may be modified if using nonlinear elasticity) [N]
+
  self.nNodes = int(nSegs) + 1
  self.cb = float(cb) # friction coefficient (will automatically be set negative if line is fully suspended)
  self.sbnorm = [] # Seabed Normal Vector (to be filled with a 3x1 normal vector describing seabed orientation)
@@ -799,7 +802,7 @@ def staticSolve(self, reset=False, tol=0.0001, profiles=0):
  #If EA is found in the line properties we will run the original catenary function 
  if 'EA' in self.type:
  try:
- (fAH, fAV, fBH, fBV, info) = catenary(LH, LV, self.L, self.type['EA'], w,
+ (fAH, fAV, fBH, fBV, info) = catenary(LH, LV, self.L, self.EA, w,
  CB=cb, alpha=alpha, HF0=self.HF, VF0=self.VF, 
  Tol=tol, nNodes=self.nNodes, plots=profiles) 
  except CatenaryError as error:
@@ -1039,11 +1042,41 @@ def getPosition(self, s):
  Zs = self.rA[2] + z
 
  return np.vstack([ Xs, Ys, Zs])
+
 
  def attachLine(self, lineID, endB):
  pass
 
 
+ def activateDynamicStiffness(self):
+ '''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)
+
+
+
+ def revertToStaticStiffness(self):
+ '''Switch mooring line model to dynamic line stiffness
+ values, including potential unstretched line length
+ adjustment. This only works when dynamic line properties
+ are used.'''
+
+ # switch to static/default stiffness value
+ self.EA = self.type['EA']
+
+ # revert to original line length
+ self.L = self.L0
+
+
 def from2Dto3Drotated(K2D, F, L, R): 
  '''Initialize a line end's analytic stiffness matrix in the 
  plane of the catenary then rotate the matrix to be about the 

moorpy/system.py

@@ -2543,7 +2543,7 @@ def getCoupledStiffness(self, dx=0.1, dth=0.1, solveOption=1, lines_only=False,
 
 
 
- def getCoupledStiffnessA(self, dx=0.1, dth=0.1, solveOption=1, lines_only=False, tensions=False, nTries=3, plots=0):
+ def getCoupledStiffnessA(self, lines_only=False, tensions=False, nTries=3, plots=0):
  '''Calculates the stiffness matrix for coupled degrees of freedom of a mooring system
  with free uncoupled degrees of freedom equilibrated - analytical appraoch. 
  
@@ -2905,6 +2905,29 @@ def checkTensions(self, N = None):
  return 
  return(ratios)
 
+
+ def activateDynamicStiffness(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.'''
+
+ for line in self.lineList:
+ line.activateDynamicStiffness()
+
+ #initialize?
+
+
+ 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.'''
+
+ 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