Updates to dynamic EA stiffness implementation

1) line.activateDynamicStiffness now takes the maximum value of the dynamic and static stiffness, at whatever line tension, to ensure that a dynamic stiffness value that is lower than a static stiffness value (usually at low tensions) is not used 2) unit error in the dynamic stiffness inputs: the beta term (in other research studies) is defined as the slope of the Krd vs. %MBL curve, where %MBL values are in units of 20,30,40...[%MBL]. So, we would have to add a factor of 100 to the mean tension value used to calculate the dynamic stiffness, or multiply the beta/slope values by 100 in MoorProps_default.yaml
NREL · Nov 2, 2023 · 1a39ae0 · 1a39ae0
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Showing 2 changed files with 5 additions and 5 deletions.
diff --git a/moorpy/MoorProps_default.yaml b/moorpy/MoorProps_default.yaml
@@ -67,7 +67,7 @@ lineProps:
     MBL_d2    :  308.2e6    # minimum breaking load per diameter^2 [N/m^2]     
     EA_MBL    :   14        # quasi-static stiffness per MBL [N/N]
     EAd_MBL   :  11.615     # dynamic stiffness per MBL [N/N]
-    EAd_MBL_Lm :  0.396     # dynamic stiffness per MBL per fraction of mean load (not %) [N/N]
+    EAd_MBL_Lm :  39.6     # dynamic stiffness per MBL per fraction of mean load (not %) [N/N] (beta term to be multiplied with the absolute mean tension)
     density   :  1380       # density of the polyester material [kg/m^3] (taken from specific gravity of 1.38, relative to 1000 kg/m^3)
     cost_MBL  :  1.65e-05   # cost per MBL [$/m/N]
 
@@ -77,7 +77,7 @@ lineProps:
     MBL_d2    :  207.0e6    # minimum breaking load per diameter^2 [N/m^2] 
     EA_MBL    :  5          # quasi-static stiffness per MBL [N/N] (can range from 1 to 10)
     EAd_MBL   :  2.08       # dynamic stiffness per MBL [N/N]
-    EAd_MBL_Lm : 0.39       # dynamic stiffness per MBL per fraction of mean load (not %) [N/N]
+    EAd_MBL_Lm : 39.0       # dynamic stiffness per MBL per fraction of mean load (not %) [N/N] (beta term to be multiplied with the absolute mean tension)
     density   :  1140       # density of the nylon material [kg/m^3] (taken from specific gravity of 1.14, relative to 1000 kg/m^3)
     cost_MBL  :  4.29e-05   # cost per MBL [$/m/N]
 
@@ -87,7 +87,7 @@ lineProps:
     MBL_d2    :  579.5e6    # minimum breaking load per diameter^2 [N/m^2] 
     EA_MBL    :  56.54      # quasi-static stiffness per MBL [N/N]
     EAd_MBL   :  59         # dynamic stiffness per MBL [N/N]
-    EAd_MBL_Lm : 0.54       # dynamic stiffness per MBL per fraction of mean load (not %) [N/N]
+    EAd_MBL_Lm : 54.0       # dynamic stiffness per MBL per fraction of mean load (not %) [N/N] (beta term to be multiplied with the absolute mean tension)
     density   :  975        # density of the hmpe/dyneema material [kg/m^3] (taken from specific gravity of 0.975, relative to 1000 kg/m^3
 
 

diff --git a/moorpy/line.py b/moorpy/line.py
@@ -1069,8 +1069,8 @@ def activateDynamicStiffness(self, display=0):
         if self.type['EAd'] > 0:
             # switch to dynamic stiffness value
             EA_old = self.type['EA']
-            EA_new = self.type['EAd'] + self.type['EAd_Lm']*self.TA  # this implements the sloped Krd = alpha + beta*Lm
-            self.EA = EA_new
+            EA_new = self.type['EAd'] + self.type['EAd_Lm']*np.mean([self.TA, self.TB])  # this implements the sloped Krd = alpha + beta*Lm
+            self.EA = np.max([EA_new, EA_old])  # only if the dynamic stiffness is higher than the static stiffness, activate the dynamic stiffness
 
             # adjust line length to maintain current tension (approximate)
             self.L = self.L0 * (1 + self.TB/EA_old)/(1 + self.TB/EA_new)