Validation mooring lines - Comparison FRyDoM / DeeplinesWind

Test case

In this benchmark, a Floating Offshore Wind Turbine (FOWT) platform with three mooring lines anchored to the bottom are considered. The platform is fixed or mooved with prescribed harmonic motion. The tension results of the three mooring lines at the fairleads and anchor points are compared to DeeplinesWind [DLW2013] simulation.

FOWT platform 3D view

Fig. 69 3D view of the FOWT platform with the three mooring lines anchored to the seabed

The water depth is set equal to 200 m, the gravity acceleration is 9.81 m/s^2 and the water density is 1025 kg/m^3.

Mooring lines properties

The mechanical properties and morison coefficients of the mooring lines are listed in the next table.

Properties Value
Diameter (m) 0.0766
Length (m) 835.5
Axial stiffness (N) 7.536E+08
Linear density (kg/m) 113.35
Drag coeff. (normal) 1.1
Drag coeff. (tangential)
Added mass (normal)
Added mass (tangential)

Failead positions

The following table gives the cartesian coordinates of the fairlead in the platform reference frame.

Fairlead Position (in body reference frame)
T1 {20.435, 35.4, -14}
T2 {-40.87, 0., -14}
T3 {20.435, -35.4, -14}

Anchor positions

The following table gives the cartesian coordinates of the anchor point in the world reference frame.

Anchor Position (in world reference frame)
G1 {418.8, 725.383, -200}
G2 {-837.6, 0., -200}
G3 {418.8, -725.383, -200}

Environment set

The following table gives the environment simulated for the different comparison studies

Motion Waves Current
Harmonic surge (T=50s ; A=10m) No No
Harmonic heave (T=30s ; A=5m) No No
No Regular wave (T=27s : A=5m ; dir = 0 deg) No
No Regular wave (T=27s ; A=5m ; dir = 90 deg) No
No No yes
No No yes

Results

Imposed surge motion

The motion of the platform is harmonic in surge, with a time period equal to 50 seconds and an amplitude of 10 meters. Other degrees of freedom are fixed.

In the next figures we compare the tension of the three mooring lines at the corresponding fairleads between FRyDoM and DeeplinesWind

Tension fairlead T1

Fig. 70 Tension of the line 1 at the fairlead with a harmonic surge motion of the platform

Tension fairlead T2

Fig. 71 Tension of the line 2 at the fairlead with a harmonic surge motion of the platform

Tension fairlead T3

Fig. 72 Tension of the line 3 at the fairlead with a harmonic surge motion of the platform

In the next figures we compare the tension of the three mooring lines at the anchor points between FRyDoM and DeeplinesWind

Tension anchor point G1

Fig. 73 Tension of the line 1 at the anchor point with a harmonic surge motion of the paltform

Tension anchor point G2

Fig. 74 Tension of the line 2 at the anchor point with a harmonic surge motion of the platform

Tension anchor point G3

Fig. 75 Tension of the line 3 at the anchor point with a harmonic surge motion of the paltform

One can observe that the non-linearties are important on the dynamic of the mooring line and well capture in FRyDoM. The results of FRyDoM are very close to the results from the benchmark model with a mean relative difference about 3.2%, 4.1% and 3.2% for respectively the tension of the mooring lines 1, 2 and 3 at the fairleads. At the anchor points, the differences are about 3.9%, 4.7% and 3.9% for respectively the mooring lines 1, 2 and 3.

Imposed heave motion

The motion of the platform is harmonic in heave, with a time period equal to 30 seconds and an amplitude of 5 meters. Other degrees of freedom are fixed.

In the next figures we compare the tension of the three mooring lines at the fairleads between FRyDoM and DeeplinesWind.

Tension fairlead T1 heave

Fig. 76 Tension of the line 1 at the fairlead with a harmonic heave motion of the platform

Tension fairlead T2 heave

Fig. 77 Tension of the line 2 at the fairlead with a harmonic heave motion of the platform

Tension fairlead T3 heave

Fig. 78 Tension of the line 3 at fairlead with a harmonic heave motion of the platform

In the next figures we compare the tension of the three mooring lines at the anchor points between FRyDoM and DeeplinesWind.

Tension anchor point G1

Fig. 79 Tension of the line 1 at the anchor point with a harmonic heave motion of the platform

Tension anchor point G2

Fig. 80 Tension of the line 2 at the anchor point with a harmonic heave motion of the platform

Tension anchor point G3

Fig. 81 Tension of the line 3 at the anchor point with a harmonic heave motion of the platform

The tensions of the mooring lines simulated by FRyDoM follow the time evolution of the tensions given by the benchmark model with a constant offset around 4E+04 N. The non-linearities are correctly predicted. The origin of the offset is investigated.

Regular waves

(in progress)

Uniform Current

(in progress)

References

[DLW2013]
  1. Perdrizet, J-C Gilloteaux, D. Teixeira, G. Ferrer, L. Piriou, D. Cadiou, J-H Heurtier, C. Le Cunff, “Fully coupled floating wind turbine simulator based on nonlinear finite element method - Part II : Validation results”, OMAE, 2013, Nantes