Analysis of wave power production systemTECHNICAL REPORT no. 2548 and extended analyses, 20.05.2015

Objectives

Fedem Technology AS (Fedem) has been invited by Murtech AS (Client) to

perform hydrodynamic and structural analyses of a floating wave power

generator concept.

The aim of the analysis is to investigate the concept by means of a dynamic

simulation model that will aim to determine the power production potential.

Scope of work

• WP1: Establish a design basis based on input from the client

• WP2: Data collection and initial analyses (normal operation)

• WP3: System model (normal operation)

• WP4: Structural integrity (storm condition)

• WP5: Reporting

Case Operating condition Wave height, Hmax [m]

1 Normal (WP2/3) 1

2 Normal (WP2/3) 2

3 Normal (WP2/3) 4

4 Normal (WP2/3) 6

5 Storm (WP4) 15

WP1: Design basis

WP1: Design basis

Pontoon properties Value Unit

Mass 7500 kg

Draft 0.65 m

Outer diameter 8.0 m

Diameter waterline 6.6 m

0.71m

0.65m

3.3m

4.0m 0.35m

mean sea level

WP2: Computational fluid dynamics (CFD)

Main conditions for the CFD simulations:

• Performed in Star-CCM+ provided by CD-adapco.

• Three-dimensional and time-dependent.

• Reynolds-averaged Navier-Stokes (RANS) turbulence model.

• Regular incoming waves with wave height according to scope of work. Wave

length calculated to avoid breaking.

• One single pontoon able to move in the vertical direction only (no friction or

generator resistance).

WP2: CFD results, pontoon movements

Vertical pontoon

translations [m]

Vertical pontoon

velocities [m/s]

WP2/3/4: Structural model

Main conditions for the structural simulations:

• Performed in FEDEM simulation software.

• Simulations predict the dynamic response of elastic mechanisms experiencing

non-linear effects such as large rigid-body rotations.

• Hydrodynamic loads from the Morison equation valid for slender beams only.

• Pontoons able to move along beam elements (no friction or generator

resistance).

WP2: Tuning of structural model

System parameters in

FEDEM must be tuned

for a realistic behavior of

the pontoons, according

to CFD results.

WP3: Complete structural model, remarks

Differences from design basis:

• As a result of preliminary studies

which showed instability issues

by using one central mooring line

only, mooring lines were

connected to the system in three

points of the structure, leading to

a connection point 20m below

the buoyancy tank.

• The truss structure of the system

was for simplicity modelled by

beams with circular outer cross

section. The transparent

properties of L-profiles, as

intended for the truss structure

by the client, were adapted by

tuned hydrodynamic properties

of the beam elements.

WP3: Structural results, pontoon movements

Vertical pontoon

translations [m]

Vertical pontoon

velocities [m/s]

WP3: Structural results, potential power production

CaseWave height

[m]

Wave period

[s]

Wave length

[m]

Average power

output [kW]

Case 1 1 2.7 12 380

Case 2 2 3.0 16 690

Case 3 4 4.5 36 805

Case 4 6 5.4 52 951

Case 5 (storm) 15 8.7 119 NA

Note:

Due to the simplified wave load calculations

based on the Morison equation, pontoon

movements were somehow overestimated in

FEDEM for cases with waves shorter about 3x

the outer diameter of the pontoon. These

movements could not be damped sufficiently in

FEDEM to match CFD results for Case 1 and 2.

In addition, the wave energy processing through

the system is not reduced in FEDEM due to

power extraction of the pontoons, e.g., the total

power estimated for the complete structural

model is somewhat overestimated in the present

analyses.

WP3: Structural results, overall system behavior

Case 3

system inclination

WP4: Structural results, storm conditions

A capacity check of the beams in the slider/truss structure has been conducted.

It is found that:

• the L-profiles meet the requirement against buckling.

• yield of the cross section will occur before buckling.

• the max. stress utilization factor for the brace is 0.02 (1 is max. allowed).

• the max. stress utilization factor for the column is 0.11 (1 is max. allowed).

It is, however, recommended to conduct a more detailed structural assessment when

more details of the truss structure is available.

WP5: Animations

Animations showing CFD and FEDEM simulations are attached according to the

following references:

CaseWave height,

Hmax [m]

Wave period,

T [s]

Wave length,

L [m]File name

1 1 2.7 12CFD_Case1.avi

Fedem_Case1.avi

2 2 3.0 16CFD_Case2.avi

Fedem_Case2.avi

3 4 4.5 36CFD_Case3.avi

Fedem_Case3.avi

4 6 5.4 52CFD_Case4.avi

Fedem_Case4.avi

5 15 8.7 119Fedem_Case5.avi

(FEDEM only)

WP5: Concluding remarks

Pontoon geometry:

• From the CFD analyses it is concluded that the pontoons follow the wave propagation

closely, especially for wave lengths greater than two times the pontoon diameter.

However, as the wave height increases, the waves tends to partly break on top of the

pontoons, limiting the vertical displacement of the pontoons and hence power

production.

Potential power production:

• The total power production estimated from the pontoon weight and velocities has

been presented and it is seen that a theoretical peak production of 1200kW is

achieved for Case 4 (instantaneous), while the average power output was 951 kW.

Structural integrity under storm conditions:

• The capacity check showed that the truss construction meets the design

requirements when subjected to the global forces and moments occurring in storm

condition (Case 5). Hence, it is assumed that sufficient capacity is also achieved

during normal operating conditions.

WP5: Recommendations and further work

Pontoon geometry:

• The geometry of the pontoons may be revised as waves tends to partly break on top

of the pontoons. In addition, a detailed CFD analysis of the complete system may be

performed in order to account for such effects as shielding from the upstream

pontoons etc.

Potential power production:

• Fedem Technology recommends an investigation of the wave power system

subjected to irregular sea to be performed. This should be conducted together with

applying realistic power generator resistance, as well as frictional contact mechanics

between pontoon and truss structure, as this will affect the pontoon response.

Mooring arrangement:

• It is advised to further revise the mooring arrangement as the center of gravity and

center of buoyancy are closely located, which will lead to instability.

• It is also believed that by improving the mooring arrangement, the lean-over effect of

the system will be reduced.

Extended analyses - models

Model Buoyancy tank BallastTotal weight

tank + ballast

0 Steel

thickness 20mm, diameter

10,3m, height 4m

weight 46,2 tons

Concrete

in bottom of tank

thickness 150 og 500 mm

weight 44,3 tons

90,5 tons

1

Steel

thickness 20mm

diameter 16,3m

height 4m

weight 97,3 tons

(relevant for models

1, 2 og 3)

Concrete

in bottom of tank

thickness 200mm

weight 95,5 tons

192,8 tons

2 Concrete

below tank

Ø 3m x 3m

weight 48,8 tons

146,1 tons

3 Concrete

below tank

Ø 3m x 5m

weight 81,3 tons

178,6 tons

Extended analyses - results

ModelSystem rotation

(max. angle)Comments

0 Case 1: 4,3

Case 2: 13,4

Case 3: 12,1

Case 4: 14,2

Model and results from first phase of project (three

mooring lines).

1 Case 1: 3,4

Case 2: 10,5

Case 3: 11,5

Case 4: 14,3

General:

The results show that it is possible to utilize only one

mooring line by increasing the diameter of the

buoyancy tank to 16,3 m. All models show maximum

angles of rotation within the upper requirement of 15

deg and Case 2 – 3 also meet the lower

requirement of 10 deg for all sea states.

Models:

Model 1 gives the biggest system rotation due to the

locations of buoyancy and mass centers being

close. The rotation is reduced for Model 2 og 3

when the center of mass was lowered together with

a reduction in the total ballast weight.

2 Case 1: 1,3

Case 2: 3,5

Case 3: 4,4

Case 4: 5,7

3 Case 1: 1,1

Case 2: 2,9

Case 3: 4,9

Case 4: 7,5