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WP 4

Model Tests and Analysis

Objectives: To design and conduct physical model tests in the wave tank of the Laboratory of Ship and Marine Hydrodynamics (NTUA) by accounting for the inertia characteristics of the W/T, the characteristics of the OWC’s device and the moorings; to construct the model and conduct experiments; to validate numerical models by comparison with experimental results.

Task 4.1: Design of a scaled-down W/T configuration

Objectives: To design a scaled-down W/T configuration for the physical model testing campaign.

Description: Given the dimensions of the wave tank facility, the rotor diameter will be in the range of 1-1.5m and so the length scaling is defined. Since modern wind turbines are all variable speed and variable pitch, it is logical to have as first priority the nominal operation point. Usually it will correspond to a tip-speed ration in the range of 6-8. Together with a) the requirements on Ma (the tip Mach should not exceed 0.3) and Re numbers,(the Re number should be as high as possible), b) the anticipated budget, and c) the minimization of the air-flow blockage of the generator the optimum combination of rotor speed and wind inflow will be defined. Preliminary analysis indicates a wind speed ~5m/s and a rotational speed of ~500rpm. It is clear that it is not possible to achieve Re similarity and therefore within this task, the aerodynamic characteristics of the blades will be corrected for the corresponding conditions of the test. To this end CFD calculations will be carried. Next the scaled down wind turbine will be analysed in order to estimate the corresponding loads on the floater.

Task 4.2: Design of experimental set up and construction of the model

Objectives: To define scaling, sea state conditions (regular, irregular and extreme sea conditions); to design and construct the floater with the OWC; to construct the W/T physical model, finalized in Task 4.1; to design and construct the fan system that will generate the wind flow.

Description: The Power Take off system of the OWC will be modelled through an approximately 2.5cm diameter opening of at the top of the air chamber. The corresponding damping of the PTO system will be modelled though a thin porous carpet tightly fitted to obstruct this opening

Task 4.3: Conduct experiments

Objectives: To measure motions of the structure; free surface elevation in the air chamber; tensions in the mooring lines; loads at the mounting place of the W/T on the floater, together with the parameters characterizing the OWC (airflow rate supplied to the chamber and the pressure drop between the chamber and the outside environment).

Description: The motions of the structure will be measured by tracking the position of three points on it. Free-surface elevation inside the chamber will be measured at two points. The pressure drop will be obtained by measuring the pressure inside the chamber, upstream of the carpet. The downstream pressure, outside the chamber, is equal to the atmospheric pressure.

Task 4.4: Analysis of the measured data and validation of the numerical results

Objectives: To analyze the experimental data and validate numerical models by comparison with experimental results.

WP 5

Geotechnical Studies

Objectives:  To numerically analyze the anchor piles’ response under combined static and dynamic loading due to the pre-tensioned mooring cables (TLP principle) in clayey and silty-sand seafloors.

Description: The multi-purpose platform will be stabilized in place with the aid of pre-stressed vertical cables which will be anchored to the sea-floor via large diameter single piles (caissons) or pile groups with a common pile head. These piles operate under tension, during calm weather conditions (due to the pre-stressing of the cables) as well as during storms. Hence, the conventional methods for pile design, under compressive static and dynamic loading, have to be revised for one at least reason: the pile tip resistance, which represents an important portion of the total bearing capacity for piles in compression, has to be ignored as tensile loads are carried by friction forces developing at the pile-soil interface. In view of the above peculiarities, the anchor pile response will be analyzed numerically, with the aid of specialized software, developed at the Foundation Engineering Laboratory of NTUA, which is based on a nonlinear, dynamic Finite Difference numerical analysis method and may efficiently account for the “macroscopic” pile-soil-platform interaction, as well as for the “microscopic” interaction between the soil skeleton and the pore fluid. The relevant numerical analyses will be performed in two discrete stages:

• 1^st Stage: The accuracy of the aforementioned software will be verified against experimental results from (large and small) scale model tests, as well as against results from simplified empirical analyses with the widely applied t-z method.
• 2^nd Stage:  A large number of parametric analyses will be performed for two commonly encountered sea floor ground conditions (clayey, as well as sandy soil), so that the effect of the prevailing soil conditions on the platform design can be evaluated. The analyses will focus upon (a) the limit axial load which can be applied on each pile head, (b) the possible permanent creep (pullout) displacements of the piles under the combined effect of pre-stressing and wave loads, as well as (c) the equivalent cyclic foundation stiffness Κ_found = Ρ/δ (Ρ= range of axial load variation on pile head due to wave action, δ = range of respective displacement variation).

The work package will be carried out in two tasks:

Task 5.1: Anchor pile design in clayey seafloor

Task 5.2: Anchor pile design in silty-sand seafloor

Both of them will include literature survey for relevant experimental data, calibration of constitutive soil models against lab test results from cyclic loading of clayey and silty-sand soil elements, respectively, pilot numerical analyses and comparison with experimental results for model anchor piles in clay and silty-sand soil layers, respectively, under combined static and cyclic loading, large number of parametric analyses for the quantitative evaluation of key parameters for the platform foundation design.

WP 5

Geotechnical Studies

Objectives:  To numerically analyze the anchor piles’ response under combined static and dynamic loading due to the pre-tensioned mooring cables (TLP principle) in clayey and silty-sand seafloors.

Description: The multi-purpose platform will be stabilized in place with the aid of pre-stressed vertical cables which will be anchored to the sea-floor via large diameter single piles (caissons) or pile groups with a common pile head. These piles operate under tension, during calm weather conditions (due to the pre-stressing of the cables) as well as during storms. Hence, the conventional methods for pile design, under compressive static and dynamic loading, have to be revised for one at least reason: the pile tip resistance, which represents an important portion of the total bearing capacity for piles in compression, has to be ignored as tensile loads are carried by friction forces developing at the pile-soil interface. In view of the above peculiarities, the anchor pile response will be analyzed numerically, with the aid of specialized software, developed at the Foundation Engineering Laboratory of NTUA, which is based on a nonlinear, dynamic Finite Difference numerical analysis method and may efficiently account for the “macroscopic” pile-soil-platform interaction, as well as for the “microscopic” interaction between the soil skeleton and the pore fluid. The relevant numerical analyses will be performed in two discrete stages:

• 1^st Stage: The accuracy of the aforementioned software will be verified against experimental results from (large and small) scale model tests, as well as against results from simplified empirical analyses with the widely applied t-z method.
• 2^nd Stage:  A large number of parametric analyses will be performed for two commonly encountered sea floor ground conditions (clayey, as well as sandy soil), so that the effect of the prevailing soil conditions on the platform design can be evaluated. The analyses will focus upon (a) the limit axial load which can be applied on each pile head, (b) the possible permanent creep (pullout) displacements of the piles under the combined effect of pre-stressing and wave loads, as well as (c) the equivalent cyclic foundation stiffness Κ_found = Ρ/δ (Ρ= range of axial load variation on pile head due to wave action, δ = range of respective displacement variation).

The work package will be carried out in two tasks:

Task 5.1: Anchor pile design in clayey seafloor

Task 5.2: Anchor pile design in silty-sand seafloor

Both of them will include literature survey for relevant experimental data, calibration of constitutive soil models against lab test results from cyclic loading of clayey and silty-sand soil elements, respectively, pilot numerical analyses and comparison with experimental results for model anchor piles in clay and silty-sand soil layers, respectively, under combined static and cyclic loading, large number of parametric analyses for the quantitative evaluation of key parameters for the platform foundation design.