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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.