Simulating monopile installation through scour protection using the Discrete Element Method

Abstract

Over 5000 offshore wind turbine generators have been installed in the past decades. 80% of these wind turbines have a monopile foundation. Monopiles are hollow steel tubular piles with a diameter of up to 10m. The first phase of the monopile installation consists of the self-weight penetration into the seabed due to the mass of the monopile. Followed by the placement of the hammer, which installs the piles to the designated depth. A scour protection is deposited near the piles to avoid erosion (scour holes) around the monopiles. The scour protection consists of rock material with different size gradations and densities. Generally, the scour protection is installed in two campaigns. The first campaign consists of the installation of smaller rocks (filter layer), followed by the installation of bigger rocks (armour layer). Between these two campaigns, the monopile is installed through the filter layer. To achieve a more time-efficient installation sequence, in some projects a full scour protection (single layer) is deposited prior to the installation of the monopile. However, the technical feasibility of monopile installation through a scour protection has not been researched. Within practice, difficulties arise during the installation of monopiles through a single layer of scour protection. This leads to timely and costly delays.
This thesis contributes by use of simulations, to study the technical feasibility of monopile installation through scour protection. With a focus on the scour protection behavior in the first phase of the installation, where the monopile penetrates the seabed due to its mass, so-called self-weight penetration.
During monopile self-weight penetration, physical processes within the subsoil, scour protection, and monopile wall create penetration resistance. The most important physical processes during monopile self-weight penetration are identified using literature:
• The development of tip resistance due to the scour protection layer.
• The development of shaft resistance due to the scour protection layer.
• The self-weight penetration due to the weight and dimensions of the monopile.
• The interaction between the monopile wall and the scour protection rock and inter-rock interaction (interlocking, rotation, breaking, and protrusion of the subsoil).
The Discrete Element Method (DEM) is considered most suitable for the representation of the scour protection behavior and is an effective method for addressing engineering problems in granular materials. The most important reason for using DEM is due to the discontinuous behavior of the scour protection material.
The Discrete Element Method (DEM) is considered most suitable for the representation of the scour protection behavior. DEM is an effective method for addressing engineering problems in granular materials. The most important reason for using DEM is due to the discontinuous behavior the scour protection materials.
To set up the simulation, calibrated DEM input parameter sets are required to recreate reliable bulk behavior. These parameter input sets are found with the use of two Key Performance Indicators (KPIs) of the bulk material: the porosity and the angle of repose. These KPIs are included because they represent the inter particle frictional resistance to movement. With the calibrated input sets combined with data from a case study, the Full-Scale Monopile Simulation is set up. Consistent with the case study, the simulation is designed using a monopile with a tip diameter of 8m and a wall thickness of 0.08m. The seabed consists of a sandy subsoil and a scour protection on top.
To achieve a realistic stable simulation with a reasonable computational time, experiments are executed. Within an experimental design, a reference simulation is developed, followed by a parameter sensitivity analysis. The most important conclusions of the parameter sensitivity analysis are:
• When using an 8m diameter monopile, a minimum simulation domain of 12*12m2 is recommended. Resulting in a minimum of 2m space around the monopile.
• A subsoil is required for successful penetration through the scour protection layer.
• The subsoil input parameters are of significant influence on the penetration behavior of the monopile, due to the rock penetration into the subsoil.
• The scour protection thickness can be included as a variable parameter.
To verify whether the reference simulation properly represents practice, a qualitative validation is executed using the case study. Successful penetration is achieved within both the reference simulation and the case study. However, the current simulation does not present a comparable depth over time correlation. An underestimation of the penetration resistance is observed. Increasing the rolling resistance and therefore limiting the angular velocity of the particles results in higher penetration resistance. Restricting the angular velocity of the scour protection completely, results in an unsuccessful penetration.
Further research should focus on representative penetration resistance of the scour protection, combined with an alternative and more representative inclusion of the subsoil. Furthermore, the validation should be elaborated using adequate data from practice.