Pile Driving Analysis
Pile driving analysis is a tool used for supporting conductor or monopile installations, either for oil & gas wells or wind turbine foundations. The analysis determines a piles suitability to be driven to a pre-determined depth below mudline (BML). The required depth BML is calculated beforehand and is based on the soil’s axial load carrying capacity, the maximum axial load expected on the conductor during its lifetime, and the conductor’s structural parameters (diameter, thickness and material grade). A hammer type is also pre-selected or assumed. For example, the IHC S-90 hydrohammer is most often used in the North Sea.
The assessment criteria include the following.
1. The allowable compressive stress of both the conductor pipe and its connectors
2. The allowable fatigue damage accumulation as a result of driving.
The drivability assessment is performed using industry standard software, GRLWEAP. It simulates the positions and forces in the conductor, during driving, using the wave equation approach.
The software computes:
- The blow count of the conductor under the soil’s ultimate axial resistance for the given hammer;
- The axial stress along the conductor;
- The energy transferred from the hammer to the conductor; and
- The pile velocity and displacements.
First of all, the software calculates the long term static resistance (LTSR), or ultimate axial capacity, of the soil as a function of depth. This calculation follows a chosen industry recommended guideline or code.
The LTSR is equal to the shaft friction on the outer conductor surface plus the end bearing capacity. To account for soil disturbance and changes to the soil’s properties during driving, the LTSR is factored down - by how much, depends on soil type. Empirical factors, suggested by the software, are used since they are based on a wealth of accumulated data over the history of the software development.
The program’s wave equation then calculates the blow count at specified depths. Details of the wave equation approach for pile driving can be found in published literature.
AS Mosley has the capability to produce charts of axial capacity vs depth and blow count vs depth. The former chart shows the depth at which the required axial capacity will be achieved whilst the latter shows the corresponding blow count per meter of soil penetration. Industry practice suggests that the conductor should be driven until refusal. The refusal limit is set to protect both hammer and pile. Refusal limits as high as 984bpm (300bpf) are mentioned in standard offshore guidance publications such as the American Petroleum Institute, Ref: API RP 2A. Industry guidance is usually around 787bpm (240bpf).
This analysis helps operators understand refusal limits. Understanding the refusal limit will help to determine whether the conductor can be driven/installed to the desired depth. If not, further adjustments can be made to the hammer specification/procedure until the design is optimised.
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