Structural vibrations induced by pile driving

Structural vibrations induced by pile driving

Zwartveld, J.

Metrikine, A.V. (mentor)
Tsouvalas, A. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

Offshore Engineering

2016-01-11

For decades, sub-sea pile driving has been performed to safely attach offshore and sub-sea infrastructure such as jackets, templates and manifolds to the supporting soil. In sub-sea pile driving, a hammer is lowered from an installation vessel and similarly descended until the (generally steel tubular) pile reaches his final penetration depth. Multiple types of driving mechanisms are used, where the mechanical impact of a heavy ram is the most common. The ram is lifted within the casing of a hammer and subsequently accelerated until it hits an anvil. This anvil spreads kinetic energy of the ram around the circumference of the pile, where the energy progresses as stress waves. Energy is radiated away from the pile in acoustic pressure and Scholte waves, elastic waves in water and soil, plastic deformation in the soil and heat. Each blow causes transverse vibrations of the pile, both due to radial expansion of longitudinal stress waves and due to misalignment between hammer and pile. If the pile is in contact with a structure such as a conductor template, it exchanges energy with this structure. Heerema Marine Contractors (HMC) conducted vibration measurements during the installation of the Britannia template, where peak accelerations rapidly increased as the pile reached its final penetration. Values up to 300 m/s2 were measured. More recently, similar conductor templates needed to be installed. Static design loads were based on these peak accelerations and the mass of the pile sleeve, which most likely causes over-designing of the templates. The aim of this thesis is to give more insight in the load transfer between pile and conductor template. Pile driving during the installation of the Britannia template is modelled, where it is attempted to validate the model with the measurements. The model describes both the pile and the template with one-dimensional vibration theories. It proved not possible to validate the model for two reasons. Insufficient quality and amount of data made it impossible to validate both sub-models independent of each other. And second; one-dimensional theories do not suffice to describe local accelerations in thick plate girders. In reality, the plate girder act as a waveguide where local variations in the motions along its height might be large. The measurement data show a large increase in the local peak acceleration, which does not necessarily imply a large acceleration of the whole cross section at once and it does not imply a large change in velocity. A follower is generally placed on top of the pile, to drive the pile up to its final penetration without causing conflict with the construction. It is likely that the presence of this follower increases the radial stiffness of the pile. A range of radial stiffness values was used to model the interaction between pile and template. The analysis showed that an increase in radial stiffness causes a large increase in the transferred force. It is therefore plausible that this is the mechanism that causes a rapid increase in accelerations as the pile reaches its final penetration.

http://resolver.tudelft.nl/uuid:782ecc4b-3d55-40ab-bb14-120a115921f3

2020-01-10

Student theses

master thesis

(c) 2016 Zwartveld, J.