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NS-VIP project -- Description

Introduction

To limit the environmental effects of offshore pile installation regulations exist to define acceptable underwater acoustic emission levels. These regulations impose limitations to the installation of monopiles for offshore wind farms in Europe. In Germany the Bundesamt für Seeschifffahrt und Hydrographie (BSH) insists on a maximum sound pressure of 160 dB at 750 metres from the installation. Anti-noise legislation in the Netherlands prohibits pile driving from January to July and restricts the building of offshore wind parks to only one per year. On the other hand, the Netherlands have the ambitious target to increase the offshore wind capacity (for example the “Energieakkoord” targets a 4450 MW capacity increase in the Dutch offshore waters until 2023). Countries surrounding the Netherlands have similar ambitions.

An alternative technique of installing monopiles which is not bound by above mentioned sound restrictions is vibrating. However, up to now the certifying bodies require impact driving at least over the last part of the installation due to the uncertainty about the capacity of vibratory installed monopiles. To close the knowledge gap with respect to vibratory driving, a consortium of energy utilities referred to as the VIBRO Consortium consisting of energy utilities (RWE, Dong Energy, Vattenfall, E.ON, EnBW), contractors (Bilfinger, IHC, PVE), consultants (GEO engineering, ARUP), monopile manufacturers (Steelwind Nordenham), authorities (BSH, BAM), certifying bodies (DNV-GL, TÜV-SÜD, TÜV-Rheinland) and universities (TUHH, TU Braunschweig NGI, Universität Stuttgart, BW Universität München) has initiated a research project to investigate the feasibility of using full depth vibratory installation of monopiles for offshore wind turbines instead of impact driven installation methods. This research project, called VIBRO, is part of the Offshore Wind Accelerator and is funded by The Carbon Trust (UK).

The main objective of the above mentioned research consortium is “to prove that the design methods applied for laterally loaded standard driven piles can equally be applied to full depth vibratory driven piles.” Therefore, the consortium is performing field tests in summer 2014 comparing the behaviour of both types of installation methods in particular in lateral capacity and stiffness (eigenfrequency). The testing goal is “to demonstrate that the lateral load bearing capacity of vibrated and hammered piles in sand is to be regarded as equivalent.”

In the field test campaign six monopiles with a diameter of 4.3 m were installed in an onshore sand pit in Cuxhaven (see Figure 1); three of which were impact driven and three were vibrated (see Figure 2). Cone penetration tests (CPT) were performed before and after installation to assess the installation effects. The soil profile at the test site at Cuxhaven consists of medium dense sand, with silty inclusions overlaying a dense sand layer. This site was selected because the soil profile should be representative for a wide range of typical offshore soil conditions to be found in the North Sea and further afield. After consolidation time lateral load tests will be performed to directly compare the bearing capacity of the three pile pairs.


Figure 1: Artist impression of test site in Cuxhaven. ©VIBRO-project by RWE


Figure 2: PVE 500M vibratory hammer (left); IHC S-1200 impact hammer (right). ©VIBRO-project by RWE

Current knowledge on the differences between vibrated and impact-driven piles is limited to the axial capacity. It is observed that the ratio between the capacity of vibratory piles versus impact driven piles is less than 1.0 for dense sands and larger than 1.0 for loose sands. Based on this experience it can be concluded that the initial density plays an important role at least for the axial capacity ratio. For lateral capacity and lateral system stiffness this dependency is unknown but will be addressed. Therefore the question arises “How to generalize results of the VIBRO project to sites with other initial densities?”

The current state-of-the-art installation methodology for foundation piles for wind turbines is for the piles to be vibrated to a maximum depth of half of the desired installation penetration depth and then hammered further to full depth. The hammering is currently precautionary as there is little documented experience of the load bearing capacity of foundation piles vibrated to full depth. The view of the certification bodies and consequently the national authorities has been not to permit full depth vibratory installation of wind turbine foundation piles until more knowledge is gathered through full/near full scale testing. In order to demonstrate that foundation piles are suitable for wind turbine loading it is also necessary to collect information on the lateral load bearing capacity of a pile vibrated to full installation depth. The certification bodies have been consulted extensively in the VIBRO project in their role independent review panel and their test requirements have been fully incorporated into the design of the VIBRO measurement campaign.

In the VIBRO project this translation is foreseen to be done by comparing CPT’s before and after installation. It is expected that the difference in installation effects in layers with different initial cone resistances can be assessed with these CPT’s at increasing distance. This is a very useful approach especially for practical applications and codes of practice. However, more insight is required into the effects of the installation methods in sands with different initial densities and the underlying processes.

To generate generally applicable insight numerical modelling techniques can be employed. In the past the finite element method (FEM) has been used successfully to model geotechnical problems. However, limitations exist with respect to the modelling of the occurring large deformations during the installation phase. Therefore, an advanced modelling technique, the so-called material point method (MPM) has been developed to overcome the limitations of FEM. With this method the full installation process of both, impact and vibratory driven piles, can be modelled, being followed by the lateral load test. During the NS-VIP project this method are further developed and applied to the site specific conditions in Cuxhaven to prove its capability of predicting the pile capacity. Afterwards for different soil conditions simulations are performed to generalise the VIBRO project results to (offshore) sites with different initial densities and different pile diameters. A key deliverable of the NS-VIP project is a database which specifies the resulting lateral capacity depending on the pile diameter, installation method and initial soil density.

Project aim and objectives

The main objective of the VIBRO-project in Cuxhaven is:

  • To prove that the design methods applied for laterally loaded standard driven piles can equally be applied to full depth vibratory driven piles.

The current NS-VIP project has the following main objective:

  • To generalize the test results of the VIBRO-project in Cuxhaven with impact and vibratory driven piles to (offshore) sites with different initial densities and significantly larger pile diameters.

Sub-objectives to achieve this are as follows:

  • Deploy a numerical modelling technique (material point method or MPM) which is capable to model the installation process of both, impact and vibratory driven piles, with subsequent lateral load test to determine the lateral capacity.
  • Apply MPM to the site specific conditions in Cuxhaven to validate the method.
  • Use MPM for different initial soil conditions to translate VIBRO-results to other sites.