Pablo Cuéllar

Pore-Pressure Accumulation and Soil Softening Around Pile Foundations for Offshore Wind Turbines

The foundation of offshore wind turbines usually involves the installation of large-diameter steel piles in the seabed, either in monopile or multi-pile configurations (jacket, tripod, etc…), which have to ensure a proper fixity of the turbine during its whole service life-time.However, such foundations raise several challenges and novel questions, partly due to the special characteristics of the offshore environment (for instance, the large numbers of load cycles from wind and waves and the possible influence of transient changes of pore water pressure around the pile) and aggravated by their large diameter, reduced slenderness and elevated ratio of lateral to vertical loads (see Fig. 1).This paper studies the effects of cyclic lateral loading on the offshore piles focusing on the possibility of a progressive accumulation of residual pore water pressure within the saturated embedding soil. As it will be shown, this can lead to significant changes of their behaviour under external loading, which can potentially compromise the foundation’s stability or serviceability.The paper will also analyse some singular effects of an irregular loading (e.g. cyclic loading with variable amplitude), in particular the so-called “order effects” and the phenomena arising during a realistic storm of moderate magnitude, and discuss their potential for transient damages to the foundation’s stiffness.All these phenomena, which can lead to a loss of serviceability of the structure, have been investigated by the authors by means of a coupled bi-phasic analytical model of the offshore foundation featuring a versatile constitutive law suitable for the soil. The constitutive model, in the frame of the theory of Generalized Plasticity, can reproduce some complex features of cyclic soil behaviour such as the tendency for a progressive densification under cyclic loading, which is responsible for the soil liquefaction phenomena in undrained conditions.Finally, some implications of these issues for the practical design of offshore monopiles will be discussed and some specific recommendations for the design procedures will be outlined.Copyright

Uncharted Territory on the Seabed

If the foundations for wind farms are oversized they cause great expense; if they are undersized they endanger the structure. One of the focuses of the RAVE project Monitoring Procedures and Assessment Model for the Foundations of Offshore Wind Turbines was a new type of foundation, the Tripile, used in the BARD Offshore 1 (80 × 5 MW) wind farm. Like a tripod, three piles are driven into the seabed, but rise several metres above the surface of the water, where they are connected with one another by a square transition piece. The aim was to do everything of significance above water. In this project a new procedure was used to determine the pile load displacement curves from the turbine’s operating data. Trials with saturated ground carried out on land showed the pile reaction: on the one hand the load-bearing capacity of the piles increased with standing time, but on the other hand it was reduced by the cyclic loads.

Please Avoid Tilting

Everything rests on the pile. Over half the planned offshore wind farms are to have monopile foundations, while over 40% will have multiple piles. The RAVE project Application-Oriented Design and Monitoring Model for Offshore Foundation Constructions investigated foundations, carried out model trials and equipped a wind turbine in Alpha Ventus with sensors. The more wind that pressed onto the turbines, the more load was experienced by its pile in the seabed. The pore water pressure – the tension in water-filled pores in the seabed – can influence the load-bearing behaviour there. After an extreme storm the foundation is most vulnerable when greater pore water pressure loosens the seabed. After a while it hardens again. Larger pile diameters ensure better stress distribution. The task remains to consider real impact scenarios for the foundation, which experiences constantly changing load directions and time sequences, from the brief peak to the long-term impact.

„Neuland“ auf dem Meeresboden

Sind Fundamente fur Offshore-Windparks uberdimensioniert, verursachen sie zu grose Kosten; sind sie unterdimensioniert, gefahrden sie das Bauwerk. Im RAVE-Projekt „Uberwachungsverfahren und Bewertungsmodell fur die Grundungen von Offshore Windkraftanlagen“ stand unter anderem auch ein neuer Grundungstyp im Mittelpunkt: Das Tripile, verwendet im Windpark Bard Offshore 1 (80x 5 MW). Wie beim Tripod werden hier drei Pfahle in den Meeresboden gerammt, ragen aber noch einige Meter uber die Wasseroberflache hinaus und werden uber ein Stutzkreuz miteinander verbunden. Ziel: „Wir machen alles Wichtige uber Wasser“. Im Projekt ermittelt ein neues Verfahren die Pfahl-Last-Verschiebungskurven aus den Betriebsdaten der Anlage. Versuche mit wassergesattigtem Boden an Land zeigen die Pfahlreaktion: Die Tragfahigkeit der Pfahle nimmt einerseits mit ihrer Standzeit zu, andererseits wird sie durch die zyklischen Lasten reduziert.

Schieflagen bitte vermeiden

Am Pfahl hangt alles. Uber die Halfte der geplanten Offshore-Windparks sind Ein-Pfahl-Grundungen, uber 40 % haben mehrere Pfahle. Das RAVE-Projekt „Anwendungsorientiertes Bemessungs- und Uberwachungsmodell fur Offshore-Grundungskonstruktionen“ erforschte Grundlagen, fuhrte Modellversuche durch und rustete eine Windenergieanlage in alpha ventus mit Sensorik aus. Je mehr Wind auf die Anlage „druckt“, umso mehr wird ihr Pfahl im Meeresboden belastet. Der Porenwasserdruck – die Spannung in wassergefullten Poren des Meeresbodens – kann dort das Tragverhalten beeinflussen. Nach einem extremen Sturm ist das Fundament am ehesten gefahrdet, wenn starker Porenwasserdruck den Boden auflockert. Nach einiger Zeit verfestigt er sich wieder. Grosere Pfahldurchmesser sorgen fur eine bessere Spannungsverteilung. Aufgabe bleibt, reale Einwirkungsszenarios fur die Grundung, die standig wechselnde Lastrichtungen und Zeitablaufe, vom kurzen „Peak“ bis zur Langzeitwirkung erfahrt, zu berucksichtigen.

A method for system identification of a structure supported by nonlinear springs using evolutionary computing

A mechanical structure supported by nonlinear springs subjected to an external load is considered. If all mechanical parameters of the system were known, the displacement of the system subjected to this load could be easily calculated. If not all of the parameters are known, but the load and the displacement are measured at one location, an inverse problem exists. In the presented problem the nonlinear springs are unknown and have to be determined. At first glance a problem needs to be solved, which is underdetermined due to the number of unknown variables. However, evolutionary computing can be applied to solve this inverse, nonlinear and multimodal problem. Sometimes a prior knowledge exists on certain system properties, which is difficult to implement into analytical or numerical solver. This knowledge can play a decisive role in identifying the system properties and it can be easily included as boundary condition when applying evolutionary algorithm. This article examines how and under what conditions the spring resistances can be identified. The procedure is exemplified at a mechanical system of a pile foundation.