Monitoring Suction Bucket Jacket

Description: 

The research project "Monitoring SBJ" is a collaborative project between the project partners DONG Energy Wind Power, Leibniz Universität Hannover (LUH) and Bundesanstalt für Materialforschung und -prüfung (BAM, Federal Institute of Materials Research and Testing). The Bundesamt für Seeschifffahrt und Hydrographie (BSH, Federal Maritime and Hydrographic Agency) is involved in the project as an associated partner.

Motivation

The central environmental policy goal of the German government is to increase the share of renewable energies to around 80 % of gross electricity consumption by 2050. An important contribution to this is the reduction of energy production costs by developing more cost-efficient foundation and support structures and deploying them in a timely manner.

State of the art

Germany is currently still in the early stages of extensive construction activities in the German North and Baltic Seas. It can therefore increasingly be assumed that several construction projects will have to be carried out in parallel in order to accelerate the expansion of offshore wind energy and thus achieve the ambitious targets. The state of the art for anchoring offshore wind turbines to the seabed is still the driven pile foundation. The installation process is associated with considerable hydro noise emissions, which generally significantly exceed the limit values for noise immissions of 160 dB re 1 μ Pa for the single event sound pressure level and 190 dB re 1 μ Pa for the peak level at a distance of 750 m, unless noise reduction measures are used. As an alternative to noise reduction methods, but largely still at the development and testing stage, foundation variants that can be constructed without impulsive pile-driving noise are currently the subject of intensive research. In this context, resolved foundation constructions such as the jacket in combination with suction buckets, which only have a minimal impact on the marine environment compared to gravity foundations, are particularly promising solutions.

In recent years, various monopod foundations have been installed for met masts (e.g. for OWP Horns Rev 2 or Doggerbank). A prototype monopod foundation supporting a 3 MW turbine has been in operation in Frederikshavn (Denmark) for more than 10 years. A particular challenge with a monopod is the vertical installation during construction. Systems with separate chambers were developed to avoid misalignment. Complex alignment techniques are also used. Another critical point is the large dimensions required, which can cause stability failures during the installation process and drive up material costs. Compared to monopods, multipods - foundations that use three or four buckets as standard - have the following advantages:

• Misalignments during the installation process can be easily avoided by changing the negative pressure of the individual buckets,
• the individual buckets have smaller dimensions and are therefore less at risk of instability,
• the installation process is accelerated.

To date, there is a lack of practical experience with such foundation constructions under the typical load situation for offshore wind turbines (high horizontal and moment loads with relatively low static vertical loads, as well as high cyclical loads with recurring tensile load peaks). As a result, there are neither standardised calculation methods nor verification concepts. There is also a lack of validated structural models that take into account the non-linear soil-structure interaction, which is also dependent on the load history, and with which the load-bearing behaviour of subsequent cyclic and dynamic loads can be calculated.

The Suction Bucket Jacket (SBJ):

DONG Energy Wind Power has developed an innovative design that combines an optimised, industrially manufactured jacket structure with suction buckets (hereinafter referred to as "SBJ" for short). The foundation structure is designed for the 5 to 8 MW class and water depths of up to 60 metres.

The SBJ was installed in the OWP Borkum Riffgrund 1 in the German North Sea in August 2014 (video). The Siemens tower and turbine with an output of 4 MW were installed in January 2015. The SBJ has been equipped with extensive metrological instrumentation. Loads and deformations of the foundation structure are determined on the jacket structure using wave radar and strain, inclination and acceleration sensors. Inclination and acceleration sensors as well as pore water pressure sensors were positioned at different depths on the individual buckets. In conjunction with strain measurements in the connection area to the jacket structure, it is possible to observe loads and the associated load-bearing behaviour of the individual suction buckets in detail. Acceleration sensors have also been installed along the tower in order to observe the dynamic load-bearing behaviour of the entire structure.

The aims of "Monitoring SBJ":

As part of the project, both the installation and the load-bearing behaviour of the foundation and overall structure are to be intensively monitored using measurement technology. The measurement results will be used to validate the calculation methods developed in the project and the structural model. With regard to the installation process, it can be assumed that the above-mentioned limit values for noise emissions will be adhered to, as high-energy, impulse-type noise is avoided. However, this premise is also to be reviewed as part of the project.

Objectives of the overall project

One of the main reasons for the project is to validate the currently available calculation methods for the design of suction bucket foundations in order to improve the verification procedures. There are still many uncertainties regarding the design parameters, which have to be compensated for by very conservative assumptions. The load-bearing behaviour under cyclic loading during operation and under the extreme load in the design storm has not been sufficiently researched. There is a great need for research into the tensile load-bearing capacity in particular, which can best be covered by measurements on a 1:1 scale prototype.

One of the aims of the project is to develop relatively complex numerical simulation methods and validate them by means of measurements in order to clarify and better understand the basic load-bearing behaviour of suction bucket jackets. Another aim is to use the findings to develop tools for pre-dimensioning and the simplest possible methods for dimensioning suction buckets in practical design. For the overarching goal of paving the way for the practical application of suction bucket jackets as an innovative and environmentally friendly foundation, such methods that can be used in practice are of great importance. At present, WTGs on SBJ foundations are not yet certifiable and authorisation can only be granted if the observation method is used. The expected eligibility for authorisation, certification and financing after the end of the project essentially depends on whether proof of in-situ installability has been provided and whether practice-oriented validated models and calculation approaches for the dimensioning of suction buckets exist in the medium term. The required models and calculation approaches are being developed in ‘Monitoring SBJ’. The extent to which these can be validated using measurements on the prototype depends on whether suitable measurement events occur. Only on the basis of validated calculation methods and models can it be expected that significant progress in cost reduction will be achieved in the long term through design optimisation. 

Scientific and technical work objectives

Both individual extreme loads and cyclical loads influence the compression and tension behaviour of the foundation. Cyclic loads lead to an accumulation of excess pore water pressures due to the alternating shear strains. This mechanism is counteracted by the fact that the excess pore water pressures are reduced by time-dependent dissipation. The load-bearing capacity of the subsoil therefore depends to a large extent on the balance between these two effects. Existing design methods attempt to estimate these effects with sufficient accuracy, but there is no verification of these estimates by means of pore water pressure measurements on a 1:1 scale prototype.

Further uncertainty exists with regard to the tensile load-bearing capacity of suction buckets. In the case of lightweight resolved support structures for WTGs, a temporary or even permanent tensile load on one of the foundation bodies cannot be ruled out. Suction buckets offer good resistance to short-term tensile loads due to the resulting negative pressure inside the bucket, but this is reduced again by dissipation under longer loads. The exact reaction of the foundation to long-term tensile load is not known, so it is particularly important to measure the negative pressure under the cover.

The prediction of deformation accumulation due to cyclic loads (and also individual extreme loads) is very difficult. It is unlikely that an accurate prediction of deformations can be made using small-scale modelling tests or FE models calibrated to these tests. In situ tests with real load situations make it possible to perform the serviceability and fatigue analyses on the basis of the measured deformations.

Efficient numerical structural models with real coupling stiffnesses, taking into account the soil-structure interaction, are required to verify stability and identify critical areas with localised stress peaks. The system stiffnesses and load-displacement reactions are of great interest both for stability and fatigue checks and for assessing the dynamic behaviour. For which load situations and under which assumptions foundation stiffnesses of suction buckets are to be calculated is a question that has not yet been conclusively clarified. The measurement of time-dependent deformations, displacements, forces and excess pore water pressures is therefore an essential aspect in order to improve the calculation methods and make more accurate predictions.

Literature

Achmus et al. (2017): „Verbundprojekt: Monitoring Suction Bucket Jacket - Monitoring SBJ : Schlussbericht zum Teilvorhaben der Gottfried Wilhelm Leibniz Universität Hannover“, Institut für Geotechnik, Leibniz Universität Hannover. (DOI: https://doi.org/10.2314/GBV:1011240475)