Kerstin Lesny

Finite-Element-Modelling of Large Diameter Monopiles for Offshore Wind Energy Converters

Monopiles for offshore wind energy converters are highly laterally loaded structures with diameters up to 6 m and more. Their design is based on the well-known p-y-method, which has been calibrated on field test results for much smaller pile diameters. The present paper provides an FE-analysis of the monopile behavior compared to the standard design method. The results indicate the influence of the pile diameter on the pile-soil stiffness and point out that the p-y-method overestimates the soil stiffness at great depth. A modification of the p-y-method is proposed which better accounts for the pile-soil interaction of large diameter monopiles.

LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures

This report develops and calibrates procedures and modifies the AASHTO LRFD Bridge Design Specifications, Section 10 - Foundations for the Strength Limit State Design of Shallow Foundations. The material in this report will be of immediate interest to bridge engineers and geotechnical engineers involved in the design of shallow foundations.

SCALE EFFECTS IN LATERAL LOAD RESPONSE OF LARGE DIAMETER MONOPILES

Monopile foundations are frequently used for offshore wind energy converters. These piles are highly laterally loaded structures with large horizontal forces and bending moments. Due to the harsh environmental conditions in the southern North Sea diameters of 4 to 8 m are required to maintain serviceability. In common practice smaller laterally loaded pipe piles are designed using the well-known p-y-method, in which the pile-soil stiffness is considered by nonlinear p-y-curves derived from field tests. An alternative design method is the strain wedge method in which the pile response is derived from the stress-strain relationship of the soil assuming a certain failure zone ahead of the pile. In the present paper, the design of a large diameter monopile foundation for typical loading conditions is presented. The pile response in cohesionless soil determined by the p-y method and the strain wedge method is compared with a finite element (FE) analysis with respect to scale effects when extrapolated from commonly used pipe pile diameters to large size monopiles.

Uncertainties in the Bearing Capacity of Shallow Foundations and the Factor N y Using an Extensive Database

A comprehensive database with 549 cases of load tests on shallow foundations, mostly in/on granular soils, was compiled as a part of two research projects (NCHRP 12-66 and NCHRP 24-31) carried out to develop the AASHTO (American Association of State Highway and Transportation Officials) Specifications for Highway Bridges. Based on this database, the uncertainties in the ultimate bearing capacity of footings in/on granular soils and under centric vertical loading were appraised. The proposal by Vesi (1963) was used for the interpretation of failure loads from load tests (measured capacity), while the equations given by Vesi (1975) was used to estimate the theoretical bearing capacities (BC) (calculated capacity). For the footings in soils with controlled compaction and with known particle sizes, it was found that Vesic (1975) gives conservative estimates. Further, the model uncertainty of the BC equation in terms of Nγ was examined for granular soils with friction angles between 42° and 46°, using the back-calculated values from 125 load tests carried out to failure for footings on granular soil surfaces and under vertical centric loadings. ABSTRACT: A comprehensive database with 549 cases of load tests on shallow foundations, mostly in/on granular soils, was compiled as a part of two research projects (NCHRP 12-66 and NCHRP 24-31) carried out to develop the AASHTO (American Association of State Highway and Transportation Officials) Specifications for Highway Bridges. Based on this database, the uncertainties in the ultimate bearing capacity of footings in/on granular soils and under centric vertical loading were appraised. The proposal by Vesi (1963) was used for the interpretation of failure loads from load tests (measured capacity), while the equations given by Vesi (1975) was used to estimate the theoretical bearing capacities (BC) (calculated capacity). For the footings in soils with controlled compaction and with known particle sizes, it was found that Vesic (1975) gives conservative estimates. Further, the model uncertainty of the BC equation in terms of Nγ was examined for granular soils with friction angles between 42° and 46°, using the back-calculated values from 125 load tests carried out to failure for footings on granular soil surfaces and under vertical centric loadings.

The Role of Favourable and Unfavourable Actions in the Design of Shallow Foundations according to Eurocode 7

For the design of shallow foundations under combined loading Eurocode 7 provides different partial safety factors for unfavourable and favourable actions and for different ultimate limit states. Several load combinations need to be checked to find the most critical combination of loading and limit state which governs design. Hence, the resulting safety is often not apparent. This paper illustrates the role of actions within the design of shallow foundations and points out the effects on the safety of the system.

Safety of shallow foundations – Limit State Design according to Eurocode 7 vs. alternative design concepts

The main objective of Eurocode 7 was to harmonise geotechnical design in Europe by consistently implementing limit state design. However, Eurocode 7 only provides a framework in which each member state defines the design approach as well as the partial factors according to national design and safety requirements. The design approaches differ in the way partial factors are applied and lead to remarkably different designs especially in the case of shallow foundations under complex loading. Hence, the actual safety of the foundation cannot be reliably determined. In the present paper the correlations between loads and resistances of shallow foundations within Eurocode 7 design are analysed by use of interaction diagrams. The effects of the design approaches on the overall safety are discussed. An alternative design method based on an unique failure criterion for the ultimate limit state is introduced, the implementation of the partial factor concept is discussed.

Design, construction and installation of support structures for offshore wind energy systems

Abstract: This chapter discusses design, construction and installation of support structures for offshore wind turbines. Structures fixed to the seabed are an appropriate choice for moderate water depths. However, with offshore wind energy expecting to move further offshore, flexible or floating structures will be more feasible than fixed ones. The design of the support structures is particularly influenced by the load characteristics resulting from the environmental impact, the variation of the site conditions within the wind farm and the available construction and installation facilities. The support structure is to be designed to withstand not only the controlling extreme event, but also the effect of continuous cyclic loading on its operating behaviour.

Offshore Wind Energy in Germany Compared to Other European Areas — Challenges and Recent Developments

Utilization of offshore wind energy in Europe is well in progress. A total capacity of about 1000 MW has already been installed. Eight wind farms with about 1300 MW are under construction and many more are planned. The existing wind farms are located relatively close to the shore in moderate water depths of up to 20 m. In contrast to this the site conditions in the planned wind farms in the German North and Baltic Sea are more demanding as these wind farms have to be built far off the shore. The present paper shows the current status of offshore wind energy in Europe and recent developments in Germany. Site conditions at various wind farm locations in the German North and Baltic Sea are illustrated and compared with the conditions in existing wind farms. Resulting consequences for foundation design and installation are discussed.