۱- Introduction

۲- Earthquake codes and guidelines

۳- Modelling for earthquake loads

۴- Summary and conclusions

References

Abstract

Interest in renewable and clean energy over the past decade has motivated immense research on wind energy. The main issues in design of offshore wind turbines in regions of recent development have been aero- and hydro-dynamic loads; however, earthquake is a design concern in seismic areas such as East Asia and Western United states. This paper reviews the state of practice in seismic design of offshore wind turbines. It is demonstrated that wind turbines are in particular vulnerable to vertical earthquake excitation due to their rather high natural frequencies in vertical direction; however, inclusion of the radiation damping could contribute considerably reduce the earthquake loads. Moreover, it is demonstrated how soil nonlinearity could lead to settlement and permanent tilting of offshore wind turbines on caisson foundations or tripods. Using these cases, the paper demonstrates that the design of offshore wind turbines for earthquake loading is driven by performance-based considerations.

Introduction

For much of the twentieth century there was little interest in using wind energy for generation of electricity. One notable development was the 1250 kW Smith-Putnam wind turbine constructed in the USA in 1941. Shepherd [1] and Divone [2] have presented the history of early wind turbine development. Their reviews include, among others, the 100 kW 30 m diameter Balaclava wind turbine in Russia in 1931, the Andrea Enfield 100 kW 24 m diameter pneumatic generator designed and constructed in the UK in the early 1950s, the 200 kW 24 m diameter Gedser machine in Denmark in 1956, and the 1.1 MW 35 m diameter turbine tested by Electricite de France in 1963. Despite some technical advances and other initiatives, there was no systematic interest in wind generation until the oil crisis in the early 1970s. The dramatic increase in the oil price motivated considerable government-funded research programs. For example, in the USA, the research led to construction of a series of prototype turbines, starting with the 38 m diameter 100 kW Mod-0 in 1975 and evolving to 97.5 m diameter 2.5 MW Mod-5B in 1987 [3]. Similar research programs were initiated in the UK, Germany and Sweden. Large turbines were constructed with two and three blades until the Danish wind turbine concept emerged of a three-bladed, upwind rotor (i.e. machines that have the rotor facing the wind) and a fixed-speed drivetrain (gearbox and generator). This design proved to be successful and was implemented on turbines as large as 60 m in diameter and at ratings of up to 1.5 MW [3]. Wind quality is better at sea than on land. The smoother surface at sea compared to land results in stronger and less turbulent wind that ensures a greater and more reliable power production. It also allows for the use of larger turbines and lower elevation above ground/water level. Moreover, areas with good wind quality for energy production are limited on land and are often far from big cities. From an environmental viewpoint, placing the turbines offshore makes them less visible and causes less noise for the public. In 1991 the first offshore wind farm was constructed at Vindeby, Denmark, consisting of eleven, 450 kW wind turbines located up to 3 km offshore. Throughout the 1990s small numbers of offshore wind turbines (OWT) were placed close to shore, until in 2002 the Horns Rev, 160 MW wind farm, about 20 km off the western coast of Denmark, was constructed. This was the first project to use an offshore substation that increased the power collection voltage of 30–۱۵۰ kV for transmission to shore. OWTs have kept increasing in size over the years. The largest OWT at the time of this publication is 8 MW, is 220 m high, has a rotor blade length of 80 m and weighs 5900 t. A wind farm, consisting of 32 of these turbines, has recently been developed at the Burbo Bank Extension offshore wind farm in Liverpool Bay, off the west coast of England.