Dynamic Stability of Suction Caisson Founded Offshore Wind Turbines
Authors: Cox James Alexander , Bhattacharya Subhamoy 
Published: April 2012

In recent years there has been a massive expansion in the offshore windengineering sector with millions of pounds being invested. Most current windturbine sites are founded on monopiles. For future developments sited in deeperwater further from shore, existing monopile technology will be insufficient. Tothis end alternative foundations have been proposed, one such arrangement isthe suction caisson.

This paper describes a series of 1:100 scale 1-g model tests conducted on asuction caisson supported Offshore Wind Turbine (OWT) founded in sand, with theaim of replicating the dynamic-soil-structure interaction effects likely to beseen in a real system. By applying a dynamically representative loading cyclethe structures dynamic response was measured by an array of accelerometers. Particular reference was paid to the First Modal Frequency (FMF) of the systemto see if it altered as a result of cycle dependent soil stiffnesschange.

From the series of tests conducted it was observed that the FMF of a suctioncaisson founded OWT has the potential to change under cyclic loading. Initiallyan increase in the natural frequency was observed. As other dynamic factors arekept constant this change corresponds to an increase in soil stiffness and adensification of the near filed sand. This apparent change in the FMF followsan approximately logarithmic profile. Under longer-term loading, the naturalfrequency appears to stabalise this behaviour is so far unclassified althoughit is believed the soil may have reached a critical state.

This has significant applications to future OWT projects. So far the long-termdynamic stability of OWT's has been underappreciated; a significant body ofinformation is available however it remains closely guarded by the developers. As new foundations are brought into service there is a necessity to understandthe behaviour of the turbine system over the lifetime of the structure in orderto prevent adverse dynamic interaction effects.

Introduction

With the ever increasing demand for renewable energy and the shifting socialattitudes toward nuclear power wind energy is becoming a viable way ofproducing large quantities of energy. It wasn't long before the advantagesposed by placing wind turbines offshore became apparent, advantages such asless turbulent wind flow, higher wind speeds, with fewer space and noiserestrictions. These advantages are however inconvenienced by impeded access andgreater environmental loading.

As the amount of knowledge applying wind turbines to offshore situationsincreased, designers started to place wind farms further from shore. Doing sothe water depth becomes increasingly great and the loads the turbine issubjected to become increasingly high. This combination of deep water andexposure starts to cause problems for conventional offshore turbinefoundations.

To date the majority of offshore wind farms use monopile foundations. Monopilefoundations consist of a large diameter steel pile that is simply driven intothe seabed. Monopiles are however generally limited to diameters in the regionof 5m by the size of the pile driving vessel used to install the foundation. Therefore for a typical turbine of 1−3 MW a monopile foundation will becomeincreasing uneconomical to fabricate in water depths beyond 30m, due to thepile wall thickness required. Further to this when monopiles are used tosupport large output turbines in the region of 5MW's the viable foundationdepth reduces to around 20m [1]. As many shallow offshore sites have alreadybeen developed or are unsuitable for further development most new sites will belocated in increasingly deep water where conventional monopile foundations willbe economically unviable.