Development of a Six Degree of Freedom Loading Rig
Dr Byron Byrne and Prof. Guy Houlsby
The response of shallow foundations subjected to general loading is an important area of civil engineering, particularly in the offshore industry, where foundations must contend with very harsh environmental conditions. These extreme conditions lead to large vertical (V), horizontal (H) and moment (M) loads on the foundations. Whilst earlier studies considered overall stability, more recent studies have, attempted to model the displacements, using work hardening plasticity. Recently, this work has focussed on a novel foundation called a "suction caisson". Like an upturned bucket, it is installed simply by sucking the water out and thus forcing the skirts into the seabed. The developing offshore wind energy industry have expressed considerable interest in using this design for offshore wind turbines. In this case the loading consists of very high moment and horizontal loads, but low vertical loads, a very different pattern of loading from that experienced by structures in the oil and gas sector. In addition the wind and wave directions may not be co-incident, so the base shear and moment are not in the same direction. Considerable uncertainty therefore surrounds how these foundations may perform.
All previous studies assume planar loading, consisting of three components {V:M:H}. This assumption means that true three-dimensional analyses of the structure cannot be carried out. The true loading involves six degrees of freedom: vertical (V), moment (M1, M2), horizontal (H1, H2) and torsion (T). Extensions of the plasticity theory from planar to general loading have been postulated, but experimental investigations are needed to validate and extend these new theories. Once validated these new theories will have a significant impact on the offshore wind sector and other applications. They will be formulated for use in commercial finite element codes, and so will also be of great interest and value within the traditional offshore oil and gas industry. Funds have been obtained from The Royal Society to develop an experimental rig for applying general loading to suction caissons. An elegant approach will be followed using six linear actuators placed in an octahedral configuration known as the Stewart platform. The rig will be used to map out the true general loading response which will be interpreted within plasticity theory. Funds have also been obtained from the Lubbock trust to enable preliminary testing and design to be carried out.