Offshore foundation systems are constantly evolving to meet the needs of new developments in the energy sector. These challenges may be created by the requirements of moving into ever deeper water for hydrocarbon recovery, or creating foundation systems for renewable energy sources, such as offshore wind farms. This type of massive turbine structure induces complex loading states on foundations as the result of combined wind, wave and self weight loading effects, all of which must be accommodated within tolerable displacement envelopes to allow the turbines to operate effectively.
As a consequence ‘hybrid’ systems involving combinations of foundation elements, such as monopiled footings, suction cassions and/or gravity structures, are generally evaluated at each potential site. This paper presents the results of a physical model study on the performance on one such hybrid system, namely a monopiled circular footing.
A series of small scale single gravity tests was carried out to evaluate the performance characteristics of a model monopiled footing. The tests were performed in a ‘sandbox’ and involved evaluation of the component elements of the system and the hybrid system under combined lateral and vertical loading.
The results of the study indicate that the hybrid system offers advantages in terms of lateral and axial load capacities, as well as increased lateral stiffness, compared to a conventional monopile foundation.
There are several types of foundation systems available to the designer of offshore wind turbines. The preferred foundation system will of course be dependent on the local seabed soil conditions and size of the turbine. Generally speaking the choice will be between surface or near surface type foundation solutions, such as gravity-based or suction caisson systems (including tripods), or large diameter monopiles. There are also instances where a combination or hybrid arrangement of foundation elements may prove the most economical.
This paper presents a laboratory study of a hybrid foundation system, which combines a monopile and circular footing or foundation plate, see Figure 1a. Despite the apparently obvious advantages of such hybrid systems, there is little information available in the literature to suggest that such systems have been explicitly studied. On the other hand, there is a significant body of literature available in respect to the component elements of a monopiled footing. In particular, research on the response of shallow foundations to combined loading1, 2 and, more recently, the use of sophisticated numerical modeling3 has led to the development of design aids for circular footings under combined loading. Similarly, several methods for the analysis of piles, and in particular their response to lateral loads in cohesion-less soils, have been developed4, 5, 6, 7, 8, 9, 10.
The monopiled footing problem investigated here can be considered analogous to that of a single capped pile. Poulos and Randolph11 developed methods for analysing the relative influence of the pile and pile cap under axial loading, and some studies of the influence of the pile cap on the lateral performance of single piles has also been reported12, 13, 14. The dimensions of pile caps are generally relatively small