suction buckets) supporting a jacket structure, the large overturning moment is transferred into vertical load pairs applied onto each of the foundations. A single skirted foundation supporting an offshore wind monopod structure will be subjected to large overturning moments, whereas for skirted foundations (e.g. The combination of loads applied to a skirted foundation strongly depends on the structural configuration. It is necessary to consider the cyclic nature of the environmental loads as well as the combination of vertical ( V), horizontal ( H) and moment ( M) loads in the design of foundations to be installed offshore. Skirted gravity-based structures (GBS), suction anchors for floating platforms, spudcans equipped with skirts, suction caissons on jackets and mono-caissons for offshore wind turbines are all examples of shallow foundations with skirts. The concept has been used in offshore foundation engineering in various soil conditions for more than 40 years. The advantages of skirted foundations compared to foundations resting on the seabed are well documented in several studies ( Andersen & Jostad, 1999 Villalobos et al., 2003 Cassidy et al., 2004 El Sawwaf & Nazer, 2005 Jostad & Andersen, 2006 Singh et al., 2007). The embedded skirts provide additional stiffness and capacity to the foundation, but give only a modest increase in the foundation weight. The framework of procedures can be used to estimate foundation stiffness and capacity and the results can serve as a basis for the development of foundation macro-element models.Ĭircular skirted foundations are, as the name indicates, shallow circular foundations equipped with skirts embedded into the seabed. The database is accompanied by a simplified procedure to account for cyclic degradation through equivalent number of cycles, different normalised load–displacement response and variation in foundation geometry. As an example, the well-defined cyclic contour diagrams of Drammen clay were utilised to demonstrate how foundation response diagrams can be established. In addition to failure envelopes, contours of displacements were computed in the three-dimensional load space (vertical, horizontal and moment load) for a more complete description of the response. It was found that the shapes of the failure envelopes are little affected by degradation expressed by the number of equivalent cycles. This paper presents a numerical study focusing on cyclic degradation and failure envelopes for skirted foundations subjected to combined cyclic and static loading. Most of these studies focus on ultimate capacity and static loading. A large number of studies on failure envelopes exist in the literature based on experiments and numerical analyses. In design, the use of failure envelopes is convenient for assessing stability under combined loading. Skirted foundations have been widely applied as offshore foundations for several decades.
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