Back analysis of foundation piles is an important aspect of a project, as it can allow for refinement of soil parameters, leading to pile holding capacity variations, and a more accurate fatigue life assessment. Back analysis of pile driving data to determine the capacity of an axially loaded pile, such as for a jacket structure, is a well-developed process, with the results of pile driving data directly applicable to axial foundation response. However, for mooring piles, where the loads are dominated by lateral forces, determining the capacity is more complex as the results of driving data are not directly relatable.
Offshore mooring piles are opened-ended steel tubular piles, with diameters typically ranging between 1 m to 2 m, with a padeye (mooring chain attachment point) located at some point along the pile, usually within the upper third. The piles are driven into the seabed with an underwater impact hammer, or vibratory hammer, and are used for mooring floating structures. This paper presents a methodology to carry out a back analysis of mooring pile capacity based on pile driving data. The process involves two stages, the first is to assess the self-weight penetration of the pile, which will allow clarification of the design shallow soil properties; the second stage is to compare the pile driving data, with the estimated soil resistance to driving, to gain an understanding of the deeper soil properties.
The procedure presented within this paper was applied to an assessment offshore South East Asia, a case where location-specific data were not available but were within 500 m of the mooring pile locations. The procedure was used to verify that the piles were suitably designed against the given loading conditions, as the soil data was not co-located with the mooring pile locations.
The process forms an important aspect when considering the future of offshore moorings, specifically looking ahead towards floating offshore wind, potentially offering refinement of design factors, verification of pile capacity, reducing offshore exposure time, and optimizing the fatigue life of piles following driving.