Ocean Systems Engineering, Vol. 9, No. 4 (2019) 349-368 DOI: https://doi.org/10.12989/ose.2019.9.4.349 349 Copyright © 2019 Techno-Press, Ltd. http://www.techno-press.org/?journal=ose&subpage=7 ISSN: 2093-6702 (Print), 2093-677X (Online) Response of triceratops to impact forces: numerical investigations Srinivasan Chandrasekaran  and R. Nagavinothini Department of Ocean Engineering, IIT Madras, India (Received March 18, 2019, Revised September 17, 2019, Accepted September 19, 2019) Abstract. Triceratops is one of the new generations of offshore compliant platforms suitable for ultra-deepwater applications. Apart from environmental loads, the offshore structures are also susceptible to accidental loads. Due to the increase in the risk of collision between ships and offshore platforms, the accurate prediction of structural response under impact loads becomes necessary. This paper presents the numerical investigations of the impact response of the buoyant leg of triceratops usually designed as an orthogonally stiffened cylindrical shell with stringers and ring frames. The impact analysis of buoyant leg with a rectangularly shaped indenter is carried out using ANSYS explicit analysis solver under different impact load cases. The results show that the shell deformation increases with the increase in impact load, and the ring stiffeners hinder the shell damage from spreading in the longitudinal direction. The response of triceratops is then obtained through hydrodynamic response analysis carried out using ANSYS AQWA. From the results, it is observed that the impact load on single buoyant leg causes periodic vibration in the deck in the surge and pitch degrees of freedom. Since the impact response of the structure is highly affected by the geometric and material properties, numerical studies are also carried out by varying the strain rate, and the location of the indenter and the results are discussed. Keywords: impact analysis; stiffened cylindrical shell; ring frames; stringers; triceratops 1. Introduction In order to extend the oil exploration and production to greater water depths, structures with adaptable structural forms are necessary. Triceratops is one of the new generation offshore compliant platforms, consisting of a deck connected to three buoyant legs by ball joints (White et al. 2005). The ball joints restrain the transfer of rotational motion and allow only the translational motion between the deck and buoyant legs. The buoyant legs are position restrained by a set of taut moored tethers. These platforms are highly suitable for ultra-deep waters under different sea state conditions (Chandrasekaran and Nagavinothini 2018, Nagavinothini and Chandrasekaran 2019, Srinivasan Chandrasekaran and Nagavinothini 2019a, b). The conceptual design of triceratops is shown in Fig. 1. The buoyant legs are usually designed as orthogonally stiffened cylindrical shells with stringers and ring frames to resist the axial load and hydrostatic pressure. The buoyant legs also have a relatively high radius to thickness ratio. Due to the large distance Corresponding author, Professor, E-mail: drsekaran@iitm.ac.in