Ocean Engineering 201 (2020) 107148 Available online 5 March 2020 0029-8018/© 2020 Elsevier Ltd. All rights reserved. Application of poroelastic layers in a semi-submersible platform: Devising an effcient heave motion response reduction method Arefeh Emami , Ahmad Reza Mostafa Gharabaghi * Department of Civil Engineering, Sahand University of Technology, Tabriz, Iran A R T I C L E INFO Keywords: Semi-submersible platform Heave motion response Poroelastic material Eigenfunction expansion method ABSTRACT This paper describes an effcient and applicable approach to decrease the heave motion response of a semi- submersible platform by introducing poroelastic layers (PELs) at the bottom of its pontoons. This study was carried out utilizing a semi-analytical method based on eigenfunction expansion, which involves determining the added mass, damping, related hydrodynamic force and heave motion response. The adequacy of the semi- analytical method was verifed by applying it to a typical GVA4000 drilling semi-submersible platform, whose heave response amplitude operator was extracted and compared with available experimental data. The extracted value and available data showed close agreement. Furthermore, the effectiveness of the PELs attached at the bottom of the GVA4000 drilling semi-submersible platform and the effect of PEL characteristics such as the thickness, intrinsic permeability, and elasticity on reducing heave motion response were evaluated. Comparing the results with the studied GVA4000 drilling semi-submersible platform without PELs indicated that the application of PELs could considerably reduce the heave motion response and nearly eliminate the risk of creating a resonance phenomenon in the semi-submersible platforms in different sea wave conditions. The approach introduced in this study could be a suitable pathway to improve the performance of semi-submersible platforms, particularly their heave motion responses. 1. Introduction A semi-submersible platform is a special type of foating platform which is used predominantly for drilling and extraction of oil and/or natural gas from deep waters. This platform has a simple geometry consisting of a hull, mooring, and riser systems. The hull is characterized by a deck, columns, pontoons, and braces, and plays the main role in wave response. The platform can be classifed as a particular type of watercraft because 70–85% of its hull is located underwater. The dis- tinguishing features of a semi-submersible platform are its large deck area; payload capacity; equal resistance to waves, winds, and currents; and ability to support mooring systems. The conventional semi- submersible platform offers motion with six degrees of freedom, namely, surge, sway, yaw, heave, pitch, and roll. Its heave motion response is signifcantly larger than the other platform types, such as spar platforms and tension leg platforms, due to its small water plane and draft. The small damping of this platform, which is predominantly hydrodynamic damping due to wave radiation, also increases its heave motion response. The large heave motion of a semi-submersible plat- form often limits its drilling operations and may damage the risers and mooring systems. Therefore, it is necessary to improve the heave motion responses of such systems and ensure to shift their natural heave fre- quency away from the frequencies of daily waves. Many researchers have attempted to improve the heave motion re- sponses of semi-submersible platforms by employing different tech- niques such as optimizing the geometrical shape (Akagi and Ito, 1984; Clauss and Birk, 1996; Birk and Clauss, 2001; Birk, 2009; Chen and Uddin, 2013; Venzon et al., 2013; Park et al., 2015; Jiang et al., 2016), altering the hull shape (Dahan et al., 1985; Nishimoto and Leite, 1993; Mansour et al., 2011; Zou, 2012; Lidekrantz, 2014; Wang et al., 2015; Muehlner and Banumurthy, 2015; Rijken, 2017), changing the draft (Bennett et al., 2001; Ye et al., 2016; Bo et al., 2017), and installing heave plates or truss-type columns (Halkyard et al., 2002; Cermelli et al., 2004; Chakrabarti et al., 2007; Chen et al., 2007; Murray et al., 2007; Srinivasan et al., 2010; Ding and Soester, 2011; Zhu et al., 2012; Liapis et al., 2016; Liu et al., 2016; Liu and Ou, 2016; Ma et al., 2018). Researchers have also created various fexible marine structures to effectively vary the wave feld. Such investigations have involved uti- lizing a submerged fexible mound (Ohyama et al., 1989), a hinged fexible breakwater (Lee and Chen, 1990), a porous and fexible break- water (Wang and Ren, 1993), a fexible circular cylinder (Lee and Lan, * Corresponding author. E-mail addresses: a_emami@sut.ac.ir (A. Emami), mgharabaghi@sut.ac.ir (A.R. Mostafa Gharabaghi). Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng https://doi.org/10.1016/j.oceaneng.2020.107148 Received 4 August 2019; Received in revised form 18 February 2020; Accepted 19 February 2020