GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.3 GNGTS 2024 sismica-nazionale ) , whose recorded data are transmited in real tme, through the EDISON network, to the INGV seismic monitoring centre. The Rospo Mare C oil platorm: arrangement and mechanical behaviour The RSM-C oil platorm (42° 14’ 8,365” N, 14° 55’ 54,682” E) is located ofshore facing the city of Vasto and is part of a group of three platorms interconnected with each other by submarine pipelines. It is characterized by a four-legged retcular structure, about 100 m high from the seabed, while around 80 m are submerged. About 40 m of the c.p. D are buried within the sof sediments of the seafoor, while around 94 m between the seafoor and the sea surface, and other 10 m from the later to the platorm. The ofshore platorms are obviously subjected to several natural forces (winds, sea currents, waves) and anthropogenic ones ( e.g. oil extracton processes), that excite the vibratonal mode shapes of their structures. All these vibraton felds can interfere with the seismic data acquisiton system, in terms of quality of the recordings, reducing the Signal-to-Noise Rato (SNR). To calculate the natural frequencies of the c.p. D, we perform a fnite element modal analysis, appropriately constrained, following the method described in Cammalleri and Costanza (2016). For the same purposes, the natural frequencies of other existng platorms, structurally similar to RSM- C, were collected from the literature (Jiammeepreecha et al ., 2008; Raheem et al ., 2012; Weldelassie, 2014). Furthermore, the frequencies of marine waves and those related to Von Karman’s vortces (caused by sea currents fowing around the c.p. D) were considered. All those frequencies are listed in Tab. 1. Tab 1. Vibraton frequencies Data acquisiton system The installaton of an OBS at the base of the c.p. D of the RSM-C has been logistcally possible, with the awareness of disturbances and relatvely high noise levels between 0.1 and 10 Hz. As to minimize the infuences of such disturbances, and also to ensure a good coupling, we developed a system which allows the sensor self-burying within the pelitc sediments of the seabed. As shown in Fig. 1, two conical caps are atached to the sensor and connected to a hydraulic circuit which conveys a fow of water from the top of the c.p. D to the seabed. A downward jet of water, coming out from below the sensor, digs a hole in the sediments, both ensuring an easier deployment and [Hz] Nat. Freq. of platorms structurally similar to RSM-C Nat. Freq. C.p. D [Jiammeepreecha] [Raheem] [Weldelassie, 2014] Marine waves Von Karman 0.3 0.8 1.1 0.25 0.005 – 0.15 0.03 0.8 0.8 1.1 0.06 1.5 2 1.4 0.12 2.5 3.6 0.15 3.7 3.6