Journal of Coastal Research, Special Issue No. 65, 2013 Wave and current forces at a bottom-mounted submarine pipeline 153 Wave and current forces at a bottom-mounted submarine pipeline Francesco Aristodemo†, Giuseppe Roberto Tomasicchio‡ and Paolo Veltri† †Dipartimento di Ingegneria Civile, Università della Calabria, Arcavacata di Rende (CS), 87036, Italy francesco.aristodemo@unical.it paolo.veltri@unical.it ‡Dipartimento di Ingegneria per l’Innovazione, Università del Salento, Ecotekne, Lecce, 73100, Italy roberto.tomasicchio@unisalento.it INTRODUCTION A fundamental step in on-bottom pipeline design is the evaluation of the hydrodynamic loads for a wide range of environmental conditions. Frequently, submarine pipelines are subjected to wave and current induced forces at finite water depths where non-linear effects occur. The approach for determining the hydrodynamic forces acting on slender structures such as submarine pipelines can be principally defined through the diffraction parameter, D/L, where D is the cylinder diameter and L is the wave length, and the Keulegan-Carpenter number, KC, depending on the ambient horizontal velocity, the wave period and the diameter. Slender submarine pipelines refer to the case of D/L < 0.2 and KC > 4 (Sarpkaya and Isaacson, 1981). In this region the incident flow separates from a wall-mounted cylinder, producing the break-up of symmetry in the flow and the suppression of the vortex shedding regime which occur conversely for wall-free cylinders (Sumer and Fredsoe, 1997). A common model for evaluating in- line hydrodynamic forces (drag and inertia components) induced by oscillatory flows and currents on slender bodies is the Morison semi-empirical equation (Morison et al., 1950). The transverse force is calculated by a similar expression of the Morison drag force. Many researchers have paid attention to the calibration of the Morison coefficients for a wide range of values of KC, adopting different geometrical configurations of cylinders. For the periodic wave case, small-scale laboratory experiments were limited to determining drag, inertia and lift coefficients for bottom-mounted cylinders subjected to sinusoidal waves (Cheong et al., 1987; Bryndum et al., 1992), also in combination with currents (Bryndum et al., 1992; Neill and Hinwood, 1998). Moreover, the ambient horizontal velocity in the Morison and transverse force scheme in the above experimental investigations was determined analytically from the measurements of a corresponding surface elevation, or far from the pipeline. This approach allows an indirect evaluation of the near kinematic field and, consequently, of the hydrodynamic forces at the pipeline. However, an accurate calibration of the flow field and hydrodynamic coefficients involved in the force models in order to analyze their performances is required. In this context, full- scale experiments were conducted in a large wave flume on a bottom-mounted pipeline subjected to non-linear Stokes and cnoidal waves and also interacting with a positive current variable along the depth. The adopted flume was characterized by the use of two separate wave channels where simultaneous measurements of surface elevations, velocities and pressures at the cylinder location were performed. The flow field was characterized by the formation of a vortex behind the cylinder in the first half period, followed by a successive motion away from the cylinder (4 < KC < 7) or by the creation of a vortex pair (7 < KC < 13) in the downstream direction (Sumer and Fredsoe, 1997). In this case, the flow separation induces drag and lift forces which become significant in comparison with those of the inertia. The present paper is organized as follows: in the next section the force models are recalled; then, the experimental investigation ABSTRACT Aristodemo, F., Tomasicchio, G.R. and Veltri, P., 2013. Wave and current forces at a bottom-mounted submarine pipeline. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 153-158, ISSN 0749-0208. The present work deals with the analysis of in-line and transverse forces for a bottom-mounted slender pipeline subjected to periodic motions characterized by non-linear Stokes and cnoidal waves, and also with a superimposed positive current. Full-scale laboratory tests were conducted in the large wave flume of CSMI at Padua, Italy, and characterized by a longitudinal vertical wall which was built in the flume to create two separate channels where simultaneous measurements of pressures, velocities and surface elevations were performed. The experimental dataset was defined by 39 tests with periodic waves using a smoother cylinder, 14 tests with periodic waves using a rougher cylinder and 11 tests with periodic waves plus currents, leading to a Keulegan-Carpenter number ranging from 4 to 13. The hydrodynamic coefficients of Morison and transverse force models were deduced from the time records of kinematics and pressures at the cylinder through the evaluation of the performances of time and frequency domain techniques such as the ordinary and the weighted least squares. The comparisons between experimental and calculated non-linear forces in terms of peaks, associated phase shifts and spectral densities show acceptable accuracy as regards the Morison force components and a poor estimation of the vertical forces. ADDITIONAL INDEX WORDS: Full-scale experiments, vortex regime, hydrodynamic coefficients, hydrodynamic forces. www.JCRonline.org www.cerf-jcr.org ____________________ DOI: 10.2112/SI65-027.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013