1 GROUP INTERACTION EFFECTS OF SLENDER CYLINDERS UNDER WAVE ATTACK Uwe Sparboom 1 , Arndt Hildebrandt 2 and Hocine Oumeraci 3 To investigate group interaction effects on nonbreaking and breaking wave loads on a vertical cylinder as a function of cylinder group configurations and spacing, extensive and systematic large-scale model tests were performed in the Large Wave Channel (GWK) of the Coastal Research Centre (FZK), Hannover. A total of 345 tests, including 15 group configurations with spacing up to three times the cylinder diameter were investigated using regular and irregular nonbreaking wave trains as well as breaking waves. The experimental set-up, the test program and some results of the experiments are reported, showing unexpected interaction effects with regard to the maximum wave loads associated with both breaking and nonbreaking waves. INTRODUCTION For the design of marine structures including cylindrical components the influence of neighboring cylinders on the wave loading (interference, shelter and amplification effects) cannot yet be estimated with sufficient engineering accuracy. Apelt and Piorewicz (1986) reported results of small-scale tests with breaking waves acting on rows of cylinders. In many cases they found that breaking wave forces on cylinders in transverse arrangements are larger than those on a single isolated cylinder. Smith and Haritos (1997) investigated group interference effects in small-scale experiments for nonbreaking waves. Morison- coefficients depending on KC-number were derived for specific cylinder group configurations. One of the main problems up to date is the uncertainty associated with the transfer of small-scale model test results to prototype conditions. At the Forschungszentrum Küste (FZK), the Large Wave Channel (GWK) offers large scale test facilities to investigate wave loads on coastal and offshore structures, thus minimizing scale effects. The main dimensions of the GWK are: length 309 m, width 5 m, depth 7 m, maximal water depth 5 m, maximal wave height 2.5 m and breaker height up to 3.2 m. Any kind of waves can be generated: Regular wave trains, solitary waves, wave packets (freak waves) and irregular waves (common and any other spectra measured in the field). Within the research program a variety of large-scale experiments with slender cylinders were performed and analyzed (Wienke et al., 2000; Wienke, 2001; Sparboom et al., 2001; Irschik et al., 2002; Wienke et al., 2004; Irschik et 1 Dr.-Ing., Senior Research Engineer, Forschungszentrum Küste (FZK) Merkurstrasse 11, D-30419 Hannover, Germany, sparboom@fzk.uni-hannover.de 2 Dipl.-Ing., Forschungszentrum Küste (FZK) 3 Full Professor, Dr.-Ing., Forschungszentrum Küste (FZK)