This paper presents results of an investigation of the modifications to the in-line Modson force coefficients on individual vertical surface piercing cylinders within cylinder groups associated with group interference effects determined from m experimental study conducted in the University of Melbourne's Michelle Laboratory wave flume. The current study investigates group interference effects at higher Keulegan-Carpenter numbers and in more detail within the previously identified critical close spacing region than previous work conducted within the department. This paper focuses on results associated with the current experimental program conducted on cylinder pairs oriented in-line and transverse to the wave direction in irregular wave conditions.
Experimental programs investigating modifications to the Morison force coefficients due to group interference effects are necessitated by the lack of a theoretical solution to the determination of the individual forces on cylinders within cylinder groups in separated flow. Computation schemes whilst evolving rapidly arc presently not able to model the complexities of separated flow on isolated cylinders over a wide Reynolds number range in "wavy" flows. Hence, the importance of small scale laboratory investigations which can provide valuable insights into group interference phenomena, although results are not directly applicable to full-scale offshore structures, such as in riser groups, due to the different Reynolds numbers regimes that small scale and full-scale structures experience. The current experimental study compared the variation in the inline Morison force coefficients between the isolated cylinder condition and closely spaced cylinder groups. Modifications to the Morison force coefficients associated with group interference effects derived from tests in regular wave conditions have previously been reported in detail for the two cylinder case with cylinders oriented transverse to the wave direction (Haritos and Smith, (1994, 1995); Smith and Haritos (1995)) and in-line to the wave direction (Smith and Haritos, 1996).