This paper analyses the results of two series of experiments concerned with the response of a single vertical cylinder in the inertia regime in steep non-breaking waves. We recorded first the loading on a cylinder when it was held stationary, and secondly, its response in the same waves when it was pivoted just above the floor of the wave flume, and supported at the top by springs in the horizontal plane. Spring stiffnesses were set to achieve natural frequencies (measured in still water) in the range between 3 and 11 times the dominant wave frequency. The experiments were repeated with cylinders of three different diameters.Peak loading on stationary cylinders was found to exceed the predictions of a Morison model (based on kinematics computed from a numerical model of the measured waves), though improvements were achieved through the inclusion of slender-body terms. Measured ringing responses are generally in good agreement with those computed on a quasi-static basis from the measured loading history, but in some conditions, particularly at low frequency ratios, there is clearly some feedback from the motion to the excitation. Peak accelerations in the steepest waves are found to be limited approximately to those that would occur if the maximum loading were applied as a step change. Particular attention is given to a rapid cycle of loading that occurs after the crest has passed the cylinder's axis, and to images of the flow around the cylinder at the water surface.

This paper derives an equation for the potential-flow wave loading on a lattice-type offshore structure moving partially immersed in waves. It is for the limiting case of small lattice-member diameter, and deals entirely in member-centreline fluid properties, so that it can be applied computationally by a simple ‘stick model’ computer program. This field is currently served by a simple two-term semiempirical formula ‘Morison's equation’: the new equation is effectively a replacement for the Morison inertial term, allowing the Morison drag term (or some refinement of it) to describe exclusively the effects of vorticity, which can in principle be calculated to greater accuracy when isolated in this way.The new equation calculates the potential-flow wave load accurate to second order in wave height, which is a great improvement on ‘Morison's equation’: such results can currently only be sought by very much more complicated and computationally intensive methods, of currently uncertain repeatability. Moreover the third-order error is localized at the free-surface intersection, so the equation remains attractive for fully nonlinear problems involving intermittent immersion of lattice members, which are currently beyond even the most sophisticated of these computationally intensive methods. It is shown that the primary reason for this large contrast in computational efficiency is that the loads are derived from energy considerations rather than direct integration of surface pressures, which requires a lower level of flow detail for a given level of load-calculation accuracy.These improvements must of course be seen against the current levels of uncertainty over the calculation of vorticity-induced loads, which in many applications completely dwarf inaccuracies in potential-flow load calculation. The conditions are accordingly established under which the improvements are comparable to the total wave load predicted by the Morison drag and inertia terms in combination. They are that the lattice member diameter is greater than its length/10, or the relative fluid motion/5, or the structure's motion radius/20, or the wavelength/30: if any one of these conditions is satisfied, the new equation is worthwhile even when used in combination with simple vorticity-induced load calculations from a Morison drag term.

Laboratory measurements of the performance of the Anaconda are presented, a wave energy converter comprising a submerged water-filled distensible tube aligned with the incident waves. Experiments were carried out at a scale of around 1 : 25 with a 250 mm diameter and 7 m long tube, constructed of rubber and fabric, terminating in a linear power take-off of adjustable impedance. The paper presents some basic theory that leads to predictions of distensibility and bulge wave speed in a pressurized compound rubber and fabric tube, including the effects of inelastic sectors in the circumference, longitudinal tension and the surrounding fluid. Results are shown to agree closely with measurements in still water. The theory is developed further to provide a model for the propagation of bulges and power conversion in the Anaconda. In the presence of external water waves, the theory identifies three distinct internal wave components and provides theoretical estimates of power capture. For the first time, these and other predictions of the behaviour of the Anaconda, a device unlike almost all other marine systems, are shown to be in remarkably close agreement with measurements.