Measured and Predicted Wave Plus Current Loading on a Laboratory-Scale, Space Frame Structure

Abstract

ABSTRACT A wave tank model test program was carried out, using a 1:47 scale model of a space frame structure, to evaluate the use of the Morison force equation for calculating:total wave plus current (towing) loads on structures andvertical loads on conductor guide frames. Prototype-scale wave heights ranged from 0-70 ft and current speeds ranged from 0-5 kt. Predicted hydrodynamic loads, based on laboratory-scale wave force coefficients, were compared to the measured loads. Results show that total wave plus current loads are predictable if proper account is taken of model-scale drag and inertia coefficients. However, the drag coefficient needed to explain the loads in the wave plus current tests is considerably smaller than expected from laboratory data on single cylinders. This may be a scale effect or it may be due to shielding. The incremental total vertical force due to the conductor guide frames is predicted reasonably well if the frames are assumed to act hydrodynamically like flat plates. INTRODUCTION Wave plus current loads on offshore structures, calculated using conventional wave force computer programs, can be significantly higher than wave only loads. These higher loads result from use of the Morison equation in which the drag force is proportional to the square of the sum of the wave-induced water particle velocity and the current velocity - In general the same, constant drag coefficient is used for waves only and for waves plus current. These loads are calculated by summing the forces on individual members of the structure without taking interference or shielding into account, except possibly for conductors if their spacing is sufficiently small. Although we had no reason a priori to question the conventional wave force calculation method, an opportunity arose to carry out tests in a wave tank using an existing, 1:47 scale model of a structure for 450 ft water depth. The model was available from a previous test series in which its upending characteristics and on-bottom stability were investigated. Accordingly, the model was tested in a series of wave and current conditions including both regular and random waves, and 0° and 45° approach directions. The wave profile, water velocity, total horizontal force, vertical force, and overturning moment were measured. During the tests, relatively large vertical forces that could not be explained at that time were noted. A decision was then made to remove the model's conductor guide frames (also referred to as guide panels) and to repeat some of the tests. As discussed below, the reason for the large vertical forces was later explained in terms of laboratory vs prototype conditions. The latter tests, however, allowed us to evaluate the incremental total vertical load due to the conductor frames. Even though the model is quite small in scale, we believed the tests would give valuable information because the model realistically represents the geometry of actual structures, with all the inherent effects of complex structural nodes, shielding, and wake disturbances on adjacent members. To our knowledge most of the previous work on evaluating Morison's equation in controlled laboratory experiments deals with single cylinders and simple multiple-cylinder models.