Experimental and Theoretical Study of Motion of a Moored Barge
Authors: Muga Bruce J. 
Published: December 2007

A 100-ft-square construction-type platform excited by unidirectional random surface gravity waves was studied in the field, at reduced scale in the laboratory, and by using the linear theory of rigid ships' motion. The platform was spread-moored in about 165 ft of water in the open Pacific Ocean. Water-level variations at three locations, ship rotations and accelerations, mooring forces, and wind velocity were measured in Sea States 2 and 3 (Neumann wind model). A Sea State 4 was simulated in the laboratory. Three recordings each (representing nearly beam-on, quartering and stem-on seas) of the prototype and model tests were analyzed using prototype and model tests were analyzed using time-series techniques to provide amplitude-response operators for all of the ship's motions and mooring forces. The amplitude-response operators were computed from a linear theory based on the slender body approximation. The in-line coupled equations of motion are formulated for the 6 degrees of freedom and solved literally and numerically. The results in the form of complex-response operators are compared with those obtained from the prototype and model measurements. Generally, the agreement in both amplitude and phase is excellent for the surge, sway, roll, heave and pitch motions. Discrepancies found for certain heading and wavelength (or wave frequency) combinations are easily explained by the presence of crosscoupling terms or values of damping coefficientsIt is found that the linear theory of ships' motion using the slender-body approximation is an adequate tool for predicting the behavior of moored platforms in Sea States of up to 3.

Introduction

Ships of conventional design seldom operate at zero speed in significant waves. As a consequence, only a trivial amount of data has been published on the motions of ships at zero speed. Zero-speed studies of models in the laboratory are difficult to conduct because of the requirement of eliminating wave reflections from the tank sides. Data on the behavior of moored ships, barges and special purpose platforms is scarcer than that for models. platforms is scarcer than that for models. Nevertheless, the recent and continuing demands for ocean construction underline the need for knowledge in this area. The present study is intended to contribute pertinent information. The present study consists of: present study consists of:Analysis of the measurements of the motions induced on an instrumented prototype barge by irregular waves as moored in the open Pacific Ocean in approximately 165 ft of water off San Clemente Island.Analysis of the measurements of the motions induced on a dynamically similar reduced-scale model barge by irregular waves.Determination of the motion response of the barge to representative sea states by application of the linear theory of ships' motion.Selected comparisons of the results obtained from Statements 1 through 3.

Interest usually centers on the forces induced in the mooring lines. These forces may be obtained from the motions, once the latter are determined.

PROTOTYPE PROTOTYPE

The craft studied is a special purpose construction-type platform designated as Fishhook, plan views of which are shown in Fig. 1. The hull plan views of which are shown in Fig. 1. The hull outline is that of two standard (34 × 110 ft) Navy lighter barges (YFNB) separated by a distance of 22 ft, which gives the appearance of a flat-bottomed catamaran. Forward of Frame 5, the center section provides additional buoyancy, while aft of Frame 5, provides additional buoyancy, while aft of Frame 5, the barges are tied together by a structural framework, leaving an open well between the barges. The composite platform supports a massive tower and recovery mechanism whose mass distribution gives the Fishhook some unusual, if not unique, naval architectural and dynamic characteristics. The general location of the study was off of Wilson Cove on the north coast of San Clemente Island, about 65 miles south of Los Angeles. Figs. 2 and 3 present details of the mooring arrangement. The craft was moored by chains in about 165 ft of water.

SPEJ

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