Steady Wind- and Current-Induced Loads on Moored Vessels
Authors: Palo P.A. 
Published: October 2008

This paper discusses methods for calculating wind and current-induced steady loads on moored vessels. An analysis of existing wind tunnel data was used to develop anew, generally-applicable calculation method for wind loads that accounts for individual vessel characteristics and the natural wind gradient. It is also shown that the accuracy of state-of-the-art methods for calculating current-induced loads is undefinable, given the large scatter among the methods and the absence of validation data. Results are presented from the first known full-scale current loads test.

INTRODUCTION

A wide range of mooring expertise and technology is required by the U. S. Navy to properly support and execute its missions. The Naval Civil Engineering Laboratory (NCEL) under Naval Facilities Engineering Command (NAVFAC) sponsorship, is contributing to this technology base by developing and validating general purpose mooring analysis computer models. Experience at NCEL using these computer models has generated questions regarding the accuracy of existing methods for estimating steady wind-, wave- and current-induced forces and yaw moments on moored vessels. This article is an overview of NCEL's continuing efforts to improve the state-of-the-art in estimating both wind- and current induced steady loads on moored vessels. These efforts have also served to establish the relative accuracy of existing calculation methods, and have provided examples of the sensitivity of mooring simulations to these steady loads. Wave-induced (drift) steady loads are not considered in this overview, nor are "a typical" vessels such as submarines, catamarans or semi-submersibles.

The coordinate system used in this report is illustrated in Figure 1. The excitation is decomposed into a longitudinal force component (X) parallel to the keel, a lateral force component (Y) perpendicular to the keel, and a yaw moment component (N) about the center of gravity. These loads are referenced to the local incident angle (8) between the excitation and the keel of the vessel.

NCEL's first mooring simulation using the recently developed mooring models is described in Reference 1. Three different sources of current- and wind induced loads (A, B and C) were used in three different simulations of a 70 KDWT tanker moored by a single hawser to a mooring buoy with four catenary legs. The static and dynamic tensions are summarized in Table 1. The differences among the static tensions demonstrated both the inconsistencies in the state-of-the-art calculation methods, and the sensitivity of the static equilibrium to these loads. The importance of accurately estimating these steady loads was further reinforced by the dynamic tensions comparison. In both the model test and the simulation, the dynamic loads only slightly increased the static tensions. Considering that this mooring system was excited by 12 foot waves, this example demonstrated that dynamic effects are not necessarily more important that the static loads. It was therefore concluded that a thorough investigation of steady loads on moored vessels was needed to have confidence in the simulations.