Validation Of A Static Mooring Analysis Model With Full-Scale Data
Authors: Karnoski S.R. , Palo P.A. 
Published: October 2008

Full-scale mooring data of a two-point moored T-2 tanker and a single-point moored Spruance-class destroyer was used to investigate the accuracy inherent in static mooring analyses using a Navy mooring program. Comparisons between computed and measured line tensions showed differences of up to ± 50%. These differences, which are attributed primarily to the effects of a vertical current velocity gradient in the computation of the current induced load, are considered inherent to any static mooring analysis computer program.

INTRODUCTION

Validation of a computer program can be difficult because of the many sources of error. These sources include: coding errors in the computer program; simplifications and assumptions made in the models which do not accurately represent the physical situation; incomplete or inaccurate input data; and mooring configurations where small variations in one of the inputs can produce drastic changes in the results.

Finally, the question arises as to when does one consider a program validated? A program which computes a moored vessel's static response to predetermined static wind, current, and wave forces and moments (entered by the user) may be considered validated if the summation of internal and external forces equals zero and the mooring configuration is stable. But, consider the same program which internally computes environmental forces based on a description of the environment and load coefficient algorithms. The accuracy of these latter results will be limited to the accuracy of the environmental description and the algorithms. It can be very difficult to produce in full-scale experiments a sufficient number of test cases that exercise the scope of the program's capabilities.

All of the above issues are usually encountered in a validation process. The comparisons made between the full-scale data and the computer program FLEETMOR discussed herein were no different. The purpose of this paper is to (1) identify problems with existing static load calculations, (2) discuss the problems which occurred during the comparison process which are inherent to any static mooring analysis program, and (3) to present a data set which one may use to make comparisons with any static mooring analysis program.

The data used in this comparison is from 8 full-scale wind and current force measurement program performed on a T-2 tanker and Spruance-class destroyer in two- and single-point mooring configurations, respectively. The data from these tests was used to validate the static mooring analysis program FLEETMOR developed by the Naval Civil Engineering Laboratory under the sponsorship of the Naval Facilities Engineering Command [1].

FLEETMOR DESCRIPTION

FLEETMOR [2] computes the static response of a moored vessel to Wind, current, and second-order wave drift forces. FLEETMOR models up to 24 mooring legs with each leg comprised serially of up to three cable segments, a buoy, and a sinker. Line stretch "is included to simulate the elongation of synthetic lines. The buoy is not confined to the water surface. Catenary equations are used to model the mooring legs.