Statistical Variability of Measured Loads and Motions for Two Types of Single-Point Moorings Subjected to Random Seas
Authors: Johnson T.L. , Collins J.I. 
Published: October 2008

Two physical model test programs were completed with several repeats of "survival sea" test conditions for a double articulated single anchor leg mooring (SALM) and a turret mooring. Seven 120 minute duration tests were conducted in random waves alone for the SALM. Thirteen 90 minute duration tests in wind alone, waves alone and wind and waves were conducted for the turret moored system. The objectives of the test programs were to provide measured test load data for review of design calculations and quantify factors of safety for important mooring components. Neither mooringsystem was particularly unique.

Wide variability of the statistics of loading extremes, mean and standard deviations were noted for the SALM, whereas the turret moored system exhibited good repeatability under all ronmental conditions. Statistics of extremes, means and standard deviations of repeat tests are presented showing the variability of the loading results. The SALM was tested in waves alone and exhibited wide variability of extremes, means and standard deviations. The turret moor was subjected to wind and waves. The wind and waves had low frequency components affecting the environmental forcing functions and were not necessarily in phase, but the tests showed repeatability of extremes, means and standard deviations with small variability. Great care was exercised to assure consistency in the reproduction of the environmental conditions.

INTRODUCTION

Two scale physical model test programs were conducted for a turret moored single point mooring system and a double articulated single anchor leg mooring system (SALM). Figures 1 and 2 are schematic representations of the mooring systems tested. Several tests were performed with similarenvironmental forcing. The objective of the test programs was to obtain sets of test data from which the repeatability of measured maxima, averages and standard deviations of important in-line loads and motions could be evaluated.

Physical model tests of non-linear mooring systems subjected to random seas in the model basin have been accepted as a tool for design guidance. Repeatability of results produces confidence in the method of analysis, but statistical variations of extremes, means and standard deviations for "similar" tests have been encountered. This should be recognized by classification societies, owners, operators and the design community as a possible sign of an under-designed system rather than variation due to imprecisely repeated environmental conditions in a physical model basin.

DESCRIPTION OF TEST PROGRAMS

The two separate test programs were conducted. A SALM mooring system and a turret mooring system were tested. Neither was particularly unique. They were realistic systems designed for the environmental conditions tested. Both tankers were able to move in all six degrees of freedom in their moorings.

Test Facility

The test facility used for the two test programs as Arctec Offshore Corporation's Wave and Current Basin. This facility is 42 meters by 26.7 meters (138 feet by 87.5 feet) and up to 1.8 meters (6 feet) deep. The water depth in the basin for the SALM tests was 1.676 meters (66 inches). The SALMfoundation was located in the pit in the center of the basin at a depth of 2.31 meters (90.93 inches) The water depth for the turret moor system was 1.545 meters (60.85 inches).