Effects of Noncolinear Environments on SALM-Yoke Loads

Abstract

ABSTRACT This paper presents the results from a series of model tests conducted to study the dynamic response characteristics of two generic Single Anchor Leg Mooring (SALM) configurations. Both configurations included a storage tanker moored to a SALM via a rigid yoke. Wave/low frequency motions and loads of the SALM-yoke-tanker systems were measured for both collinear and noncolinear environmental conditions, with wind directions varying from 0 to 120 degrees relative to the waves. INTRODUCTION The dynamic response of a tanker mooring system depends upon the combination of wind, wave, and current environmental conditions acting on the system. Both the magnitude and directionality of the wind, waves, and current affect the response. A colinear environmental condition is when all dominant directions of wind, waves, and current are coming from the same direction. A noncolinear environmental condition is when the wind and/or current directions are different from the dominant wave direction. Due to their specific site conditions, past design studies of SALM systems were mostly conducted for colinear environmental conditions and for a limited range of noncolinear conditions with maximum wind angles of 15 to 30 degrees with respect to the waves (References 1 - 3). Under these environmental conditions, dynamic effects due to nonco1inear environment had relatively insignificant differences from those under colinear conditions. Significant dynamic effects resulting from nonco1inear environmental conditions on the SALM-yoke loads will impact the system's integrity during its design life and they should be accounted for in the structural design (Reference 4). The purposes of this study were:to investigate the dynamic effects of nonco1inear environmental conditions on SALM-yoke-tanker systems, andto provide technical data for the validation of analytical tools for predicting SALM system response under nonco1inear environments. There are two basic configurations of SALM-yoke systems currently used as offshore production terminals: a one-piece, buoyant-tower type SALM, and a two-piece, articulated SALM. The first type is normally used for water depths below 450-500 ft. The second type has a middle articulation between the buoy and the riser, and it can be applied for water depths greater than 450-500 ft. The purpose of the middle articulation is to minimize the bending moments on the SALM structure. However, the dynamic behaviors of the two SALM configurations could be different due to the extra articulation. DESCRIPTION OF MODEL TEST PROGRAM A series of model tests were carried out during 1986 to study the two basic SALM-yoke systems. Both systems included a storage tanker moored to the SALM via a rigid yoke. The objective of the model test program was to obtain technical data on dynamic responses (both low/wave frequency motions and reaction loads) of the SALM-yoke-tanker systems when subject to various co1inear and noncolinear combinations of wind, waves, and current conditions. The model tests were conducted in the Ocean Basin at the Norwegian Marine Technology Research Institute (Marintek) in Trondheim, Norway. The Marintek model testing facility has a dimension of 80 meters long, 60 meter wide, and a variable water depth up to 10 meters.