Field Measurement Of Very Large Crude Carrier (VLCC) Mooring Loads
Authors: Fang S. 
Published: June 2014

The proper mooring system design for very large crude carriers (VLCCs) is necessary to assure safe loading or unloading operations. The design of a conventional pier mooring system involves the predictions of static wind and current loads acting on the tanker using theoretical analysis and/or model test data. In order to verify these prediction procedures, Exxon Research and Engineering Co. conducted a series of experimental studies at two tanker piers handling vessels up to 280,000 deadweight tons. The experimental studies utilized two of the earliest operating mooring load monitoring systems for VLCC piers. The force in each mooring line was monitored and recorded during the measurement period. Fender deflections and environmental conditions such as wind and current were also recorded. From the measured mooring line and fender force data, the resultant load acting on the tanker can be evaluated. The resultant load is then compared with the design load based on the measured environmental condition. In addition, the experimental study included the recording of the tanker dynamic response when subject to natural wind.

This experimental study demonstrated that industry wind and current load data (OCIMF Prediction of Wind and Current Loads on VLCCs) are adequate in predicting the static wind and current load acting on a moored tanker. It further indicated that analytical procedures based on theory and small scale model studies are applicable in prototype situations. However, more research would be beneficial in understanding the dynamic response of a moored VLCC while subjected to natural wind. Better understanding of this phenomenon could result in more efficient/economical mooring systems for large tankers.

INTRODUCTION

Full scale measurements at piers has been a continuing research and development program of Exxon Research and Engineering Company since 1970. One of the objectives of the full scale investigation was to verify ER&E's VLCC mooring load prediction procedures against actual prototype loads. The mooring load prediction procedures are based on theory and model test data. The procedures verified in this field experiment are the prediction of wind and current loads acting on the tanker. In addition, the dynamic response of a moored tanker when subject to high natural wind was investigated.

Vessel mooring line forces and pier fender deflections were measured along with the corresponding environmental conditions at the terminal. The data on vessel mooring line forces were collected with the mooring hook load monitoring systems installed at two VLCC terminals of Exxon affiliated companies. In addition to providing useful operating information to the terminals mooring hook load monitoring systems made an accurate full scale measurement study possible.

This paper summarizes the data collection procedure and the results of the analysis of the mooring force records. Resultant environmental loads based on recorded mooring forces were compared to those loads calculated by the present prediction methods for the same environmental and mooring conditions. The adequacy of the prediction method was evaluated by this comparison.