Fatigue Considerations for Ocean Towage

Fatigue damage of the structure, sea fastenings, or barge during ocean towage can be a critical consideration in the choice of barge and tugs and of tow route and marine operations. This paper describes the procedures that were followed to assess the risk of fatigue damage and the practical precautions that were taken to control its incidence during the tow of the jacket for Amoco Norway's Valhall production platform from Morgan City, LA, to Stord, Norway, during autumn of 1980. Damage criteria are identified and calculations are demonstrated for survival of the unit in possible extreme conditions, exposure along the tow route, and derivation of the critical motion curve to limit the rate of fatigue damage to an acceptable level. The analysis of motioninduced stresses is explained. The theoretical survival life of the unit in maximum seas is shown to be very sensitive to the values determined for parameters such as the stress concentration factor (SCF) and the wave period. This paper reports the marine procedures adopted and describes the fatigue audit that was maintained throughout the tow, which indicated that the theoretical fatigue damage was controlled to a low level successfully.