Fatigue Considerations for Ocean Towage
Authors: Hambly Edmund C. , Edwards Anthony J. , Kohli Colin K. , Miller Barry L. 
Published: December 2007

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 motion-induced stresses is explained, including the rapid methods of calculation adopted to derive hot-spot stresses around a complex multichord overlapping joint, and to adapt standard S-N curves for direct use for Rayleigh-distributed stresses. 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.

Introduction

The jacket for the Valhall production platform was loaded out onto the barge Intermac 600 on Aug. 9, 1980 at Morgan City. Fig. 1 shows the 4,737-long-ton-jacket on the 500- X 120- x33-ft barge. The tow began on Aug. 25 when all weather forecasts were favorable, and the barge was towed by four river tugs to Bayou Boeuf. From there the 5,000-hp Dallas J. Adams towed the unit 20 miles offshore to hand over to the deep-draft, ocean-going salvage tug Atlantic, 140-ton bollard pull. (Throughout the paper the term "unit" is used to describe the total structure of the jacket, tie-downs, and barge.)

The tow route is shown in Fig. 2. The tow proceeded without serious barge motions until Sept. 8, when Hurricane Earl passed nearby, and the tug had to heave to for a period to limit barge motions to below the limits previously set for control of the rate of fatigue damage. previously set for control of the rate of fatigue damage. Subsequently, steady progress was maintained until a storm was encountered on Sept. 18 near 44 deg. N 21 deg. W, when the tow line broke for reasons not fully understood. This was close to the predetermined point at which the Atlantic was to be joined by the tug Caribic, 84-ton bollard pull, for the remainder of the tow. The unit was subjected to uncontrolled motions for 32 hours, after which the tow continued with both tugs until Stord was reached on Sept. 30. At Stord the unit was inspected thoroughly and was found in good condition.

The following sections, through Sensitivity of Fatigue Calculations, describe the studies carried out before the tow to assess the risk of fatigue damage and to determine the critical motion curve for the unit to limit the rate of fatigue damage to an acceptable level. Fatigue Audit gives details of the fatigue audit maintained during the tow, and Implications for Marine Operations indicates the implications of fatigue damage control on marine operations.

Fatigue Damage Criteria for Tow

Three sets of fatigue loading conditions were considered for the tow.

JPT

P. 2967