Influence of Wave-Induced Iceberg Heave Motions on Risks to Subsea Equipment

Abstract

Abstract In offshore regions where risk of iceberg contact with subsea equipment is unacceptably high, protection options include reduction of the probability of impact by placing the equipment below the seabed in an excavation, and use of protective structures to reduce the probability of damage given an impact. An initial estimate of the probability of impact from a freely drifting or scouring iceberg may be determined based on the chance that an iceberg with draft greater than the top of the subsea equipment drifts directly over it. A more detailed analysis might consider the increase in probability of impact as a result of iceberg heave and pitch motions. In this paper, a methodology is presented for assessing the importance of heave motions of freely floating icebergs on probability of impact with subsea equipment protruding above the seabed. The method incorporates numerical and statistical modeling of random icebergs motions in irregular seas, including shallow water effects. Consideration is given to the mean clearance of the iceberg above the equipment, the random iceberg wave-induced heave motions, the geometries of the structure and iceberg and the time for the iceberg to pass over the site. Analyses are conducted on the effect of wave-induced heave motions of icebergs in wind-generated sea states and swell in water depths of 95 m and 60 m. A number of iceberg shapes are considered. It is found that for the conditions considered, heave motions do not have a significant influence on impact probability. In evaluating the previous methods for estimating wave-induced iceberg motions, some deficiencies in previous iceberg heave and surge response amplitude operators were discovered, and improvements are recommended when estimating iceberg impact loads with gravity based structures and floating systems.