Iceberg grounding and scouring on the Labrador Continental Shelf

Abstract

Icebergs drifting in seas of the eastern Canadian Continental Shelf present serious hazards to offshore drilling operations. Damage to offshore structures may be caused by direct collisions with a floating or gravity-based structure. Icebergs whose keels touch and plough through or scour the soft sediments of the seabed may crush and rupture seabed installations such as wellheads, anchoring/mooring systems, pipelines and telecommunication cables. Observations made at exploration wells on the Labrador Shelf from 1973 to 1981 are used to delineate areas of active iceberg scouring and to quantify the incidence of grounding and scouring icebergs. The term grounded is used to describe icebergs whose keels have contacted the seabed and which have thus been halted. Scouring icebergs are those whose keels have contacted the seafloor, but which continue to move forward. Observations of more than 1500 icebergs from twenty-two well sites have been analyzed and criteria for identifying grounded and scouring bergs have been established. Over fifty icebergs have been observed to ground and scour in eleven areas. Over the observation periods, the average grounding frequency for Makkovik and Saglek Banks were 3.3% (data collected in seven years) and 4.3% (data collected in six years), respectively. It appears likely, however, that these frequencies are an underestimate and that many more “possible” grounding and scouring icebergs will eventually be included in the data set. We show that many scouring icebergs can move over large (60 km) distances and are able to traverse significant (up to 45 m) ranges in bathymetry. We suggest that they may accomplish this through increases and decreases in draft by continual gradual rotation about a horizontal axis normal to the movement direction.