A pipeline laid on an erodible seabottom introduces a disturbance in the sea water flow which can lead to the onset of scouring below the pipeline itself and to the development of scouring induced non-stationary free spans.

Scouring induced free spans have a variable length, generally increasing with time, and may reach not acceptable conditions before closure due to the mid-span sagging into the scour hole. Hence, both the integrity management and the inspection plan have to face with span length evolution.

Inspection surveys performed during various years, from start up on 2003 to 2015, allowed to identify two different locations of the Blue Stream pipeline system presenting scouring induced free spans.       

A pipe erodible seabed interaction model has been applied to reproduce the observed evolution. The model has been calibrated using the actual wave history experienced during the years by the pipeline applying the NOAA hindcasted wave model. The general behavior of the free spans has been well reproduced. The fatigue cumulated by the free spans during their evolution with the actual climate experienced has been evaluated.

The assessment of the possible further fatigue damage during the rest of the free spans or pipeline life has been probabilistically assessed by applying different possible climate scenarios as resulting from the whole NOAA hindcasting going from 1979 to 2016.

As expected, free spans subject to shorter life and larger metocean loads have been found to be those cumulating the larger fatigue damage.

The expected total fatigue was largely lower than the allowable, hence no remedial action has been planned. The following external inspection has allowed to verify if free span lengths are evolving according to model predictions.