Abstract:
Internal corrosion has long been acknowledged as one of the dominant forms of
deterioration process that contribute to the containment loss for marine steel pipelines. Aging and
deteriorating pipelines under influence of corrosion threat may experience a serious reduction of
their structural integrity and can lead to eventual failure. To secure pipeline safety for a long and
profitable life, the operators need to develop their own risk-based inspection schedule for future
inspection and maintenance activities. A deterministic methodology for predicting the remaining
strength of submarine pipelines subjected to internal corrosion using a capacity equation as included
in the DNV RP-F101 (Part A) code is described in this paper. The equation can be used to estimate
the maximum allowable operating pressure of the corroding pipelines based on a series of pigging
data, which represents the corrosion pit location and dimension. The introduction of partial safety
factors in the Part A of the DNV RP-F101 code is tailored to minimise the effect of uncertainties
due to defect sizing. The authors have added prediction capabilities to the capacity equation by
introducing a standard deviation model of future predicted defect depth. By doing so, the variation
of safety factors of the capacity equation has been manipulated to that extend where prediction of
future pipeline remaining life-time becomes feasible. The paper demonstrates derivation of time function standard deviation equation, Std(d/t) of tool error, calculation and prediction of pipeline
remaining lifetime subject to internal corrosion. The increment of standard deviation of corrosion
depth, Std(d/t) was addressed since it can affect the value of partial safety factor as corrosion
progresses, hence amplifying the conservatism of time to failure. The prediction results shows that
the dynamic of safety factors has successfully downgraded the structure resistance as corrosion
progresses to reflect the actual condition of the pipeline on site. The technique to evaluate future
pipeline remaining lifetime can effectively assist pipeline operators to evaluate future safe operating
strategies including re-inspection and appropriate maintenance schedule. As a result, it can minimise
the likelihood of pipeline failures until it reaches its designed lifetime.
