Reliability Methods are presented for assessment of environmental hazards to submarine pipelines. The focus is on the hazard associated with free spans induced by sediment instability. Considered limit states are fatigue and overload failure with consequences as leak and rupture. Failure probability is calculated using both the complete and simplified reliability approaches. Strategies for in-service inspections are considered. Methods for reliability model updating are also presented, which can be practically applied in determining optimal inspection intervals or burial measures based on target safety levels. The methods presented are illustrated with a case study.
Environmental hazards are of primary concern in the design of submarine pipelines which are exposed to a combination of severe meteo-marine conditions, relatively shallow water and potential sediment instability. This is the case of pipelines crossing the Southern North Sea, i.e. the Dutch, German and Danish territorial waters where free spans developing in a short period have been experienced, in spite of the efforts made during installation, sometimes even after extensive post-trenching works. A pipeline lowered into the seabed by post-trenching works with concrete coating designed to guarantee on-bottom stability to the exposed pipeline in the early stage of construction could during later days be subject to exposure, e.g. as a consequence of liquefection of backfill or activity of bedforms. Consequences of excessive lateral displacements might lead to serviceability problems such as interference with neighboring structures and seabed obstacles or excessive ovalization at the excessively strained section, however, not expected to evolve in the short time to leaks and rupture, Sotberg (1990). This could be a potential evolution of unexpected exposure or a possible scenario foreseen in the design stage, however, not properly quantified to formulate any remedy in the short term as expected not to give rapid evolution to critical conditions.
