Interaction of dragging anchors with offshore pipelines is one of the most serious accidents the pipeline may experience. In case the risk of a rupture with leakage is considered not acceptable, a trench and a cover so deep and protective to reduce significantly the risk of interaction, has to be envisaged. In this paper main parameters of concern for risk of pipe damage from dragging ship anchors are discussed, in particular the ones which give a significant contribution to the assessment of the frequency of pipe-anchor interaction and of the pipe damage.
The risk of anchor damage to offshore pipelines is a major subject in offshore pipeline technology, as it' can influence a decision about the burial depth and protection cover. A wide bibliography is available concerning ship traffic analysis and accidental scenarios, studies on the mechanics of anchor-soil interaction, experiments on the pipe capacity to absorb impact and hooking loads, design applications to define the minimum measures which could ensure an acceptable level of risk of damage. Brown (1974) has discussed the main design parameters for minimum depth recommendations vs. estimated costs of repair, while Mousselli et al. (1978) have highlighted the importance of mechanics of anchor-soil interaction and of pipe capacity to absorbe energy. Colquhoun (1985) provides a synthesis of the studies carried out in the early '80s for the Danish Great Belt Gas Trasmission Crossing Project, which is still considered to be the most complete experience on the risk of ship interference with submarine pipelines. The interest on this subject has increased following de la Mare and Andersen (1980) studies which gave a synthesis of pipeline failure history, indicating a significant contribution of the anchor damage to failure statistics and stressing its importance for an assessment of safety of pipelines.
