Mooring lines, risers and umbilicals are critical components of subsea oil and gas field developments. Indeed, they have to carry out their respective duties over several decades, facing extreme storms, without any maintenance. A key issue to address is interference, implying that the systems have to be arranged in a fashion that ensures the various components will, as much as possible, not clash with their neighbors. The present paper proposes a probabilistic approach to assess the criticality of overlaps involving a mooring line and a riser/umbilical.
In order to assess, the relevancy of riser or umbilical configurations, a wide range of analyses are conducted. Principally, these analyses include extreme analyses to confirm the design can sustain violent storms, fatigue analysis to verify the design is in line with the design life of the field and interference analyses. Regarding the last one, common approaches consist in ensuring that lines remain sufficiently far from one another. If a "no-contact criteria" cannot be met then probability of impact scenario as well as severity of impact shall be properly documented to confirm no damage occurs. When mooring lines are involved, DNVGL RP F203 states that "to avoid damage on risers due to mooring line failures it is normally not allowed that a mooring line crosses above the riser in any condition." Then, one could wonder where to draw the line between allowable overlaps and unacceptable ones. For instance, would an overlap induced by a 10 year return period storm be acceptable? What about an overlap due to a 100 year return period storm?
In a tentative to rationalize the discussion, the present paper proposes a probabilistic approach to define what is acceptable and what is not. The governing approach of most offshore standards is to associate an acceptable probability of failure with a given consequence, as presented in DNVGL ST F201. A similar approach can thus be applied to a riser damage due to a mooring line failing while overlapping it. The principle of the proposed approach is the following:
A safety class is associated with the riser based on its service type;
From the safety class a nominal acceptable failure probability is derived;
An assessment is then carried out in order to determine the failure probability related to a simultaneous overlap and mooring line failure;
The computed probability of failure is then compared to the allowable one to confirm whether overlaps are acceptable or not.
