Applying Partial Safety Factors in Mooring System Design

The DNV offshore standard for position mooring, DNV-OS-E301 [1], was issued June 2001 based on the result from a joint industry project. A new revision was issued in October 2004. The consequences for mobile units when applying the new standard, compared to the old class rules, have been a major concern for operators of mobile units. A comparison study has therefore been initiated, where four relevant units are considered. We have applied the mooring design for existing mobile units that operate in Norwegian waters. Two different water depths are covered. The new standard, DNV-OS-E301, applies a partial safety factor format. However, the main difference when applying DNV-OS-E301 compared to the old POSMOOR [2] rules is that low frequency (LF) motions will have to be taken into account when calculating the line tensions. The results for the four mobile units are compared also to other relevant codes, i.e. the Draft International Standard ISO 19901-7 [4] and the present Norwegian regulations for offshore structures, NMD [5 & 6]. It should be noted that the present NMD regulations still do not require that LF motions are taken into account. Due to the partial safety factor format in DNV-OS-E301, more or less all of the units fulfill the requirements even though LF motion is accounted for. However, when comparing the results to the NMD regulations, the introduction of LF motion is crucial for almost all of the mobile units studied, as they have problems in fulfilling the requirements when this response is accounted for. Simply including LF motion in design would tend to increase the required strength of the resulting mooring line designs, and thereby raise the safety level if nothing else is done with the NMD regulations. Thus, provided that the present safety level for mobile units is sound, the present NMD safety factors could either be reduced or the partial factor format in DNV-OS-E301 could be adopted in order to maintain the safety level for mobile units when LF motion is taken into account.