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ABSTRACT: The paper describes the background for the safety factors required by the two codes and show, by FE calculations, how the use of the different codes may affect the anchor size. Typical soil strength profiles and anchor chain loads for deep water West Africa, Gulf of Mexico and North Sea have been used as input for the calculations. The results are presented and show that the DNV code was more conservative for two of the three cases but that the difference depends in particular on the ratio between mean tension and total tension and the target probability of failure one aim at for a particular design case.
1 INTRODUCTION
1.1 General
Suction anchors have frequently been used as anchoring for floating production systems at offshore oil and gas fields around the world since the early 1990s. The anchors are considered reliable with no reported failures due to loss of holding power or dragging to date. Two common anchor design codes, APIRP2SK and DNV-QS-E303, have been compared with respect to anchor sizes required to fulfill the respective code safety factors.
1.2APIRP2SK-background and requirements
The current recommendations in APIRP2SK were formulated in the early 2000s and is based on "expert opinion" from industry engineers and an initial target annual probability of failure of 10-4. Safety factors are given as lump factors with no distinction between load factor and soil material factor. Recent reliability work (Clukey et al., 2013) have shown that with current API practice for an intact mooring system the annual probability of failure for a single suction anchor could range from 10-5 to 10-6 depending on design (intact vs. 1-line damaged), type of mooring system (taut vs. catenary) and loading conditions (wave vs. loop current).
Table 1 summarizes the lump safety factors required by API for permanent mooring of floating structures.
1.3DNV-OS-E303-background and requirements
DNV-OS-E301 /5/ uses a safety format based on partial safety factors on the mean and dynamic component of the line tension combined with a material factor on the anchor resistance.
In addition, two consequence classes (CC1 and CC2) are defined depending on the consequences of a failure. The load and material factors will vary depending on these consequence classes as presented inTable 2. These partial factors have been calibrated...