scope:

Scope and Application

This Recommended Practice applies to the geotechnical design and installation of fluke anchors in clay for catenary mooring systems

The design procedure outlined is a recipe for how fluke anchors in both deep and shallow waters can be designed to satisfy the requirements by DNV.

According to this recommendation the geotechnical design of fluke anchors shall be based on the limit state method of design. For intact systems the design shall satisfy the Ultimate Limit State (ULS) requirements, whereas one-line failure shall be treated as an Accidental Damage Limit State (ALS) condition.

For the ULS, the failure event has been defined as the inception of anchor drag. Subsequent drag of any anchor is conservatively assumed to imply mooring system failure in the ALS. This avoids the complexity of including uncertain anchor drag lengths in the mooring system analysis. Thus, the ALS is formulated to avoid anchor drag, similarly to the ULS.

The line tension model adopted herein splits the tension in a mean and a dynamic component, see background in /4/, which differs from the line tension model adopted in the current DNV Rules for Classification of Mobile Offshore Units /5/

Traditionally, fluke anchors have been designed with the mandatory requirement that the anchor line has to be horizontal (zero uplift angle) at the seabed level during installation and operation of the anchors. This requirement imposes significant limitations on the use of fluke anchors in deeper waters, and an investigation into the effects of uplift on fluke anchor behaviour, as reported in /1/, has provided a basis for assessment of an acceptable uplift angle.

Until the design rule presented herein has been calibrated based on reliability analysis the partial safety factors will be tentative.

This recommendation is in principle applicable to both long term (permanent) and temporary moorings.