Scope and application

This Recommended Practice applies to the geotechnical design and installation of fluke anchors in clay for catenary mooring systems. However, the principles for design and installation of fluke anchors are applicable also to other types of soil; see section [5.2.3] and App.B. The basis for calculation of the minimum anchor installation tension, which meets the governing safety requirements, is addressed in section [5.4].

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 anchor resistance following a one-line failure shall be treated as an accidental damage limit state (ALS) condition.

For the anchors in a mooring system to satisfy the safety requirements, the anchor drag must be tolerable both during installation and during the governing design event. In section [5.2.3] the focus is set on the significance of the soil conditions for the potential consequences of anchor drag during extreme environmental events.

If anchor drag may lead to unacceptable consequences for various reasons, as discussed in section [5.3], the prediction of anchor drag during the ULS or ALS condition becomes a design issue.

The anchor failure related to excessive drag has been defined as either of the following events:

— Anchor failure

Continuous anchor drag experienced before the required anchor resistance is reached;

— Excessive additional drag

The additional drag required to resist the design tension in any of the lines leads to a breach of the safety factor with regard to breaking strength of the adjacent mooring lines;

— Threat to adjacent installations

Predicted anchor drag length violates the safety distance between the moored structure/anchor/mooring line and adjacent structures after dragging;

The line tension model adopted herein splits the tension in a pretension and a environmental component; see background in /4/ and /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.

The design rule presented herein has been calibrated based on reliability analysis of one test case as documented in /9/. The partial safety factors are considered to be tentative until further calibrations have been carried out.

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