DNV RECOMMENDED PRACTICE RP-E301 DESIGN AND INSTALLATION OF FLUKE ANCHORS IN CLAY

1. General
1.1 Introduction This Recommended Practice features a substantial part of the design procedure developed in Part 1 /1/ of the joint industry project on Design procedures for deep water anchors, and it was developed further through a pilot reliability analysis in Part 2 /2/.  An overview of this project is given in /3/.
1.2 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.
1.3 Structure of the RP Definition of the main components of a fluke anchor is given in Chapter 2, followed by a description of the general behaviour of fluke anchors in clay in Chapter 3.
In Chapter 4 a design methodology based on calibrated and validated analytical tools is recommended in lieu of the current use of design charts. The recommended procedure for design and installation of fluke anchors is presented in Chapter 5.  The close and important relationship between the assumptions for design and the consequential requirements for the installation of fluke anchors is emphasized. General requirements to soil investigations are given in Chapter 6. The intention has been to make the procedure as concise as possible, but still detailed enough to avoid misinterpretation or misuse.  For transparency details related to the various design aspects are therefore found in the appendices. A number of Guidance notes have been included as an aid in modelling of the anchor line, the anchor and the soil.  The guidance notes have been written on the basis of the experience gained through the joint industry project, see /1/ and /2/.
1.4 Definitions Dip-down point Point where the anchor line starts to embed. Fluke Main load bearing component. Fluke angle Angle between the fluke plane and a line passing through the rear of the fluke and the shackle (arbitrary definition). Forerunner Anchor line segment being embedded in the soil (preferably wire, but may also be a chain). Inverse catenary The curvature of the embedded part of the forerunner. Shackle Forerunner attachment point (at the front end of the shank). Shank Rigidly attached to the fluke. Touch-down point Point where the anchor line first touches the seabed.