Design Procedures for Deepwater Anchors in Clay
Authors: Dahlberg R. 
Published: October 2008

This paper gives an overview of an ongoing Joint industry project (HP) on design procedures for deepwater anchors in clay. A new procedure for design of fluke (drag embedment) anchors is described. Ongoing work towards a design procedure for drag-in plate anchors, or vertically loaded anchors (VLA), is briefly addressed.

Introduction

Presently many parallel efforts are being made to develop procedures and analytical tools required for design of mooring/anchoring systems for the deep waters. Anchors represent a vital component for the safety of the mooring system as installed, and the safety requirements for floating production units arc higher than for drilling units. The traditional use of nuke anchors is for temporary mooring of mobile drilling units. More recently, however, fluke anchors have been used also for permanent anchoring of floating production units. Until recently, anchors in a catenary system were not allowed 10 take inclined loads, the mooring line must intersect the seabed under zero angle for the design load. l1le rules have been relaxed on that point, such that nonzero uplift angles at the seabed can be accepted under certain conditions. The current methods for design of fluke anchors are highly empirical, using power formulae with the ultimate anchor resistance being related to the anchor weight (resistance used instead of holding capacityin this paper). These relationships plot as straight lines in a Jog-log diagram, see e.g. Figs. 5.2 and 5.3 of the API RP 2SK (Ref. I). It is recommended that this design practice is replaced with a design approach, which utilizes recognized theoretical models and geotechnical design principles. According to the design procedure for nuke anchors developed as a result of the current HP the characteristic anchor resistance Rclrar is made up of two main contributions (see glossary and definition of terms under Nomenclature).The measured installation resistance Rdip. which is equal to the target installation load Fdip at the dip-down point.11le predicted contribution from sliding resistance along thc length Ls of anchor line at the seabed during design loading and the post-installation effects due to consolidation ?Rcons and cyclic loading ?Rcy,(mathematical equation)(available in full paper)

Provided that the uncertainty in the load measurements is accounted for and that the target installation load Fdip is reached and maintained during a prescribed period of time, the uncertainty in the installation resistance Rdip can be disregarded. The main uncertainty lies then in the predicted post-installation effects. The design resistance Rd will then be(mathematical equation)(available in full paper)

where ?m, is the material coefficient. For floating production units with catenary mooring systems the choice has been between fluke anchors, suction anchors and pile anchors. As the exploration for and production of oil and gas has reached ever-deeper waters the conventional catenary systems have become too heavy. This has led to the introduction of taut mooring systems (TMS) with synthetic fibre ropes instead of wires and chain, In a taut mooring system the anchors will be subjected to significant vertical load components, which the anchor needs to be designed for.