Geotechnical Design of a Grillage and Berm Anchor
Authors: Erbrich C.T. , Neubecker S.R. 
Published: October 2008

This paper describes the geotechnical design of a novel anchor system for a catenary CALM buoy mooring termed a 'Grillage and Berm Anchor'. This anchor system comprised a steel grillage (to which the mooring line is attached) with a rock berm built on top of and around the grillage to provide the necessary holding capacity. This paper describes the first known application of this type of anchor system, which wason the North West Shelf of Australia. For this case the anchor system was placed upon a seabed comprising a thin layer of carbonate sand overlying calcarenite. The design posed several unusual challenges which will be discussed, including global stability and local failure modes of the system under both static and cyclic environmental loads, as well as design of the berm to resist erosion under cyclonic storm conditions.

Introduction

The grillage and berm anchor was devised as an alternative to a standard gravity box anchor. On the North West Shelf of Australia most readily available construction vessels are relatively small and have cranes of modest lifting capacity. This poses considerable problems for a standard gravity box design since these are generally too heavy for installation in a single lift and would therefore need to be constructed out of several pieces and assembled on the seabed, which is a costly operation. Abundant supplies of iron ore are available locally and can be supplied and placed at relatively low cost in order to provide the necessary ballast. The basic principal of the grillage and berm anchor is therefore to use a smaller amount of fabricated steel work (the grillage) and a greater amount of iron ore (the berm) than in a traditional gravity box anchor.

Grillage and Berm Anchor-Geometry

For the application considered in this paper, the grillage and berm anchors were configured such that two mooring lines were attached to one grillage and berm anchor. Six mooring lines were used in total, connected to three grillage and berm anchors.

The general arrangement of a typical grillage and bermanchor is shown on Fig. 1. The steel grillage is characterised by a strong composite beam at the front, configured in the form of a truss arrangement of individual elements. This is equired to distribute the anchor line loads throughout the grillage and in order to mobilise the berm material placed in front of the grillage. Behind this beam arrangement is a simple square grillage of beams which effectively forms a mattress that is dragged behind the front beam, whilst mobilising the weight of berm material above.

The geometry of the iron ore berms was optimised at each of the three berm locations, in order to allow for different loading conditions appropriate to each case. Berm slopes of 1:3 were adopted throughout, since this was considered achievable during an underwater rock dumping operation.

One grillage geometry was adopted at all anchor locations for ease of fabrication and installation.

Design Loads and Factors of Safety

The grillage and berm anchors were designed to resist loads generated during a 100 year cyclonic event. Mooring line loads were developed assuming all lines remained intact and for the one-line broken condition. Considerable directionality of the prevailing weather conditions meant that substantially different loads acted on each of the three anchors.

For the broken line case, a small global torsion was also induced on the anchor due to the eccentricity of the padeye location from the centreline of the grillage.