Unique Onshore Tests of Deepwater Drag-In Plate Anchors
Authors: Dahlberg Rune , Strøm Pål J. 
Published: October 2008

Instrumented drag-in plate anchor tests have been performed under controlled conditions in an onshore well-documented clay deposit. Detailed measurements of the load-anchor-soil interaction have given an improved insight into the behaviour of drag-in plate anchors and a basis for design and specification of installation criteria for this type of anchors. The test anchors, type Denla (Bruce Anchor) and Stevmanta (Vryhof Ankers), were about 30-40% of prototype size and offshore installation and testing procedures were adopted.

Introduction

Drag-in plate anchors have been developed for use in combination with taut mooring systems and they can resist both the vertical and the horizontal loads transferred to the anchors in such a system. This anchor is installed as a conventional fluke anchor, see Ref. 1, and when the target installation load Fdip, equal to the horizontal component of the line tension in the dip-down point, has been reached it is triggered to receive the line tension normal to the fluke. In this normal loading mode the anchor acts as an embedded plate with a high pullout resistance Rp.

The testing of drag-in plate anchors has so far concentrated on obtaining experience from offshore handling and installation of such anchors, whereas the test data as required for a geotechnical interpretation of the results have been too crude. Also, the soil information is often lacking detail, which increases the uncertainty in the assessment of the anchor behaviour.

Therefore, the driving argument for proposing drag-in plate anchor tests on an onshore site was to eliminate some of the inevitable uncertainties, which are attached to the parameters measured in offshore tests. The intention was to monitor the anchor-soil interaction versus the line tension both during installation and pullout of the anchor, and in addition to measure several other parameters that would be asked for in a back-fitting analysis of the results. Besides giving more reliable and conclusive results, onshore testing is much less expensive than offshore testing, and practically weather independent.

Drag-in plate anchors, previously called vertically loaded anchors (VLA), are available in two versions, the Denla anchor from Bruce Anchor and the Stevmanta anchor from Vryhof Ankers. These anchors have been under continuous development during the last 5 years through co-operative efforts between the anchor manufacturers and the offshore industry. Significant work on drag-in plate anchors has been carried out through the Aker Omega JIP (1994), the Aker Maritime JIP (1995), the DeepStar project (1996) and the Petrobras tests (1994-1997). The results from these offshore tests, and the onshore tests described herein, are being analyzed by Det Norske Veritas (DNV) as part of the deepwater anchor project, see Ref. 1, where one objective is to develop a design procedure for drag-in plate anchors.

Petrobras installed 12 Stevmanta drag-in plate anchors as part of a taut mooring system (TMS) for a semi-submersible production unit at the Marlim field in 1998, which is the first commercial use of drag-in plate anchors, see Ref. 2 and Ref 3.

The anchor performance ratio Pr is normally defined as the ratio between the installation pullout resistance Rpi and the horizontal component Fdip of the line tension in the dip-down point, i.e. Pr = Rpi/Fdip, as measured in an offshore test. This parameter has shown some scatter in the tests reported, see e.g. experience with the Denla anchor in Ref. 4. This scatter is to a great extent explained by the uncertainty in the