New Calibrated Design Code for Plate Anchors in Clay
Authors: Dahlberg Rune , Ronold Knut O. , Strøm Pål J. , Mathisen Jan 
Published: October 2008

This paper describes the features, advantages and limitations of a new design code for plate anchors in clay, which is applicable to both production and mobile platforms. The code is the final result of a joint industry project on plate anchors. The design code prescribes the use of partial safety factors that are calibrated on the basis of structural reliability analysis to meet specified annual failure probabilities. For the geotechnical design of plate anchors, two limit states and two failure consequence classes are specified with partial safety factors on load and resistance given for each combination. The allowance for use of reduced safety factors when the failure consequence is small is an advantage particularly for mobile platforms, as discussed in the paper, at the same time as higher safety factors can be applied for production platforms. For anchors in clay subjected to storm loading, the effect of cyclic loading on the clay undrained shear strength normally leads to an increase in the anchor resistance compared with the resistance during static loading. A methodology for calculation of the cyclic shear strength for use in the anchor design is presented in the paper.

It is emphasised that the design of plate anchors must account for the accuracy by which a plate anchor can be installed and to ensure that the design assumptions are met by use of adequate means for control of the plate anchor installation. It is emphasised in the paper that installation aspects must be addressed already at the design stage.

Introduction

The development of design codes for plate anchors has gone hand in hand with the testing of various types of plate anchors over the years starting with the work by the US Navy on direct embedment anchors for shallow water applications many years ago, see /1/.

The current edition of the API RP2SK /2/ is under revision and the anchor part will be significantly revised and updated. An ISO code for stationkeeping system design /3/ is also under development, which will address anchors. In addition, the anchor manufacturers have their own in-house design methods, e.g. Vryhof Ankers /4/. The DNV-RP-302 /5/, which is the subject of this paper, addresses all types of plate anchors.

The first plate anchors developed for deepwater application were of the type drag-in plate anchors, or Vertically Loaded Anchors (VLA). Since 1994 the Denla /6/7/ and the Stevmanta /8/ plate anchor concepts have been tested both offshore, near shore and onshore /9/, and design methods for drag-in plate anchors evolved based on a combination of theory and empiricism. Later other types of plate anchors were developed, which are driven into the seabed either by suction, e.g. Sepla /10/ and SEA /11/, or by pushing, e.g. Pader, which is briefly described in /5/. Plate anchors have been designed and installed offshore for commercial purposes since 1998, when Vryhof installed 16 Stevmanta VLAs for an FPSO at the Voador Field (P-27) for Petrobras /8/, which was followed up by four more Stevmanta VLA installations for FPSOs in Brazil, the latest in 2003.