SS: MODU Anchor: Improved Predictions of MODU Fluke Anchors Increases the Confidence in Mooring System Design
Authors: Dahlberg Rune , Resseguier Sebastien 
Published: July 2009

An overview of the current Norwegian and international requirements for design and installation of fluke anchors is given. Important design and installation issues are discussed with special attention on the specification of the anchor installation tension, which is the single most important factor for the reliability of the as-installed anchors.

The paper describes the features of a design tool, which can assist in the specification of the minimum installation tension needed to meet the governing safety requirements.

Examples of analyses of fluke anchors both in deep deposits of soft clay and in layered soil illustrate how results from anchor analyses are used to document the design and to assist in the planning of an anchor installation.

Introduction

The current international guidelines for design and installation of fluke anchors for MODUs are developed with focus on anchors installed in soft to medium clay and for design situations where potential uncontrolled anchor drag during the design event is assumed to have no serious consequences for adjacent installations or the environment. Unfortunately, this design philosophy is frequently seen adopted also when designing fluke anchors in design situations where only limited or no drag can be tolerated, which may lead to serious consequences.

However, after the devastating hurricanes Ivan in 2004 and Katrina and Rita in 2005 in the GOM the API Interim Guidance for the 2006 hurricane season [3] was issued, which set a high focus on the need for improving the MODU mooring design practice. Among the recommended actions were that the number of lines should be increased, the anchors should be upgraded, and the mooring installation plan should give more attention to the installation control of fluke anchors. The anchor part of the current APIRP2SK [4] has been significantly updated compared with the previous edition, although no significant changes are observed in the recommendations for fluke anchors, which continue to focus on design situations characterized by soft soil and the governing threat to mooring systems from hurricanes. Uncritical use of the recommendations in [4] for design of fluke anchors in other design situations can lead to serious consequences. The 2SK work group states [19] that the present 2SK methodology is developed for low consequence operations, whereas no guidance is given for high consequence operations, although for the majority of the MODU operations in the GOM the high risk is the more important case. It is therefore essential to provide design guidelines also for these cases. The fluke anchor part of the existing edition of ISO 19901-7 [13] is largely copied from [4], but the forthcoming (2nd) edition [14] is expected to provide more guidance for how to design and install fluke anchors.

The governing criteria for design and installation of fluke anchors should be defined as early as possible in a project. The design of fluke anchors, and the installation planning, benefit significantly from access to relevant site-specific soil data. The focus should be on assessment of the minimum installation tension required to meet governing safety requirements for the anchors. The capacity of the installation equipment may sometimes be insufficient to meet the prescribed minimum installation tension requirement, which means that the as-installed anchor safety factors will be less than required for the most severe environmental conditions during the predicted operational period. Norwegian national requirements request in such cases that the additional drag in case of overloading is predicted and checked against the project tolerance level.

The results from fluke anchor predictions at an early stage will provide a basis for identifying critical design and installation issues well ahead of the installation, which may avoid discussions at a later, more time-constrained, stage.

In layered soil, information about the thickness and strength of all significant soil layers is needed to predict the optimal combination of anchor size and fluke angle to assist in the analysis and planning of the anchor installation.