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Design and Installation

of Fluke Anchors

MAY 2012

The electronic pdf version of this document found through http://www.dnv.com is the officially binding version



DET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life, property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and consultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries out research in relation to these functions.

DNV service documents consist of among others the following types of documents:

— Service Specifications. Procedural requirements.

— Standards. Technical requirements.

— Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:

A) Qualification, Quality and Safety Methodology

B) Materials Technology

C) Structures

D) Systems

E) Special Facilities

F) Pipelines and Risers

G) Asset Operation

H) Marine Operations

J) Cleaner Energy

O) Subsea Systems © Det Norske Veritas AS May 2012 Any comments may be sent by e-mail to rules@dnv.com This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document, and is believed to reflect the best of contemporary technology. The use of this document by others than DNV is at the user's sole risk. DNV does not accept any liability or responsibility for loss or damages resulting from any use of this document.

Recommended Practice DNV-RP-E301, May 2012 Changes – Page 3


General This document supersedes DNV-RP-E301, May 2000.

Text affected by the main changes in this edition is highlighted in red colour. However, if the changes involve a whole chapter, section or sub-section, normally only the title will be in red colour.

Main Changes

• General Total revision of the document with same title from May 2000; i.e. all text is considered new text, and appears

as clean black text. In addition to further clarification of the previous text the following have been added:

— New design chart allowing for anchor drag under certain conditions.

— Tentative guidance for design of fluke anchors in sand.


–  –  –


1. General

1.1 Introduction

1.2 Scope and Application

1.3 Structure of the RP

1.4 Definitions

1.5 Abbreviations

1.6 Symbols and explanation of terms

2. Fluke Anchor Components

3. General fluke anchor behaviour

4. Methodology for fluke anchor design

4.1 General

4.2 Design charts

4.3 Analytical tools

4.4 Anchoring risk assessment

5. Recommended design procedure

5.1 General

5.2 Alternative design procedures

5.3 Tentative safety requirements

5.4 Minimum installation tension

5.5 Step-by-step description of procedure

6. Requirements for Soil Investigation

7. References

Appendix A. Analysis tool for fluke anchor design

Appendix B. Anchors in layered clay or stiffer soil

Appendix C. Installation and testing of fluke anchors

Appendix D. Setup effect on anchor friction resistance

Appendix E. Effect of cyclic loading

Appendix F. Uplift angle at the seabed

Appendix G. General requirements for soil investigations

–  –  –

1. General

1.1 Introduction General references are found in Section 7 and given the format /no./.

This Recommended Practice features a substantial part of the design procedure developed in Part 1 /1/ of the joint industry project (JIP) on Design procedures for deep water anchors, and it was developed further through a pilot reliability analysis in Part 2 /2/. An overview of this JIP is given in /3/.

The experience gathered through a more recent JIP, which focused on the analytical procedure for design of fluke anchors in clay /13/, has led to significant improvements, which have been implemented in this revision of the RP.

The experience gathered through a number of anchoring projects for mobile drilling units and production platforms has also been considered.

1.2 Scope and Application This Recommended Practice applies to the geotechnical design and installation of fluke anchors in clay for catenary mooring systems. However, the principles for design and installation of fluke anchors are applicable also to other types of soil; see Section 5.2.3 and Appendix B. The basis for calculation of the minimum anchor installation tension, which meets the governing safety requirements, is addressed in Section 5.4.

The design procedure outlined is a recipe for how fluke anchors in both deep and shallow waters can be designed to satisfy the requirements by DNV.

According to this recommendation the geotechnical design of fluke anchors shall be based on the limit state method of design. For intact systems the design shall satisfy the Ultimate Limit State (ULS) requirements, whereas anchor resistance following a one-line failure shall be treated as an Accidental Damage Limit State (ALS) condition.

For the anchors in a mooring system to satisfy the safety requirements, the anchor drag must be tolerable both during installation and during the governing design event. In Section 5.2.3 the focus is set on the significance of the soil conditions for the potential consequences of anchor drag during extreme environmental events.

If anchor drag may lead to unacceptable consequences for various reasons, as discussed in Section 5.3, the prediction of anchor drag during the ULS or ALS condition becomes a design issue.

The anchor failure related to excessive drag has been defined as either of the following events:

— Anchor failure Continuous anchor drag experienced before the required anchor resistance is reached;

— Excessive additional drag The additional drag required to resist the design tension in any of the lines leads to a breach of the safety factor with regard to breaking strength of the adjacent mooring lines;

— Threat to adjacent installations Predicted anchor drag length violates the safety distance between the moored structure/ anchor/ mooring line and adjacent structures after dragging;

The line tension model adopted herein splits the tension in a mean and a dynamic component; see background in /4/ and /5/.

Traditionally, fluke anchors have been designed with the mandatory requirement that the anchor line has to be horizontal (zero uplift angle) at the seabed level during installation and operation of the anchors. This requirement imposes significant limitations on the use of fluke anchors in deeper waters, and an investigation into the effects of uplift on fluke anchor behaviour, as reported in /1/, has provided a basis for assessment of an acceptable uplift angle.

The design rule presented herein has been calibrated based on reliability analysis of one test case as documented in /9/. The partial safety factors are considered to be tentative until further calibrations have been carried out.

This recommendation is in principle applicable to both long term (permanent) and temporary (mobile) moorings.

1.3 Structure of the RP Definition of the main components of a fluke anchor is given in Section 2, followed by a description of the general behaviour of fluke anchors in clay in Section 3.

A brief overview of fluke anchor design methodologies is presented in Section 4.

The recommended procedure for design and installation of fluke anchors is presented in Section 5. The close and important relationship between the assumptions for design and the consequential requirements for the installation of fluke anchors is emphasized.

General requirements to soil investigations are given in Section 6 and in Appendix G.

–  –  –

The intention has been to make the procedure as concise as possible, but still detailed enough to avoid misinterpretation or misuse. For transparency, details related to the various design aspects are therefore found in the appendices.

A number of Guidance notes have been included as an aid in modelling of the anchor line, the anchor and the soil. The guidance notes have been written on the basis of the experience gained through the joint industry projects, see /1/, /2/, /3/ and /13/, and practical design and installation experience.

1.4 Definitions Dip-down point Point where the anchor line starts to embed.

Fluke Main anchor load bearing component (see Figure 2-1) Fluke angle Angle between the fluke plane and a line passing through the rear of the fluke and the shackle (arbitrary definition).

Forerunner Anchor line segment being embedded in the soil.

Inverse catenary The curvature of the embedded part of the forerunner.

Shackle Forerunner attachment point (at the front end of the shank).

Shank Rigid connection between the fluke and the shackle (see Figure 2-1).

Touch-down point Point where the anchor line first touches the seabed.

–  –  –

2. Fluke Anchor Components

The main components of a fluke anchor (Figure 2-1) are:

— the shank — the fluke — the shackle — the forerunner

–  –  –

The fluke angle is the angle arbitrarily defined by the fluke plane and a line passing through the rear of the fluke and the anchor shackle. It is important to have a clear definition (although arbitrary) of how the fluke angle is being measured.

Normally the fluke angle is fixed within the range 30° to 50°, the lower angle used for sand and hard/stiff clay, the higher for soft normally consolidated clays. Intermediate angles may be more appropriate for certain soil conditions (layered soils, e.g. stiff clay above softer clay). The advantage of using the larger angle in soft normally consolidated clay is that the anchor penetrates deeper, where the soil strength and the normal component on the fluke is higher, giving an increased resistance. However, when a larger angle is used in stiffer soils, the anchor could experience difficulties in penetrating the seabed.

The forerunner is the line segment attached to the anchor shackle, which will embed together with the anchor during installation. The anchor penetration path and the ultimate depth/resistance of the anchor are significantly affected by the type (wire or chain) and size of the forerunner, see Figure 3-1.

The inverse catenary of the anchor line is the curvature of the embedded part of the anchor line, see Figure 3-1.

3. General fluke anchor behaviour The resistance of an anchor depends on its ability to penetrate and reach the target installation tension (Ti), together with its ability to penetrate further and gain a necessary additional resistance within a tolerable additional drag length during a potential overloading situation.

The penetration path and ultimate penetration depth is a function of — the soil conditions (soil layering, variation in intact and remoulded undrained shear strength, geotechnical properties of the soil in general) — the type and size of anchor, — the anchor’s fluke angle, — the type and size of the anchor forerunner (wire or chain) — the line uplift angle α at the seabed level.

— Installation procedure and execution (installation speed, start penetration of the anchor, end position of the anchor, ratio dynamic vs. static load during installation…) In clay without significant layering a fluke anchor normally penetrates along a path, where the ratio between incremental penetration and drag decreases with depth, see Figure 3-1. At the ultimate penetration depth zult the anchor is not penetrating any further. The anchor is “dragging” with a horizontal (or near horizontal) fluke and no general increase in anchor resistance can be seen. At the ultimate penetration depth the anchor reaches its ultimate resistance Rult.

Since reaching the ultimate penetration depth is associated with drag lengths in the range 5 to 10 times the penetration depth, it is impractical to design an anchor under the assumption that it has to be installed to its

–  –  –

ultimate penetration depth. A more rational approach is to assume that only a fraction of the ultimate anchor resistance is utilized in the anchor design, as illustrated by the intermediate penetration depth in Figure 3-1.

This will also lead to more predictable drag, and should drag occur, the anchor may have reserve resistance, which can be mobilized through further penetration.

The cutting resistance of a chain forerunner will be greater than the resistance of a steel wire, with the result that a chain forerunner will have a steeper curvature (inverse catenary) at the anchor shackle than a wire forerunner, i.e. the angle θ at the shackle is larger. This increases the upward vertical component Tv of the line tension T at the shackle with the consequence that a fluke anchor with a chain forerunner penetrates less than one with a wire forerunner, and mobilizes less resistance for a given drag distance.

It has been demonstrated in the JIP on deep-water anchors /1/ that a non-zero uplift angle α at the seabed see Figure 3-1, can be acceptable under certain conditions as discussed in Appendix F. If the uplift angle becomes excessive during installation the ultimate penetration depth may be reduced. The anchor resistance R(z) is defined as the mobilized resistance against the anchor plus the resistance along the embedded part of the anchor forerunner. However, for anchoring systems with a high uplift angle at the seabed the contribution from the anchor line to the anchor resistance will be greatly reduced, see Eq. (F-1).

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