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Anchor Manual 2010 The Guide to Anchoring

2 anchor manual 2010 Vryh of Anchors BV P.O. Box 109, 2900 AC Capelle a/d Yssel, The Neth er lands www.vryhof.com vryhof@vryhof.com Copyright © Vryhof Anchors B.V., Capelle a/d Yssel, The Netherlands 2010. No part of this book may be reproduced in any form, by print, copy or in any other way without written permission of vryhof. Vryhof, Stevin, Stevpris, Stevshark, Stevtensioner and Stevmanta are registered trade marks. Vryhof reserves all intellectual and industrial property rights such as any and all of their patent, trademark, design, manufacturing, reproduction, use and sales rights thereto and to any article disclosed therein. All information in this manual is subject to change without prior notice. Vryhof Anchors is not liable and/or responsible in any way for the information provided in this manual. First edition published 1984. Print run 7,500 copies. Second edition published 1990. Print run 7,500 copies. Reprint second edition. Print run 5,000 copies. Third edition published 2000. Print run 2,500 copies. Reprint third edition print run 1,500 copies. Second reprint third edition print run 1,000 copies. First print fourth edition published 2006 print run 1,000 copies. Second print fourth edition print run 1,000 copies. Third print fourth edition print run 1,000 copies. Fourth print fourth edition print run 1,000 copies.

Company profile Since the beginning of exploration of oil & gas offshore the art of anchoring has taken a dramatic turn. Vryhof Anchors was one of the pioneers and achieved holding powers up to 25 times an anchor's own weight. Consequently the company soon emerged as market leader in anchor design and manufacturing and with over 7500 units sold, its anchors are the most applied offshore. Vryhof understood that clients’ needs cannot be satisfied by supply of standard hardware alone. With best-in-class sales services it shared technology to create fit-for-purpose mooring systems; it initiated lease/purchase concepts and introduced an alliance of leading mooring component manu- facturers in order to more efficiently serve a changing offshore industry in the 1990s. Exploration in ever deeper waters and more remote locations has encour- aged the development of more advanced mooring solutions. Consequently Vryhof anchor holding powers now reach beyond 75x their weight. Once again, the industry welcomes new ways of cooperation to include supply of complete mooring systems and lease concepts. This sets the stage for Vryhof, who under new private ownership and with a presence in major offshore arena’s will continue to responsibly introduce quality designs in pace with industry's requirements; to explore opportunities to approach clients in an open, sincere, professional manner and take its world class services well into a new era for the benefit and satisfaction of its clients.

Introduction 4 A stone and something that looked like a rope. For millennia this was the typical anchor. Over the last 25 years of more recent history, vryhof has brought the art to a more mature status. They have grown into a world leader in engineering and manufacturing of mooring systems for all kinds of floating structures. In doing so the company has secured numerous anchor and ancillary equipment patents, and shared its experience with others. The company understands that the needs of the industry can not be satisfied by the supply of standard hard-ware only. Universal and tailored solutions rooted in proven engineering should be based on long practical experience. Vryhof has been and will be introducing new and original anchor designs well into the 21st century. With their products, advice and this manual, it shares this knowledge with those who are daily faced with complex mooring situations. This manual is intended as a means of reference for all who purchase, use, maintain, repair or are in any way involved with anchors. Though written from one anchor manufacturer’s standpoint, the information contained herein is applicable to many types of anchors. Total objectivity is, of course, impossible. It is hoped this manual will contribute to the work and success of all who work with anchors. They are the only fixed reference point for many of the floating structures on the world’s often turbulent waters.

1 General

Mooring systems 6 Mooring systems have been around just as long as man has felt the need for anchoring a vessel at sea. These systems were used, and are still used, on ships and consisted of one or more lines connected to the bow or stern of the ship. Generally the ships stayed moored for a short duration of time (days). When the exploration and production of oil and gas started offshore, a need for more permanent mooring systems became apparent. Numerous different mooring systems have been developed over the years, of which a short selection is presented here. Semi-submersible drilling rig - generally the semi-submersibles are moored using an eight point mooring. Two mooring lines come together at each of the columns of the semi-submersible. CALM buoy - generally the buoy will be moored using four or more moor- ing lines at equally spaced angles. The mooring lines generally have a cate- nary shape. The vessel connects to the buoy with a single line and is free to weathervane around the buoy. semi-sub mooring SALM buoy - these types of buoys have a mooring that consists of a single mooring line attached to an anchor point on the seabed, underneath the buoy. The anchor point may be gravity based or piled. typical turret mooring Turret mooring - this type of mooring is generally used on FPSOs and FSOs in more harsh environments. Multiple mooring lines are used, which come together at the turntable built into the FPSO or FSO. The FPSO or FSO is able to rotate around the turret to obtain an optimal orientation relative to the prevailing weather conditions.

Mooring systems 7 catenary system fig. 1-01 taut leg system fig. 1-02 Spread mooring - generally used on FPSOs and FSOs in milder environ- ments. The mooring lines are directly connected to the FPSO or FSO at both the stern and bow of the vessel. When oil and gas exploration and production was conducted in shallow to deep water, the most common mooring line configuration was the catenary mooring line consisting of chain or wire rope. For exploration and produc- tion in deep to ultra-deep water, the weight of the mooring line starts to become a limiting factor in the design of the floater. To over- come this problem new solutions were developed consisting of synthetic ropes in the mooring line (less weight) and/or a taut leg mooring system (fig. 1-01 and fig. 1-02). The major difference between a catenary mooring and a taut leg mooring is that where the catenary mooring arrives at the seabed horizontally, the taut leg mooring arrives at the seabed at an angle. This means that in a taut leg mooring the anchor point has to be capable of resisting both horizontal and vertical forces, while in a catenary mooring the anchor point is only sub- jected to horizontal forces. In a catenary mooring, most of the restoring forces are generated by the weight of the mooring line. In a taut leg mooring, the restoring forces are generated by the elasticity of the mooring line. An advantage of a taut leg mooring over the catenary mooring is that the footprint of the taut leg mooring is smaller than the footprint of the cate- nary mooring, i.e. the mooring radius of the taut leg mooring will be small- er than the mooring radius of a catenary mooring for a similar application.

fig. 1-03 fig. 1-04 Mooring components 8 A typical mooring system can be divided in three different components, the mooring line, the connectors and the anchor point. Mooring line Chain The most common product used for mooring lines is chain which is available in different diameters and grades. Two different designs of chain are used frequently, studlink and studless chain. The studlink chain is most common- ly used for moorings that have to be reset numerous times during their life- time, for instance semi-submersibles, while studless link chain is often used for permanent moorings (FPSOs, buoys, FSOs). A chain mooring line can be terminated in either a common link or an end link (fig. 1-03). Wire rope When compared to chain, wire rope has a lower weight than chain, for the same breaking load and a higher elasticity. Common wire ropes used in off- shore mooring lines are six strand and spiral strand. The wire rope is termi- nated with a socket (for instance open spelter, closed spelter, CR) for con- nection to the other components in the mooring system. Generally wire rope is more prone to damage and corrosion than chain (fig. 1-04). Synthetic fibre rope A recent development is the use of synthetic fibre ropes as mooring line. Typical materials that can be used are polyester and high modulus polyeth- ylene (Dyneema). The major advantage of synthetic fibre ropes is the light weight of the material and the high elasticity. The synthetic fibre rope is generally terminated with a special spool and shackle for connection to the other components in the mooring system.

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