Concerning Wire Ropes for Anchoring Semisubmersibles
Authors: Acaster S.M. 
Published: December 2007

If you're about to build yourself a semisubmersible, don't assume that your old Boy Scout Manual will tell you all you need to know about the anchor lines. There's more to the rope tricks than meets the eye, so call on an expert. And call on him early

Introduction

The expansion of offshore drilling operations during the past few, years has greatly increased our knowledge of anchoring requirements and techniques for offshore drilling units. This experience has been gained under many and varied conditions throughout the world, and particularly in the North Sea, where the severest conditions continue to be encountered. It is true that far more violent conditions are sometimes experienced in other parts of the world, but they are comparatively short-lived; it is the accumulative effect of sustained poor weather that makes the North Sea one of the worst operating areas for offshore units. As a result of earlier experiences with rigs in the North Sea, a new breed of bigger and better rig has been designed and is under construction. Naturally, as the general fabrication increases in size and efficiency, so too must all the component parts. Most of the rigs of this new breed are of the semisubmersible type and therefore mooring members, whether of chain or rope, will obviously play a major role in the total efficiency of the unit play a major role in the total efficiency of the unit and so must be designed with care.

Requirements for Good Anchor Lines

In the much earlier days of the offshore rig, mooring systems involving wire rope were similar to those used for mooring conventional ships. However, the new sophisticated rigs designed to operate in heavy seas and at greater depths have presented far different problems. It has been necessary to carefully reconsider those designs, fabrication methods, and physical properties. physical properties. Experience has shown that the most important attributes to incorporate into the design of mooring lines for offshore rigs are (1) high strength, (2) high elongation coupled with high elastic limit, (3) corrosion resistance (using maximum effective galvanized coating), and (4) environmental protection (using suitable lubricants and service dressings). The choice of rope for a given application is generally a matter of compromise. In the case of anchor lines, one must also consider resistance to fatigue, abrasion, and deformation.

Achieving Strength

The strength of a wire rope derives principally from (1) the cross-section of steel area and (2) the tensile strength of the material used. The diameter of anchor line to be used to anchor a given offshore unit will naturally depend upon the size and deign of the unit and upon the forces that will be imposed upon this structure by the wind and the sea. Anchor lines presently being used in the North Sea vary between 2 1/2 and 3 1/2 in. in diameter, and the construction details referred to here are based on this size range. The tensile strength of the material used should always be as high as possible, depending on the construction of the rope employed and the application and work involved, and commensurate with good rope making.

JPT

P. 659