Vessel Motion Line Control Through Anchor Springiness

Abstract

ABSTRACT This paper concerns the catenary anchor lines used to anchor floating work vessels such as pipe laying barges, drilling vessels, and derrick barges. An anchored vessel in a sea-way behaves somewhat like a mass suspended between springs and acted upon by an oscillating force field. The mass will vibrate; the amplitude and frequency of this vibration depends upon the natural resonant frequency of the mass spring system, the frequency of the exciting force field, the size of the mass, the springiness of the springs, and damping. In a similar way a vessel in a sea-way will surge, pitch, and heave depending upon the wave periods, the size of the vessel, and the springiness of the anchor lines. For such a vessel, we do not have any control over the exciting force field, the size and frequency of the waves; nor do we have control over the mass of the vessel. But we can control the springiness of the anchoring system. The analysis developed in this paper suggests that the springiness of an anchoring system can be controlled by changing the pretension in the anchoring system, by changing the weight of the anchor lines (through use of wire line or chain), and by using a compound anchor line consisting of an upper light weight, wire line section and a lower heavy weight, chain section. The relationship between vessel motion and the anchor lines is shown by means of a specific example using an 880 ton vessel for which response functions have been previously published. The calculations illustrated in the paper suggest that the use of chain anchor lines results in less surge than use of wire lines and that inappropriate design of an anchor line consisting of wire and chain can lead to very large surge motions. INTRODUCTION Several years ago I visited many of the floating drilling vessels operating in the Gulf of Mexico in order to try to learn how the operators of these vessels adjusted their anchoring systems to keep the vessel on station and to minimize anchor line damage during storms. The things that I learned confused me. Some barge captains advocated slackening all lines before a storm other captains advocated tightening the lines. Some of the operators maintained that chain anchor lines were far superior to wire lines; others preferred wire. In order to reduce this seeming chaos to a degree of rationality, I attempted to devise a mathematical analysis of vessel motions as they might be affected by the anchoring system. Then, I discovered a Rand Corp. memorandum written by J. J. Leendertse1 that appeared to cover some aspects of the situation. This present paper is based in part upon Leendertse's analysis. As usual, when we attempt to apply formal mathematics to a complicated problem, we learn that many assumptions and simplifications are necessary to reduce the complexity of the mathematics to a point where some practical calculations can be made. This paper is no exception.