Method for Predicting Geometry and Load Distribution in an Anchor Chain from a Single Point Mooring Buoy to a Buried Anchorage

Abstract

ABSTRACT Procedures are developed for estimating geometry and load distribution in a chain connected from a floating object to a fixed anchor beneath the seabed. Previously published information on this subject has been restricted to a discussion of resistance of soil to movement of chain normal to the chain path. This paper expands the theory to include procedures for considering tangential resistance of soil to movement of the chain, the effective weight of chain in soil, and the general case of chain entering the s oil at an angle. The more rigorous treatment of chain behavior in s oil yields shriller Toads on anchor s. These smaller loads could result in savings on installation costs for anchors. A parameter study is included to snow the influence of several variables on chain loads at the anchor. INTRODUCTION Anchorage for a single-point mooring system, SPM, is provided by chains connected to conventional anchors or piling. For locations with weak soils, the connection to the anchor or piling should be at some distance beneath the seabed in order for the anchor to provide optimum resistance to the vertical and horizontal components of load from the chain. In this paper it is as sunned that the chain is connected to a pile, although the analytical treatment could apply to a conventional anchor that has been jetted or pulled to some distance below the seabed. Published information on loads from chain to anchor points beneath the seabed is restricted to a consideration of the component of soil resistance that is normal to the chain path. This results in the conservative assumption that chain tension is constant from the seabed to the point of connection to the pile. This cons erratum is believed unwarranted for connection points at considerable depths beneath the seabed. Analytical procedures presented in this paper are more complete in acknowledging the total resistance of soil to chain movement. For many installations, these more complete procedures will predict significantly lower loads at the pile. GEOMETRY AND EQUILIBRIUM OF CHAIN Korkut and Hebert1 have presented the basic derivations and equations for catenaries as they apply to marine anchorage problems. They discuss the effect of currents on catenaries and they include discussions of geometries of anchor chains related to the operations of running and dropping anchors. Collipp 2 has presented no dimensional parameters which define mooring line catenaries when the chain is anchored on the ocean floor. Stanton, Tidwell, and Lloyd3 have investigated effects of sloping sea floors on anchor line loads and chain geometry when the anchor is on the sea floor. Hsu and Blenkarn4 have developed procedures for calculating peak mooring forces of a vessel in a random or irregular sea. Reese5 has discussed the equilibrium of a chain in soil. Reese s discussion is restricted to chains which are tangent to the seabed and his equilibrium equations do not include effects of weight of chain in the soil or resistance of soil tangent to the chain path.