On the Slow Motions of Tankers Moored to Single-point Mooring Systems
Author J.E.W. Wichers (Netherlands Ship Model Basin NSMB)
Title On the Slow Motions of Tankers Moored to Single-point Mooring Systems
Conference/Journal Journal of Petroleum Technology
Paper no. SPE-6242 / OTC-2548
Month June
Year 1978
Publisher Society of Petroleum Engineers / AIME
Pages 947 - 958

Abstract
A single-point mooring (SPM) system used by a tanker generally is designed for a maximum operational seastate condition. The sea state at which the down-time starts (disconnecting bow hawser and floating hoses) is determined for the most part by the force level in the bow hawser.
The behavior of a tanker moored to an SPM system is determined mostly by the slow motions of the tanker in the horizontal plane. Because of this slow-motion behavior (fish-tailing and galloping) of the tanker, a slowly varying force occurs in the bow hawser. Superimposed on this force in the hawser are the forces caused by buoy and tanker motions with wave frequency. The combination of these forces can lead to high peak loads in the bow hawser force.
These phenomena that occur while the tanker is moored are complicated and will be influenced by many parameters, such as the size of the tanker and its loading conditions, the elastic properties of the buoy and bow hawser, the length of the hawser, and the forces on the vessel exerted by wind, current, waves, astern propulsion, and the underkeel clearances.

This study shows that a relatively simple linear model can be used to obtain a good conception of the stability in wind and current of the equilibrium position of a tanker moored to a single point with a bow hawser. Some preliminary model tests showed the applicability of the method and good correlation of measured with computed natural periods of the horizontal motions of the ship.
Computations and model tests show that a ship moored to an SPM system can perform slow oscillating motions in the horizontal plane in wind and current only without a slowly oscillating, external drifting force resulting from waves. These slow motions occur only when the "equilibrium position" of the ship in wind and current is unstable. In that case, the magnitude of the oscillations can be determined only by accounting for nonlinear damping terms when describing equations of motions.
When the equilibrium position of the tanker in wind and current is stable, slow motions will be damped after some time unless an oscillating external force affects the ship. This force is present in irregular waves and is called the wave drifting force. Estimates of these wave drifting forces for the right sides of the equations of motions will be investigated further.

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