Load on Anchor due to wind, current and waves


The external elements that affect an anchored vessel are wind, wave, tides and the currents present in the Anchorage area. The effect of those elements that usually act in combination are forces transferred to the anchor. The estimation of these factors is not a simple task, but the force due to the pressure of wind and current on the vessel is relatively straightforward to approximate, while it is much more difficult to determine the intermittent load caused by wave action.

Wind: Gusts of wind cause a vessel to sheer back and forth on its rode, falling off first one way and then the other. The bow is blown off until the rode comes tight, snubbing the bow back into the wind. The ship them surges forward, responding to the weight and elasticity of the anchor rode, until the next gust blows the bow off once more . The load induced by the force of the wind on an anchored vessel depends on two factors:
1-The wind speed and the exposed surface area of the vessel. Measuring the wind speed is straightforward; estimating the effective exposed surface area is more difficult.
2-The vessel’s length, beam, and height above the waterline, as well as the vessel design influence the estimate of the exposed surface. Loads caused by wind on vessels can be estimated by conducting a wind-tunnel test.
Waves: The kinetic energy accumulated by the vessel caused by waves can generate a peak load on the anchor of up to several tons when the anchor rode becomes tight. Wave action causes the pitch and roll movement of an anchored vessel, this continuous action results in a backward and upward movement of the vessel, and the anchor rode comes tight and slows down the vessel’s surge. This motion can result in broken anchor-chain 7 connectors and bent anchor shanks. To prevent these failures, seamanship guidelines suggest that the energy related with this type of force be absorbed by catenary in the rode ensuring the presence of loads on the anchor will be decreased. Hence, having plenty of sag in the rode decreases shock loads and helps the anchor remain embedded in the sediment by lessening the angle between the rode and anchor as the loads increases. In severe situations, the angle becomes positive as the weight of the rode is overcome by the tension. This positive angle creates a force component acting on the anchor to pull it out increasing until the catenary becomes a straight line, ultimately dislodging the anchor. If the catenary allows vessels to move a some distance before the chain tightens, the associated force will be reduced proportionally to the distance achieved

Current: Loads caused by currents are relatively insignificant in many protected anchorage areas, but deserve consideration when anchoring areas are in river estuaries or areas subject to significant tidal currents. This is the case especially during the tidal changes when the anchored vessel will swing successively in one direction, and then the other.

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