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Mooring of ships - forces

Mooring of ships - forces. Kapt. K. De Baere. Purpose of mooring configuration. To bring the ship alongside To keep the ship alongside To assist the ship when un-mooring. Design criteria of mooring configurations. Based on the forces acting upon the ship Wind Current Waves Swell

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Mooring of ships - forces

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  1. Mooring of ships - forces Kapt. K. De Baere

  2. Purpose of mooring configuration • To bring the ship alongside • To keep the ship alongside • To assist the ship when un-mooring

  3. Design criteria of mooring configurations • Based on the forces acting upon the ship • Wind • Current • Waves • Swell • Other ships passing by (suction effect) • Location of the berth – Protected or sea berth • Types of ship – size, displacement, draught etc.

  4. Protected berths • Design criteria – limiting values • Cross wind up till 15m/sec (6-7 Beaufort) • Tidal current of 3 knots in longitudinal direction • Cross current of 1 knot • Cargo- and container ship are normally moored along well protected berths => Mooring winches are designed to pull the ship alongside with 1 headline and 1 stern line against a cross wind of 5 Beaufort

  5. Sea berths – designed for >wind • Design criteria – limiting values • Cross winds up till 20m/sec or 8 Beaufort and gust of winds up till 10 Beaufort • Tidal current of 3 knots in longitudinal direction • Cross current of 1 knot • Waves and swell • Waves and swell with a short period have a limited influence

  6. Fetch • The size of a wave depends on its fetch. The fetch is the distance a wave travels (see next slide). The greater the fetch, the larger the wave. • If the wind is blowing for a longer period of time in the same direction => long fetch with a high wave height and a longer period => important dynamic effect on the ship

  7. Fetch – DefinitionGrowth rate of wind generated waves • The distance that wind and seas (waves) can travel toward land without being blocked. In areas without obstructions the wind and seas can build to great strength, but in areas such as sheltered coves and harbours the wind and seas will be calmer.

  8. Mooring of VLCC’s • Often moored outside the harbours along sea berths • Forces are so great that no winch is capable of bringing the ship alongside • Tugs are always used when mooring and leaving berth • The only criteria is the holding force of the winches • The ship must be maintained in position related to the shore manifold (chiksans)

  9. Relation maximum pulling power – Displacement () • Figures are used to design shore facilities (bollards, bits ……….. Etc.) • 25% safety margin to be added 1 kN = 1 ton pulling power (not scientific)

  10. Mooring winch with undivided drum

  11. Mooring winches – Divided drum-polyprop octopus

  12. Chicksan

  13. Chicksan • One of the biggest problems with the fixed loading/discharging systems is the restricted liberty of movement of the ship • If one of the limits is breached => ESD-system activated

  14. Assessing the forces • Forces due to wind and current are proportional to the square of their speeds. f.i. the force caused by a wind of 40 knots is 4 times the influence of a wind of 20 knots • The wind speed increases with the height above the ground. A wind of 10 knots at 2 meters increases till 60 knots at 40 meters => importance of the freeboard (height of the structure). To obtain comparable figures all winds are recalculated to a standard height of 10 meters

  15. Maximum wind limits (400.000 dwt ship) in function of the breaking power of the winches

  16. Wind limits • The previous pictures learns us that; • The wind limit is determined by the holding power (breaking power) of the winches • The wind limit is determined by the material of the mooring lines

  17. Assessing the forces • Influence of a cross current is inverse proportional with the keel clearance. In case of a small keel clearance the current is obstructed by the ships hull and searches way out via the stem and the stern. A Suction effect is created trying the move the ship away from the berth.

  18. Theoretical example of the influence of the keel clearance • A ULCC with a draft of 15 meters is moored alongside a berth with 16.5 meters of water => relation water depth/draft = 1.1 • Relative resistance factor in case of cross current = 5.6 • In case of unlimited water depth a cross current of 1 knot produces a force of 60 tons

  19. Theoretical example of the influence of the keel clearance • In case of a limited water depth (example) this force is increased till 5.6 x 60 ton = 336 ton • This equals 9 steel mooring ropes of 40mm diameter

  20. Theoretical example of the influence of the keel clearance • The relative proportion of the different elements has to be considered • Ballasting decreases the keel clearance but also reduces the lateral wind surface. The wind effect is of greater importance than the the clearance effect (see next slide).

  21. Example of cross and longitudinal forces • 18.000 & 70.000 SDWT: Wind 60 knots (30m/s), current 5 knots longitudinal and 1 knot cross current • 200.000 SDWT: Wind 60 knots, current 3 knots longitudinal and 1 knot cross current

  22. Conclusions • In ballast condition the most important forces are wind generated • In loaded condition the most important forces are current generated • The total force on the ship (alongships + athwartships) is greater in ballast condition than in loaded condition => influence of the wind is of greater importance

  23. Different materials • 3 different configurations • All steel wire ropes (equipped or not equipped with tails) • All ropes are synthetic • Mixed systems (synthetic + steel wire rope) • New materials

  24. Steel wire rope + tail (ralonge de la touline) • Purpose of the tail is to add elasticity to account for change in tidal heights • Always use 8 strands nylon with an MBL 25% > steel wire rope • To protect against chafing cover splice of the tail with leather or plastic • The tail is connected to the steel wire rope by means of a Tonsberg shackle or a Mandal shackle • In case of frequent use tails are changed every 18 months

  25. Steel wire rope + tail • Steel wire rope have a high MBL and are not elastic. • Steel wire rope are stored on winch drums with a manual brake • Steel wire rope are relatively easy to handle up to 40mm  ???? • Steel wire ropes last longer than synthetic ropes • Price steel wire = synthetic

  26. Tonsberg shackles

  27. Mandal Shackle

  28. Full synthetic mooring configuration • Biggest problem is elasticity • This elasticity can give an important « sway » (balancer) to the ship (breaking out) • 3 mooring ropes – different materials – same length (50 m), MBL and load • Steel wire – 0.3m elongation • Polyprop – 5m elongation • Nylon – 8 m elongation

  29. Breaking out

  30. Effect of the hawser elasticity on the restraint capacity • Materials with the smallest elasticity take the biggest load • Short rope = big load • Relation -  is not linear

  31. Full synthetic mooring configuration • Synthetic fibres loose tensile strength (force de traction) if submitted to cyclic tensions attaining 30 to 50% of their MBL. • Those cyclic tensions are not constant, due to resonance high tensions occure during short periods of time

  32. Full synthetic mooring configuration • Because of; • Cyclic tensions • Internal friction • Exposure to the marine environment • Tensile strength of synthetic ropes will diminish after 1 year • Tensile strength of steel wire rope will diminish after 5 years => more durable

  33. Full synthetic mooring configuration • Another side effect is sagging (affaissement) • The « sag » is function of; •  m-n • Weight of the mooring line • Tension in the line • Water depth (suction effect)

  34. Full synthetic mooring configuration • Consequence of the sagging is that a synthetic rope can never be pulled as stiff as a wire rope. • A wire rope will « react » faster on a breaking out of the ship. • A synthetic rope will compensate the the sag before reacting • Max. allowed distance between berth and ship is normally limited to 6% of the water depth

  35. Mixed mooring systems • Mix of wire ropes and synthetic ropes • Certainly NOT the best configuration but the most common one. • If possible use steel wire rope as springs and breasts and use synthetic ropes as head- and stern line

  36. New materials • Composite materials • Expensive but excellent mooring system • Kevlar –Aramid ropes are very strong, light and show little sagging. They react fast in case of breaking out of the ship.

  37. Efficient mooring • The efficiency of a mooring rope depends on the following factors • Material (steel wire or synthetic – elongation & MBL) • Length • Angles with longitudinal and transversal axis in the horizontal plane • Angles with the horizontal in the vertical plane

  38. Function of the different ropes • Head- and stern lines & the springs are stabilising the ship alongside • Breast line will prevent the ship to break free from the berth • Breast lines must be as perpendicular as possible to the ships longitudinal axis • Springs must be as parallel as possible to the berth

  39. Recommendations • The function of springs and breast lines is clear. Springs are preventing longitudinal movement while breast are opposing transversal movements. • The function of head and the stern lines depends on their angle with the longitudinal axis. Great angle => they serve mainly as breast line while small angle => stopping longitudinal movement

  40. Recommendations • The ideal configuration will rarely be achieved. • To obtain a perfect mooring configuration their must be a perfect harmony between the ships equipment and disposition on board and the configuration ashore • Berthing ships is always a matter of compromises

  41. Recommendations • Following recommendations have been published by the OCIMF = Oil Company International Maritime Forum • The recommendations are valid for a tanker moored alongside a T-berth

  42. Recommendations based on OCIMF – Effective mooring • The horizontal angles of head-, stern- and breast lines < 15°

  43. Recommendations based on OCIMF – Effective mooring • The vertical angle with the horizontal plane must be < 25° • The effective force is proportional to the cosine of the angle • If the angle is 25° the line is effective for 91% • If the angle is 45° the efficiency is reduced to 71% • => Springs & breasts must be long enough and not to steep

  44. Springs & Breasts

  45. Recommendations based on OCIMF – Effective mooring • Breast lines are most effective is  on the longitudinal axis. If  is 45° we have to increase the force in the breast line till 141 ton to obtain an effective transversal force of 100 ton

  46. Recommendations based on OCIMF – Effective mooring • Springs offer the greatest holding power in the longitudinal direction. Their length is  60 meters

  47. Recommendations based on OCIMF – Effective mooring • The impact of the head and the stern lines on the total holding power of the mooring configuration is less important than the influence of springs and breasts. This mainly because these lines are too long. • Never the less they are important to compensate the dynamical forces. • Length  110m = ½ coil

  48. Recommendations based on OCIMF – Effective mooring • Very short lines must be avoided. They always take the most important part of the load, especially when the ship is moving Short length = important vertical angle

  49. Short breast lines • Long breast line: 52ton load is sufficient to obtain an effective holding power of 50 ton • Short breast line: Load has to be increased till 88 ton to obtain same result

  50. Recommendations based on OCIMF – Effective mooring • All the mooring ropes in the same group (working in the same direction)must have a same tension. If not, the weakest line will break first. Total load will have to be received by the remaining lines => increased risk of breaking (chain reaction) • Groups are f.i. aft spring + head lines, Stern lines + forward spring, breast lines

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