SWIVEL

As a safety precaution, a swivel is inserted in the system to release stress, turns and torsion in steel wires.

The swivel is inserted between the dead man wire and the PCP, to ensure no stress, turns and / or torsion in the wire, enabling the deck crew to safely disconnect the systems.

Use of swivel can however give a reduction in the breaking load with up to app. 30%, depending on the type of swivel in use.

It is strongly recommended not to use a swivel with too low friction coefficient allowing the wire end to freely rotate during normal operation. This will decrease the fatigue life dramatically.

The MoorLink swivel has a high friction coefficient and will not allow the wire to rotate when under load.

T.O. has delivered a MoorLink swivel to all AHTS vessels.

Please observe the enclosed table / drawing (page 5) showing breaking strength when the swivels are on wire drums and stern rollers.

Please read the following pages together with chapter 8 for further information.

MoorLink Swivel

Subject: Theory - Swivels versus Wire torque

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Background

Six-stranded wire rope behaves different in different applications or operations, which could lead to potential problems for the user.

In theory a six stranded rope should not be allowed to open up (swivel) under load to achieve longest lifetime of the rope. This is normally only possible in a perfect world, where no external operational criteria are present. An all wire moored drilling or accommodation rig might achieve this by perfect anchor handling and spooling off / on from / to a winch. In reality the winches are not spooling perfectly and if the wire is dragged over or in seabed the geometry of the wire could lead to induced torque.

Safety

Torque can cause severe damages to personnel and equipment. This normally occurs when an anchor handling wire is spooled in with high tension and disconnection shall occur. The torque has been transferred to the end of the rope disconnection can be impossible or lead to a kink in the rope. This also happens during cross over operations on combination mooring systems.

Combination Mooring Systems

For drilling rigs equipped with combination chain /wire system swivels would assist during the cross over operation and bolstering of anchors. When hauling in the wire, the torque moves towards the end of the rope. In order to remove the torque from the wire to prior to disconnection the swivel positioned in the cross over point should absorb the torque at a relative low tension.

It is strongly recommended not to use a swivel with too low friction coefficient allowing the wire end to freely rotate during normal operation (when moored). This will decrease the fatigue life dramatically.

The wire also introduces twist to the chain during normal operation and when hauling in anchors. The chain has a relative high torsion stiffness when under tension (nil when stored in a pile onshore or in the chain locker). This means that the wire will induce a number of turns over the length of the chain, which is not causing any damages to the chain. However, when the chain is hauled in and the AHT is coming closer to the bolster these turns will be present on a short piece of chain, potentially leading to problems bolstering the anchor properly. By installing a swivel close to the anchor end this torque could be absorbed.

Anchor handling

Anchor handling can be divided into two different main categories:

1.The usage of vessel’s own anchor handling wire or tow wire, which is permanently installed

(and replaced when damaged) and kept with high tension on the drum.

2.The usage of external supplied anchor-handling wires (normal for deep-water operations).

These wires are normally not spooled on to the winches with any high tension before commencement of work.

The problem that occurs during anchor handling is that the torque induced in the wire is transferred to the end of the rope and if the axial stiffness in the connected part is low the torque is transferred further.

This means that a swivel can absorb the torque and avoid any twist to be transferred.

Bearing Systems

1.Slide Bearing System

Bearing system is bronze aluminium type running on a polished stain less steel washer. The material is often used in high load / low speed bearings in many offshore applications (very good corrosion and wear resistance in seawater).

The bearing is self-lubricating with embedded sold lubricant. The base material is high-grade bronze alloys and has finely finished surface with pockets in which a specially formulated solid lubricant is embedded. During operation a very fine, but very strong lubricating film is deposited automatically over the complete moving area. This film remains intact at all times, even immediately upon starting. The construction is also being equipped with grease inlets in order to secure and guarantees a well-lubricated moving surface.

2.Roller Bearing System

The roller bearing swivels are equipped with a cylindrical thrust roller bearing system (either single or double row).

Summary

What is best? The usage of roller or slide bearing swivel?

It depends on your operation. The main issue is that most operations are different. The operation can be normal anchor handling, or installation of chain, polyester ropes or spiral strand, anchor proof loading, towing etc.

The slide-bearing swivel should not rotate under tension until the induced torque is exceeding the start friction. This enhances the fatigue life of the wire. Typical operation is anchor handling and inserts in combination mooring systems

The roller bearing systems would rotate under tension, as the friction moment is lower than the induced torque. This could be benefit if you do not want to transfer the torque from your wire to the object lowered. Bear in mind, fatigue life of the wire will decrease after continuos use of roller bearing swivels. Typical operation is installation of sub sea equipment, anchors or proof loading of anchors.

Theory of Torque versus Friction:

Based on our past experience and information provided by two large steel-wire rope manufacturers: ScanRope and Haggie Rand the induced torque by a six-stranded wire rope is:

6-8% of the diameter of rope x tensions.

Example

Induced torque:

Resulted induced torque: 0.07 x 0.089 x 200.000 x 9,81 = 12.223 Nm

Break Out Torque Comparison:

1.Friction moment Roller Bearing System: 0.015 (0.005 in rolling mode)

As can be seen above the resistance (friction moment) in the slide bearing system is HIGHER than the induced torque in the wire. The swivel will not rotate when the tension is increased.