Force Management in the Dual Capability Two Tension System

The Dual Capability Two Tension System is based on having two independent but linked systems, both of which have 100% load capacity (minimum of 20 kN per MPD focal point even in the event of one main anchor failure). During normal operation, both sub systems carry equal load. Should either subsystem fail the other subsystem is capable of picking up the full load.

This is the default system for high-angle rescue, and LAMRT’s equipment and methodology have been selected on this basis.

To qualify as a capable and competent mainline or belay, the system employed must pass the revised 2016 Belay Competence Drop Test Method criteria. This test assumes a worst-case scenario of a 1m drop on 3m of rope using a 200 kg mass (fall factor 0.33). This requires:

• A maximum arrest force of no more than 12 kN.
• A stopping distance (pre-rebound) of no more than 1 m.
• That the system remains competent to function effectively during and after a worst-case event.
• Residual rope strength greater than 80%.
• That the system must not fail a fall factor 0.5 drop test (i.e. 1.5 m drop onto 3 m rope with 200 kg mass). This test ensures that there is enough strength margin above and beyond the minimum FF 0.33 drop test criteria. For our purposes this requires a 20 kN system. (More information on FF risk management is included in the Advance User section of this manual.)

To be capable and competent to meet these criteria, the descent control device in use requires the following attributes:
• force-limiting capability in the range of 6–12 kN (with post-slip functionality).
• minimum breaking strength of 20 kN.
• self-braking (i.e. a hands-off action by the operator results in ‘automatic’ rope stopping).
• descent control friction that can handle working loads of up to 4 kN.
• rope tailing techniques that must be proven to work with no more than 0.1 kN force and that must stop the load with no more than 1 m stopping distance. Rope tailing safeguards against human factors (e.g. the descent control operator not allowing self-braking to occur).

By building and operating two parallel systems that meet these criteria, we provide redundancy and reduce the potential peak forces acting on the system. This also has other advantages:
• it reduces the risks from human factors.
• it reduces the risk of damaged or cut working lines.
• it reduces force acting on individual anchor.
• it reduces stopping distances.
• it allows for rapid transfer into a guiding line.

Anchors

The standard Dual Capability Two Tension System assumes at least four main anchor points arranged in an even spread behind two MPD master points. In many cases, several smaller anchors may need to be placed to establish each main anchor, e.g. multiple equalised nuts. The objective is that each of the MPD focal points will have a minimum strength of 20 kN even if an anchor is lost or one of the rigging lines is cut.

Load Multiplication Effect

When selecting suitable anchors, it is important to consider the load-multiplying effect of increasing the angle between anchors. As the angle increases, the force on each anchor increases rapidly. After 120 degrees the force experienced by each anchor becomes >100% of the original force.

This must also be kept in mind when setting up tensioned lines or where a horizontal line could suddenly be tensioned, e.g. a hand rail (which is in effect a high-angled belay and therefore generates high loads on its anchor points).

Useful measures of angle are:

Equalisation of Anchors

The Two Tension System relies on multiple solid, equalised, redundant anchors with minimal extension. Ideally, this means that every anchor shares an equal proportion of the load, is backed up by a second anchor, and if either anchor fails the system will not release or be subjected to a shock load due to extension. However, it is very difficult to fully equalise all anchors and to avoid any extension.

The LAMRT Two Tension System assumes that only trees or rock anchors are used. Snow stakes, dead-men and ice screws are not suitable anchors for use with this system. More information on anchor equalisation is included in the Advanced User section of this manual.

Additional Information for Advanced Users

The purpose of this section is to provide additional information and directions for team members who have a specific interest in Rope Rescue and are likely to take leading roles in rescue situations.

Fall Factors

Fall Factor (FF) is the ratio of Fall Height to Length of Rope, e.g. a 2m fall onto a rope length of 4m gives a fall factor of 2/4 = 0.5. The fall factor influences the severity of the load placed on the falling object and the belay system. A long fall onto a short length of rope gives a high fall factor and is likely to cause injury or failure of the system.

The Two Tension System is designed to keep FF to a minimum by always maintaining the load below the belay with the ropes in tension. During normal operation of the system, high FF shouldn’t be an issue and any high resultant forces will be shed by slipping of the MPDs. FF could be an issue in some scenarios however, e.g. during a haul where one rope jams close to the stretcher and the load switches to the other, moving, rope and the jammed rope becomes slack. If hauling were to continue, the potential FF on the jammed rope will rise rapidly and a failure of the moving rope would result in a high shock load, which can’t then be absorbed by the MPD because the rope is jammed.

Another potentially very high FF is if the Attendant for some reason gets above the stretcher, e.g. climbs up to free a stuck rope. In this scenario, the length of rope from the Attendant to the tie-in point is only about 1m, hence simply standing on the stretcher would give FF=1 and climbing 1m above the stretcher would give FF=2.

There are also multiple scenarios where the Edge and Vector could potentially get above their tie-in points. These persons must be made aware of this, and should be adjusting their prusik and attachment points constantly to maintain a tight rope and a position below the belay point.

Obviously, any lead climbing will introduce FF, e.g. climbing up to secure a belay location. In the unusual situation that lead climbing is required, this must be done using a dynamic lead climbing rope.

Equalisation of Anchors

The Two Tension system relies on multiple solid, equalised, redundant anchors with minimal extension. Ideally this means that every anchor shares an equal proportion of the load and is backed up by a second anchor, so that the system will not release or be subjected to a shock load due to extension should either anchor fail. However, it is very difficult to equalise all anchors fully and to avoid any extension.

For example, the system below is equalised when pulled directly downwards; however a small movement to the left places all the load on a single anchor.

 The system below appears to be equalised; however, the difference in the length of each leg means that when loaded the short leg will experience a greater percentage of stretch and hence be in more tension than the other two legs. This potentially becomes very significant when considering long legs from the main belays to the MPD focal points.

Where possible, slings (which have almost no stretch) should be used to extend the main anchors such that the belay rope lengths are approximately equal.

Similarly, in the example below, the two right-hand legs have double strands and hence will be considerably ‘stiffer’ and take more of the load than the leg with the single strand. This method can be used to deliberately direct more force to the best of the three anchors.

All these illustrations are essentially fixed directional anchors, and are vulnerable to a sudden change of direction of the load e.g. should one of the adjacent main anchor points fail.

A potential improvement on these fixed directional anchors is a self-equalising system, the simplest of which is the Sliding X. In this system, the twist in the sling is critical such that the system does not completely fail if one anchor fails.

The main shortfall of this system is the uncontrolled extension of one leg should the other fail. This would place a high shock load on the remaining anchor. Self-equalising anchors are not normally used in the Two Tension system.

The Sliding X can be greatly improved by the addition of limiting overhand knots. This method is may be useful as a personal belay when some movement may be required, e.g. top-roping a scrambler.

Winter Anchors

The LAMRT Two Tension system assumes that only trees or rock anchors are used. Snow stakes and ice screws are not suitable anchors for use with this system. The reason for this is that the Two Tension system places constant tension on the anchors throughout the duration of the rescue. At typical Lakeland winter temperatures snow and ice has a high water content and as a result behaves more like a super-cooled liquid. As a result, loaded ice screws, ice threads and bollards will tend to melt out.

Large dead men or snow anchors constructed from buried rucksacks may be used for protecting snow/ice slopes in scrambling-type situations, or for stretcher back roping and similar situations.