MoorGuard Fuse Guidelines
A MoorGuard fuse application is designed in three steps. First determine the Safe Working Load of the application. Second match the Minimum Elongation Load of the fuse to the Safe Working Load of the system. Third recalculate the Minimum Break Strength of the replacement rope.
Minimum Break Strength (MBS) is the minimum force required to break a new rope in laboratory conditions.
Working Load (WL) is the force applied in a given application.
Safe Working Load (SWL) is a guideline for maximum allowable capacity and should not be exceeded. Applied loads higher than the SWL can overstress and damage fibers, resulting in premature rope failure.
Design Factor (DF) is the ratio between the MBS and WL, historically the margin of safety for an application.
Minimum Elongation Load (MEL) is the force required for a MoorGuard® fuse to activate.
Rope Design Factor Decision
Prior to the introduction of MoorGuard, commercial and industrial users had to determine the DF based on actual service conditions taking into account the degree of risk to life and property plus other factors affecting rope behavior. The more severe the application or higher the risk the greater the DF required. Experience has been the best guide for determining a DF. Industry standards are a good starting point when including an increased DF for:
High or continuous dynamic load
Extensive cyclic loads
Long periods of tension
Potential for injury, death or loss of property
Poorly trained operators
Poorly defined procedures or controlled operations
Used rope of unknown properties
Un-inspected or infrequently inspected rope
Unknown or long service life rope
The DF has historically been the safety factor in an application. With the invention of the MoorGuard® fuse most of the critical parameters requiring a DF increase can be mitigated. With MoorGuard® the DF can safely be reduced to 3 for most applications and fiber types, and to a 2 for Aramid and UHMWPE ropes. The reduction in DF allows for a corresponding reduction in rope size.
Safe Working Load Calculations
The first step in designing a MoorGuard application is to calculate the Safe Working Load of the system if not already known. The SWL is obtained by dividing the Minimum Breaking Strength by the Design Factor.
MBS ÷ DF = SWL. For example, an application with an MBL of 42 tons and a DF of 7 equals a SWL of 6 tons.
(42 ÷ 7 = 6). Or the SWL can be found in industry publications such as the ‘Cordage Institute Rope Specifications’.
The DF must reflect added risk in extreme cases, like towing, where the dynamic load on a rope may exceed the static load by 3x or more. Also, the dynamic effects are greatest on low elongation ropes and more significant on shorter segments than on longer ones.
A MoorGuard fuse helps prevent line overload and snapback in case of parting and will negate the need for a DF increase. To choose the correct size fuse we start with the rope system currently in use. After we have determined the SWL we next determine which MoorGuard fuse configuration is best matched to the SWL. We determine this by first deciding how the fuse will be used, as a loop or as a quad configuration. If a loop is used we match the fuse MEL to the SWL of the current system. If the quad is used we use a fuse with an MEL of ½ the SWL. Each leg of a fuse carries the same load. So when a 3 ton loop (two legs) is used as a quad its load capacity doubles because there are now four legs securing the line at 6 tons.
In our example from No. 7 the SWL is 6 tons. Two MoorGuard fuses could be used. From table 1 we determine that a type 3T5935 could be used as a loop rated at 6 tons. Or a type 3T3275 could be used as a quad rated at 6.6 tons. (3.3 x 2 = 6)
Line Resizing Calculations
After determining the SWL of our system and which fuse we will use we can now calculate the smallest line to accomplish the task. We do this by multiply our SWL by 3 (the new DF) to arrive at the new MBS.
SWL * new DF(3) = new MBS. In our example the SWL of 6 tons is multiplied by the new DF of 3 to give a new MBS of 18 tons. (6 * 3 = 18)
Our example 42 ton MBS rope is a three strand nylon and has a diameter of 2 inches (51 mm). By reducing our new MBS to 18 tons a nylon three strand rope of 7/8 inches diameter (22 mm) can be substituted.
If we replace our 42 ton MBS three strand nylon with a spectra rope we could reduce the new DF to 2. With our 6 ton SWL multiplied by our new DF of 2 we have a new MBS of 12 tons. (6 * 2 = 12) A corresponding 12 ton MBS spectra, twelve strand braid has a diameter of 9/16 inches (14 mm).
MoorGuard Estimated Rope Size
Table 1 lists the approximate size of the AkTiv rope used to fabricate MoorGuard fuses for various Safe Working Loads in loop and quad configurations.
MoorGuard Estimated operating tonnages, MEL’s and diameters:
|MoorGuard type||MEL Loop tons||MEL Quad tons||Diameter inches||Diameter mm|
All figures are estimates based on a 3 strand construction and calculated from both sample runs and current production.