When IMO Circular 1175 determined the MBL (Line Minimum Breaking Load), it was noted that many Masters and Companies, based on the thought that the higher the MBL the safer the ship would be, purchased lines with significant higher breaking force than required. However, this was a risky approach, because the fittings were constructed to support (Safe Working Load) the construction of MBL.
This means for example, that if a ship had MBL (based on EN) 70t, the fittings were constructed to support such value, and Company had purchased lines 90t, then the ship would be safe only if moored under good environmental conditions.
What would happen if environmental circumstances got close to or went over the boundaries that were taken into account when building?
Additionally the construction reinforcement of the winch base would be 1.25 x MBL = 87.5 t. The breaking force of the line 90t is higher than this value which means that in extreme weather condition close to those calculated for construction the winch based would be damaged before the line break.
The Ship Design MBL (as per OCIMF MEG-4 and IMO Circ. 1175 rev.1) has brought the structure base for all fittings and lines used on board. Additionally, the LDBF (Line Design Break Force) means the minimum force that a new, dry, spliced, mooring line will break at, has set the upper limit to the strength of a line used on board.
Why it is important to have the correct value for Ship Design MBL?
Winches have to be set to have Operational brake holding load at 60% of Ship Design MBL. However, if the operator sets the operational brake holding load to 60 % of MBL of the lines used on board and the lines have significant higher MBL than required by SDMBL, then serious hazards may occur.
Life service and MBL
Each line when constructed should followed by an estimation for life expectancy (under the consideration of safe use and normal environmental conditions) and each line manufacturer is responsible for providing such value. After purchasing the line, every Ship Manager should follow all relevant requirements and tests to determine how long same will be used.
OCIMF MEG-4 recommends mooring lines to be replaced when their residual strength reaches 75% of the Ship Design MBL. This reduction can be ascertained through a combination of destructive testing and historical record. The problem lies when manufacturers or third party experts require a part of the line to be removed from used in order to be tested in laboratory.
Best practices
An effective line management plan which will include retirement criteria , end for end policy, periodical inspections and Wear zone management in order monitor the line is vital.
If the line is about to reach the limit of life expectancy as set by the manufacturer, to be replaced without using any extensions .
Companies may also set their own criteria for line service life for example set a time limit like, each line will be used for 5 years maximum unless retirement criteria are in place for line’s replacement.
Source: Safety4Sea
