Marine Safety Forum warns that a MODU was moored with eight mooring lines and connected to the well. A loud noise was heard originating from aft, port side column and it was observed on the tension monitoring that the mooring line no. 5 lost its initial tension of 145mT down to 45mT.
The MODU got an excursion of 12 meters from initial position and the MODU tilted 2,3 degrees. Angle on lower flex joint was less than 2 degrees. Ballasting system was run to stabilize the MODU and the thrusters operated in manual mode to re-locate the MODU back to its initial position prior to the mooring line failure.
It was identified some time thereafter that the fiber rope insert in mooring line no. 5 had failed. The triggering cause was that the subsurface buoy shackle/chain came into contact with the fibre rope insert and lead to loss of integrity of the fiber rope insert.
Critical Factors (CF) that lead to the incident:
CF1: Subsurface buoy shackle and chain fastened too close to the fiber line
CF2: Rotational movement of the mooring line lead to the subsurface buoy arrangement getting tangled up into the fiber line (Fig 2)
1. Install subsurface buoy to the bottom chain segment by “snotter” shackle in a safe distance to avoid the subsurface buoy to reach the fiber line segment connection point
2. Install high tension swivels in both ends of the fiber line insert
3. Evaluate use of swivels during test tension to avoid twist in pre-installed anchor lines
4. Assess the use of ROV survey when the MODU has achieved work tension in the mooring lines
5. All parties involved in the rig move process, is recommended to make themself familiar with industry learning related to mooring line failures and by doing so, bring learning forward in risk assessments and point-out potential weaknesses in rig move documentation issued for review
Rig Specific Corrective Action Plans to be developed, tracked and closed.