Loss of mooring chain to FPSO

A planned ‘routine’ maintenance operation was in progress, to individually move each of eight anchor chains, to a pre-arranged plan, to ensure that the chain links were not subject to wear and fatigue at the same points over the life of the mooring chain. This operation had been repeated successfully on an annual basis for the four previous years.

The linear tensioner assembly equipment was pressured up to allow for the stowing of two links of chain for tensioner number 5. The chain was lifted by energising the rams to raise the chain gripper and the chain stopper was opened. When the rams reached their full extent, the operator tried to close the chain stopper, to engage the chain in its new position. The chain stopper would not fully engage.

The rams were lowered back to the start position and a second attempt was made. Once again, the chain stopper could not be engaged, due to an incorrect chain position (i.e. they tried to close on the side of the link, rather than on the shoulder of the link).

After the second failed attempt, the operator made the decision to lower the rams to stow the chain in its original position and report the problem. As the rams were being lowered, the operator heard a loud noise, realised there was a major problem and abandoned the immediate worksite. The failure of the equipment assembly resulted in the gypsy wheel being torn from the deckhead. The upper assembly sheared off of the rams and the chain grippers sheared their retaining bolts, allowing the chain to fall freely and pay out of the locker. The bitter end shackle pin broke in the chain locker and the chain paid out to the seabed.

As a result of a lengthy investigation, including the company involved and the equipment manufacturer, the follow lessons learned have been noted:

• The size variation of the individual chain links had had a major impact on the (re)design of chain tensioning equipment, which had not been fully understood or taken into account in the original design. This had had a significant impact on both the chain gypsy wheel design and the chain gripping arrangement;
• Changes made during the design stages had not been fully evaluated for their effect on the installed equipment, resulting in the need to redesign and change out the gypsy wheel and to extend the full hydraulic ram lifting movement, which was insufficient to allow for the tolerances on the mooring chain and correct engagement of the stoppers at certain points in the chain;
• The angle of engagement between the mooring chain and the upper grippers during the chain moving operation was critical. If there was an increased angle, there was the potential for the chain weight to force open the upper grippers.

The company involved has made the following recommendations:

• The gypsy wheel and its support structure are to be redesigned, to ensure that bearing and chain snagging are eliminated. The planned maintenance regime will be redeveloped, to ensure the necessary maintenance tasks are carried out;
• The tensioner assembly is to be modified to lock chain grippers and stoppers in place hydraulically, to ensure correct alignment and to prevent movement and chain release due to chain loading;
• A modification will be made to increase the ram travel distance, to ensure that correct stopper engagement can be achieved when a chain is lifted, taking into account chain link tolerances and variations;
• A remote control device is to be fitted, to allow the operator to be well clear of the equipment when chain movement operations take place.