Loss of Heading, Mooring System failure and subsequent loss of position

Cause & Consequence

• Failure of vessels or pipework

Incident Location

Onshore office, support base, heliport

Incident Activity

Drilling, workover

Incident Info Source

Neil Smith - Maersk

Specific Incident Equipment

The vessel has a Tentech turret just forward of midships equipped with a 10 point

mooring system with 84mm diameter, K4 grade chains with a design Break Load

(BL) of about 730 tonnes. The turret is maintained in a fixed orientation, the vessel

rotates around it.

Lessons Identified

The incident highlights the need for duty holders to be aware of the potential for

loss of heading & position on vessels fitted with Position Mooring (PM) systems.

1. PM System

The Class driven maintenance and functionality checks for a moored PM vessel are

less stringent and comprehensive than those implemented by full dynamically

positioned vessels such as DSVs. It may be prudent to consider employing a similar

philosophy for moored vessels.

During this incident a number of inputs to the PM model were inaccurate,

contributing to error forces building up within the model and resultant incorrect

forces being applied by the PM system.

Duty holders may wish to review their processes and procedures to ensure that:

the data recording system is sufficient; the model is reset or refreshed at a suitable

frequency; the maintenance and functionality checks are suitable and sufficient;

operators are drilled in the actions necessary in all foreseeable emergency

scenarios; and inputs to the PM system are accurate, reliable and have sufficient

redundancy.

2. Mooring System.

The basis of design of a PM vessel mooring system assumes that the vessel will not

lose heading control, so will remain within approximately 10 degrees of head into

the environment.

Analyses of the mooring system showed that the forces placed on the anchor lines

would have approached or exceeded the break load of the chain at angles seen

during the incident.

Duty holders may wish to review their mooring system’s failure mode analysis in

order to ensure that the forces applied to the vessel, at reasonably foreseeable

angles and environmental conditions are understood and appropriate procedures

and processes are in place to manage the risks from loss of heading and those

forces.

3. Anchor Chains

During this incident anchor chain number 7 failed at the flash butt weld of one of its

links. This failure mode is unlikely to be picked up by visual inspection.

Duty holders may wish to review their inspection and discard criteria for anchor

chain, to ensure they remain appropriate in the light of this failure type.

4. Power Management

During the incident, the power management set up was not optimal for the

prevailing weather conditions.

Duty holders may wish to review their power management procedure and processes

to ensure that they are appropriate for all reasonably foreseeable operating and

weather conditions.

Incident Recommendations

See above