NOBLE REV 02 FPSO MOORING SYSTEM INTEGRITY STUDY

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Eric Scott
5 years ago
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1 DENTON INTEGRITY STUDY APPENDIX C INCIDENT REPORTS

2 Incident One FPSO Questionnaire Failure of gripper used to rotate turret Several months after installation This FPSO has an internal partially rotating turret utlising a drag chain to control the speed of rotating. In order to change the heading of the FPSO a set of grippers were design in that lock onto the turret and push the hull around it. During one of these movements one of the grippers failed and so the vessels heading could not be changed. Overloading on the hydraulic rams used in the grippers and the grippers associated locking pins. The vessels heading could not be changed and so the vessel was at increased risk of greenwater damage until such time as the unit could be changed out and full heading control restored. Loss of heading control system via alarms in control room. Damaged gripper was replaced with spare and damaged gripper was subsequently repaired. There is an ongoing program to try and improve the design of the grippers as they are known to be continuously overloaded when in use. 72 hours

3 Incident Two FSU Connecting Socket detail between chain and wire element(s) of the mooring system. 2-3 years after installation. On a turret moored floating storage unit connection detail between the chain and wire elements consisted of a wire end socket, a tri-plate and a d-type shackle connecting to the ground chain. The wire open end socket was connected to the tri-plate by a round connecting pin that was held in place by an end plate secured to the socket by 3 bolts around its circumference and to the pin by three bolts in a line. During a subsea survey it was found that the end plate had dropped off and the pin dropped out. It was also noted that end plate bolts had failed or backed off on a number of the other socket connections although the pin(s) had not yet dropped out. The subject connection was in the seabed working section of the catenary and as the wire socket repeatedly picked up and set down (preliminary) there was a large relative motion between the socket and the heavier tri-plate and ground chain section that remained on the seabed. It is thought that this introduced a large torsional/friction load between the pin and the body of the socket that could not be accommodated by the end plate retaining bolts and these failed allowing the end plate to drop off and the pin drop out. There had also been a failure to properly insulate the wire section from the chain section and the cathodic protection on the wire was drained down by the ground chain section resulting in a corrosive environment that might have contributed to the failure. Temporarily restricted offloading operations and repair costs. Subsea ROV survey. Replacement of pin(s) modification of retention bolts, re-instatement of cathodic protection. Initial repair approx 12 weeks total refurbishment several months

4 Incident Three Description: FPSO / Questionnaire Chain link failed single line failure. Several months after installation Having been on location for several months the crew felt that the motions of the vessel had changed and so suspected that a line had failed, this was reinforced by the fact that one of the line tension monitors was reading zero but the monitors were uncalibrated and unreliable and the zero reading was not initially believed. The line tension data was therefore examined on the beach and after 3 weeks of signal processing a step change of 10 tonnes was noted in the remaining lines thus indicating that the line tension monitor was indeed working correctly and the line had failed. Luckily, during and after the failure the conditions were benign and so the rig was not left exposed to a storm in the one line failed condition. On examination of the failed line it was found that the chain had failed at the fairlead. Fatigue failure of the anchor chain due to one of the chain links continuously slapping against the sharp edges of the fairlead (which was a fabricated item rather than a cast pocket). The failure was accelerated due to the detailed design of the fairlead, the large relative motion between the fairlead and the mooring line and the lack of movement of chain through it to spread the wear zone and fatigue loading. Production was not suspended, but examination of the remaining lines found similar damage to all the other lines. To prevent the incident from re-occurring the top sections of all the lines were changed out and the fairlead design was modified. : Changes to IRM The top sections of the chains are now routinely rotated through the fairlead to spread the wear zone and so reduce the fatigue loading. In addition to which, these sections of chain are now routinely inspected to look for damage or excess wear. Lessons Learnt: The lessons learnt from this, was that detailed design of the fairlead should be carefully undertaken and that chain should be routinely rotated through fairleads to spread any damage and so lengthen the working life of the chain. In addition to which, the fact that the top section of each mooring line was designed to be changed out facilitated the quick resolution of the problem. 6 weeks for initial repair plus 3 weeks for the initial identification of the problem. Modifications to the fairlead several months and only feasible by dry docking the vessel.

5 Incident Four FPSO Questionnaire Turret control system (intermittent) This is an intermittent problem and has been occurring since soon after installation The turret in this installation is not directly coupled to the FPSO, instead it floats inside the FPSO moon pool and the vessel centres itself on it by means of a number of horizontal and vertical hydraulic rams that each push 3 graphite type pads onto polished bearing surfaces. Domestic PCs and a specially written piece of software control these rams, and on several occasions the system has lost the plot and locked the turret up solid. Integrity of domestic PCs in an industrial control situation. The vessel becomes stuck on heading when the system flips out and so is at increased risk of greenwater damage until such time as full heading control can be restored. Lack of rotation of the FPSO about its turret. The system is unreliable and so the control system has to be changed out for a system utilising industrial computers. N/A

6 Incident Five FSU Mooring to turret connection detail. Within 2-3 years of initial installation. The connection between a mooring wire and the turret was made by an open ended socket with a central pin. The pin was retained by a bolted detail that became subject to corrosion and fatigue and subsequently failed. Failure to correctly appreciate complex corrosion/fatigue mechanism at critical wire socket to turret detail. Temporary loss of availability and repair costs Annual ROV survey Initial repair with similar item followed by redesign and long term replacement of all similar details with more robust arrangement in more compatible materials. Not Known

7 Incident Six : FPSO Thruster Within first 2-3 years of operation. One of two thruster motors burned out due to Lack of manufacturing integrity of motor windings causing short and burn out of the motor. Loss of availability to thruster, limited operational restriction to ST loading operations and repair and replacement costs. Loss of motor function. Motor removed to beach for repairs. 3 months

8 Incident Seven : FSU Line tension monitoring via sensor on pipe hawser(s) Line tension signals lost Not knowm Not known Loss of line tension read-out Loss of line tension read-out None Not re-instated

9 Incident Eight FPSO Questionnaire Line Tension Monitors Several Years after installation The line tension monitoring system on the unit consists of electronic inclinometers connected to the chain out with the chain pipes. During routine inspection of the chain pipes by an air diving team one of the divers inadvertently knocked off several of the inclinometers. Air diver Loss of line tension monitoring capability Line tension gauges ceased to show any tension even though system was known to be intact. Line tension monitoring was dropped in favour of position monitoring to monitor the status of the mooring system. It has not been repaired.

10 Incident Nine FSU Thruster Several months after installation Due to the loss of the main electrical system the unit was unable to use its electrically driven thruster. Electrical blackout. As the thruster is only used for heading and surge control during shuttle tanker, supply boat and helicopter operations the consequences were not high. N/A None with regard to the mooring system. 5-6 hours

11 Incident Ten FPSO (MOBILE EWT) Mooring Chain 10 years after initial construction After being hit by a 20-25m high wave the vessel lost two moorings lines, subsequent to this over the following 8 hours a further two lines failed. Resulting in the loss of half the mooring system, but the vessel managed to stay on station by utlising its main engine and the remaining 4 mooring lines. It should be noted that although this was a multiple line failure there had been previous single line failures in the unit(s) previous operating history. Excessive wear/motion at the fairlead mooring line interface, chain cables working at high utilisations. Temporary interruption to production, no major loss of position, repair costs. Larger excursions after extreme event. Mooring system upgraded, procedure introduced for inspection, rotation and change out of chain cables in the fairlead working zone. Initial repair one week.

Allspeeds Ltd. Royal Works, Atlas St Clayton le Moors Accrington Lancashire England BB5 5LW. Tel +44 (0)

Allspeeds Ltd. Royal Works, Atlas St Clayton le Moors Accrington Lancashire England BB5 5LW Tel +44 (0)1254 615100 www.allspeeds.co.uk WIRE ROPE GRIPPER WGO135 (without manifold and intensifier) PRODUCT