Dynamic Positioning Station Keeping Review – Incidents and Events Reported for 2020 (DPSI 30) IMCA M 256 February 2021 The International Marine Contractors Association (IMCA) is the international trade association representing offshore marine contractors, service companies, and the industry’s supply chain. IMCA’s mission is to improve performance in the marine contracting industry. Our value proposition is to influence our industry in key technical, contractual, policy and regulatory matters that are in the collective best interest of the marine contracting industry. For over 25 years IMCA has maintained an important body of knowledge to assist our industry in the form of published guidance documents promoting good practice across a wide range of technical and professional disciplines. Documents have a selfexplanatory title and are catalogued using a code containing letters and numbers. The letter indicates the discipline, and the number is simply sequential within that discipline. 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Feedback – If you have any comments on this document, please email us: feedback@imca-int.com IMCA M 256 – Version History Date Reason Revision January 2021 Initial publication Rev. 0 DP Station Keeping Review Incidents and Events Reported for 2020 IMCA M 256 – February 2021 1 Introduction ................................................................................................................... 1 2 DP Station Keeping Summary for 2020 .......................................................................... 2 3 2.1 Participation....................................................................................................................2 2.2 Summary of Causes 2020 ................................................................................................3 2.3 Comparison of DP Incidents and DP Undesired Events/ Observations ..............................9 2020 DP Event Trees and Case Studies ......................................................................... 11 1 Introduction This volume of DP station keeping events, reported during 2020, has been prepared by the IMCA secretariat and continues to maintain a high degree of confidentiality and security. Each report received by IMCA is treated on an individual basis. The person submitting the report is contacted by email and only once the information contained in the report is anonymised is it used as feedback to the industry. The importance of receiving reports from the industry cannot be over emphasised; the information the reports contain is used across all IMCA’s DP activity. This includes new and revised guidance documents, feedback to client and regulatory organisations and to users through the IMCA DP Bulletin. The review and analysis of received reports is ongoing with the aim of encouraging and improving reporting throughout the industry and providing valuable lessons learnt and meaningful analysis and feedback. Statistically, authoritative conclusions about the safety of DP operations cannot be drawn from analysis of the reports received; however, trends or patterns may be determined over time. In addition to this review, initiating events, causes and comments reported by users have been incorporated into a summary table for easy comparison. This enables individual DP vessels to readily complete an onboard comparison of actual events occurring in the industry with the situation on-board their vessel. The IMCA 2020 DP Station Keeping Event summary table is available here. DP station keeping reports are welcome from both IMCA members and non-members alike. Reports can be accepted in company or other format providing that the analysis can be carried out from the information received. The IMCA DP station keeping event report form is available here. IMCA M 256 1 2 2.1 DP Station Keeping Summary for 2020 Participation Between 1 January and 31 December 2020, a total of 144 DP station keeping reports were received from 104 different DP vessels; all these reports have been analysed and included within this report. Participation in the scheme remains high; this is a positive reaction from the industry recognising the importance placed on sharing data for the purposes of DP incident prevention and the safe and efficient operation of DP vessels. However, there is still an awareness that DP incidents and undesired events are occurring and not being reported. IMCA will continue to provide lessons learnt from received reports and engage with vessel operators and other industry bodies to ensure the industry receives the maximum benefit and value from participation in the IMCA DP station keeping reporting scheme. The 144 reports submitted by 104 vessels gave an average of 1.4 reports per vessel. As in previous years, the average rests between one and two reports per vessel. Twenty-eight vessels reported more than one DP station keeping event; 76 vessels reported just one event. If the rate of 1.4 reports per vessel were to be repeated throughout the global DP fleet, there should be a much higher number of reported DP station keeping events. Reports received 5 4 3 2 1 0 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 101 Reporting vessels Figure 1– DP station keeping reports per vessel It is important to understand the principle behind the reporting of DP station keeping events as not all reports received indicate an inability to maintain automatic DP control. The reports are used by IMCA primarily to inform the DP industry of lessons to be learnt, therefore contributing to the safe and efficient operation of DP vessels. Three categorisation levels are used by vessel operators when reporting a DP station keeping event: DP Incident A major system failure, environmental or human factor which has resulted in loss of DP capability. DP Undesired Event A system failure, environmental or human factor which has caused a loss of redundancy and/or compromised DP capability. DP Observation An event that has not resulted in a loss of redundancy or compromised DP operational capability but is still deemed worthy of sharing. During 2020 there were 41 DP incident, 79 DP undesired event and 24 DP observation reports submitted. 2 IMCA M 256 Figure 2 – Regional split of DP reporting The above chart represents the number of reports received from each region by IMCA. It should not be used to suggest which region is having more DP incidents but could indicate where the reporting of DP undesired events and observations should be further encouraged. 2.2 Summary of Causes 2020 The IMCA DP station keeping event scheme requires the reporter to provide both a main and secondary cause of the event. The main cause is the event that caused the vessel to lose redundancy or position keeping ability; the secondary cause is the reason why that main cause occurred. This is explained in the following simple example: A DP equipment class 2 vessel configured with four thrusters, bus tie open, has one stern and one bow thruster on each bus. One thruster stops, and the root cause was found to be a power module failure on the thruster frequency drive. Given this example, the IMCA DP reporting scheme would record the main cause as thruster/propulsion; because that was why redundancy was lost. The reason the thruster stopped, and thus the secondary cause, would be ‘electrical’. 50 45 40 35 30 25 20 15 10 5 0 Figure 3 – Number of DP station keeping events by main cause IMCA M 256 3 Figure 4 – Main cause – all DP station keeping events It can be seen from Figure 4 that the largest percentage of the main causes reported for 2020 was thruster/propulsion at 33% (47); this continues to be the most reported main cause since 2012. It is worth noting that 11 of the 47 (23%) resulted in the vessel not maintaining automatic DP control, this is an increase on last year when only 9% resulted in a loss of automatic DP control. Power and Computer were two other main causes making a large contribution to reporting, 18% of these reported events culminated in a loss of automatic DP control. 60 50 40 30 20 10 0 Figure 5 – Number of DP station keeping events by secondary cause 4 IMCA M 256 2% 2% 1% 2% Computer Electrical 10% 13% Environment External factors Human factor Mechanical 41% 26% Position references Power Sensors Thruster/Propulsion 2% 2% Figure 6 – Secondary cause – all DP station keeping events A secondary cause was identified in 129 of the reports; the most frequent causes remain the same as last year electrical 41% and human factors 26%. These causes together with the newly introduced cause of mechanical (13%) represent more than 75% of all secondary causes. The category of ‘human factors’ is broad in nature however all 30 reported causes could be categorised as ‘unintentional behaviour’. There may be many reasons for such unintentional behaviour, these have been subdivided into four categories as follows: 1) Sensory Error – e.g., errors caused by difficulty distinguishing functions, controls, colours, labelling, etc. 2) Memory Error – e.g., errors caused by forgetting to make a selection or setting 3) Decision Error – e.g., errors where a clear decision was made to operate in a particular way 4) Action Error – e.g., errors where a function or control was selected incorrectly 9 8 7 6 DP Incidents 5 DP Undesired Events 4 DP Observations 3 2 1 0 Sensory error Memory error Decision error Action error Figure 7 – Secondary cause – Human Factors These errors may be because of design factors, ergonomic factors, familiarisation issues, training issues, documentation issues, competency issues and so on. It can clearly be seen that IMCA M 256 5 decision and action errors led to proportionately more events leading to loss of DP control than any other error. The category of ‘electrical’ & ‘mechanical’ is also broad in nature. Reviewing these secondary causes, we can subdivide the total number of 73 reported causes into two categories as follows: 1) Lack of / insufficient maintenance 2) Natural Failure 30 25 20 DP Incidents 15 DP Undesired Events DP Observations 10 5 0 Lack of maintenance Natural Failure Figure 8 – Secondary cause – Electrical & Mechanical Natural failure has been assigned where the received report details a simple component failure. Further investigation into the cause of these component failures may have indicated lack of or insufficient maintenance. 2.2.1 DP Incidents Event 20118 20005 20080 20108 20036 20086 20071 20079 20060 20091 20092 20041 20072 20096 20115 20043 20084 20094 6 Main Cause Computer Computer Computer Computer Computer Computer Computer Environment Environment Environment Environment Environment Environment External Factor Human Factor Human Factor Human Factor Human Factor Secondary Cause Computer Computer Computer Computer Electrical Human Factor Human Factor Environment Human Factor Human Factor Human Factor Human Factor Position References Mechanical Computer Human Factor Human Factor Human Factor Operation Cable laying Cargo Gangway ROV Standby Cargo ROV Cable laying Geotechnical drilling Geotechnical drilling Geotechnical drilling Well stimulation ROV Pipelaying ROV Cargo Saturation Diving Surface Diving DP Class 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 IMCA M 256 20101 20134 20031 20029 20102 20116 20077 20046 20130 20042 20088 20033 20082 20140 20078 20017 20028 20061 20093 20011 20089 20023 20128 2.2.2 Event 20037 20074 20047 20070 20063 20105 20014 20129 20131 20006 20059 20113 20020 20081 20095 20098 20138 20035 20039 20045 20099 20104 20110 20004 20040 20126 20127 20120 IMCA M 256 Human Factor Human Factor Position References Position References Position References Position References Position References Power Power Power Power Sensors Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion External Factors Human Factor Human Factor Human Factor Electrical Human Factor Mechanical Thruster / Propulsion Electrical Electrical Electrical Environment Human Factor Human Factor Human Factor Human Factor Human Factor Mechanical Cargo ROV Cargo Cargo Cargo Cargo Pipelaying Cargo Standby Standby Drilling ROV Cargo Cargo Cargo Cargo Cargo Cargo Geotechnical drilling Saturation Diving Cargo Cargo Cargo 2 2 2 2 1 2 2 3 2 2 2 2 2 2 2 1 2 2 2 2 2 2 2 Operation Cargo Cargo Standby ROV Saturation Diving Cargo Cargo Drilling Cable laying Standby Standby Geotechnical drilling Saturation Diving Standby DP Trials Cargo Pipelaying Cargo Cargo Cargo Cargo Cargo Cargo Cargo DP Trials DP Trials Geotechnical drilling Heavy lift DP Class 2 3 2 2 2 2 2 3 2 2 2 2 2 3 2 2 3 2 2 3 2 2 2 2 2 3 2 3 DP Undesired Events Main Cause Computer Computer Computer Computer Computer Computer Computer Computer Computer Electrical Electrical Environment Environment Human Factor Position References Position References Position References Power Power Power Power Power Power Power Power Power Power Power Secondary Cause Computer Computer computer Computer Computer Electrical Electrical Electrical Electrical Electrical Human Factor Human Factor Power Mechanical Position References Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical 7 Event 20073 20015 20038 20107 20075 20132 20114 20065 20008 20085 20044 20049 20052 20083 20109 20025 20121 20112 20117 20021 20050 20067 20002 20097 20007 20026 20027 20058 20003 20030 20032 20076 20034 20064 20069 20087 20122 20124 20013 20119 20012 20111 20125 20141 20142 20068 20053 20022 20090 20133 20018 8 Main Cause Power Power Power Power Power Power Power Power Power Power Power Power Power Power Power Power Power Power Sensors Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Secondary Cause Electrical Electrical Electrical Electrical Electrical Electrical Human Factor Human Factor Human Factor Human Factor Mechanical Mechanical Mechanical Mechanical Mechanical Mechanical Mechanical Computer Computer Computer Computer Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Electrical Human Factor Mechanical Mechanical Mechanical Mechanical Mechanical Mechanical Power Thruster / Propulsion Operation Pipelaying ROV ROV ROV Standby Standby Cargo Pipelaying Standby Standby Cargo Cargo Cargo Cargo Cargo Geotechnical drilling Pipelaying Standby ROV Cargo Cargo Standby Saturation Diving Cable laying Cargo Cargo Cargo Cargo Cargo DP Trials DP Trials FPSO heading control ROV ROV ROV ROV ROV ROV Saturation Diving Standby Saturation Diving Cargo Cargo Cargo Cargo Well stimulation Standby Cargo ROV Cargo ROV DP Class 2 2 2 3 3 2 2 2 2 2 3 2 2 2 2 2 3 2 2 2 2 2 3 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 2 2 2 2 2 2 2 2 2 2 2 IMCA M 256 2.2.3 Event 20136 20137 20066 20001 20100 20139 20019 20010 20055 20051 20009 20143 20106 20062 20016 20048 20054 20057 20103 20024 20144 20123 20135 20056 2.3 DP Observations Main Cause Computer Computer Computer Computer Computer Computer Computer Electrical Electrical Electrical Position References Position References Position References Power Power Power Power Power Power Sensors Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Thruster / Propulsion Secondary Cause Electrical Electrical Electrical Sensor Human Factor Human Factor Electrical External Factors Human Factor Electrical Mechanical Mechanical Mechanical Mechanical Power Mechanical Electrical Electrical Electrical Human Factor Operation Pipelaying Pipelaying Saturation Diving Saturation Diving Cargo Pipelaying ROV Cargo Saturation Diving ROV Cargo Saturation Diving Cargo Cargo Cargo Cargo Cargo Pipelaying Cargo DP Trials DP Trials Saturation Diving Standby Saturation Diving DP Class 3 3 3 3 2 3 2 2 3 2 2 2 2 1 2 2 2 3 2 2 2 2 1 3 Comparison of DP Incidents and DP Undesired Events/ Observations A comparison has been undertaken of the number of DP incidents (loss of automatic control) and the number of DP undesired events/DP observations (potential loss of automatic control), for each of the nine recorded DP event main causes. Of the 144 DP events reported to IMCA during 2020, 41 of these resulted in loss of automatic control and 103 were DP undesired events or DP observations. For any given DP event main cause, if effective barriers are in place, there should be a low ratio of DP incidents to DP undesired events and DP observations. Whereas if there is a high ratio of loss of automatic control to DP undesired event and DP observations, this might indicate that barriers are not being effective. IMCA M 256 9 40 35 30 25 Loss of automatic DP control 20 15 10 5 DP undesired event & observation 0 Figure 9 – Reported DP incidents/DP undesired events & DP observations by main cause 2020 A high ratio of loss of automatic control to DP undesired event and DP observations was recorded for environment, human factor and position references. This suggests that barriers might not have been so effective for these events. When considering the secondary cause for these 17 recorded events 13 of them had human factor or no conclusion as the cause. 45 40 35 30 Loss of automatic DP control 25 20 15 10 5 DP Undesired event & observation 0 Figure 10 – Reported DP incidents/DP undesired events & DP observations by operation 2020 Only limited conclusions should be drawn from the above. The proportion of DP undesired events and DP observation to loss of automatic DP control is more than 2:1 for all types of operation except cable laying, drilling and other. There were only two DP station keeping events reported by DP drilling operators. Operations included in the ‘other’ category are geotechnical drilling, often in shallow water, gangway transfer, FPSO heading control, heavy lift and well stimulation. Since 2015, Cargo operations have shown a relatively high number of DP incidents (loss of automatic DP control); this is thought to be because the operation was generally not as position sensitive as some other DP operations and can, on most occasions, be quickly terminated. It is pleasing that there is a good spread of reporting across all sectors of the industry. 10 IMCA M 256 3 2020 DP Event Trees and Case Studies All IMCA DP Event Bulletins can be found and downloaded here. DP Undesired Event Due to Online Maintenance Work DP Class 2, on DP in 1900m water depth, engaged in ROV operations 6 thrusters online, nil on standby 4 generators online, nil on standby, bus tie open, 4 redundant groups 3 DGNSS & 1 HPR online 3 Gyros, 3 MRUs and 3 wind sensors online 0249 Local alarm Remote MultiPurpose input/output card frozen 0400 risk assessment completed, RMP module to be replaced 0413 Generator 2 circuit breaker reconnected automatically Two ROV in water 0412 During replacement the circuit breaker for generator 2 tripped Power restored to thruster 3 and other affected equipment Main crane with load in water, not connected to seabed Thruster 3 tripped and supplementary power lost to the crane Vessel maintained position on DP Wind 12kts 187°, current 2.5kts 149°, wave height 1.4m, visibility good Comments from the report: The vessel reported that they had changed these modules in the past whilst on DP without any issues. The risk assessment had identified that the consequence would not lead to loss of equipment greater than worst case and proceeded due to the status of the vessel operations, open water and not connected to the seabed. Considerations of the IMCA Marine DP Committee from the above event: Incident validates the redundant capability of the vessel, but also potentially highlights a hidden failure leading to generator 2 failure; The report did not identify the root cause for the loss of equipment whilst replacing the RMP module, a successful online activity undertaken previously; This event highlights the benefit of considering mission equipment during risk assessments; It also event highlights that expected results of repairs are not always guaranteed; As far as is possible, maintenance or replacement of critical DP equipment should not be done while conducting DP operations; In this instance, it would have been prudent for ROVs to return to their TMS and the crane made safe prior to undertaking the work. IMCA M 256 11 Change of Heading Leads to DP Incident DP Class 2, on DP in +3000 m water depth, engaged in ROV operations 6 thrusters online, 2 on standby 3 generators online, 2 on standby, bus tie closed, 2 DGNSS & 1 HPR online 3 Gyros, 3 MRUs and 3 wind sensors online 12:35 ROV operations ended and crane was deployed to recover dead ROV 12:47: T 2 started & selected to DP and load shared with the other two Thusterrs successfully 12:51 ON MAIN CONTROL UNIT, stop button was pressed allowing restart of all 3 bow thrusters 12:43 Vessel heading was changed 15˚ away from the wind to accommodate recovery of ROV to deck 12:49: Thrusters 1, 2 & 3 stopped with No DP or Thruster alarms Vessel drifted off position 200 m 12:46 Change of vessel heading caused high load on bow thrusters 1&3 On Thrusters Main Control Unit showed T 1, 2 & 3 Start Interlock 12:52 Bow thrusters 1, 2 & 3 all online again and operations resumed Wind 40 kts, wave height 2.5 m, visibility moderate Comments from the report: The vessel had been on DP for the whole day and in order to lift the ROV on deck with the crane, vessel heading had to be changed 15 degrees away from the wind. This caused a high load on two running bow thrusters and the third bow thruster was started, selected to DP and successfully shared the load. Increased windward force on the vessel due to high wind velocity acting on it as it changed heading, caused all the three bow thrusters to trip on overspeed with no DP or thrusters’ control system alarms! Further investigation revealed the overspeed settings were too tight and had to be increased in accordance with manufacturer’s recommendation. Considerations of the IMCA Marine DP Committee from the above event: It is not clear why the additional bow thruster was not put online before the heading change; A simulated alteration of heading and or referring to the vessel’s capability plots should have been considered to ensure that the heading change could have been performed within vessel’s DP capability; The overspeed trip of all thrusters during DP operations shows inadequate initial commissioning and full power testing during the FMEA proving trials and annual DP trials and following any modification; This incident further highlights the importance of equipment protection settings with respect to DP redundancy concept, in addition to equipment safety. DP FMEAs require to consider the effects of equipment protection system on the wider DP redundancy concept. Reference Guidance on Failure Modes & Effects Analysis (FMEA) (IMCA M 166). 12 IMCA M 256 Unreliable Component Causes a DP Undesired Event On DP in 500m water depth, standby awaiting cargo operations 5 thrusters online, nil on standby 2 generators online, 2 on standby, bus tie open 2 DGNSS online 1 laser system on standby 3 Gyros, 2 MRUs and 2 wind sensors online Standing by 400m from drill ship awaiting instruction One stern and one bow thruster unavailable Vessel in drift off position Task Appropriate Mode Vessel maintained position on DP Port switchboard blacked out Departed the 500m zone for investigation Wind 5kts 240°, current 1.2kts 220°, wave height 1.0m, visibility good Comments from the report: Investigation found that a 24v bridge rectifier supplying voltage to the main generator switchboard breakers 1 and 2 had burned out. The age and quality of the rectifier was thought to be the main reason for failure. Planned maintenance routines were updated and shared with the fleet. It was recognised that the system was vulnerable due to a single 24v bridge rectifier supplying the entire port switchboard. Considerations of the IMCA Marine DP Committee from the above event: The vessel validated that it could maintain position following the worst-case failure of a complete switchboard. Though redundancy is a must in DP 2/3 designs, components quality may impact the likelihood of such DP events occurring. IMCA M 256 13 DP Incident Caused by Software Glitch DP Class 2, on DP in 1501-3000 m water depth, engaged in Cargo operation 6 thrusters online, nil on standby 2 generators online, nil on standby, bus tie open, 2 redundant groups 2 DGNSS & 1 Laser based online 3 Gyros, 3 MRUs and 2 wind sensors online 1330 With hose connected the vessel was asked to move a short distance 1333 DP system showed the new position as zero latitude and longitude 1338 Vessel holding position and was instructed to retrieve the hose 1332 2 meters to Stbd was initiated in two instalments of 1m each 1335 Control was taken by independent joystick to maintain position 1345 Vessel headed out of the 500m zone in DP mode for investigation The vessel suddenly commenced an uncontrolled drive off 1336 Vessel in control and DP mode selected as position readings back to normal Wind 4kts 47°, current 1kts 51°, wave height 1m, visibility good Comments from the report: The two meters move command was given in two one-meter steps to the DP system by the operator with a time lapse of a few seconds. This was investigated by the DP system vendor and was identified as a software glitch. Two consecutive incremental move commands before the window screen has time to refresh resets the DP system to a zero position automatically. Instructions were put in place on-board this and similar vessels until the vendor remedies the software issue. Considerations of the IMCA Marine DP Committee from the above event: This and similar incidents highlight the need for extensive testing of the software at all stages; Many vendors continuously update their software with patches, revisions, etc. Vessel operators must ensure a robust management of change (MoC) procedure and suppliers should provide an appropriate rigorous testing program; The DP operator(s) appear to have initially responded well to the incident and this is a credit to their training and competence; Reference IMCA documents Guidance for developing and conducting DP annual trials programmes (IMCA M 190) and Guidance on failure modes and effects analysis (FMEA) (IMCA M 166). 14 IMCA M 256 DP Incident with Assisted Mooring DP Class 1, on DP in 100m water depth, engaged in Cargo operations 5 thrusters online, nil on standby 3 generators online, one on standby, bus tie status not mentioned, no redundant groups 2 Gyros one online and one on standby, 1 MRUs and 2 wind sensors online 2 DGNSS online 0715 Vessel in DP assisted mooring involved in cargo operations 0732 Mooring line aft breaks due to PS propeller pitch goes to 100% 0733 Sway control taken in joy stick mode to reduce t hrust commands preventing vessel bow to come close t o the install ation 0729 DP alarms initiated due to insufficient thrust Vessel swings out of position with load connected 0734 Vessel back in posi tion wi th sway control in joystick, hook disconnected, and v essel set up i n auto DP mode Heading out of limits and crew already fastened crane hook to the load Crane driver maintains slack on the crane cable until position restored 0740 Vessel all fast again and resumes operation Wind 10kts 260°, current 2.5kts 292°, wave height 1m, visibility good Comments from the report: During cargo operations in DP assisted mooring, due to undefined information being fed to the DP system, the pitch on the port propeller increased to maximum. The vessel started to drive off the installation with the crane connected to the load, dimensions 3.5*10.5*6.5 m weighing 11 M Tons. All deck crew were already in safety area behind crash bar ready with tug lines for the lift. Considerations of the IMCA Marine DP Committee from the above event: There appears to be no consideration to the external force being applied to the vessel due to the use of mooring lines when in DP; If an external force is applied to a DP vessel without feedback and correct configuration within the DP system, it will have unintended consequences; It appears an appropriate Activity Specific Operating Guideline (ASOG) was not in use. IMCA M 256 15 DP2 Semi-submersible MODU DP Incident Case narrative: A DP2 Semi-submersible mobile offshore drilling unit (MODU) was working in field preparing for drilling operations on location. The environmental conditions at the time were benign. Cargo operations were ongoing with a DP2 platform supply vessel alongside. The vessel experienced a total loss of all online thrusters leading to the MODU starting to drift. The power management system standby started offline generators; however, it could not re-engage thrusters to DP automatically. The supply vessel was ordered to relocate out of the 500m zone, and the crane driver instructed to lower the crane boom to rest. With no thrusters available the vessel drifted before the DPOs were able to start three previously offline thrusters manually, which were then selected to DP. Subsequently, the Electrical Technical Officer reset a further two thrusters locally allowing them to be selected to DP control. Within 3 minutes a sufficient level of thrusters were online and station keeping was stable. Within 17 minutes, all remaining thrusters were reset and operational. The vessel management team commenced a failure investigation thereafter. At the time of the event, the vessel was being operated on automatic DP2 mode with 3 of 8 generators and 4 of 8 thrusters online. The main 11kV switchboards were being operated with closed bus tie, each side of the main switchboard powering one forward and one aft thruster as per the proven redundancy concept. The investigation revealed that, as a result of an incorrect maintenance action, the initiating event was caused by a human factor – an offline generator was accidently connected to one of the main switchboards which resulted in an instant severe power instability, causing a significant active and reactive power demand and subsequent voltage and frequency drop. The generator in question then tripped on its reverse power protection resulting in significant voltage spike as a result of removing the large inductive load from the network. The large voltage and frequency drop caused the thrusters to phase back. It is not clear from the investigation whether this was a function of the power management system or the fast acting blackout prevention function of the thruster drive (or a combination of both). The subsequent voltage spike caused by the removal of the stopped generator led to the online thrusters tripping offline. Although the investigation did not detail the reason for this, it is assumed that the thruster drives were protecting themselves from internal damage by tripping offline. Most modern thruster drives have sophisticated monitoring and protection systems measuring internal DC voltage and ensuring that this voltage does not increase or decrease beyond limits that would otherwise result in internal component damage. The lessons: 1. The investigation report highlighted a number of findings and subsequent actions as follows: a) Undertake a full review of maintenance procedures on high voltage circuit breakers to ensure that this work is undertaken at suitable windows of opportunity and not during DP operations; b) Undertake refresher crew training and review switchboards for any need for additional safety or warning labelling; c) Engage with the voltage regulator and thruster drive vendors to ensure their products reacted as would be expected in such an event; d) Investigate the ‘locking off’ of the manual close buttons of ‘key’ HV circuit breakers including the permit to work system; 16 IMCA M 256 e) Share lessons learned internally within the vessel owner’s fleet and externally within industry. 2. The investigation report did not consider bus tie position and the overall redundancy concept of the vessel. There are well documented arguments for open or closed bus tie position; however, this event clearly demonstrates the risk of a failure on one redundant group affecting the other redundant group through the common point of the bus tie. It is not inconceivable to assume that had environmental conditions been greater, necessitating more thrusters online at the point of failure, then this failure may have resulted in insufficient thrusters offline available to be selected to DP thus causing a significant delay manually resetting thrusters and a significant drift off. 3. Continuing with the theme of closed bus operations, the investigation report did not discuss the DP FMEA specifically considering the suitability of the overall protection scheme coupled with the power management system and thruster blackout prevention functions, such that a fault cannot be transferred from one redundant group to another. 4. The investigation report detailed the desire to ‘lock off’ manual controls on the switchboard. Careful consideration of such an action is needed to ensure that emergency functionality is not inhibited. For example, manual circuit breaker controls may be required for emergency synchronisation of generators. This case study demonstrates the risks of undertaking maintenance of critical DP components or systems while undertaking DP operations. The case study also highlights the challenges that exist for vessels operating in closed bus mode, i.e., where the otherwise redundant groups are connected via a common point and the risks presented as a result. Such factors should be considered at the design stage of DP vessels and fully analysed within the vessel’s DP FMEA, confirming through FMEA proving trials and subsequent DP Annual Trials Programmes. IMCA M 256 17 DP Incident Caused by Human Factor DP Class 2, on DP in 30 m water depth, engaged in Geotechnical drilling 5 thrusters online, nil on standby 3 generators online, 2 on standby, bus tie open, 2 redundant groups 3 Gyros, 2 MRUs and 3 wind sensors online 3 DGNSS & 1 Taut wire online 09:55:16 Alarm on DG#1 – Fuel differential pressure 09:59:37 DG#1 is disconnected from the Port Swbd for repairs Decision made to start DG#4 and stop DG#1 10:02:47 After 3.5 minutes running, DG#4 trips for unknown reasons causing partial blackout 09:59:28 DG#4 started and connected to Port switchboard Due to the partial blackout, port azimuth thruster and bow tunnel T1 are lost Wind 20kts 225°, current 1.6 kts at 250° wave height 4.2 m, visibility moderate Vessel drifts off 3.1m and then regains position Comments from the report: It is mentioned in the report that DG#4 tripped out for unknown reasons. Elsewhere in the report the cause of this incident is mentioned as complacency or lack of knowledge with DP redundancy concept. Considerations of the IMCA Marine DP Committee from the above event: • The set-up of the PRS was such that the DGNSS had the majority of the “PRS Weight” calculation in the DP controller, which is not recommendable. • When a fuel filter differential pressure alarm is received, ensure that the correct procedures for filter changeover and filter replacement are carried out as soon as reasonably practicable. Always ensure that standby diesel generator is checked to ensure continuous service. • The excursion was 3.1m but it should be noted that this is 10% of the water depth and could have been critical depending on the industrial mission. • There is no mention of the DP watch circle in the report. It is particularly important that an Activity Specific Operating Guidance (ASOG) document is compiled and utilised for all DP operations, in accordance with IMO Circular 1580, chapter 4, see IMCA M 220 Operational Activity planning. 18 IMCA M 256 Lack of Operational Planning Leads to DP Incident DP Class 2, on DP in 60m water depth, engaged in saturation diving operation 5 thrusters online, nil on standby 2 generators online, nil on standby, bus tie closed 2 DGNSS online 1 Radar based & 1 HPR system on standby 3 Gyros, 2 MRUs and 2 wind sensors online 07:56 Diver in water 08:09 No available reference system and vessel lost position Vessel made contact with installation 07:58 DGNSS No. 1 alarmed on high noise, deselected by DPO Joystick selected by Master/DPO 08:14 Diver was recovered, and vessel clear of installation 08:00 DGNSS No. 1 restarted exhibiting the same alarm Dive control asked to surface the diver 08:35 Vessel clear out of 500m zone for investigation 08:08 DGNSS No. 2 alarmed Invalid HDOP (Horizontal Dilution of Precision) 08:11 Vessel getting close to the rig due to wind effect Wind 13kts 180°, current 0.6kts 296°, wave height 1.2m, visibility good Comments from the report: It was concluded that the loss of both DGNSS were due to huge superstructure of the installation interfering with satellite signals. This phenomenon is called reflection meaning DGNSS picks up reflected signals from the surroundings. Considerations of the IMCA Marine DP Committee from the above event: • The vessel was not being operated as a DP Equipment Class 2, as the requirement is at least three independent position reference systems should be installed and simultaneously available to the DP control system. Reference IMCA M 252 Guidance on Position Reference Systems and Sensors for DP Vessels. • Loss of more than one DGNSS position reference system is a well-known issue due to external common cause failures, for example shielding of reference satellites and/or differential corrections. • Operational planning and decision support tools, such as Activity Specific Operating Guidelines (ASOG), were clearly not in use or were inadequate. Reference IMCA M 220 Rev 1 provides Guidance on Operational Activity Planning. • • It appears that the DP Alert system was not used. It is mentioned in the report that at 08:09, master takes control of the vessel by the IJS. It is unclear from the report why the vessel then made contact with the installation. Reference IMCA M 117 The Training and experience of Key DP personnel. This event clearly demonstrates the benefit that can be gained from regular DP exercises and drills. IMCA M 256 19 Loss of Gyro Input Causes Loss of Position Case narrative: A DP 2 vessel was involved in ROV operations in deep water with good visibility. The environmental conditions were moderate with a wind speed of 15 Knots and a current speed of 1.2 knots during the time of event. The vessel was operating in open bus tie mode with two redundant groups. There were five (05) thrusters and four (04) generators installed on the vessel and all were selected to DP. There were three (03) gyros, two (02) position reference systems and two wind sensors available and all selected to DP. The following diagram depicts the system configuration prior to the event: G1 Bow tunnel 2 G2 Port Azimuth G3 Bow Tunnel 1 Stbd. Azimuth G4 Bow Tunnel 3 Figure 11 Vessel power system configuration The event: An alarm “Gyro 2 Not Ready” appeared and gyro 2 was automatically deselected from the DP system. Immediately after this, gyro 1 also dropped out with alarms, “Gyro 1 Not Ready” and “Gyro 1 Heading Dropout”. Within one second the third gyro failed with the following alarms in succession, “Gyro not enabled”, “All Reference systems rejected” and “Position Dropout”. It was noted that gyro 1 repeater on the bridge continued to display the correct heading, while heading input from gyro 1 was missing at the DP operator station (OS). Concurrently, it was further noticed that gyro 2 repeater was in alarm with no heading display and gyro 3 was displaying the heading with 10 degrees offset. Orders were given to recover the ROVs on deck and the vessel control was transferred to full Manual mode. 20 IMCA M 256 The crew managed to get the vessel back to full DP control within twenty-eight (28) minutes from the onset of the first failure with one hundred and fifty (150) metres of uncontrolled movement during that time. The following actions were executed: • Gyro 2 restarted, rebooted and realignment completed • DP OS 1 & OS 2 soft rebooted • Controllers A & B of OS 1 rebooted after which all PRS and DP sensors online and enabled The investigation revealed that gyro 2 was lost due to the loss of its Interface and Power Supply Unit. After replacing this unit, a test was carried out to prove the independence of gyro 1 from gyro 2. On disconnecting the power supply to gyro 2, it was observed that gyro 1 was also lost. Further system analysis concluded that the gyro 1 ready signal was linked with that of gyro 2 in the DP controller cabinet. This cross-connection issue caused a detrimental effect on both gyros’ performance. In addition, following a malfunction on any gyro, the No. 1 Control and Display Unit would force the gyro 1 ready signal to disappear at the DP operator station. These issues were corrected and tested to the satisfaction of all involved parties. During the time of the incident gyro 3 sensor value was found to be approximately ten (10) degrees different from gyro 1 and gyro 2 headings. When both gyros 1 and 2 were deselected from DP, gyro 3 was selected to the DP system. However due to the large deviation in the heading value of gyro 3 compared with the estimated heading, which in this case should not have been more than two degrees, the DP controller rejected gyro 3. The lessons • • • • If a gyro has a static deviation from other gyros, this should have been identified, investigated, and rectified so that all gyros’ measured headings are within the DP control system limits to avoid any such problems The common mode failure related to the ready signal of gyros’ 1 and 2, should have been identified during commissioning, FMEA proving trials and DP annual trials programme. IMCA documents M 166, “Guidance on Failure Modes & Effects Analyses (FMEAs) and M 190, “Guidance for Developing and Conducting DP Annual Trials Programmes”, underline the importance of having robust FMEA and set of annual trials IMCA M 252 “Guidance on Position Reference Systems and Sensors for DP Operations” provides general information on use of reference systems and the good practice of multiple references and sensors Importance of proper investigations and robust testing of cross connections in redundancy groups of DP vessels is highlighted in this case study This case study demonstrates the importance of robust testing procedures during FMEA proving trials, annual trials or 5-yearly trials program. The case study also highlights the importance of conducting daily compass checks and completion of all other operational checklists so that differences in redundancy groups are identified early and can be corrected. IMCA M 256 21 Simultaneous activities on the bridge caused a DP Incident DP Class 2, on DP in 125m water depth, engaged in saturation diving operations 5 thrusters online, nil on standby 5 generators online, nil on standby, bus tie open 1 DGNSS, 1 HPR & 1 laser-based system online, 3 DGNSS & 2 taut wire on standby 3 Gyros, 3 MRUs and 3 wind sensors online Vessel on DP preparing to start the diving operations, DPO engaged in completing 6 hourly checklists The vessel was taken in joystick control auto heading to move away from the platform Heading change inadvertently selected unnoticed Joystick mode set to manual heading continuing to sway clear The master observed the stern of the vessel coming close to the platform and alerted the DPO Joystick auto heading with the exact heading selected The amber status was immediately activated DP selected clear of the platform and vessel stepping away on DP Wind 22kts 225°, current 1kts 060°, wave height 1m, visibility moderate Comments from the report: The DPO, though experienced, was new to the vessel and the DP system. He had undergone familiarisation of the system but misinterpreted 30 o heading increment as being a 30o / min rate of turn. As the 6-hourly checklists were being completed, the vessel was making moves and deploying references. Multiple operations were initiated just prior to watch handover which could have been delayed by a few more minutes. Considerations of the IMCA Marine DP Committee from the above event: Bridge resource management was not properly exercised. In principle, the second DPO should fill out the checklists and the DPO at the desk should concentrate on vessel position keeping only; The familiarisation process should be reviewed to be more robust to ensure new DPOs are completely familiar with the desk and functions; Reference should be made to IMCA M 117 “The training and experience of key DP Personnel; It should be noted that a 30o per minute rate of turn was considered to be far too high whilst engaged in diving operations. 22 IMCA M 256 Inadequate Position Reference Systems Caused DP Incident DP Class 2, on DP in 40 m water depth, engaged in cargo operations 4 thrusters online, 1 on standby 2 generators online, 1 on standby, bus tie open, 2 redundant groups 2 DGNSS online & 1 laser based offline 2 Gyros, 2 MRUs and 2 wind sensors online The vessel was involved in a SIMOPS with a PSV Interference from a faulty RADIUS of the PSV caused the loss of the signals to both DGNSS During the operation, the vessel lost inputs to both her DGNSSs Switched to manual control and manoeuvred clear of the platform The third PRS was out of order during the incident Vessel drifted off 5m while transferring to manual mode Wind 8 kts at 260°, current 0.8 kts at 060°, wave height 1m, visibility good The vessel lost automatic control due to the loss of all its position reference systems Comments from the report: It is mentioned in the report that task appropriate mode (TAM) and activity specific operating guidelines (ASOG) were in use, with the limitation identified for 2 DGNSS subject to common mode failure. Considerations of the IMCA Marine DP Committee from the above event: Use of only 2 DGNSS was contrary to the MSc.1 / Circ. 1580 requirements. This meant the vessel was not set up in accordance with DP class 2 requirements of 3 position reference systems based on at least two different principles; Vessel sensors selection did not appear to comply with DP class 2 requirements, though this noncompliance was not a factor in the incident; Task appropriate mode was in place, indicating that a loss of position was acceptable. However, the incident indicates that an adequate risk assessment was not done and TAM and ASOG were not thoroughly reviewed prior to their approvals; When the vessel lost its automatic DP control due to the loss of all its PRSs, the position could have been maintained for a period of time using the DP mathematical model; The report states that interference from a faulty laser-based system on the PSV caused the loss of signals to both the DGNSSs. However, this could have been caused by interference of the differential correction signal receivers caused by shielding or reflection; Information on radio interference is provided in IMCA M 252 Guidance on Position Reference Systems and Sensors for DP Operations, Section 4.6 Operational Consideration Summary. IMCA M 256 23 Lack of Experience in Dealing with Environmental Conditions Caused Loss of Position Case narrative: A DP 2 vessel with two redundant groups and operating in open bus mode, was involved in drilling operations in water depths of approximately 38 metres. The vessel was performing cone penetration testing (CPT) at a location with a drilling depth up to 90 metres. There were six thrusters and four generators available onboard and all were selected to DP. The available position reference systems (PRS) onboard were three DGNSS, one taut wire and one HPR, out of which only two DGNSS were selected to DP. Also, out of the available four wind sensors, three of them were selected into DP, along with three gyros and three motion reference systems. The visibility was reported to be good with wind speed of fifteen knots at 045˚, current speed of four knots at 040˚and swells of 2 metres height with a duration of ten seconds at 220˚ were recorded during the operation. At the time of the incident, the vessel was drilling at a depth of 54 metres. N The vessel was encountering strong currents from the stern, as expected, and experienced by the DPO’s during the earlier spring tides. For this reason, the vessel receives location specific current modelling from a remote centre during its operations. These models display peak marine current’s direction and speed at any given time. During these strong currents, it is prudent to change the vessel heading regularly to follow the currents direction and to keep them at the stern. The Event At 20:00 hrs the senior DPO took over the watch at the DP desk and at 20:10 noticed the current was changing direction and intensity. The vessel heading was changed from 020˚ to 024˚ to keep the current at the stern. After the heading change, the loading on the vessel’s thrusters were observed to be within normal operating limits and the vessel maintained its position. At 20:45, the SDPO noticed another change in the direction and speed of the current, and further observed that the load was increasing on all thrusters. The drilling control was advised of the need to make a further heading change as the CPT operation was ongoing. At 20:54 the DP consequence analysis advised that the load was higher than one of the two redundancy groups could tolerate post worst-case failure. Ten seconds later, power consumption was within limits again. The SDPO advised the drilling control of a further heading change which was confirmed at 20:56, 24 IMCA M 256 and the heading was changed from 024˚ to 028˚. At 21:00 the DP consequence analysis warning indicated that the power online was in excess of the capacity of one redundant group, post worst-case failure. This alarm lasted for one and a half minutes. After completion of the heading change and at 21:05, the loading on the vessel’s thrusters were observed to be within normal operating limits and the vessel maintained its position. The SDPO handed over the watch to the DPO. At 21:15 the DPO alerted the SDPO that the load on thrusters T3 and T4 were increasing. The SDPO, immediately attended the DP desk, noticing that thrusters T3 and T4 loads were increasing and that the vessel was struggling to maintain its position. At 21:19 the heading was changed by 2˚ to 030˚. At 21:21 the DP system indicated high loads on T3 and T4 followed by a consequence analysis “Off Position” warning alarm. Simultaneously, there were speed feedback faults on both T3 and T4. At 21:22 a DP yellow alert was given to the drill operators as the vessel excursions overshot the ten percent of the water depth limitation. The drillers immediately stopped their drilling operation, lifted the drilling equipment, and hoisted it back at the moonpool. The heading was changed to 040˚ where the current was at the vessel’s stern. At 21:25 the vessel settled down and could maintain position. At this time, the drilling assembly was inspected, and the pipe and the bottom hole assembly were observed to be in good condition. After a complete inspection, it was confirmed that there was no damage to the drilling equipment. Distance of uncontrolled movement during the event was nine metres and the duration of the event was recorded to be six minutes. The lessons It needs to be fully understood that the current indicated was the “DP current” and not a “measured current”. A DP current value could be affected by other disturbing factors and needs to be relied on with care with regard to magnitude as well as direction. It was mentioned in the report that the current strength was above the thresholds specified in the vessel ASOG. It is essential the ASOG is continually reviewed to ensure it covers all possible hazards during a specific operation. IMCA M 220 Guidance on Operational Activity Planning, discusses the importance of various documentations such as CAM, TAM and ASOG. DP and emergency response drills discussed in the document, highlight the importance that all DP personnel must know what actions to take and what to expect when operating parameters are exceeded. It is concluded in the report that current must be included as a factor in the vessel’s manual of permitted operations. The effect of the stern thrusters, running at near full power and throwing the wash against the hull, will be reduced due to thruster-hull interaction. The vessel DP capability plots should be consulted. It needs to be highlighted that only two position reference systems of the same kind were being used. Therefore, with regard to position reference systems, the vessel was not set up according to DP class 2 requirements. A minimum of three position reference systems with one being of different type are required in a DP class 2 or 3 operation. The contents of the ASOG for this operation was not disclosed. However, it was stated in the report that immediately after the yellow alert, drilling operations stopped, and the drilling equipment lifted and hoisted up in the moonpool. It is not clear whether these actions were precautionary measures or mandates arising from the ASOG. In this instance a Yellow alert was indicated to the drill operators, if a loss of position was occurring a Red light should have been given to the drill operators. It is not clear in the report as to why the speed feedbacks on both T3 and T4 became faulty. One possibility was that the speed feedback discrepancies of T3 and T4 could have been caused by the strong current inflow and thrusters running at near or above full speed. IMCA M 256 25 This case study demonstrates the importance of fully understanding the vessels capability and normal operating limits, and also ensuring that this level of detail is clear within the ASOG. The case study also highlights the significance of training in emergency situations for key DP personnel. 26 IMCA M 256
