-
Lessons from Structural Failures in Offshore Structures and Pipelines
-
OutlineIntroductionLiterature ReviewType of Offshore StructureCauses of Failure on Offshore StructureType of Offshore PipelineCauses of Failure on Offshore PipelineCase Study and DiscussionBravo EkofiskAlexander KiellandPiper Alpha, North SeaWest GammaSleipner AMumbai High North PlatformLesson LearntConclusion
-
Literature ReviewType of Offshore Structure
For the purpose of exploitation of oil and gas under the seabed,various type of offshore structurehas been built:Fixed offshore structureConcrete gravity base structureTension leg platformFloating production storage and offloading system (FPSO)Shutter tankerSemi-submersible vessel Self-elevating jack-up
Example: Fixed offshore structure are only suitable on the shallow waterFPSO system will be more suitable for deepwater
-
Causes of Failure on Offshore Structure
Failure resulting from statistical variations in loads and structural load bearing capabilitiesFailure due to accidents Failure due to a human error during design, fabrication and operation of structures
Arne Kvitrud (2001) claimed that human errors are the most important contributor to the risk of failure.This conclusion is drawn based on the studies on the four major structural accidents in Norway
-
Type of Offshore Pipeline
Offshore pipelines can be categorized into four groups:
Flow lineGathering lineTransmission line (trunk line) Distribution line
Flow line carries untreated hydrocarbon products directly from reservoir to platform or subsea manifold. (Diameter 5 - 25cm)Gathering line connects from one platform to another platform and usually used to transmit oil and gas from processing field and storage facilities to a large storage tank where it is gathered for pumping to another long distance. (Diameter 10 - 92cm)Transmission line is used to carry the combined flow from one or many platforms to onshore. (Diameter range up to 142cm)Distribution line used to transfer oil and gas to the nearest cities and used as the combustible material.
-
Causes of Failure on Offshore PipelineExpansion and Global Buckling Due to the restraint provided by the seabed fiction, a pipeline expansion only occurs at the end .At undisturbed sections of the pipeline, the restraint against thermal and pressure induced expansion may cause a compressive pipeline force which would result in a global buckling mechanism The resulting buckling configuration such as mode, wave length and amplitude depends upon the frictional resistance between the pipe and soil.
External or internal corrosionCorrosion is actually a chemical mechanism that corrodes the steel and later weakens the strength of itCorrosion on the internal wall of a natural gas pipeline can occur when the pipe wall is exposed to water and contaminants in the gas, such as O2, H2S, CO2, or chlorides.
-
Casting PorosityCasting porosity can be related to insufficient liquid being fed to the mold, low liquid metal temperature, improper mold and gating design, etc.
Mechanical damage Mechanical damage normally consists of gouges and dents - created by excavation or handling equipment during construction.
Metal fatigue Metal fatigue is caused by repeated cycling of the load.The process of fatigue consists of three stages i.e. initial cracking, progressive crack growth across the part and final sudden fracture of the remaining cross section.
Equipment failure
-
Case Study and Discussion
Offshore industry has overcome several accidents.The most severe ones are Frigg DP1, Bravo Ekofisk, Alexander Kielland, Piper Alpha, West Gamma and Sleipner A
-
Bravo Ekofisk
Blow out 22 April 1977No fire, but 22 500 tonnes oil releasedAccident happened during removal of valve for maintenance and well stabilisationunsatisfactory installation of down hole safety valve during nightmud started leaking out next morningsafety valves (BOP) on deck were not closedOil recovery equipment mobilised took several days. Only 4% of the oil recovered.A total spill estimate between 13,000 m3 and 20,000m3
-
Alexander Kielland
Fatigue crack in one of its six bracings (bracing D-6).Rupture / collapse in the other5 braces.Loss of column DThe rig immediately listed to one side at an angle of 35 degreesInitial collapse occurred within a minute but the Kielland remained floating for another 14 minutesEvacuation - only two of the sevenlifeboats launched successfullyAfter around 15 minutes, Kiellandcapsizing89 survived and 123 fatalities
-
Piper Alpha, North Sea
Gas audibly leaked out at high pressure, ignited and exploded, blowing through the firewallsFire spread through the damaged firewalls, destroyed some oil lines and soon large quantities of stored oil were burning out of controlAutomatic deluge system had been turned offAfter 20 minutes, fire had spread and become hot enough to weaken and then burst the gas risers from the other platforms. All routes to lifeboats were blocked by smoke and flames, and in the lack of any other instructions, they made the jump into the sea hoping to be rescued by boat.Explosion result a total insured loss of $ 3.4 billion and 167 men died.
-
West Gamma
On 20 August 1990, the West Gamma accommodation jack-up ran into a gale (with waves up 12 meters and winds gusting 60 knots) The rig first lost its helideck to a large wave and then lost its tow with the Normand Drott during the stormAs night fell, one of the deck lifeboats broke loose, damaging vent pipes and access hatches and causing down-flooding in the rig's hullEvacuation by helicopter (not possible) - due to the damage sustained to the helideck and helicopter winching was not possible due to the high windsThe reasons contribute to the sinking of the West Gamma including the bad weather, loss of the towline, structural failure and flooding.
-
Sleipner A
In August 1991, prior to the mating of the hull and the deck unit, the hull was towed into Gandsfjord where it was to be lowered in the water in a controlled ballasting operation at a rate of 1m per 20 minutes.As the hull was lowered to the 99m mark, rumbling noises were heard followed by the sound of water pouring into the unit. A cell wall had failed and a serious crack had developed, and sea water poured in at a rate that was too great for the deballasting pumps to deal with.Within a few minutes, the hull began sinking at a rate of 1m per minute.Total loss of about $700 million.
-
Causes - inaccurate finite element approximation during calculations in the design of the structure. Stresses on the ballast chambers were underestimated by 47% and some concrete walls were designed too thin. Upon reaching a given pressure, these walls failed and cracked.
-
Mumbai High North Platform
The fire occurred on 27 July 2005 - a multipurpose support vessel (MSV), Samundra Suraksha,100m long, hit one of the MHN platform risers.The fire was so intense that the MHN was abandoned in accordance with the disaster management plan of offshore operatorsWithin two hours, the whole platform collapsed into the sea with a few foundation piers leftA Pawan helicopter positioned on it was also lost11 people died and 11 others were reported missing
-
Summary of Accident Causes
PlatformCausesBravo Ekofisk (1977)Human Error - mechanical failure of the safety valve during earlier maintenanceOperational Error Lack of safety planning and procedure for maintenanceAlexander Kielland (1980)Fatigue failure of one braceInadequate evacuationFabrication defect due to bad welding and inadequate inspectionNo fatigue design check carried outLack of life boats, survival suitsLong mobilizing time for rescue vesselsPiper Alpha (1988)Leak from partly demounted pumpEscalation after first gas explosionNo evacuation rescue vessel was not alarmedThe condition of the pump was not reported to the control roomFire pumps/sprinkler system were not automatically initiated - because they were in a manual mode
-
Summary of Accident Causes (Continued)
PlatformCausesWest Gamma (1990)Bad weatherLoss of towlineStructural failureFloodingSleipner A (1991)Caisson wall fractured due to low strengthFlooding and sinking in 18 minutesInadequate reinforcementInadequate internal and external control of designCodes did not specify requirement to pumping capacity or watertight subdivision to limit flooding under such conditionsMumbai High North Platform (2005)Risk assessment processes did not control the threat to the risersProcedures to manage vessel vessel operate near riser location No structural protection for riser
-
Lessons Learnt
-
Bravo Ekofisk
Investigation focused on lack of safety planning and procedures during maintenance a general need for better organisation of safety by operatorsContributed to develop and implement specific regulations for oil companies own control by the oil directorateIntensified focus by authorities and industry to improve organisation and equipment for oil recovery after blowoutsRescue vessel mobilization requirement established on the spot within 25 minutes of an accident
-
Alexander Kielland
Cracks introduced during construction must be detected before the unit is launched.When fatigue cracks might grow, means to detect such cracks before they grow to a critical size must be implemented.If a floating unit develops severe listing, there should be a last barriers (i.e. buoyancy volume or a righting force) in order to allow time for organise and safe evacuation of personnel.Conventional lifeboats are not satisfactory in bad weather conditions. The experience from this accident was the driving force behind the development of free fall lifeboats for offshore applications.It was clearly demonstrated that the rapid and steep inclination angle makes orderly escape and evacuation very difficult.It was realised that the rescue of survivors from lifeboats by traditional vessels was impossible in bad weather conditions.The role and the capabilities of the standby vessel were questioned after the accident, when it was realised that it took the vessel 1 hour before it could attend the scene of the accident.
-
Piper Alpha
Regulatory control of offshore installationsAdherence to Permit-to-Work SystemDisabling of protective equipment by explosion Need for safety trainingAuditing is vitalProper isolation of plant for maintenanceLimit inventory on installation and in pipelines Emergency Shutdown ValvesTemporary Safe Refuge (TSR)Evacuation and EscapeUse of wind tunnel tests and explosion simulations in design
-
West Gamma
When the jack-up is in transit, the legs have been retracted, and may pose a very large obstruction for helicopter approach, if the helideck has not been located with this in mind.The most critical aspect was a sufficiently high speed for the lowering and retrieval of the Fast Rescue Crafts (FRC).The crane used for deployment and retrieval of the FRC should also be located as close to midships as possible, where movement are least.A need was also demonstrated to be able to retrieve the FRC with more than nine persons onboard.
-
Sleipner A
Personnel play an extremely important role in promoting safety competency, experience and knowledge is important.We need to ensure that the safety barriers are maintained so as to control / mitigate accidents. Never be overconfident - we should always make sure all the calculation and design have done properly and accurately.Design changes to be verified against original design.Allow time and resources for independent 3rd party verification with detailed scope to be defined by verification contractor.Do not have blind faith in computer models use other programs or program versions for verification.Never think of a job as just routine.
-
Mumbai High North Platform
Critical Barriers (managing the threats) Properly designed fenders, addressing all credible threatsInstall risers within protective sleeves such as caissons or J tubesLocate risers away from platform loading zonesProtect risers from hazards by location, barriers, or other meansAvoid vessel operations near riser locationsProvide subsea isolation valves (SSIVs) to limit consequences of riser damageThe Need for Better DesignThe Need for Incident Reinforces Development, implementation and maintenance of associated risk management measures Adoption of collision avoidance and protection measures which at least meet current good practice as described in Oil & Gas UK Management arrangements to ensure that the risk management measures are effective and observed in practice.
-
Conclusion
The lessons that still need to be remembered is that human factors play a decisive role in safety and that proper safety culture and management are required in the involved organisations.The requirements in the standards should be:compatible with available design tools, such as finite element analysis programs easy to understand for engineers, in order to avoid gross errorsspecific and not open to interpretation.Engineers should have relevant education, also including education in preparation of design documentation that can be verified by others. Organisations must take a responsible attitude to competence planning and quality, applying the principles of triple bottom line and Corporate Social Responsibility.We must identify possible failure scenarios that may lead to critical situations and perform independent verification.
-
THANK YOU!!
