Lessons from Structural Failures

in

Offshore Structures and Pipelines

Outline Introduction Literature Review Type of Offshore Structure Causes of Failure on Offshore Structure Type of Offshore Pipeline Causes of Failure on Offshore Pipeline

Case Study and Discussion Bravo Ekofisk Alexander Kielland Piper Alpha, North Sea West Gamma Sleipner A Mumbai High North Platform

Lesson Learnt Conclusion

Literature Review Type of Offshore Structure

For the purpose of exploitation

of oil and gas under the seabed,

various type of offshore structure

has been built:

• Fixed offshore structure

• Concrete gravity base structure

• Tension leg platform

• Floating production storage and offloading system (FPSO)

• Shutter tanker

• Semi-submersible vessel

• Self-elevating jack-up

Example: Fixed offshore structure are only suitable on the shallow water FPSO system will be more suitable for deepwater

Causes of Failure on Offshore Structure

Failure resulting from statistical variations in loads and structural load bearing capabilities

Failure 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 line Gathering line Transmission 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 Pipeline Expansion 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 corrosion

• Corrosion is actually a chemical mechanism that corrodes the steel and later weakens the strength of it

• Corrosion 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 Porosity

• Casting 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 1977 No fire, but 22 500 tonnes oil released Accident happened during removal of valve for maintenance and well stabilisation unsatisfactory installation of down hole safety valve during night mud started leaking out next morning safety valves (BOP) on deck were not closed Oil 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 other

5 braces. Loss of column D The rig immediately listed to one

side at an angle of 35 degrees Initial collapse occurred within a

minute but the Kielland remained

floating for another 14 minutes Evacuation - only two of the seven

lifeboats launched successfully After around 15 minutes, Kielland

capsizing 89 survived and 123 fatalities

Piper Alpha, North Sea

Gas audibly leaked out at high pressure, ignited and exploded, blowing through the firewalls

Fire spread through the damaged firewalls, destroyed some oil lines and soon large quantities of stored oil were burning out of control

Automatic deluge system had been turned off After 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 storm

As night fell, one of the deck lifeboats broke loose, damaging vent pipes and access hatches and causing down-flooding in the rig's hull

Evacuation by helicopter (not possible) - due to the damage sustained to the helideck and helicopter winching was not possible due to the high winds

The 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 operators

Within two hours, the whole platform collapsed into the sea with a few foundation piers left

A Pawan helicopter positioned on it was also lost 11 people died and 11 others were reported missing

Summary of Accident Causes

Platform Causes

Bravo Ekofisk (1977) • Human Error - mechanical failure of the safety valve during earlier maintenance

• Operational Error – Lack of safety planning and procedure for maintenance

Alexander Kielland (1980) • Fatigue failure of one brace• Inadequate evacuation• Fabrication defect due to bad welding and inadequate inspection• No fatigue design check carried out• Lack of life boats, survival suits• Long mobilizing time for rescue vessels

Piper Alpha (1988) • Leak from partly demounted pump• Escalation after first gas explosion• No evacuation – rescue vessel was not alarmed• The condition of the pump was not reported to the control room• Fire pumps/sprinkler system were not automatically initiated - because they

were in a manual mode

Summary of Accident Causes (Continued)

Platform Causes

West Gamma (1990) • Bad weather• Loss of towline• Structural failure• Flooding

Sleipner A (1991) • Caisson wall fractured due to low strength• Flooding and sinking in 18 minutes• Inadequate reinforcement• Inadequate internal and external control of design• Codes did not specify requirement to pumping capacity or

watertight subdivision to limit flooding under such conditions

Mumbai High North Platform (2005)

• Risk assessment processes did not control the threat to the risers• Procedures 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 operators

Contributed to develop and implement specific regulations for oil companies own control by the oil directorate

Intensified focus by authorities and industry to improve organisation and equipment for oil recovery after blowouts

Rescue 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 installations Adherence to Permit-to-Work System Disabling of protective equipment by explosion Need for safety training Auditing is vital Proper isolation of plant for maintenance Limit inventory on installation and in pipelines Emergency Shutdown Valves Temporary Safe Refuge (TSR) Evacuation and Escape Use 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 threats Install risers within protective sleeves such as caissons or J tubes Locate risers away from platform loading zones Protect risers from hazards by location, barriers, or other means Avoid vessel operations near riser locations Provide subsea isolation valves (SSIVs) to limit consequences of

riser damage The Need for Better Design The 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 errors specific 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!!