Structural integrity management – from cradle to grave

Mohammad Nabavian
Friday, October 11, 2013

Properly implemented, structural integrity management can provide structural integrity assurance from design to decommissioning, explains Mohammad Nabavian of Wood Group PSN.

The concept of structural integrity management (SIM) for offshore installations is maturing, underpinned by the need to assure the reliability of ever-aging infrastructure, cope with the demands of complex life extension projects, and cater for safe and successful decommissioning. The systems and tools to support SIM need to be robust, practical, and flexible to adapt to what is known about the structure and account for its individual risk profile and ageing characteristics.

Image: Aging assets in mature basins like the North Sea means asset integrity is a key industry issue.

Development of SIM systems

Fixed offshore structures are typically designed using industry standard codes (ISO-19902, API, AISC). On the UK Continental Shelf, governmental regulations also play a part in the design process, ensuring hazards, such as the loss of structural integrity, loss of containments, and fires & explosions are identified, and appropriate mitigation measures put in place.

SIM systems can ensure longer term asset integrity, taking into account the operations, life extension, and decommissioning phases of the asset life cycle. Thus, a SIM system should be in place from the cradle-to-grave of an asset.

  • A robust SIM system is designed to ensure:
  • Safety of those working offshore
  • Continued production
  • Protection of the environment
  • Legislative compliance
  • Industry best practice.

An integrated SIM system accounts for the development of inspection strategies, work scopes, inspection results reviews, and the maintenance of inspection record databases. It also makes for easier identification and management of defect assessments and the development of repair solutions for both topsides and subsea structures.

Change management is required when modifications are made to an installation, normally involving changes in the physical and operational parameters of the asset. These modifications should be designed with an understanding of the complete brownfield environment. At the heart of the SIM system is engineering competence.

Ageing assets

Driven by the UK Health and Safety Executive (HSE), Key Program 4 (KP4) is the next stage in the HSE’s aging and life extension inspection program. This means operator’s assets are regularly subject to scrutiny by independent safety inspectors to ensure standards are maintained according to UK regulations. The program, which was due to run until September 2013, has the potential to impact the way operators manage the structural integrity of their assets globally.

On average, more than 30% of nearly 7000 platforms in operation around the world have been in operation for more than 20 years. In the North Sea, more than 50% of assets fall into this category. With so many operating beyond their original design life, and yet more discoveries being made, operators are under pressure to extend the life of their offshore assets without compromising asset integrity, reliability, productivity, and more importantly, safety. KP4 is designed to stimulate a common industry approach to managing ageing installations that will ensure safe offshore operations for the long term.

The UK-HSE states that KP4 is not simply about how old an asset is, but about how it is ageing and what is known about that process.

Figure 1 shows a simple “bath tub” curve, which illustrates that, as aging mechanisms—corrosion, erosion, accumulated damage—take effect, there is an inevitable increase in the likely hood of failures occurring.

KP4 has focused operators to take responsibility for managing the risks associated with asset integrity and life extension projects through the use of SIM systems. Operators are also keen to increase the value of their assets, and a robust SIM system is seen as a part of achieving this. If this approach to structural integrity management was adopted globally, there is an opportunity to transfer SIM best practice from the North Sea to operating assets around the world.

Life extension

As assets age, trends show decreasing revenues and increased operational costs. Life extension projects and the implementation of an appropriate SIM system can counter these trends and increase the value and viability of an asset, Fig.2.

For example, asset value may be increased by enhanced oil recovery or tie-back projects, both of which would rely heavily on structural analysis to demonstrate additional capacity from the existing structure. Likewise, elements of operational expenditure could be optimized; such as subsea inspections targeted on critical components, if the behavior of the structure and its risk profile is understood.

Demonstrating the structural integrity of aging assets is of paramount importance when considering life extension. In Norway, an application for life extension is a formal requirement, and in the UK, a safety case must be reviewed when installation design life has been exceeded.

The effects of the Deepwater Horizon tragedy in the Gulf of Mexico led to an announcement by the Bureau of Ocean Energy Management, Regulation and Enforcement that all operations and drilling facilities on the outer continental shelf would be required to develop and maintain a safety and environmental management system (SEMS). The SEMS final rule is designed to improve workplace safety offshore.

The rule will hold operators accountable for overall facility safety, including ensuring that all contractors and subcontractors have safety policies and procedures in place that support the implementation of the operator’s SEMS program, and align with the principles of managing safety, set forth in API RP 75, the recommended practice for offshore oil, gas, and sulfur facilities and associated equipment.

Applying a SIM system can help operators manage these challenges, and ensure that the required systems and tools are available for asset life extension or demonstration of enhanced structural capacity beyond the original design values.

The realization of enhanced structural integrity can involve advanced structural analysis techniques such as:

  • In-place assessments  
  • Reserve strength ratio (RSR) assessments
  • Boat impact assessments  
  • Finite element analysis
  • Dynamic and post-buckling assessments
  • Blast assessments  
  • Fatigue/fracture mechanics
  • Reliability assessments

There are various assessment triggers which can be used to inform the need for such analyses.

Decommissioning

The requirement for SIM does not stop at cessation of production. SIM plays a vital role in the safe and successful decommissioning of offshore infrastructure.

The SIM system provides a central location for platform knowledge, containing information about the structure, its weight, configuration, changes and modifications, and current condition. The retention and availability of this information forms a critical part of decommissioning planning. When considering removal options as part of the decommissioning process, SIM is used to take into account the integrity of the topside and the jacket, so options can be assessed according to the robustness of the structure for safe removal, transportation, and load-in, Fig. 3.

If effectively implemented and maintained, the SIM system is there to provide the required structural integrity assurance from design to complete decommissioning. OE

Mohammad Nabavian
 is head of advanced integrity at Wood Group PSN. His experience includes design and analysis of new offshore structures for the North Sea, Middle- East, and deep waters of the Gulf of Mexico, as well as, appraisal of existing mature platforms, involving damage assessment and engineering beyond the confines of current codes. He earned an MSc and PhD from Cranfield University.

 

Categories: Europe Maintenance North America Construction Installation

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