E.S.Lim, firstname.lastname@example.org, Universiti Teknologi PETRONAS
M.S.Liew, email@example.com, Universiti Teknologi PETRONAS
I.Toloue, firstname.lastname@example.org, Universiti Teknologi PETRONAS
Asset integrity is often an integral topic in the books of any oil and gas operator. Typical offshore structures are designed for a pre-specified duration of service and this can vary from five (5) years for marginal field structures up to thirty (30) years for larger fields. Due to the exposure of harsh and deteriorating conditions offshore, without proper maintenance, many of these structures will fail to withstand the test of time. This is asset integrity philosophy and campaigns organized by the operator are critical in ensuring its continued safety and operational assurance. Moreover, the maturation of numerous conventional oil sources has forced many operators to innovate oil extraction methods in order to increase or maintain yield levels. In addition, the advancement of exploration technologies have unveiled new oil pockets in existing fields to justify the need to further extend the operations of the facilities. All these factors combined drive further the need for thorough asset integrity campaigns to be deployed on existing assets.
Understanding the durability, serviceability, safety and sustainability of a structure has never been a straightforward topic for most. More often than not, it is a cost and labour intensive exercise that sees the option of conducting it limited to only severe cases or after an unexpected event has occurred on the structure. While this may be a straightforward solution when one is an operator of a few assets, it could become an enormous undertaking for organisations and companies that own multiple assets that are all ageing simultaneously. In the case of Malaysia in particular, we have approximately 300 offshore structures located within the Malay, Sarawak and Sabah Basin with nearly 57% of them falling within the “high” and “very high” risk categories. In Sarawak alone, there are nearly 66 platforms with an operating life that has exceeded 25 years. In order to address the management of assets, a Risk Based Underwater Inspection (RBUI) regime was introduced to evaluate the maintenance intervals of structures on the basis of consequence of failure and probability of failure. One of the key elements of this approach is the detection and cataloguing of anomalies in a structure that requires intense manpower mobilization to perform a variety of tasks and inspection procedures such as flooded member detection, corrosion monitoring and dent profiling.
Figure 1: Risk distribution of fixed platforms in Malaysia
The inspection campaign can be triggered by a variety of issues including age-driven effects such as fatigue and corrosion or event-driven effects such as fires and vessel collisions. While age-driven effects can be addressed as part of the RBUI campaign in the form of interval-based inspection, event-driven anomalies tend to be more drastic that could result with a significant impact on the offshore structures within a short span of time. These often have to be addressed in an in-situ inspection campaign accompanied with detailed desktop studies to quantify the damage extent and most importantly, the survivability of the structure. This exercise which is not optimal resource-wise, is manpower-intensive and dependent on costly marine spreads and specialized inspection procedures.
Figure 2: Different sources of damage on offshore structures
Figure 3: The costs involved in anomaly detection and inspection campaigns
For the past 15 years or so, automation of inspection methods have started to grow exponentially due to the increasing size of facilities as well as the number of ageing structures. The guiding concept was to reduce the dependency of manpower for certain structural non-destructive tests (NDT) especially at inaccessible and remote locations. Popular implementations saw the automation of inspection and the monitoring of bridges, pipelines, buildings and dam structures. The collective nature of monitoring using sensors and analyzing data in an integrated and automated manner is also known as Structural Health Monitoring (SHM). More often than not, the autonomy provided by the SHM systems allows operators to make pre-emptive decisions on events that could possibly compromise the global integrity of their structures. Thus, operators would not needlessly expend their manned resources to assess and verify the global integrity of a structure.
As an example, the Wind and Structural Health Monitoring System for Bridges (WASHMS) in Hong Kong is a milestone implementation of the SHM system. It is a system that has seamlessly integrated both static and dynamic monitoring in a live setup to aid the Highway Department in assessing the health of its cable-stay bridges under operational and extreme states. Besides having a set of sensors to check for strain, temperature and wind speed, it also has accelerometers that record information which is relayed live to the Highway Department and interpreted via real-time engineering models. In the event of a typhoon, the entire system would be able to monitor the model characteristics of the different spans to gauge the occurrence of a shift that could possibly lead to a loss of component stiffness. Consequently, the survivability of the bridge post-typhoon could be ascertained for the operator to make an objective and informed decision which would warrant further manned and detailed inspection.
Figure 4: The process flow for SHM system in assessing global structural integrity
The implementation of the SHM systems in an offshore environment is no different than for its onshore counterpart. Sensors capturing the dynamic properties of the offshore structures are recorded and acquisitioned in a topside unit. This is then further analysed through updated analytical models in real-time to provide operators with in-situ knowledge on structural thresholds. This is particularly useful in events such as member loss due to boat collision or after freak storms resulting in damaged members. The operators would be able to pre-emptively receive condition-based reports of the structure from the SHM system and subsequently decide if a more detailed manned inspection is required. Without a significant impact on the global integrity of the structure, the maintenance schedule could be stretched to other more critical facilities. This is applied similarly to the RBUI inspection campaigns which are time-dependent based on the individual risk rating of each offshore structure. A positively indicating SHM output during the inspection milestone can provide sufficient engineering justification to operators to reallocate their resources to more critical offshore structures.
While the SHM systems excel at providing pre-emptive decision-making to operators of assets, they presently serve mostly to verify the integrity of the global and major components of a structure. This SHM tool provides great value addition to existing asset integrity philosophies and campaigns to reduce manpower requirements at the pre-emptive stages. However, the possibility still remains in the further development of the SHM systems to monitor components at micro levels, thus possibily eliminating altogether the need for any manned inspections. But for now, we can rest assured that automation will not be displacing the necessity for detailed manned inspections. It merely serves as complementary tools in the face of growing ageing assets.
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