A Structured Approach to Rejuvenate Service Life of Marine Structures

Abstract

Extending the life of aging assets is a demanding task, especially if they have been exposed to harsh marine environment. Jetty berthing structures commissioned in 1980s for exporting hydrocarbons in one of the gas processing plants were required to be operational beyond their design life to sustain business requirements. Rejuvenation of these critical assets was therefore embarked upon to safely and reliably extend their life by twenty more years. This paper discusses challenges confronted and structured approach followed in restoring the integrity and extending the service life of the aged structures. Special assessment strategy was devised to overcome tough challenges such as limited availability of existing drawings, non-existent original design documents, absence of loading & equipment data, thick marine growth enveloping the structural members, lack of underwater inspection history, etc. To ascertain the current condition of structures, a detailed inspection plan was developed. Close Visual Inspection of splash zone and under water regions of jetty structures viz., loading platform, breasting & mooring dolphins, utility platform, pipe / access trestles, etc., was performed to detect corrosion, fatigue cracks, damaged members, marine growth, etc. In areas of identified or suspected damage, Non-Destructive Testing viz., LRUT, PEC, FMD techniques were employed to detect potential defects. Structural models reflecting as-is condition were developed based on inspection findings and through rigorous structural analyses viz., In-Place, Push-Over, Fatigue and Seismic analysis, the fit-for-purpose fitness of structures was evaluated for life extension. Obstruction to visual examination of the structures due to excessive algae growth, inclement weather conditions to conduct surveys, limited berth availability, etc., presented further challenges that were successfully mitigated by unique inspection strategies. Inspection revealed localized corrosion of nearly 30% metal loss particularly in splash zone, while sub-sea segments exhibited a general metal loss of 10-15%. Wrapping system previously employed to mitigate corrosion had suffered damage. Structural members under water were found completely swamped with thick marine growth. Structural capacities of members, joints and pile foundations were evaluated as part of In-Place analysis. Push-Over analysis was performed to ascertain the available reserve strength (RSR) up to collapse limit under extreme storm condition. Through Fatigue analysis, fatigue life of joints due to cyclic loading from oscillating waves were checked for any integrity concerns. Based on the assessment results, various potential vulnerabilities in the structure were identified. Cost effective and fit-for-purpose corrective actions consisting of strengthening, modification and repair schemes were finalized. Modern techniques / devices to remove / prevent marine growth, and durable protective wrappings in highly corroded regions were implemented. With these considerations, integrity and reliability of the aged near-shore structures was assured for its extended lifespan. In today's challenging economic situation, the demonstration of fitness of ageing assets for prolongation of service life is gaining significant attention. In this regard, this paper presents various challenges faced, unique assessment strategy followed and cost effective remedial techniques adopted for life extension of aged critical near shore structures in order to assure their safety, integrity and reliability. The requirements are common for similar ageing assets and hence the implemented strategy and techniques can be adopted across the industry.