In today's world of new construction slots at shipyards being fully booked and the high cost of steel, prudent owner/operators are looking for ways to get the most out of existing assets while assuring the asset is fit for the intended service and maintaining safety as a key performance indicator.

ABS Consulting uses proven methods to evaluate the risks associated with extending the useful life of tankers and existing floating production, storage and offloading (FPSO) platforms to allow these assets to remain in their present field service, or converting and moving these assets to new operating locations. This assistance comes in the form of a program that addresses the present condition of the asset; analyzes the fatigue and strength characteristics of the steel structure and equipment, plans the necessary modifications and repairs, and develops a risk-based inspection (RBI) plan that allows both the operator and the regulator to monitor the critical functions and structures on these assets.

Find a baseline

In order to accurately reflect the baseline condition of the asset, a review of the Classification Society records is carried out. The review identifies all noted fatigue fractures and suspect locations documented in the class files and surveys. To make this information useful, the baseline review includes mapping the locations of known fractures and areas of corrosion on the vessel combined with drawings of present details. The fractures/corrosion areas are then categorized based on location, detail and observed damage.

In addition to the class records review, third party inspection documents also used to identify and categorize the fractures.

This task serves multiple purposes. First, it provides necessary information to initiate a detailed repair and/or inspection plan and, second, it provides means to compare fatigue analysis results with detailed analysis.

Assess and review

This task involves an engineering analysis to screen the fatigue-sensitive regions and identify fatigue life for the most sensitive longitudinal end connections identified in the record reviews.

It is important to note that this analysis method is a simplified analysis tool that allows relatively quick modeling when compared to detailed finite element analysis (FEA) modeling.

Using the analysis results, comparisons are made with the observed damage, which was summarized in the baseline review. If the results are correctly identifying the observed fatigue prone areas, then these regions can be targeted for remediation. Additionally, the results are used to further refine the survey plan such that specific suspect regions of the hull are inspected in detail during the condition survey. If the remaining fatigue lives are in excess of planned service life, details likely do not require remediation.

Using the last thickness measurements taken for special survey of hull or intermediate survey, a structural evaluation is carried out. The objective of this evaluation is to assess the current gauged structural thicknesses.

The surveyor, in preparing his inspection plan prior to vessel attendance uses this information. The structural evaluation encompasses the following:

• Strength assessment of the hull girder, main supporting members and local structure for the as-built vessel.

• Assessment of the current gauged condition of the hull girder, main supporting members and local structure compared with the renewal requirements for the vessel.

• Identification of areas where the current gauged condition is approaching or below the threshold renewal requirements.

Condition survey

A team of experienced marine surveyors carries out a condition assessment survey. It includes a close-up survey of the fatigue-sensitive areas identified in the previous tasks to identify any suspect areas with substantial or excessive corrosion. The surveyor will examine the machinery and electrical systems to assess their current condition and identify areas requiring repairs or upgrading to meet the desired service life. This survey and subsequent report will identify equipment issues related to long-term reliability and associated mean time between failures. It will rank equipment for possible replacement. A detailed survey report along with photographs is developed to be used in conjunction with the information from the previous tasks to help develop the steel renewal plan. The report will include an assessment of the life extension requirements for the machinery, piping, electrical, etc.

Detailed assessment

If warranted to define better solutions for structural modifications to fatigue-prone locations, a more thorough assessment can be conducted. Figure 1 shows an example of a full ship model used for such an assessment.

From the global model, detailed fine mesh FEA models of select locations can be created. These models can be generated automatically with ease via the coarse mesh global model. The fine mesh fatigue analysis provides a more accurate estimate of the expected fatigue life for the different connections of interest when compared to the simplified methods. Additionally, this method allows details other than longitudinal connections to be evaluated. For connections that do not meet the target life, these analyses can be used as a tool to develop remediation methods.

Steel renewal/upgrade plan

Using the results of the prior tasks, previous ultrasonic thickness measurements taken in conjunction with Class surveys, and measurements of suspect areas taken during the condition survey, a general plan is developed. It identifies locations where detail fixes and remedial actions should be implemented to meet the design fatigue lives required for the proposed service. For connections identified as warranting design changes, new details will be developed.

The renewal/upgrade plan includes write-ups describing the proposed remediation steps as well as diagrams indicating the locations and steel detailing required to meet the service life requirements and a list of the machinery and electrical systems requiring renewal or upgrading. Estimated costs for these items are provided for based on the geographic region at which the work will be carried out.

Risk-based inspection plan

To appreciate the benefits to be gained from RBI for hull structures, it is valuable to review the current trends and philosophies in the oil and gas industry. One area where this is clearly visible is that of maintenance and inspection philosophies. Specifically, within the area of plant maintenance and inspection there has been a dramatic shift towards adoption of RBI programs rather than following traditional preventive maintenance methods and prescriptive inspection intervals. There are many and various reasons for this trend, but the primary benefit that asset owners and operators have gained is tangible cost reductions (for both manpower and parts) and improved asset uptime. In partnership with asset owners, ABS Consulting has been actively participating in both research and actual projects involving RBI for hull structures and process systems. The advantages to be gained from this type of program are readily apparent.

The main focus within RBI is to examine the mechanisms/modes of deterioration that apply to a specific structure. Largely, critical areas such as low fatigue life welds and high stress locations are available from the analysis work mentioned above.

To successfully implement an RBI plan the operator must step back from the maintenance and inspection activity itself, and initiate a process that studies the processes that deliver higher levels of understanding the asset. This study must identify the key factors and underlying risks that are likely to threaten or undermine the integrity of the asset are recognized and understood. Although industry derived breakdown/failure rates for equipment types, are singularly specific in targeting greater knowledge for risks specific to the actual plant and equipment owned, the same outcome is true for setting the inspection activities for the hull through the RBI effort.

To finalize an RBI for the hull structure, it is necessary to determine the consequence and the likelihood of potential failures/events. This provides insight into the risks for the specific component or compartment. When dealing with a large hull structure, this allows the operator the emphasize inspection schemes that address safety and commercial performance as desired.

Cost savings

The program described above has resulted in the following saving for operators:

• Developed an FPSO for service in West Africa in 9 months versus an 18-month new construction program.

• Resulted in typical reductions in the number of required tank inspections for FPSOs on the order of 35% to 40%.

For a typical very large crude carrier (VLCC) size hull, these reductions in inspections could equate to an operations expense savings of US $3.5 million to $4.5 million over a 20-year field life, based on typical tank cleaning, de-mucking, surveyor, and inspection crew fees. These savings do not even consider HSE risk reduction. Nor do the savings consider the potential for lost revenue should the vessel have to slow down its tank loading rates to accommodate for the lost storage space when larger tanks are out of service for inspection.

It is apparent from this work that incorporating integrated analyses and inspection efforts into an operator's planning for converting or achieving a life extension of an existing asset pays for itself in the long term by cost savings and reduced deployment time. Such a program allows the operator and the engineering service partner to identify the issues up front in the project, plan for the mitigation of these during the conversion or modification, and develop an evergreen inspection plan to ensure the asset stays on a safe path to full utilization.

Comments