Systematic evaluation of platform life

Getting to grips with the condition of older assets can be a difficult task. Stefano Copello outlines RINA’s approach to revitalization.

The re-used jacket of the Nenè platform, owned by Eni Congo, during transportation from the US Louisiana yard to offshore Congo. Images from RINA Services.

There are a lot of aging oil platforms around the world, many of them reaching the end of their design life.

But, the fields beneath them still contain viable reserves. And, there are new fields to be exploited where an existing platform from another field could be reused economically. With the squeeze on capital expenditures, there is now incentive to find economical ways to extend the life of existing platforms, either in situ or for reuse in a new location.

Each case is different and each platform different, but it is important that the system-approach to assessment of residual platform life is the same, so that operators and regulators can have confidence in the asset in its new location or extended life.

The system approach

The starting point is to create an adequate picture of the platform in its current state, which captures the design strength and possible reductions in strength due to corrosion and fatigue. This has to be set against the site-specific conditions and operational requirements, which may not be the same ones against which the platform was originally designed. Next, determine the risk of structural failure, which could lead to unacceptable consequences. Then, determine the acceptable level of risk and set the safety target.

In simple terms, newbuilds are made to withstand a 100-year event at the original work site. For reuse or life extension projects, engineers must work backwards. Measure what you have, figure out the maximum event that the structure can withstand, then figure out how long you have left by taking into account the likelihood of an extreme event.

Setting the standard

The Vega platform, operated by Edison, about 12mi from the southern Coast of Sicily, Italy.

In the 1990s, the American Petroleum Institute developed standards for the Assessment of Existing Platforms to evaluate the fitness-for-purpose of old existing fixed offshore platforms. These rules were updated in 2005. The International Standard Organization developed the 19900 suite of guidelines to address design requirements and assessment of all types of offshore structures, including fixed steel structures, covered by the issuance of ISO 19902 Standard in 2007.

These standards are important. In the last 10 years, there have been more engineering works related to fitness-for-purpose assessments of existing platforms than design of new platforms in the Gulf of Mexico. RINA Services has built these standards into the updated Rules for the Classification of Fixed Offshore Platforms, which provides a cradle-to-grave framework for the structural and process safety of the entire platform. They build on RINA’s experience with offshore platforms in the Mediterranean, Red, and Caspian Seas, as well as the Indian and Atlantic oceans. The rules facilitate life extension while giving owners more choice and control over their design, operation and maintenance strategies.

Environmental protection is central to the new rules, which have been developed with the aim of significantly reducing the risk of accidents and environmental damage. Platform operators can choose from and mix three approaches: classification, certification and verification.

Under each of the approaches RINA’s rules now allow for load and resistance factor design, a semi-probabilistic approach to structural assessment. The rules set out clear guidelines and requirements for assessing fatigue and corrosion issues to determine what must be done to allow platforms to continue to operate beyond their design life. The life extension approach incorporates risk based inspection (RBI) and risk-based maintenance planning.

LAM 28 platform, operated by DOTL, located in the Caspian Sea, approx. 25-35km west offshore Cheleken peninsula, Turkmenistan.

Reassessment

All these standards start with a requirement to assess the platform so that the actual present condition is properly known. Every reassessment starts with data gathering, including general information such as date of installation, original and current platform use and function, location, orientation, water depth, number of wells and manning level. Original design data, including design drawings and material specifications, environmental data, operational criteria, soil foundation data, equipment and appurtenances, must be collated and if needed transferred into a digital format.

Screen shot of Nenè platform 3D model used for system approach.

Construction and fabrication data including as-built drawings, fabrication, welding and construction procedures, etc., are needed, as is the platform history data, including environmental loading history and performance of the platform during past extreme environmental storms, changes in topside layout and weight, collisions or other accidental events and possible damages reported, survey and maintenance records and a list of any repairs and modifications.

Next, conduct the platform as-is survey, above and below the water, and include actual deck size and geometry, existing deck loading condition and equipment arrangement, field measured deck clearance elevation and layout of wells, with visual and non-destructive examination. All the data and survey information is used to build the platform model, which shows what you have, but it doesn’t tell you the loads the platform may face. It is important to use current methods of environmental calculations described in the updated design codes, rather than the original design loads, and this may produce surprises. There is also a good argument for using meteo-marine data collected on site during the life of the platform if the platform is to continue on the same site. RINA used this approach in the life extension of the Vega A platform with some success. In the cases where this approach was used, RINA showed that actual loads in service varied by 15% less than the design loads (particularly for extreme wave loading), which has a major impact on the expected life ahead of the platform.

The structural safety of the as-is model now needs to be assessed relevant to the expected loads. What is peculiar in the strength assessment of existing platforms is that it is permissible to have predicted limited individual component failures, provided the reserve against overall system failure remains acceptable.

Sometimes strengthening to meet the applicable standards is not a viable option, in which case decreased reliability of the overall system could be acceptable, provided that the consequences of failure are acceptable for both the life and the environment, e.g. de-manning the platform to prevent loss of life, or providing evacuation in relation to forecast of environmental conditions exceeding the one the platform is actually capable of withstanding.

We get to those probabilities by the required safety target, which can be related to the actual system capacity of the platform, measured by the residual strength reserve of the whole jacket. That is evaluated in the simplified system reliability assessment introduced by RINA for the certification of the life extension of many offshore platforms in different offshore areas.

RINA uses elasto-plastic analysis linked to statistical evaluation. The structural model is subject to environmental loads, which increase up to the whole system collapse. The statistics then take into account strength and the environmental load, in order to finally evaluate the yearly probability of collapse in storm conditions.

A notional yearly probability of collapse of the platform may be evaluated by a simplified procedure, starting from the evaluation of the environmental forces acting on the structure, corresponding to wave, current and wind conditions with return period of one-year and 100 years, which may be represented by one-year and 100 years base shear global values determined by the quasi-static analysis normally carried out for the in-place conditions of the platform.

Then, for steel truss offshore structures, where the hydrodynamic load in storm conditions is dominated by the drag contribution, it is accepted that the maximum yearly load is reached in relation to the maximum annual wave height. It is assumed that the calculated values of the environmental forces are characterized by a yearly probability of exceeding equal to those of the associated wave height.

In order to solve the reliability evaluation, it is useful to evaluate the statistical distribution of the environmental load extremes according to a lognormal distribution, since it allows solving of the reliability evaluation in a simple closed form. An evaluation of the approximation introduced in relation to this assumption has been made in a joint industry project carried out by Italian offshore operators relevant to the life extension of existing offshore structures in the Adriatic Sea and it has been shown as negligible for the final result.

The reserve strength ratio and its relationship to the target values of notional yearly probability of failure can then be calculated. Note that there are guidelines on what this ratio should be in the ISO standards for new platforms, but these can be reduced for life extension so there is no set acceptable answer.

RINA used this approach for the LAM 28 platform offshore Turkmenistan in 27m water depth, consisting of different simple jacket modules joined at their topside by a latticed module support frame. This platform was subject to a reassessment analysis in 2013 with a target life extension of 10 years. The reassessment showed that the platform was compliant with the required safety level for the desired extension life target, provided that the platform shall be evacuated, and subsequently subject to a special survey, in case that a sea state characterized by maximum wave height greater than or equal to about 10m happens.

We used the same system to determine the expected life of the Nenè platform that had been used in the Gulf of Mexico for 10 years and was then to be modified and installed on a new site off Congo. The analysis with the structural modifications and new site conditions showed that an in-service life with acceptable safety levels of 10 years was obtainable without strengthening.

Stefano Copello is product development manager at RINA Services. He graduated with first class honors in civil engineering at Università degli Studi di Genova. At RINA since 1991, he has worked on technical activities inherent to the design, construction and installation of offshore structures, with particular reference to safety, reliability and maintenance problems of fixed offshore platforms and the development of reliability tools.