Life extension of ageing assets is moving firmly up the agenda for oil and gas operators in many regions. Factors driving this include: high oil prices; technology advances; regulatory requirements; updated design standards and knowledge; a need to manage risk and ensure safe operation; an enhanced focus on safety; and changes in reservoirs.
“A significant number of offshore platforms world-over are approaching, or have already exceeded, their original design life,” said Anupam Ghosal, who is DNV GL’s service line area manager for verification, Middle East and India. “DNV GL is helping several operators in the Middle East and India region to address the challenges that ageing brings to offshore structures.”
India’s multinational Oil and Natural Gas Corporation (ONGC) provides an example of how the issue can be tackled. It has more than 250 fixed offshore platforms, designed to API-RP-2A guidelines. The oldest was installed in 1976, and most had an original design life of 25 years.
“More than 35% of our offshore platforms have outlived their design lives and many more are approaching it,” said Dinesh Kumar, general manager – head structure, at the Institute of Engineering and Ocean Technology (IEOT), the company’s engineering arm. “However, we still require these to be producing hydrocarbons until 2025 to 2030, 15 to 30 years beyond design life.”
ONGC is therefore pursuing the requalification of platforms that have exceeded a design life of 25 years.
Issues with standards
Using methods from API-RP-2A for reassessing platforms, ONGC’s IEOT reviewed the structural integrity of its offshore platforms. Some did not meet present day API-RP-2A 21st edition requirements on design level and fatigue.
“This was mainly because of a change in hydrodynamic coefficients over the period of time; additional loading due to risers, clamp-on wells, deck extension and so on; and damage from incidents such as collision and dropped objects,” Kumar explained.
Failure of soil, piles, members and joints were major reasons why structural integrity could not be documented. Other issues included the very low fatigue life of some joints, and the fact that results of fatigue analysis and actual inspection did not match.
ONGC has planned to use section 17 of API-RP-2A 21st edition as its framework for action. This deals with assessment of existing offshore structures. Section 17 allows reduced metocean loading for platforms designed around API-RP-2A 20th or earlier editions, and permits the use of ultimate strength analysis for a given reserve strength ratio.
Platforms not passing design level or ultimate strength analysis need strengthening or mitigation measures such as load reduction. However, APIRP-2A does not explicitly deal with life extension of offshore structures.
ONGC considered what DNV GL could offer to facilitate its platform requalification requirements.
“We had extensive discussions with DNV GL experts in Oslo to firm up the proposal,” Kumar said. “In 2008, it became the first organisation to bring out a standard (N-006 for NPD) on Life extension of offshore structures. The company had carried out extensive studies and had vast experience in the life extension of both fixed and floating structures.”
DNV GL has been contracted by ONGC to assist IEOT in developing a requalification methodology for the life extension of 34 platforms, specific to conditions offshore India. ONGC is now interested in implementing a risk-based inspection (RBI) plan for all its ageing offshore structures in the near future.
RBI analysis quantifies risk in terms of the probability and potential consequences of different degradation modes for overall structural integrity, including fatigue. It also aims to establish an inspection schedule for individual joints, so as to maintain risk below the acceptable level and at all times throughout the considered service life of a platform. Inspection planning identifies a strategy to ensure that legislative and operator requirements on safety are met, and that inspection efforts are prioritised from an overall risk perspective.
With support from specialists in Norway and Denmark, DNV GL’s team in Mumbai, India, leads and delivers the work on the ONGC project.
Scoping the project
The project involves developing both a philosophy and a methodology alongside IEOT. This includes the screening and grouping of structures, and the reassessment of platforms; particularly where some structural drawings or soil and pile data are missing.
DNV GL has verified in-place and fatigue analysis; conducted independent in-place and fatigue analysis; identified structures requiring advance analysis, such as non-linear pushover analysis; established the scope and extent for further assessments; carried out advance non-linear pushover analysis for platforms to understand failure mechanisms and to identify potential mitigation measures; and has performed advance analysis for low fatigue joints. DNV GL is also developing suitable RBI plans including mitigation strategies for those structures whose life is extended for up to 15 to 20 years as applicable. The RBI plan for selected structures will be applicable to all 34 platforms.
What is innovative about these approaches? “For one thing, we have developed a smart methodology that groups structures to minimise duplicate analysis. This reduces the total number of analyses carried out,” said Varadaraj Salian, DNV GL project manager for the ONGC work.
While initial work was carried out on grouping, it became apparent that selected representative structures were failing through joints with a high degree of complexity, Salian explained.
As the project moved ahead, the high degree of complexity of different platforms required a higher number of advanced non-linear analyses. Nonlinear advance analysis helps to lower substantial expenditure by eliminating or reducing the need for structural modifications to be made offshore.
“Lack of adequate data availability is another common problem for old assets globally,” Salian said. “Real value is added through developing a procedure for the reassessment of platforms where some structural and pile drawings and/or soil data are unavailable. Additional value comes through implementing this procedure for further studies.”
Another feature of note in the ONGC project is that requalification methodology has been developed and implemented specific to local climatic operating conditions, Salian added.
Approach has wider use
ONGC is confident that the programme will achieve its aims. “The approach taken with DNV GL will definitely meet the objectives we have envisaged,” Kumar said. “Using this approach, we would be able to requalify our platforms up to 2030. RBI would be a cost-effective tool considering the extended life of the structures.”
Despite the tailoring of some elements of the ONGC project to local conditions, the strategy, technologies, systems and processes involved could be applied widely, Kumar added.
“The issue of life extension of offshore structures is being faced by a number of operators in this region. I am sure that the approach adopted in our present project would definitely be a starting point for such platforms.”
Middle East tackles asset integrity head on
Approximately half (47%) of the world’s oil and gas companies are actively planning for far longer asset life spans than before, a survey found earlier this year. This is a particularly pressing issue in the Middle East. “There are nearly 750 offshore fixed platforms and bridges in the region. Out of these, the United Arab Emirates (UAE) alone accounts for about 450 structures, and more than 70% of these are older than 25 years with some beyond 40 years,” said Anupam Ghosal, DNV GL service line area manager for verification in the Middle East and India.
Recent news from the region underlines how the need for life extension is driving real action. Company announcements indicate a growing appetite among operators in the Middle East to find intelligent ways of prolonging the productive life of their oil and gas assets. Ghosal highlights four such developments:
The UAE’s Abu Dhabi Marine Operating Company (ADMA-OPCO), a major producer of oil and gas offshore Abu Dhabi, has launched various life extension projects. It has initiated structural integrity assessment of offshore facilities; integrity assessment of ageing wells; and plans to upgrade and replace all its ageing pipeline networks by 2030.
Meanwhile, Bunduq Oil Company, Abu Dhabi, has taken up structural integrity assessment and risk-based inspection (RBI) for its ageing offshore facilities.
The Al Khafji Joint Operations (KJO) joint venture organisation in the neutral zone between Kuwait and Saudi Arabia is on course to implement an asset performance management (APM) programme. This is to better manage existing aged facilities, and additional new ones, by enhancing availability, reliability and strategy.
DNV GL is working with RasGas in Qatar to set up a structural integrity management system (SIM) for all the operator’s offshore structures.