FPSO Structural Integrity - A TWI Collection of Case Studies
Dr Marcos Pereira
Regional Manager, Latin America, TWI Ltd
Group Manager, Structural Integrity, TWI Ltd
John Still, Lochead Still Associates, formally of Amerada Hess London
Proceedings of OMAE 2004: Speciality Symposium Houston, August 30 - September 2 2004
An oil and gas producer, contemplating the development of offshore fields, has to establish the most economical way of recovering the hydrocarbons from the reservoir. One option is the use of a Floating Production Storage and Offloading (FPSO) vessel. The selection of a suitable FPSO vessel has to take into account the water depth, environmental conditions, estimated life of the field, and the most effective way of exporting the produced hydrocarbons. Several FPSO vessel types are available which involve one of the following: a purpose-built FPSO, a conversion from an existing tanker, and an interception of a new build tanker converted to operate as a FPSO. To ensure that the vessel can operate without coming off station during the life of the field, it is essential that the integrity of the hull structure and marine systems be designed and built to an appropriate standard so that the vessel survives the operating and environmental conditions.
The design requirements will in many cases, surpass the Class requirements to ensure structural integrity. Ensuring integrity involves the selection of a suitable hull material to meet environmental conditions; the development of realistic defect acceptance criteria for both hull and marine systems; undertaking fatigue and fracture assessment of hull material, hull welds, marine piping joints and other critical welds in, for example, the mooring systems; and the development of adequate NDT requirements of both hull welds and the marine piping systems. It may be necessary to apply fatigue life extension techniques to critical hull parts in order to meet the requirements of the specific project, or to apply and inspect coating systems to the vessel hull, cargo tanks and marine piping, or to develop specific in-service inspection programmes for critical hull details during the operational life of the FPSO.
By adopting a structural integrity programme from the early stages of design and construction, the operator can have confidence that the FPSO will operate satisfactorily, with the minimum of lost production incidents. This paper describes some case studies of FPSO structural integrity assessment carried out by TWI, the methodology used and the general implications for the FPSO's construction and operation.
Floating production systems range from multi-hull vessels, to production spars, to tension leg platforms. The multi hull floating production systems fall into primarily two categories, Floating Production Storage and Offloading(FPSO) vessels to Floating Storage and Offloading (FSOs). In 2003, over 65 FPSO units were in operation and with FSOs this total increases to over 89 units in service.  FPSOs operate in the North Sea and the Gulf of Mexico, as well as offshore Brazil, West Africa, Australasia, etc. The required storage capacity of FPSOs is increasing and some are operating with storage capacity over 280,000mton DWT. This demonstrates the importance given to FPSOs in oil and gas developments to accommodate particularly, the strategic shift from first oil/marginal field exploration to a longer-term operation for FPSOs worldwide.
The use of FPSO has therefore generated much interest from offshore operators and the shipbuilding industry. Today the option to use an FPSO is attractive because of the relatively low construction cost and flexibility of these systems. The use of oil tankers as FPSOs has provided many shipyards with an opportunity to enter into the offshore oil and gas supply industry. However, the shipbuilding industry must guarantee that FPSO structures will not fail in service, and operators need to ensure that the integrity of the vessel will satisfy their project requirements.  In the series of case studies presented in this paper, TWI was contracted as a third party organization to assist in structural integrity assessment projects.
Design for integrity
FPSOs were originally converted from single hull oil tankers, and these were fitted with external mooring systems. Over the last 20 years, the preferred design of new FPSO and other vessels has changed significantly from single hulls to double hulls. The use of converted single-hull tankers is still practiced particularly in Brazil, Africa and Australasia. The selection of mooring systems is dependant on the water depth and weather conditions, and turret-mooring systems remain the preferred option by many operators.
A number of operators particularly on the North Sea have constructed FPSOs without propulsion units, removing these units prior to installing on location. With the introduction of the UK Design and Construction Regulations (DCR),  operators in the North Sea also have taken the option to drop vessel Class for FPSOs and apply the DCR instead (for these 'semi-permanent' floating installations).
The International Maritime Organization (IMO) has recently announced stricter regulation governing the design of hull structures.  This new regulation, effective from April 2004, involves the gradual phasing out of single hull tankers of over 15 years of age, while vessels less than 15 years old will be subject to more rigorous, regular surveys. A recent survey of FPSO's  found the following hull configurations in service:
- Double-sided hull
- Double hull
- Single hull
- Hull type unknown
The IMO's changes raise an obvious question: 'How should operators account for these new regulations for single hull FPSOs?' If the new IMO regulations are to be applied to FPSOs, operators may need to demonstrate the continued integrity of the hull via an engineering critical assessment (ECA) route, to deal with prior history and in-service fracture and fatigue issues.
TWI's approach for FPSO hull integrity is summarized on Fig.1. During design and construction it is important to select and characterize the appropriate materials to be used in the hull. This includes the selection of appropriate weld consumables to ensure adequate fracture toughness and corrosion resistance of the weld joints in critical hull locations. It is recommended that in additional to the minimum Class requirements for mechanical qualification testing, projects should consider undertaking fracture toughness tests in order to identify the fracture initiation and arrest resistance of the materials applied. This will allow the application of ECAs in order to define appropriate flaw acceptance criteria for the critical components, which will consequently determine the appropriate inspection requirements of such components during fabrication and operation.
Fig.1. Structural integrity programme
The non-destructive testing strategy adopted in the construction of the new build, intercept or conversion project also needs to be reviewed, to identify critical components where high inspection reliability is essential. An appropriate inspection strategy should be based on the criticality of the hull details and the consequence of failure in service.
The project specifications may subsequently need to be revised to reflect a construction based on a design to ensure structural integrity. For example, the results of any ECAs may also indicate the need to apply different parent metal and consumables for certain parts of the hull.  The use of fracture assessments should ideally be considered at an early design stage to avoid compromising fabrication schedules.
This design-to-avoid-failure approach may later help to determine areas where in-situ repair might be necessary during the FPSO's service life. To this end, TWI is working with industry to develop underwater repair techniques (that minimize operational disruption), to gain approval from regulators and the Classification Societies for the use of these techniques.  This will provide operators with the opportunity to design effective repair plans in order to decrease or minimize disruption on production and potential capital loss.
Hull integrity case studies
Typical FPSO hull components that present problems during operation are commonly similar to those that fail in service on oil tankers. However, FPSOs have different loading conditions compared to oil tankers and in many cases are stationed in locations were the hull will experience more severe loads. In addition, FPSO also have larger loads on the deck structure and the structural response in various sea conditions is also affected by the mooring system, risers connected to the structure, tank configuration and cargo movement. In other words, the integrity of FPSO hulls is even more complex to manage than for oil tankers. The following cases demonstrate how operators have worked with TWI to resolve problems with FPSOs.
Case 1: Amerada Hess Triton Project
Amerada Hess' Triton FPSO, Fig.2, was an intercept oil tanker under construction in a shipyard in the Far East. The FPSO was designed to operate in the North Sea near the Shetlands Islands where environmental conditions such as low sea temperature are more severe than those normally seen by an oil tanker. TWI was involved during construction to assess the fracture and fatigue capacity of the FPSO hull and other critical components.
Fig.2. FPSO TRITON during conversion work
The approach used was the development of a robust hull assessment programme that identified critical components via hot spot stress analysis, Fig.3. This information was subsequently used to undertake an engineering critical assessment for the hull. The ECA was performed using TWI CRACKWISE TM assessment software which automates the procedures of BS 7910 (a widely accepted methodology for assessing the acceptability of flaws in welded structures)
Fig.3. Hot spot stress analysis of the FPSO hull
An example of the resulting flaw acceptance criteria is presented in Fig.4, for two different welding process selected for the welding of the deck plates. Flaw acceptance criteria developed for plates welded by Flux Core Arc Welding (FCAW) and Electro Gas Welding (EGW). The result of the ECAs applied to several hull structural components helped the operator to make decisions on the areas where materials or fabrication processes required to be modified in order to guarantee the hull integrity in service. In other words, inthe event the tolerable flaw sizes were found to be impractically small, enhanced materials and welding consumables were recommended. These ECA results also identified the components where more rigorous fabrication inspection was required and helped the operator to develop an enhanced fabrication flaw acceptance criteria, based on fracture mechanics and not on minimum workmanship criteria required by Class design rules.
Fig.4. Acceptance criteria developed for deck plates
Subsequent pre-service inspection of critical components found many fabrication defects, Fig.5, which were corrected to guarantee the life of the FPSO and avoid dry docking.
Fig.5. Flaw at weld toe found after re-inspection of the hull
Case 2: Two Petrobras Projects
PETROBRAS has converted two single hull VLCCs for FPSO service for its Barracuda and Caratinga project. During conversion, several cracks were found in the cross-tie structure, Fig.6, in several frames inside cargo tanks. Although the vessel was inspected prior to conversion, this inspection was primarily visual so the cracks were only identified during an advanced stage of the conversion stage.
Fig.6. Fatigue cracking due to loading during prior service
These two FPSOs are intended for 20 years service life without dry docking and a zero fatigue life approach was applied during the re-design stage. For this purpose the fatigue life of the hull components were checked against the prior VLCC service life and the intended FPSO service life. However, the assessment was based on Class design rules requirements which did not identify the cross-tie as an area where the fatigue life was critical. TWI was requested to undertake a fatigue review of these cracked details, to explain the reasons for the cracking based on a review of prior service history, and identify possible reasons why the Class-based assessments did not identify these details as being critical. TWI's investigation was based on hot spot stress analysis of the cross-tie ( Fig.7) and the critical details of the component were identified.
Fig.7. Hot spot stress analysis at the cross-tie
TWI determined that the Class allowable stresses did not explain the failure and developed modifications to the Class rules to correct the problem. The fatigue assessment demonstrated that the cross-ties would fail in service unless modifications to cross-tie details were made in order to decrease the local stresses.
Fig.8. Critical weld detail in fatigue testing machine
Fatigue life improvement techniques, such as burr machining, were recommended for the fillet welds identified as critical. These recommendations were verified by fatigue testing, Fig.8. The project adopted a methodology of repair by welding followed by burr machining to correct the problem. TWI's fatigue tests on fillet weld details demonstrated that an improvement factor of three on life could be justified, Fig.9
, in line with IIW recommendations. [7,8]
Fig.9. Fatigue tests results on simulated cross-tie fillet welds
TWI subsequently developed the procedures for the welding repair, burr machining and inspection of the cross-ties and trained and qualified over 100 people to execute the job on over 10,000 joints in two FPSOs. To ensure the success of this exercise, it was critical that the inspection adopted during the repair could identify small flaws in a reliable way. There was a concern that the ultrasonic testing (UT) operators would misinterpret lack of fusion of the fillet welds or flaws introduced in the joints due to the repair undertaken, Fig.10. TWI therefore modified the UT procedures for the project so that this potential problem could be largely avoided.
Fig.10. Repair area flaws to be identified during inspection
This was a substantial exercise, however, the controls implemented during the project guaranteed higher joint integrity and significantly increases the likelihood of safe future operation of the FPSOs in service.
Case 3: Emergency Turret Repairs
Two FPSOs operated by a major oil company were found to have manufacturing embedded flaws in the girth (butt) welds of the turrets. At first one of the FPSOs was taken out of service for about a week with considerable loss on production. Initial ECAs carried out by the operator were not accepted by the Classification authority because there was insufficient data to support the safety case. (During weld qualification testing only Charpy impact toughness testing was specified as a measure of the fracture initiation toughness of the joints).
TWI was requested to assist the operator during the urgent weld repair work on the turret welds of the two FPSOs. TWI's initial ECA, using existing fracture toughness data for similar materials (from TWI's extensive materials database), demonstrated that the flaws were not critical. However, it was recommended that a more accurate inspection of the flaws was required as well as the measurement of fracture toughness of the welds. The welded joints were re-inspected using state of the art ultrasonic techniques including phased-array UT and TOFD.  The results of the inspection helped to accurately size the flaws, Fig.11.
Fig.11. Phased-array UT, sectional view of a surface flaw
The investigation included the simulation of the conditions applied during manufacturing of the turret welds, using the same materials and welding consumables. TWI carried out welding trials, adjusting the welding parameters in order to reach the same hardness levels measured on-site. Two different welding restraint conditions were subsequently investigated, namely low restraint and high restraint.
These welds were mechanically tested after further comparison with the actual turret welds. Fracture toughness specimens were extracted and the resulting data were subsequently used in a CRACKWISE TM ECA to develop an envelop of critical flaw sizes. This work demonstrated that all the flaws found in the turret were not critical and the turrets were therefore fit-for-service. Figure 12 shows the dimensions of some of the flaws sized during phased-array UT inspection, and re-characterized for the ECA in accordance with BS7910:1999. After almost a year of ongoing investigation, the Classification authority waived the requirement for the repair of the turret welds indefinitely, requesting only that in-service monitoring of the flaws be made at a regular basis.
Fig.12. Flaw re-characterization done for the ECA
FPSOs are a viable option for long term oil and gas production. However, the challenges faced by operators are related to the fatigue and fracture control of the hull, tank corrosion and in-situ repair. The study cases presented demonstrates the effective use of fitness-for-service (FFS) assessments during various phases of oil field development projects involving FPSOs. TWI's experience with FPSO integrity has demonstrated that applying FFS techniques in the early stages of the FPSO fabrication, potential problems during operation can be minimized or mitigated. It also shows that considerable savings can be made with enhanced safety for the structure. However, the costs of applying FFS atlater stages can be much higher than applying it at the design and fabrication stage. Nevertheless FFS can define the level of criticality of any flaws found during fabrication and operation, and allow operators to make appropriate decisions concerning need for repair, appropriate inspection schedules. TWI will continue to work actively to further develop FFS methodologies and approaches that will help industry to improve safety of FPSO construction and operation, whilst optimizing the costs associated with FPSO inspection, maintenance and repair.
The authors wish to thank Amerada Hess and Petrobras for their kind permission to publish the details of their FPSO structural integrity assessment case studies.
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