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LIFE EXTENSION OF A FIXED OFFSHORE PLATFORM STRUCTURE BASED ON MONITORING RESULTS

S. Copello, RINA Services, P. Castelli, Edison.

This paper was presented at the 11th Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 20-22, 2013. It was selected for presentation by OMC 2013 Programme Committee following review of information contained in the abstract submitted by the author(s). The Paper as presented at OMC 2013 has not been reviewed by the Programme Committee.

ABSTRACT

The structure of the existing offshore platform Vega A, operated by Edison in the Sicily Channel, has been subject to a reassessment process in order to extend its operating life beyond the original design life. Such requalification analysis has been focused on a fatigue verification of the jacket structure with the target of life extension, as well as other reassessment issues such as the actual status of structural components, present topside configuration, etc., all considerations aimed to eventually update a proper inspection and maintenance plan, everything considered as normal practice in the offshore field where the number of existing platforms subject to reassessment process due to expiration of the original design life is increasing. What is peculiar in this case is the availability of a large amount of significant information recorded during the occurred service life of the platform by the monitoring system mounted on the structure since early phases of installation, which has definitively increased the level of reliability in the new structural assessment. In particular, it has been possible to re-evaluate the platforms response to environmental loads (the governing loading for structural safety) whose characterization has been reviewed and updated according to a large amount of wave, wind and current data measured on site for a long term and, what is more, to calibrate the calculated dynamic response, which is the basis for the fatigue assessment, with respect to the actual jacket accelerations continuously recorded on field by relevant monitoring devices. In the following the different steps of the reassessment process carried out through the calibrated structural response are described, by highlighting how the monitoring effort, along with a proper maintenance, has facilitated the achievement of the goal of life extension.

INTRODUCTION

The Vega field, operated by Edison, is located at approximately 12 miles South from the southern coast of Sicily. It includes a fixed platform, Vega A, and a floating storage offloading unit (FSO), located at 1,5 miles from the platform, and connected to the platform through sealines. The FSO is moored to the seabed through an arc-yoke articulated system composed of (Figure 1):

- A column connected to the sea bottom and extending above sea water level; - A yoke connecting the column tip to the FSO bow tanker beam.

The offshore facility was installed in August 1987 with the FSO unit being the 250,000 DWT converted tanker Vega Oil. Due to international double hull requirements, in July 2008 the FSO Vega Oil was disconnected and replaced in September 2009 by the converted 110,000 DWT Aframax tanker named Leonis. In relation to the FSO substitution, a new yoke has been designed and installed so that both the FSO and the SPM have been subject to renewal of classification by RINA, to comply with the operator requirement for a field life extension.

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In the same field, the structural design of the Vega A platform (Figure 2) was originally certified by RINA for an operating life of 25 years, thus an assessment of the fixed platform has been also required by the operator in order to extend the jacket life beyond its original design life, according to a prescribed target of further 25 years of service.

Fig 1: Vega field arc-yoke mooring system Leonis FSO

Given that target, RINA has carried out, as normal practice /Ref. 1/, the structural analyses of the jacket with the main purpose of conducting a fatigue assessment of the tubular connections and consequently to define the relevant inspection and maintenance plan to comply with for the extended service. To this aim, an updated structural model of the Vega A jacket has been built by integrating original design data with the information relevant to the present status of the platform (such as topside weight distribution) and data collected during the past service life via inspection campaigns carried out and, what is more, via a monitoring system which has been installed on the structure since the early phases of its operating life.

Fig 2: Vega A Platform

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DESCRIPTION OF THE PLATFORM AND MODEL UPDATING

The submerged structure of the Vega A platform is a 8-legs steel jacket structure in a water depth of about 138m, framed by seven plans, each at elevation -120.8, -96.0, -75, -54.0, -33, -13, +7m with respect to the mean water level, starting from a rectangular base measuring 70m x48m. up to the top side of 50mx18 m. The pile legs are tubular members with variable section from 2000mm outside diameter (OD) x 50mm wall thickness (WT) from the bottom to 1700mm OD x 50mm WT at the upper levels, provided with horizontal bracings at each plan and rows tubular brace members between the plans themselves. The jacket is fixed to the ground by 20 foundation piles (3 for each of the 4 leg corners and 2 for the internal legs), whose outside diameter is 2590 mm, penetrating 63.5m below the sea bottom, while the connection between piles and legs is made by sleeves filled by concrete grouting in the pile-sleeve section interface. The jacket of the platform is completed by 2 boat landings and other appurtenances such as bumpers, casings and risers that are not structural members but are to be considered in the model because their contribute to the hydrodynamic loading, as well as the launch runners used during the platform installation phase (carried out by launching) and still present along some jacket legs. For the appropriate evaluation of the structural dead loads (weight and buoyancy) it is worth noting that along the 2 lowest jacket plans sections the legs are still flooded, due to launching design procedure. Finally, for the appropriate model of the jacket hydrodynamic response, the specific presence of the sacrificial anodes on the structural members has been taken into account.

By considering all available up-to-date information relevant to the present conditions of the platform, an updated structural space frame 3D model of the Vega A jacket has been carried out by RINA licensed software NSO (Figure 3). In particular the jacket geometry and material from as built drawings /Ref. 2/ and structural steel specification /Ref. 3/ has been validated by fabrication data book. As far as the foundations model is concerned, the pile-soil interaction has been modelled by appropriate characterization of the soil layers along the pile depth as well as pile tip bearing capacity, all deduced by original soil geotechnical report /Ref. 4, 5/ and validated by installation driveability analysis /Ref. 6/.

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Fig 3: Vega A Platform Structural Model

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In order to complete the updated model of the platform so to reflect the present conditions as much as possible, both the generated structural weight of the jacket (about 17820 tons) and the topside modules masses and positions have been validated by their values available from project final weight control report /Ref. 7/ in addition to information obtained from topside survey and operators record, from which, e.g., it is reported that the drilling rig equipment has been present on the platform up to the end of the year 2002. Relevant mass has been considered in the dynamic and fatigue analyses carried out accordingly. In order to complete the updated model of the jacket, very important information have been obtained from inspection reports available /Ref. from 8 to 12/ for all the surveys carried out for the submerged structure during the service life of the platform, in terms of general and close visual inspection outcomes, particularly:

- Wall thickness measurements; - Cathodic protection measurements; - Marine growth measures and cleaning policy; - Non-destructive examination of the welded joints.

THE MONITORING SYSTEM

Since the early phases of its service life, on the platform it was installed a monitoring system, which, even if subject to modifications and upgrading through the years, has continuously provided the following data from 1988:

- Directional wave motion; - Current velocity and direction; - Sea level variation: - Wind velocity and direction; - Air temperature.

The present system, installed on Vega-A by the company DEAM srl at the end of 2001, is made by: - Wave meter, based on the measurement of water column pressure; - Sensors for measurement of wind velocity (anemometer) and direction; - Current meter, to record the 2-components current velocity; - Sensors for acquisition of meteorological data. Inside these categories, there are different timeframe for measurements, i.e. the meteorological parameters are detected any ten minutes inside each hour, while waves, sea level and current are detected along 17 minutes each hour. All data collected on the platform, whether following a first elaboration on the platform (reduced data), or by rough data, are sent to the Department of Civil Engineering (DICeA) of the University of Florence, which has the role of validating the samples and following statistical interpretation of the data relevant to the environmental parameters /Ref. 13/. As regards to wave characterization, that post processing phase provides:

- Significant wave height; - Maximum wave height; - Significant wave period; - Associated period to maximum wave height; - Zero-crossing period; - Peak period; - Wave incoming direction.

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Similar process is undertaken for the structural behavior, which has been monitored since the commissioning of the platform by installation of strain gauges and accelerometers on some structural bracings. At present, 6 accelerometers are employed for accelerations measurements (6 linear and 3 angular). In total, 17 sensors are present on the platform, in the positions indicated in Figure 4, with the characteristics shown in the table reported as Figure 5, where it is reported the duration, the frequency and the number of samples for any set of data collection.

Fig 4: Sensors on Vega A Platform

Fig 5: Sensors properties

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UPDATING OF ENVIRONMENTAL DATA AND STATIC ANALYSIS

After completion of the updated structural model, and propaedeutical to the following dynamic and fatigue analysis to be performed for platforms life extension, a static analysis has been carried out in relation to the two typical design conditions that are to be verified according to the international standard API code used for fixed offshore structures /Ref. 14/:

- Operating condition (i.e. with wave, current and wind loading characterized by 1-yr return period);

- Extreme condition (i.e. with wave, current and wind loading characterized by 100-yrs return period);

This latter condition has been particularly analyzed to check the accuracy of the model in comparison with the original design results /Ref. 15/, in terms of global loads resultant at the structures base or pile loads, by taking into account of course the occurred and modeled variation on environmental actions and weight distribution. Indeed a first important benefit for the structure obtained from the monitoring results has been the reduction of the environmental loads acting on the platform: for instance in the following Table 1 the values of wave heights (for both operating and extreme conditions) adopted at design stage as a result of the former meteomarine study /Ref. 16/ are reported in comparison with the new values obtained as main outcome of the elaboration of more than 20 years of recorded data on the platform /Ref. 17/. Also, new data are more refined with respect to the incoming direction (original design referred to 4 sectors only); from the comparison it can be seen how new data are more homogeneous and with a significant reduction in extreme values, which eventually results in a favorable contribution for the purpose of extending the platforms life. Furthermore, from the specific analysis of wave heights raw data /Ref. 18/, it has been possible to get an appropriate calibration of the theory that best represent the actual values of recorded wave velocities and acceleration, specifically the adoption of a 3rd order wave theory better describe the wave parameters distribution than the Stokes 5th order theory used in the original design and, in any case, the theoretical prediction overestimated the actual recorded values. That conservative assumption does result in a margin of safety actually present in the designed structural members, safety gained for the present reassessment analysis. Similar comparison and observations can be obtained from the analysis of Table 2 as regards to prediction of wind velocity.

Tab. 1: Maximum values of wave heights comparison

Operating Conditions Extreme Conditions Direction () Hmax design (m) Hmax 2012 (m) Hmax design (m) Hmax 2012 (m)

0 3.7 6.5 5.5 6.5 30 7.2 7.5 60 7.0 8.4 90 10.3 7.3 15.5 13.0 120 7.9 15.2 150 6.1 11.3 180 10.3 7.1 15.4 12.8 210 7.3 12.3 240 7.3 12.1 270 11.4 8.8 17.1 15.0 300 9.6 15.8 330 9.7 15.7

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Tab. 2: Maximum values of wind velocities comparison

Operating Conditions Extreme Conditions Direction () v

design (m/s) v 2012 (m/s) v design (m/s) v 2012 (m/s) 120 27.3 21.1 43.4 31.0 150 27.3 21.7 43.4 36.2 210 27.3 26.5 38.0 39.5 240 27.3 30.7 48.7 41.5 300 27.3 25.5 48.7 34.2

The static analysis carried out for both environmental and operating conditions showed non critical elements with respect to the structural members and joints checks performed according to API requirements. These results are shown in graphical form (e.g. in Figure 6 it is reported the output for member checks in extreme conditions) in terms of unity check (UC) of the structural components, which can be represented as the ratio between the loads demand and the resistance capacity of each component (by also accounting for appropriate safety factors according to the rules /Ref. 14/), therefore resulting in a safe state if UC < 1.

Fig 6: Outcome of structural members check in extreme condition

In particular, the following maximum values of the UC factor resulted for the platforms structural components in the two analyzed conditions:

- UC = 0.942 for the members in operating condition (for a compression + bending + hydrostatic collapse limit state);

- UC = 0.708 for the members in extreme condition (for a compression + bending + hydrostatic collapse limit state);

- UC = 0.303 for the joints in operating condition (for punching shear check); - UC = 0.489 for the joints in extreme condition (for punching shear check); - UC = 0.339 for the pile bearing capacity under compression + bending.

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DYNAMIC ANALYSIS AND VALIDATION ACCORDING TO MONITORING RESULTS

The dynamic analysis of the platform in the present conditions has been performed to evaluate the natural periods and modal shapes of the structures. In particular the calculated first 3 frequencies have been compared to the values of the same dynamic parameters obtained as main outcome from the monitoring of the structure and, consequently, a proper calibration of the model is reached. A prediction model of the structural dynamics as close as possible to the real response is of paramount importance for the following fatigue spectral analysis, which is driven by the vibration analysis results and is the definitive assessment to be performed for the life extension of the jacket. The jacket and the topside models adopted for the dynamic analysis have been the same ones used for the static analysis, apart from the pile section below the mud-line which cannot be treated for the dynamic response analysis with the full non-linear foundation model used for the static in-place analysis. The foundation pile behaviour must be linearized by evaluating the characteristics of a special pile-stub element. This element simulates the pilesoil interaction during the dynamic analysis, taking into account for the characteristics of axial load transfer along the pile shaft and the pile response to lateral loads. The dynamic mass model carried out includes:

- Jacket masses accounting for spatial orientation and distribution; - Topside masses accounting for spatial orientation and distribution (structures,

equipment, pedestal cranes and live loads); - Appurtenances (such as risers and boat landings); - Added and entrapped masses below the sea level; - Marine growth weight; - Mass of the piles.

The dynamic analysis has been carried out through eigenvalues (natural periods) and eigenvectors extraction, for a sufficient number of modes of vibration, so that a minimum 90% mass participation is achieved. As mentioned above, the results obtained from the dynamic analysis carried out on the updated structural model have been eventually validated in relation to data from monitoring, which, in turn, reflects the actual behavior of the structure. From the final analysis of the monitoring data (carried out on yearly basis), the structural response derived by the records of the deck accelerations is very useful for the purpose of definition of the global dynamic behavior. In particular the outputs of the 9 accelerometers (both linear and angular) presently installed on the structure have been processed to get average spectral response on a 3-months basis. The comparison between elaborated average spectra and raw data collected during the past platforms life shows a substantial stability in the dynamic response of the structure, which means that no significant variation in both mass and stiffness distribution have occurred along the lifetime. From the examination of the elaborated spectra /Ref. 13/ it can be drawn that prevailing harmonics are reported with a frequency of about 0.45Hz in x-direction and 0.50Hz in y-direction. This latter is slightly higher than the value of 0.48Hz reported in 2001, but equal to the value reported from 2002 to date. By the way, such peak values of the spectral response are quite higher than the values estimated at design stage in the platforms seismic analysis (i.e. fx = 0.300Hz and fy = 0.316Hz, /Ref. 19/), probably due to underestimation of the foundation stiffness in the design soil-structure prediction model. Given the actual natural periods obtained from the monitoring, the present dynamic analysis has been calibrated accordingly.

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In order to reach the same vibrations response, the sensitivity of the natural periods to the following parameters, which were uncertain or affected to some extent by model assumptions, has been investigated:

- Marine growth thickness; - Contribution of the conductors to global structural stiffness; - Foundation stiffness,

with the final determination of the structural frequencies as reported in the following Table 3:

Tab. 3: Evaluated structural frequencies

Shape of vibration Frequency from monitoring (Hz)

Calculated frequency (Hz)

1st mode 0,45 0,42 2nd mode 0,50 0,51 3rd mode 0,78 0,78

The corresponding 3 modes are reported in the following Figure 7.

Fig 7: First 3 modes of vibration of Vega-A jacket.

FATIGUE ANALYSIS AND CHECKS

A stochastic spectral fatigue analysis has been performed to evaluate the fatigue damage, at the welded tubular connections of the jacket through the following calculation steps:

- Stress range transfer function; - Environmental load spectrum; - Stress response spectrum; - Fatigue damage evaluation.

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The wave response analysis has been used to determine the system transfer function. This approach assumes that an infinite train of repeatable wave form are stepped through the structure and the response is established. In order to accurately define all the peaks and valleys inherent the sub-structure response transfer function, a sufficient number of frequencies and the corresponding wave heights are to be selected. The selection of such frequencies is based on the dynamics of the structure, thus the more realistic is the dynamic response, the more reliable is the fatigue assessment. The wave data for the fatigue check is provided on a statistical basis, where the normal parameters are the significant wave height and the zero up-crossing period, as detailed in /Ref. 17/. Then, Jonswap energy spectrum with the peak enhancement factor appropriate for the site has been used. Each connection of the jacket, that is each welded tubular joint, has been checked at 8 points around the circumference of the joint. The stress distribution all around the tubular joint connections has been defined considering hot spot stresses calculated on the basis of parametric formulation of the stress concentration factors available in literature for tubular joints, whereas the S-N curve for the evaluation of the fatigue life, applicable for tubular connections as well, is available from API rules /Ref. 14/. The evaluation of fatigue damage, and the corresponding calculation of the fatigue life of each tubular joint of the jacket, has been performed by comparing the summation of damages relevant to the various stress range sets, following the Miner-Palmgren model, with the allowable S-N curve. The results of the fatigue assessment, in terms of fatigue life for each connection, are reported in the following Table 4, for the connections with the lowest fatigue lives (lower than 200 years): all the jacket joints satisfy the requirement of fatigue life greater than 50 years (25 years of target extension life multiplied by the safety factor 2, adopted for joint connections in and below the splash zone, according to /Ref. 14/).

Tab. 4: Fatigue analysis results

Node Chord Brace Life(years) Side 448 1391-1392 2235 87 Chord 454 1369-1370 2224 113 Chord 448 1391-1392 2232 113 Chord 201 937-938 2199 116 Chord 454 1369-1370 2223 118 Chord 201 937-938 2200 120 Chord 673 1806-1807 1805 121 Brace 647 1809-1810 1805 123 Chord 463 1387-1388 1363 129 Chord 463 1387-1388 1345 131 Chord 464 1389-1390 1354 149 Chord 682 1794-1795 1994 153 Chord 681 1796-1797 1997 168 Chord 682 1794-1795 1993 170 Chord 464 1389-1390 1362 171 Chord 184 559-560 2171 172 Chord 184 559-560 2170 177 Chord 666 1811-1812 3936 193 Chord

As shown in Table 4, the most critical joint in terms of fatigue life (87 years) is located at elevation -33m below the sea level.

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UPDATING OF IMR AND RELIABILITY BASED INSPECTION PLAN

Notwithstanding the engineering assessment results, the life extension of the jacket shall be subject to the provision of an appropriate inspection, maintenance and repair (IMR) plan, which, in turn, can be updated by using the newly performed fatigue analysis, as well as the outcomes of the inspections actually carried out. Indeed, during the jacket past service life, besides the monitoring, the platform has been subject to regular maintenance and inspection activities, as usual for offshore platforms. The same joint (at node 448) which has been found as most critical in the present assessment has been subject to non-destructive examination by MPI (magnetic particle inspection) in the 1993 with no cracks detected. In general, 23 out of 110 main structural connections have been inspected by NDE in the offshore campaigns carried out from 1989 to 2011. A specific IMR plan is considered for the appropriate maintenance of the jacket, reporting the schedule of close visual inspections of welded joints as well as other controls such as wall thickness measurements, cathodic protection measurements and marine growth cleaning. During the past years most of the planned MPI have been replaced by surveys performed by flooded members detection (FMD) method, particularly for the deeper jacket structural plans (i.e. plans at el. -54m and -75m) where the NDE inspection is more difficult and expensive. All the inspected joints did not reveal defects inside the welds, apart from a micro crack detected during the 1989 inspection on a connection of the plan at -13m depth. In 1990 the same weld was subject to further MPI without showing any defect. In relation to the extension life issue, a new IMR plan has been developed to be applied to the jacket structure starting from the year 2013. The new updated IMR plan is based on the above discussed jacket analysis, particularly the fatigue analysis results have been considered to select critical and representative joints to be inspected according to the new schedule, which is covering the period of extended life. Moreover the actual inspections outcomes can be considered and fully combined with the fatigue evaluation at each jacket joint by using a reliability approach, which allows the planning of future inspections on a rational basis. In particular, by using the reliability approach, the probability of failure or, correspondingly, the safety margin, with respect to the fatigue limit state can be expressed by an index, properly called safety index , whose evaluation can be determined in closed analytical form by using, e.g., the lognormal format for the different statistical parameters which contributes to the fatigue life evaluation for a given tubular joint of an offshore platform /Ref. 20/. Such reliability evaluation allows to determine the safety margin with respect to the fatigue failure as a function of time; thus, it is possible to represent the trend of this safety margin, which, of course, is decreasing with increasing age of the structure (green line in Figure 8). The same margin can be updated by using the inspection events /Ref. 20/, which, in any case, represents a factor of knowledge on the structure, reducing uncertainty and, therefore, increasing the safety margin, particularly in case that no cracks are detected. For instance, the safety margin evaluated for the above reported critical joint 448 is reported as basic case (green line) by supposing that it will reach the safety target (i.e. the minimum allowable safety, red straight line in Figure 8) at the time of its planned next NDE inspection, that is in the year 2015. However, by considering that the same joint was inspected in the 1993 with positive outcome, the safety margin is consequently increased (yellow line) in such a way that it could be actually not inspected till the year 2027; moreover, by actually inspecting the joint with positive result in planned 2015, relevant index will be further updated (pink line) by maintaining the safety margin far greater than the minimum required one: it is highlighted how the reliability evaluation provides further confidence on the interpretation of the engineering assessment results for planning future inspection scheduling.

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Fig 8: Fatigue reliability index evaluation

CONCLUSIONS

The common process of reassessment of an existing offshore platform, in order to extend its operating life, which typically implies a structural reanalysis of the jacket with particular attention paid to the fatigue issue, is significantly improved by availability of monitoring data, gathered by a measurement system installed on the platform since the early phases of its operating life. The large amount of significant information recorded has eventually increased the level of reliability in the new structural assessment, by allowing:

- The revision of statistics of wave, wind and current data with relevant reduction of the characteristic meteomarine loadings acting on the platform;

- The calibration of the estimated structural dynamic response, which is the basis for the fatigue assessment of the jacket welded joints, with respect to the actual jacket accelerations continuously recorded on field by mounted accelerometers.

No critical situations have been highlighted by updated structural analyses carried out; in particular the performed fatigue assessment has shown that the jacket original design life can be extended up to the requested target of 25 years, provided that the operator will continue with the regular inspection and maintenance measures carried out during the whole platforms service life. To this aim a rational updating of the IMR plan has been prepared on the basis of both new engineering analyses and inspections results.

ACKNOWLEDGEMENTS

The authors would like to thank all the Edison and RINA colleagues, based in Siracusa and Genoa respectively, as well as the personnel of DEAM and professors and researchers at DICeA of University of Florence, without whose effort and contribution the job described in the paper wouldnt have been possible and, above all, it wouldnt have been possible to reach the goal of extending the operating life of the Vega A platform.

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REFERENCES

/1/ RINA, Guidelines for Requalification of Existing Offshore Platforms, Rev. 1, 7.2.2011; /2/ Vega A Platform Project Approved for construction set of drawings, issued by

Tecnomare; /3/ Vega A Platform Project Doc. No. 656513-SPE-E020-C101 - Structural Steel

Specification, Rev. 4; /4/ DAppolonia Project No. 81-958, July 1982, Offshore Geotechnical investigation Vega

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investigation Vega 4 Site; /6/ DAppolonia Project No. 86-317, December 1986, Review of Pile Driving and Redrive

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interpretazione dei dati ambientali e strutturali della piattaforma Vega A nel periodo 1988-2010 Relazione conclusiva, 2010;

/14/ API, American Petroleum Institute, API RP2A Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms - Load and Resistance Factor Design, 1st edition amended with Supplement 1, Effective Date April 1, 1997;

/15/ Vega A Platform Project Doc. No. 656513-REL-J010-S529, In Place Static Analysis, Rev. 1;

/16/ Glenn A.H. & Ass. (1984) Valutazione degli effetti ambientali relativi allattivit offshore del Campo Vega;

/17/ DEAM Report No. 400204-VEGA-EDI-AV-r0, Aprile 2011, Valori Estremi dei Parametri meteo-marini e numro di onde singole (basati sui dati rilevati dal sistema di monitoraggio fino al 31/12/2010);

/18/ DEAM Report No.400204-VEGA-EDI-AR-r0, Giugno 2011, Estensione della vita operativa Piattaforma Vega A, Sezione 2, Analisi dei dati raw e confronto con i valori teorici;

/19/ Vega A Platform Project Doc. No. 656513-REL-J020-S533, Earthquake Structural Analysis, Rev. 0;

/20/ R. Facciolli, C. Ferretti, R. Piva, S, Copello, System Fatigue Reliability Updating for Offshore Structures, Proc. OMAE 1995, Paper 1241.