reports that, “for structures with the same geometrical and structural properties, harmonization in safety levels (as are in GOM), hence requires location dependent partial action factors”. Environmental load factors have been determined for GOM, Northern North Sea, Southern North Sea, Central North Sea, China, Mediterranean Sea, Australia, Gulf of Guinea and they should be determined for regional environmental conditions [2], [9], [26], [27]. Though many studies have been conducted on the efficiency of different codes with regard to the load factors, still work is under progress in many parts of world [21]. In GOM, Graff et al. [14], showed that 19% i.e. 5500 tons would be saved on total weight of Jacket of 27,800 tons of steel. Thomas and Snell found reduction of weight of Jacket by 0.75% at one particular level by using LRFD method in North Sea [28]. The cost of Jacket could be saved by 15% if change of location dependent LRFD load factors is applied [6] in Java sea where Jackets are not dominated by wave loads but by gravity loads. In the light of above facts, it becomes very essential to research on environmental load factors for Jacket platforms in offshore Malaysia region.
Most of Jacket platforms in Malaysia have already completed their design life or will soon be completing. The reassessment will be required for extension of life, and ISO code requires a load with a return period of 104 should be applied and Jacket strength evaluated. Only probability of failure is considered in present day assessment which may show that Jacket cannot take a required load. If Bayes theorem for updating of probability of failure is applied for the same Jackets it gives us reduced probability of failure at higher loads and thus modification work can be avoided.
probability of failure was found based on First Order Reliability Method (FORM).
The component and joint reliability was determined, followed by system reliability.
Subsequently environmental load factors, based on component, joint and system reliability were determined. Probability of failure was updated based on Bayesian updating technique using Monte Carlo simulation.
Platform designed by API RP2A WSD and representing all the three regions were analysed in this study. Platforms with four and six number of legs and with different water depths were selected. Availability of SACS model was considered essential so that actual resistance and load effects should be evaluated. SACS loading models were changed as per the requirements of this study.
1.4.1 Research Objectives
The research objectives are to evaluate the environmental load factor for Jacket platforms in Malaysia using component, joint, system reliability analysis and to check the methodology for extension of Jacket life. Following are the main objectives of this research.
1 (a) To determine the geometric, material and model uncertainties to be used for calculation of resistance uncertainty. This was used for the analysis of component, joint and system reliability evaluation of Jacket platforms in all the three regions of offshore Malaysia.
1 (b) To determine the wave, wind and current uncertainties, to be used for calculation of load uncertainty. This will be used for analysis of component, joint and system load evaluation of Jacket platforms in all the three regions of offshore Malaysia.
2 To propose environmental load factors, to be used for the design of Jacket platforms in all three regions of offshore Malaysia. These factors are to be determined based on the reliability index calculated for component, joint and system analysis.
3 To propose improved calculation of probability of failure during reassessment of platforms with a view to extend service life without any modification. It will also be
applicable to change of loading conditions or damage to the platform members. This will help us to extract the remaining hydrocarbons available at the site.
1.4.2 Limitations
Environmental load considered was based on 10 and 100 year omnidirectional maximum values. Loads such as earthquake, boat impact and corrosion were not considered in this study. Dynamic analysis was not considered because Jacket platforms, in shallow water depth i.e. less than 100 m, (which is the case in Malaysia) are stiff in nature. It is required when natural period of vibration exceeds 3 seconds such as deepwater platforms (>300 m) [29], [30]. During the extreme storm conditions, dynamic nature of loads does not play a major role for ultimate limit state performance for Jacket platforms [31]. Fixed steel Jacket platforms response to environmental loading is basically quasi-static. This is due to the reason that Jackets are structurally rigid and natural period of vibration are short. Structures respond to the repetition of wave loads as though they were a series of static loads acting on the Jacket [32]. This comprises the space frame slender tubular which do not influence the gross characteristics of incident waves i.e. no wave diffraction [33]. Four platforms were selected for reliability analysis in this study. The same numbers were used for research elsewhere [26]. This research covered only four out of 250 platforms from three regions of Malaysia, this was due to non-availability of data for other platforms. The increase of data points may reduce epistemic uncertainty, to cater this We/G ratio ranging from 0.1-40 is used. For system load factors minimum RSR i.e. range of 1.5 to 2.25 is used to find optimised load factors as suggested by ISO 19902 and API 21st edition, this will cover all types of geometry of Jackets and topsides. Cost benefit analysis was not made in this study.
1.4.3 Key Assumptions
i) Data collected for uncertainty of resistance is from one representative fabrication yard in Malaysia.
ii) Four platforms are considered for the calibration representing each region of offshore Malaysia. For GOM and NS the number of platforms considered for calibration are three (3) and six (6) respectively [10].
iii) Primary members are selected for component reliability analysis which includes leg, diagonal, external horizontal at periphery and internal horizontal bracing.
iv) In this study only ultimate limit state is considered in consistency with ISO, API LRFD and NPD (Norwegian Petroleum Department).
v) Like API WSD and API LRFD [1], omnidirectional wave, wind and current values are used for this study.
vi) 100 year load return period for environmental loads is considered for calibration of Jackets as per guidelines used by API and ISO.
vii) Stoke’s 5th order wave Equations (1.1) and (1.2) were used for the selection of wave theory as can be seen from cross lines superimposed in Figure 1.1 [34].
H/ (gTapp2) = 9.7/ (9.81x10.82) = 0.01 (1.1)
d/(gTapp2
) = 61/(9.81x10.82) = 0.05 (1.2)
Figure 1.1: Wave Theory used in SACS for Analysis of Jacket Platform [34].