The semi probabilistic codes, API LRFD and ISO 19902 have environmental load and resistance factors based on calibration in GOM or North Sea as shown in Table 1.1.
These are the areas of hurricanes (typhoons in Pacific Ocean) and severe winter
storms respectively. Structural safety requires that, required strength (R) should be greater than the design loads (Q). The uncertainties of load and resistance considered were based on the local conditions of these regions. To cater for the requirements for other regions of the world it is necessary to develop local factors considering their own geographical environment. This was the reason the probabilistic evaluation of environmental loads have been done recently in China and Indonesia [5], [6] to check the influence of LRFD code. Therefore it is high time that this issue should be looked into for offshore Malaysia. Malaysia lies within 7 degrees north from equator which is considered to be safe against extreme storms. Using local geographical and fabrication uncertainties, this work proposes the modified environmental load factors for components and joints. The reassessment of platforms as per requirement of ISO and API nowadays is based on evaluating probability of failure of 10,000 year return period of load. There is a need to update this probability of failure considering probability of survival using Bayesian updating.
Table 1.1: Load factors used for Calculating the Internal Forces [7]
Governing Conditions Partial Action Factors
γD γL γW
Operating 1.3 1.3 1.0
Extreme 1.1 1.1 1.35
1.3.1 Problem Background
An efficient design of a structure needs a balance between material and risk cost [8].
Platforms are designed to resist three kinds of loads to which they are subjected namely: a) Environmental loads i.e. wave, currents and wind, b) Dead loads i.e.
weight of structure, and c) Live loads i.e. weight of consumable supplies and fluids in pipes and tanks. API RP2A WSD forms the basis of offshore steel Jacket platform design all over the world and has proved to be accepted design standard since it was first issued in 1969 [9]. WSD is based on factor of safety which is derived from working stress design theory and reduces the ultimate resistance strength to allowable stress for safe design. In WSD, minimum resistance is based on test results of yield strength and for load it is based on past experience, thus safety factor was inherent in these codes though not apparent. Since the loads/resistances are varying, the
assumptions used in WSD design process i.e. a single factor of safety for all load combinations cannot maintain a constant level of structural safety [10].
WSD and LRFD codes differ essentially in that LRFD uses more factors of safety which produces more uniform safety levels [9], [11]. In modern day structural design, LRFD codes have replaced WSD codes like AISC, API, ACI and AASHTO. The load and resistance factors in LRFD need to be checked for site specific conditions due to change of geography and material fabrications. Thus LRFD method brings out regional differences in variation to design based on extreme and operating conditions.
This is more relevant in case of offshore structures where the environmental loads are much varying in nature and are most of the times not normally distributed. This results in variability of loads and affects the structural reliability, measured by reliability index (β).
API and ISO code use GOM and North Sea, geographical environmental parameters for calibration with severe environmental conditions. When this code is used for design of Jacket platforms in less severe environment the design becomes uneconomical. API RP2A WSD is the design code in practice for design of offshore Jacket platforms in Malaysia. PETRONAS Technical Standards (PTS) provides necessary input with regard to metocean parameters for offshore Malaysia [12].
Therefore design environment criteria for platforms in South East Asia is taken based on GOM criteria and thus there is amplification of 60% during platform design due to scarcity of data [13]. High environmental load factors used in this region, due to short lead time between discovery of hydrocarbon and platform design, can result in waste of economical resources. Due to these factors it is extremely essential that actual environmental load factors should be ascertained for this region using component, joint and system reliability.
The change of loading and resistance conditions and need for extension of life of Jacket currently requires checking for probability of failure of 10-4. This method considers only failure probabilities and thus if a Jacket cannot take a load of this magnitude, re-strengthening is required which may incur huge cost. If probability of survival is also included in this analysis, the re-strengthening may not even be required.
1.3.2 Problem Motivation
It has been accepted worldwide, that LRFD method is not only more reliable but also allows the environmental load factors, to be established based on geographical locations [7], [9], [14], [15], [16], [17]. The critical part in structural design of members is assigning the properly evaluated environmental load and resistance factors. The motivation for the present study has come from the need to establish the load factors for Jacket platforms in Malaysia keeping in view the local environmental and fabrication consideration. Such factors can contribute for ISO 19901 and 19902 regional annex in particular and offshore industry in general for the efficient design of Jacket platforms in offshore Malaysia.
Bayesian updating has been suggested by Ang, Nowak and many other authors [18], [19]. This is a useful tool where low probability of failure is of importance. It considers probability of failure by taking into consideration probability of survival.
Its benefit for Jacket platforms have been highlighted in a recent work in North Sea [20].
1.3.3 Problem Description
Proper evaluation of metocean parameters is still being investigated in GOM and North Sea, so that their prediction can be made effectively. The metocean data being used has still large coefficient of variation (COV) in GOM and North Sea. The metocean data bank in Malaysia is still in its infancy. ISO 19901 is blank with regard to variables of environmental load parameters for South China Sea. The code proposes that data should be collected by each country itself. There is a great need not only to analyse this data but also to check the environmental load factor. To evaluate the component and joint environmental load factors for Malaysia, we have to find seven types of component stresses and four types of joint stresses specified by API and ISO codes.
Bay’s theorem is very useful for updating of probability of failure using probability of survival. When we apply the environmental load on Jacket platform, the responses can be determined. Using these responses, the probability of failure could be
evaluated. If this load is higher than what ISO code recommends, and if the Jacket can still survive, this information could be used to find updated probability of failure.
1.3.4 Previous Work and Limitation of Existing Studies
The load factors have been evaluated in GOM, North Sea and work on establishing metocean parameters is still in progress in this area, as more data becomes available [21]. The work in two region of Jawa and Makassar (Indonesia) has also been reported [6]. In China, Duan et al. [22], has done research on developing combinations of environmental load factors for China. Sakrit [23] has done reliability analysis of Jacket platforms in Gulf of Thailand using onshore data. The work on reliability index for Jacket platforms is reported in PMO region of Malaysia [24], [25]. There is a need for an extensive study covering all the three regions of Malaysia to determine the environmental load factors for components, joints and system.
Offshore industry practice for reassessment of Jacket is based on finding probability of failure of Jacket using ISO and API code requirement. An extrapolated 104 years environmental load is applied and probability of failure is calculated. If this probability of failure gives a return period less than 10,000 years, modifications or re- strengthening of Jacket is required. This method can be improved if not only probability of failure is considered but also probability of survival is taken into consideration. When both are combined, the probability of failure decreases considerably at higher loads [20]. The application of Bayes theorem has only recently been finding application for reliability analysis. Bayesian updating of probability of failure on Jacket platforms in this region has never been conducted and there is a need to conduct this study to avoid costly modifications.
1.3.5 Justification of the Research
The justification for finding environmental load factors for Jacket platforms for Malaysia can be attributed to the following reasons. The main justification came from the ISO 19900-1 which says that for each geographic region, environmental load factor should be evaluated specifically for that region. ISO 19902 clause A.9.9.3.3
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.