Environmental Load Factors and System Strength Evaluation of Offshore Jacket Platforms

Jacket platform acts as base for overall structure which is used to extract hydrocarbon from oceans. They are generally suitable for shallow and intermediate waters with a depth of (<150 m), but they have been built for depths like 529 m such as Bullwinkle by Shell in Gulf of Mexico. Structural design methodology of civil engineering design codes has changed from allowable stress design to limit state (load and resistance factor) design. Codes for structural design go through changing process, and whenever a new finding is reported and verified, it is incorporated in the code.

Offshore platforms are only 65 years old and are fairly new compared to other types of civil engineering structures. The first steel platform was installed in Gulf of Mexico (GOM) in 1947. In this chapter, brief overview of the past work done in this area is outlined. American Petroleum Institute (API) was the first to publish the code for offshore Jacket platforms, namely API RP2A WSD in 1969. API LRFD was published in 1993 with errata in 2003 and has not yet been revised. ISO 19902 was published in 2007 and is the most updated LRFD code available for steel Jacket platform design today.

Probabilistic models are defined for uncertainty, variability and probability distribution functions. These depend on measured data and statistical procedures. After defining basic variables, which have influence on failure of components, joints and system, failure functions are defined for each one of these in the form of ultimate limit state equation. Finally, system strength is evaluated at design and extrapolated higher load.

Resistance or strength parameters in LRFD are taken as random variables. Uncertainty modelling is the most important step for reliability analysis. The random variables are analysed, and range of type of distributions, mean values and standard deviations are discussed.

This chapter deals with statistical data analysis for load variables. The random variables are analysed with statistical distributions evaluated. The parameters of load distributions like mean and standard deviations are determined. An extrapolation up to 1,000 years of return period is made using Weibull and Gumbel distributions along with their parameters.

Offshore platforms are only 67 years old and are fairly new compared to other types of civil engineering structures. Offshore Jacket platforms in Malaysia are designed using API RP2A Working Stress Design (WSD) code. API WSD code has proved its effectiveness and has been in use for long time, but it needs to be changed into load and resistance factor design (LRFD)-based code which is being followed by all building code agencies. In place of WSD, limit state design or LRFD has proved to be more rational as it considers probabilistic models. The reliability of Jacket platforms is maintained in API RP 2A-LRFD by setting target safety factor the same as that provided in WSD, which means structures designed as per LRFD code will have the same reliability as API RP 2A-WSD (which has already provided safe structures and the best available practice for design). When adopting LRFD methodology, the appropriate load and resistance factors can be optimised through the process of calibration. Knowledge of the strength equations in the different codes and the similarities and differences between them is useful for the calibration. The first step in the calibration process is the determination of reliability of structural tubular members of the Jacket designed as per existing practice of WSD and LRFD code. In this text, API RP 2A-WSD code and International Standard Organization (ISO 19902) (LRFD-based code) are taken into consideration for the reliability analysis. The relevant strength equations of three codes are identified and compared, and the similarities and differences are determined for tubular members which are the main part of Jacket structures.

Behaviour of structure can be measured by probability of failure or reliability index. Target reliability of components is found as per API RP2A WSD. Load factors are developed in such a way that the reliability index of Jacket is at predefined target level. The reliability indices of ISO LRFD design for a range of load factors are determined. When ISO load factors are plotted against the corresponding API target reliabilities, the intersection point gave the proposed load factor. Not only the Jacket reliability designed as per new load factors will be higher than the target, but also it will ensure safer Jacket.

Due to critical nature of joints, API and ISO code recommend them to be stronger than components. The joint types are K-, T/Y- or X-joint, and they are classified as based on the geometry and loads acting on the member. The joints are analysed for four types of stresses. Their respective environmental load factors have been determined and reported.

Codes applicable to Jacket platforms, such as API WSD, API LRFD and ISO 19902, are based on component and joint design. If Design codes are followed properly, the strength of member will always exceed the load effect as utility ratio is always maintained less than one while designing the Jacket. Codes consider overall structural integrity, redundancy and multiple failure paths only indirectly by using structural integrity assessment methods. Before going for reassessment in this chapter environmental load factor has been evaluated using system reliability.

Jacket platforms are frequently checked when loading and resistance parameters are changed or at the end of design life and if hydrocarbon reserves are still there to be extracted, it must be checked for extension of life. Therefore, the probability of failure is used to check its strength at all the stages. Bayesian updating is a technique to be used for updating probability of failure taking into consideration probability of failure.

The codes of practice for API WSD and ISO LRFD are used in this book to find the reliability of Jacket platforms in Malaysia. ISO code uses LRFD methodology and its benefits are shown. Its advantages are highlighted for design of Jackets in offshore Malaysia. ISO code requires that environmental load factors should be location dependent. This code also requires that resistance and load uncertainty should be determined. Based on actual uncertainty, environmental load factors are ascertained using component, joint and system reliability. Reassessment of Jacket strength is performed using design load probability of failure with 10,000-year return period load. Probability of failure is updated using Bayesian updating technique with higher loads. The main conclusions, findings and achievements are listed in the following sections.