New Insights on Ageing Asset with Wave-In Deck Due to Subsidence Challenges
Authors: Chow W. , Hoo C. , David D. , Lim J. , Ho D. 
Published: February 2024

Abstract
Wave-in deck (WID) due to subsidence has long been a concern to many ageing assets in Malaysia. As the oil and gas business continues to mature, so are the assets, and often, simple solutions are not adequate to address the issues due to the complexities involved. Therefore, there is a need to utilize the latest life-extension solutions to ensure the hubs are fully operational without compromising on safety and integrity. This paper describes the implementation of the latest methods on Structural Reliability Assessment (SRA) and WID action, involving probabilistic assessment and creation of computer programs capable of performing time-history WID simulations. Platform life-extension involves several unique challenges, especially when it involves platform subsidence leading to WID action. One of the latest computational techniques currently available is Lagrangian Momentum Absorption (LMA) method, (Swan, 2018). This method is then combined with non-linear structural dynamic responses in terms of dynamic amplification factors to estimate the platform probability of collapse using integration of load & resistance variability method. SRA is used to assess the risk of structural collapse where a step-by-step procedure from simplified linear elastic analysis to detailed nonlinear pushover analysis and subsequent probabilistic estimation of platform probability of collapse are discussed. With WID action, itself a huge topic, SRA becomes even more complicated. The presence of WID actions changes the wave loading acting on the overall structure completely and this could then result in a different platform failure mechanism compared to the conventional pushover approach. Some established methods recommended by ISO 19901-9:2019 to study the WID behavior are through wave-basin tests and using Computational Fluid Dynamics. They are however requiring an elaborated set up and computationally expensive. This paper present analytical LMA approach, (Swan, 2018) using scripted MATLAB, (Mathworks, 2019) spreadsheet. This method is not only quick but allows various parametric sensitivity studies to be performed. LMA method can consider various topside features in computing the WID loads as the wave passes through the deck. Consequently, a greater understanding of the level of influence from various topsides features on the magnitude of the load absorbed by the topside and its failure mechanism can be established. However, there is important things to be noted that LOADS JIP is still under development. The SRA methodology discussed in this paper utilizes a combination of the latest available technologies in evaluating ageing platforms e.g., deriving WID loads using LMA method and deriving global inertial loading for jacket dynamic response using nonlinear dynamic analysis. Although the technologies itself are not novel, the methodology is one of the first being implemented in evaluating ageing platforms in Malaysia. Through this paper, the authors aim to share this methodology with the industry and improve the reliability of these ageing assets.