Advanced Structural Integrity Assessment of MOPU Structure
Authors: Kar S. , Ahmad R. S. , Rahman A. , Zubair A. U. 
Published: November 2024

The site-specific assessment analysis has been performed on the Jack-up Rig converted from a 3-legged Mobile Offshore Drilling Unit (MODU) to a Mobile Offshore Production Unit (MOPU). The MOPU will connect to eight conductors with wellhead structures. The main goal of this study is to verify the reliability of the existing rig current structure can manage the modified topside weight after conversion to a production platform. Additionally, we aim to ensure the adequacy of the MAT foundation to support leg stresses under extreme environmental conditions.

Normally stick-beam model of hull, leg and mat structure is done in the industry for structural analysis which have lots of assumptions. Also, the bottom of mat use pin boundary condition ignores the soil effects. The leg to mat connection does not model properly and fatigue in this high stress areas becomes a problem. Application of hydrodynamic loads in the submerged part also a significant problem due to multiple number of load cases.

At present three-dimensional detailed finite element model featuring shell and beam elements has been developed for the structural analysis using Finite Element Analysis (FEA) software. The rig structure has been modelled in Structural Analysis Software for environmental load computations. An automated macro-based system developed in-house is then used to transfer the environmental load into finite element model. Significant engineering efforts worth noting includes the design of the A-shaped MAT foundation, whereby the non-linear behaviour of load displacement within the foundation system is integrated into the overall finite element model, where the unrealistic assumptions of conventional pinned and linear spring foundation is able to be avoided. The dynamic response of an offshore MOPU to environmental loads has been investigated where the model includes realistic representation of topside loads, added mass, geometry of legs and MAT connection and non-linear soil elements.

The dynamic amplification factor (DAF) is estimated with respect to various wave time-periods. Fatigue life assessment for welded tubular joints of a jack-up leg has been numerically assessed as part of mitigation for platform lifetime. The analysis procedures are presented for numerical fatigue assessment methods based on the S-N curve approach for ABS standard utilizing the spectral (stochastic) method. The objective is to verify the leg of a jack-up platform has the appropriate fatigue life referring to the expected design life. The results are interpreted to ensure the structures fitness for purpose.

The study also addresses the overturning stability of the unit, preloading conditions and the structural integrity of the legs under storm loading. The evaluation, however, did not cover the strength of the hull because the jack-up is currently in compliance with ABS Class and its Class certification remains in effect. The site-specific structural strength has been evaluated considering all relevant load conditions and foundation behaviour, including leg strength for transportation condition in dry and wet tow. The overall integrity of the MOPU has been analysed for in-place, dynamic, fatigue and transportation. The details of the findings are presented further in this paper.

This paper presents an effective integrity assessment procedure for MOPU that satisfies class and code requirements. The assessment ensures that the structure is fit-for-purpose and optimizes modification requirements. A notable feature and achievement of this work is the software program development for load transfer between two different software, detailed shell modelling, and foundation soil modelling using spring. Additionally, the software program development for fatigue analysis is a significant accomplishment. The integrity of the new MOPU structure achieved for specific life extension after modifying the weak parts of the structure such as gussets and part of the legs.