Purpose: This paper aims to propose a practical method of performing maintainability analysis of an offshore system at operation phase having some improvement trend. Design/methodology/approach: The analysis follows a systematically developed method of analyzing maintenance data, identifying critical factors affecting system performance, and developing suitable downtime distribution model with some applications of statistical analysis techniques and expert opinion. Findings: Improvement in spare part logistics is found significant in reducing downtime thus should be well feedback to design and plant engineers so that it can be incorporated in new offshore system. The downtime models developed based on the steady state analysis and expert input are found to be practical for prediction of the system maintainability performance. Research limitations/implications: The analysis focuses on the downtime which includes the repair, logistics and administrative delay time. At the operation phase, plant personnel are mostly interested in the availability and downtime performance of the system thus this study provides an excellent example of how the analysis can be done practically and effectively. Practical implications: Maintainability analysis at operation phase is crucial to assess and predict the maintenance system performance and provide valuable feedback to the design phase and existing plant for further improvement. The methodology developed here is practical hence can assist plant personnel to perform maintainability analysis effectively. Originality/value: This paper present a generic method of analyzing maintainability at the operation phase. The proposed distribution method via steady state and expert input approaches provides a practical method for formulating downtime distribution model when improvement trend exists. © Emerald Group Publishing Limited.