A novel dropped object risk assessment method for offshore operations

Dropped object risk assessments quantify the risk caused by accidentally dropping objects from topsides of a fixed offshore platform onto the seabed or to the platform itself or from an installation vessel to subsea. The risk assessment evaluates both the likelihood of the dropped object scenario and its consequence. Often a risk matrix is used in reaching mitigation decisions, i.e. high impact frequency and high consequence events require more attention. This paper presents an overview of the dropped object risk in offshore lifting/drilling operations and current practice of risk assessment in oil and gas industry. A practical approach is proposed in which a two-stage Monte Carlo simulation is used to estimate the impact frequency for potential targets such as upper decks, jacket legs, risers, mooring lines, and pipelines on seabed. The proposed approach is an extension to the DNV approach which does not take into account the randomness of dropped locations on the sea surface. The two-stage Monte Carlo simulation estimates impact probability at different levels along the depth of the platform from the sea surface to the seabed. The consequence analysis is carried out using advanced nonlinear finite element analysis to reduce conservatism in simplified approaches. A case study of dropped objects to demonstrate the proposed methodology is presented. This paper also discusses asset risk assessment for dropped objects in offshore drilling and installation and proposes recommendations to common practice.