Dropped Object Risk Assessment for Fixed Offshore Platforms
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Date
2018
Authors
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Mary Kay O'Connor Process Safety Center
Abstract
Dropped object risk assessment quantifies the risk caused by accidental dropped objects on potential targets from topsides of a fixed offshore platform to seabed. The risk assessment evaluates both the likelihood of the dropped object accident and its consequence. Often a risk matrix is used in mitigation decision, i.e. high impact frequency and high consequence events require attention. The potential targets from platform deck to seabed pipelines define three types of dropped objects analysis (DOA): Topsides DOA, Appurtenance DOA, and Subsea DOA. Topsides DOA involves the risk assessment for platform structural components and equipment while Appurtenance DOA includes any potential targets from the sea surface to the seabed, such as jacket legs. Subsea DOA is often of concern because of the high environmental and economic consequences as well as loss of human life, particularly gas release close to an offshore facility. This paper will give an over view of the dropped object risk in offshore lifting/drilling operations and how the risk is assessed in current practice of the oil and gas industry. Next, it will discuss a practical approach 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 two-stage Monte Carlo approach is an extension to DNV approach which does not take into account the randomness of dropped location on the sea surface. The two-stage Monte Carlo simulation estimates impact probability at different levels along the depth of the platform from sea surface to seabed. In the first stage, a random variable pair based on the drop point distance and angle with respect to the crane position is used. Crane extension is sampled from normal distribution, constrained by crane minimum and maximum radii. Crane rotation is sampled from uniform distribution, constrained by crane lifting arc. In the second stage, a probability distribution on level Z of the sea depth (including seabed) is used that is centered at the drop point on the sea surface. The point of impact is sampled using a normal distribution of the extension based on DNV-RP-F107 approach and a uniform distribution for the rotation angle. For the second stage, the parameters for the normal distribution of the extension radius change based on water depth, weight, and shape of dropped objects. The frequency of impact due to each dropped object is calculated by adding drop frequency and number of lifts per year. The accumulative impact frequency for jacket legs or pipeline is estimated by summing values along the length (taking integral). The consequence analysis is done by means of advanced nonlinear finite element analysis which is believed to remove the conservatism in simplified approaches. The paper seeks to discuss the asset risk assessment for dropped objects in offshore drilling operations in the oil and gas industry and proposes recommendations to common practice.
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Presentation
Keywords
Fixed Offshore Platforms