A statistical method for the evaluation of offshore heavy lift operations characterized by a low hoisting velocity, is presented. The resulting expressions give the probability of undesired events occurring during the operation. These are identified in this case as snatch loads from the lifting cables and/or post lift-off contact with the transport barge. The probabilities have been expressed in closed form, in terms of Gaussian probability integrals. The method has been applied to a lifting operation, not commonly used for offshore lifting operations, where a bridge is hoisted by linear jacks. The expression for the probability of no undesired events, which can be computed in the design phase of the operation, is given.
For many offshore heavy lift operations, particularly those carried out by large semisubmersible vessels, the limiting seastate for the operation is generally governed by initial lift-off from the transport barge. The limitation is introduced to avoid serious snatch loads and/or post lift-off contact between the barge and the object being lifted. In order to achieve the maximum possible lift-off speed during this phase of the operation, a combination of crane hoist speed and high capacity ballast dumping systems are commonly used. However, there are still cases when the separation speed and distance are "not large enough to ensure with any degree of confidence, that such undesired events will not happen. For example when lifting extremely heavy loads, close to the hook capacity, the separation speed produced by the combination of ballast change and crane hoist may be too low. Or similarly, when using alternative hydraulic lifting arrangements, such as the method described in the present paper. With low lifting speed in marginal sea conditions the number of snatches and/or contacts may be considerable and a rational method for estimating this number of events is required.
