Multi-Directional Seas - A practical Tool for Evaluating the Feasibility of Marine Operations

Abstract

The timing and feasibility of installing skyscraper sized structures in the ocean depends strongly upon limitations dictated by motion response of complex combinations of heavy 11ft vessels, barges and the structures to be installed, all Joined by elastic tethers. Exposure of these systems to long crested and Irregular seas in a model test does not produce a complete picture of the response to a realistic environment. In several recent model test programs to support the assessment of the feasibility of Installing large structures, short crested seas have been generated and imposed on combinations of semi-submerged vessels and jacket structures in partial contact with the bottom. In some cases, the expectation that short crested seas would result in less severe system loads and motions was not realized. This paper will discuss the methodology employed to set up the complex physical simulation of lowering a 10,000 + ton offshore tower and mating It to a predrilled template in a North Sea environment. The techniques used to generate the multi-directional seas will be presented and the resulting functions characterizing the measured model sea will be presented and compared to the desired. The responses of a complex moored system of structure and semi-submersible lift ship to irregular and short crested seas will be presented and compared.