Prediction of Motions and Loads for Floatover Installation of Spar Topsides

Abstract

There are several substantial advantages to installing integrated topside onto a Spar using floatover method, particularly for large topsides which exceed the single lift capacity of the available heavy lift derrick barge fleet. These advantages include schedule and cost savings for the integration and commissioning of modules on land rather than at sea. Uncoupling the deck fabrication schedules from the availability of heavy lift vessels is another advantage. The performance of a successful floatover installation requires adequate design and analysis of each phase of the floatover installation, and a sufficient weather window in which to perform each phase. Design of floatover installation includes: a) Global motions / mooring analysis to determine motions and loads on mooring lines, fenders, and structural members, b) Structural design including structural integration of the topsides with the barges and shock cell design on the Spar and barges, and c) Operational procedures for mating and barge separation. Validated analysis tools are essential to ensure adequacy in the design of all stages in the floatover operation. This paper presents data from floatover installation model tests, performed at OTRC (Texas A&M University, College Station, Texas, USA), and results from numerical analysis tools for motion and load predictions. The scale of the model tests was 1:60, and the simulated topside was approximately 18,000Te. The simulated environmental conditions included expected upper limit operational sea states for the Gulf of Mexico. The details of the model tests are described in Ref [1]. The analytical challenges related to floatover installation simulations are several and include multi-body hydrodynamics, and prediction of relative motions and interface loads during the mating operations. Available numerical analysis tools include the time domain multi-body proprietary code MLTSIM, and WAMIT, a frequency domain potential code that is widely available in the industry. The validation of MLTSIM involves viscous damping, multi-body hydrodynamic interaction, and simulation of impact forces. This paper presents the results from the validation on the basis of full scale, and quantifies the accuracy of predictions by comparing the measured and predicted motions and loads.