Gulf of Mexico's First Ship Shape FPU: Overview of the Global Strength Analysis
Authors: Al-Sharif Majid M. , Chakrabarti Partha , Bose Kaushik 
Published: May 2011

Normal procedure for assessing the hull strength of a ship shaped Floating Production Unit (FPU) is to use a rule based approach recommended by the classification societies. In this approach the vertical and horizontal wave bending moments and shear forces are calculated based on formulae prescribed in the rules. The difference between the hull's total carrying capacity and the rule prescribed wave related quantities provide the envelope of the still water moments and shear forces. The loading conditions are developed such that the still water envelope is not exceeded. Although this method can reasonably predict the longitudinal hull strength the following important issues are not addressed.The transverse strength of the hull.The stresses at the interface of the hull and add-on structures like living quarters, riser porch, heavy production modules etc which are common in a FPU.The stresses at the corner of large deck openings.

The above issues can only be addressed effectively through a Finite Element (FE) based approach. In this approach a coarse mesh integrated model of the hull including the add-on structures is prepared. The envelope values of the wave dynamic pressures, accelerations etc. are obtained from the formulae presented in the rules. These are then multiplied with appropriate factors also prescribed in the rules to obtain their simultaneously occurring values. These loads along with the gravity loads are first applied on a beam model representing the hull to obtain the actual moments at the target locations. The difference between the target moment and actual moment is applied as concentrated end moments to achieve the target values. The areas of high stress reported in the coarse mesh analysis are further refined and studied in a local analysis by employing boundary displacement methods.

In this paper the above procedure has been discussed in detail and selected results have been presented to establish the effectiveness of this analysis procedure.

A detail hydrodynamic based analysis is necessary for an optimum site specific strength assessment of a FPU. But a cost-effective, although slightly conservative assessment (since the rule based design strength quantities correspond to north Atlantic conditions) can be done by the rule based approach using finite element.

The authors believe that the methodology and results reported in this paper for a specific FPU will help in understanding the rule based analysis procedure of other ship type FPUs.

Introduction

A primary requirement of a floating vessel is to ensure the global longitudinal and transverse strength adequacy of the structure to resist the gravity, buoyancy, environmental and motion induced loads for the site specific environment.

For an ocean going vessel it is normal practice to use the Rule Based approach specified by the classification societies. The longitudinal strength of the vessel is characterized by the maximum moment and shear force at any cross-section of the vessel considering both the Stillwater and the Wave effects. In the rule-based approach, the longitudinal wave effects of the vessel are calculated using essentially prescribed empirical formulae without performing any hydrodynamic analysis for the waves.