Analysis and Design Consideration of Greenwater Impact on Decks and Topsides of FPSO
Authors: Wang Mark , Leitch John , Bai Yong 
Published: October 2008

Investigation of greenwater occurrence on the Terra Nova FPSO, included a model test program, extensive analysis as well as a series of numerical simulations. Wave period (spectral peak period), Vessel heading, and Vessel draft were the three most important parameters identified to have significant effect on greenwater occurrence. Their effects on greenwater occurrence frequency and height are extensively analyzed. Variation in greenwater height and the frequency of occurrence along Vessel's longitudinal and transverse direction was also investigated. The findings were used for design and engineering of the Terra Nova FPSO and may be useful for design consideration of greenwater problem for other FPSOs.

To further assess the possibility of greenwater occurrence, a simple probability analysis was performed. Such analysis can help decision making in a quantitative way. The analysis effort concentrated on the heading distribution which is a critical part of the probability assessment.

A practical procedure is outlined for the design consideration of greenwater. The procedure is based on current industry practice and experience with the Terra Nova FPSO design. Some future possible improvement and development to this procedure is also proposed and discussed.

Introduction

Greenwater has been recognized by the offshore industry and the offshore research community as a particular problem for FPSOs (References 1 through 5). The damage caused by greenwater includes that to walkway ladders, deck equipment pipe insulation, equipment room, switch room compartments, structures, watertight doors, cable trays, etc (Reference 1).

The Terra Nova Alliance has paid great attention to the greenwater problem in the course of design and engineering of the Terra Nova FPSO. This was necessary because of the harshness and complexity of the environment on the Grand Banks of Newfoundland, where the FPSO will be moored for 20 years of her operation life. Details of the environment at the Grand Banks can be found in References 7 and 8. Figures 1 and 2 show pictures of the Terra Nova FPSO and Figure 3 shows an illustration of the Terra Nova field.

A procedure for design consideration of greenwater impact on FPSO was developed, which is a combination of model tests, analysis, numerical simulation and probability analysis. This procedure was used for the Terra Nova FPSO project. A condensed version of this procedure is presented in this paper

The Model Test Program

A series of model tests were carried out at the National Research Council Canada's Institute for Marine Dynamics in the Spring of 1998 with a model at scale of 1:44. The principal dimensions of the FPSO are given in meters in Table 1. The test model is shown in Figures 4 and 5. Like the FPSO, the test model was equipped with a thruster assisted mooring systems, consisting of 5 azimuthing thrusters and a disconnectable turret mooring system with nine lines. A total of 13 flexible risers were also modeled in the model tests. References 9 and 10 present some details of the model test program.