Green Water Evaluation for FPSOs in the Gulf of Mexico
Authors: Buchner B. , Voogt A.J. , Duggal A.S. , Heyl C.N. 
Published: October 2008

Estimation of green water occurrence and the associated loading on exposed structures is an important design consideration for FPSOs [Floating (Production) Storage and Offloading] in harsh environments. As FPSO concepts are being developed for deep and ultra-deep water Gulf of Mexico, this phenomenon must be understood for hurricane conditions and designs developed to alleviate its impact.

This paper provides a mechanism for understanding and evaluating the green water phenomenon for FPSOs in harsh environments, both at the bow of the vessel and along its sides. The paper utilizes recently developed analysis and design methodologies, and presents criteria specifically developed for the Gulf of Mexico hurricane environment. A case study is used to discuss an FPSO for the Gulf of Mexico designed to resist green water occurrence, and the development of design environmental criteria from a longterm response analysis. The case study is also used to illustrate the design methodology developed, and to show the sensitivity of the green water loading as a function of freeboard exceedence allowed. Finally the paper presents guidelines that provide a strong foundation for the evaluation of green water for FPSO concepts in the Gulf of Mexico.

Introduction

With the expansion to deep and ultra-deepwater developments in the Gulf of Mexico, several Floating Production System (FPS) concepts are being considered to safely and effectively exploit the fields. FPSO [Floating (Production) Storage and Offloading] systems are a mature floating system technology for harsh environments, having been successfully deployed in the North Sea, the Grand Banks, and the South China Sea. These systems are attractive for the deepwater Gulf of Mexico, especially where the pipeline infrastructure is limited or non-existent. This is demonstrated by the interest in this floating production technology over the past five years and the fact that they are considered in most concept selection studies for the deepwater Gulf of Mexico.

The main focus of most floating deepwater production facilities is on the subsea, mooring and riser systems, and other challenges associated with deep and ultra-deepwater. However, some important aspects in the design and operation of Floating Production Systems (FPS) are still related to the interface with the environment, especially in harsh environments. For FPSOs this applies to the weathervaning performance, tandem offloading with shuttle tankers, vessel motions, and green water loading.

Obviously offloading to shuttle tankers and weathervaning performance are FPSO specific issues and do not apply to other FPS. However, vessel motions and green water related issues are shared between all floating systems, albeit to different degrees. A major design consideration for platforms in the Gulf of Mexico is the wave run-up and deck overtopping of deep draft caisson and semi-submersible platforms, and the airgap between wave elevation and the bottom of the deck for Tension Leg Platforms (TLP), semisubmersibles, and jacket platforms.