Liuhua 11-1 FPS Dry-dock Upgrade and Life Extension - Fatigue Assessment Challenges and Solutions
Authors: Yin Han Jun , Liu Bo , Cai Yuanlang , Wang Jim , Zhong Wenjun 
Published: May 2013

The Liuhua 11–1 FPS is a converted SEDCO 700 series semi-submersible mobile offshore drilling unit (MODU) buit in 1970's. It was converted to a floating production unit and installed in the Liuhua 11–1 field in 1995. The development of a nearby marginal field through a subsea tie-back to the FPS requires additional topsides capacities and life extension of another 15 years for this aged semi-submersible. This dry-dock upgrading project presented unique technical challenges, particularly for fatigue life assessment of the floater hull and deck structures which has been well beyond their originally intended design service life.

The fatigue life assessment must consider the various loading conditions of past service histories of the semi-submersible platform, which include fatigue life prediction with increased deck payload loads and updated metocean criteria. In addition to the challenges of the complex loading histories, the existing conditions of hull and deck structures, which have various degree of local corrosions, and some details significantly differ from the as-built drawings, must be accurately represented in the global and local structural elements. Detailed field survey was conducted to obtain the actual wall thickness of the structural elements, which were used along with the 1995 conversion as-built drawings to develop a detailed finite element model of the entire hull and topside structures. This paper discusses these challenges and the solutions from structural fatigue evaluation philosophy, analysis methodology, past and current design codes interpretations to fatigue design premise and analysis results for the Liuhua 11–1 semi-submersible FPS life extension project.

Introduction

The LH 11–1 oil field is located about 200km Southeast of Hong Kong in South China Sea at approximately 310m water depth. It was one of the major deepwater developments adopting new technologies with unique features in early 1990's in offshore industry. LH 11–1 FPS, which named ‘Nanhai TiaoZhan’, is a major component of the LH 11–1 oil field development. This FPS was intended to perform the vessel station keeping function in a severer environment, to provide the drilling and well completion functions of horizontal wells, the power supply function for downhole electrical submersible pumps, and the supporting function for the installation and maintenance of LH 11–1 subsea production system. The FPS was converted from a Sedco 700 series mobile offshore drilling unit (MODU) West stadrill. The west stadrill had provided service in North Sea as drilling unit for 18years from 1975 to 1993 till it was selected as the basis for the Nanhai Tiao Zhan. Now 15 years later, the Liuhua 11–1 field is still producing. A new oilfield Liuhua 4–1, about 11km away from Liuhua 11–1, was planned to be produced by the Liuhua 11–1 FPS facilities via subsea tieback, thus requiring additional topsides capacities and life extension of another 15 years for this aged semi-submersible FPS. The FPS has been planned to be dry-docked for major upgrade in 2011. Once again before dry docking a Life time extension strategy is established and the comprehensive detailed structural fatigue analysis is implemented.