Liuhua 11-1 FPS Dry-dock Upgrade and Life Extension - Integrity Evaluation and Strengthening Solutions
Authors: CAI YUAN LANG , WANG JIM , YANG LIANG , LIU BO , ZHANG YI GONG 
Published: May 2013

The original Liuhua 11–1 project was the first deepwater development project in China located at more than 1000ft water depth. It established one of the major deepwater developments adopting new technologies with some unique features in early 1990's in offshore industry. 15 years later in 2011, Liuhua 11–1 field is still in production. At the same time, a new oilfield Liuhua 4–1, about 11km away from Liuhua 11–1, was in planning with the intention to utilize Liuhua 11–1 FPS topsides facilities via a subsea tieback scheme. Therefore, it was required to increase the topside capacities plus a life extension of another15 years for this aged semi-submersible FPS. There were significant technical challenges in this life extension and dry-dock upgrading project for the project team. Comprehensive strength and fatigue analyses of the hull and the topsides structures were performed and local structures, that required to be strengthened, was identified for the dry-dock repairs and refurbishments.

The strength assessment considered all loading conditions such as wet tow, docking, inplace conditions in which the FPS would experience the whole procedures from the disconnection to the reconnection at the field. The field survey results, including actual corrosion status and fatigue damages, were considered. Detailed hydrodynamic and structural finite element models of the FPS hull, deck and tubular braces were developed to ensure that the global behavior of the entire structure has been captured. Super elements and sub-modeling techniques were used to build the overall integrated model of the structure using DNV SESAM program. The strength analysis calculation results were evaluated for the extended service requirement. Some structural elements, dissatisfying the strength requirements, were identified and strengthened or renewed in this dry-dock upgrading project. This paper mainly introduced the strength analysis results.

Introduction

The Liuhua 11–1 oil field is located about 200km Southeast of Hong Kong in South China Sea at approximately 310m water depth. Liuhua 11–1 FPS, which named ‘Nanhai TiaoZhan’, is a major component of the Liuhua 11–1 oil field development. The FPS was a modified drilling semi-submersible converted from a Sedco 700 series semi-submersible mobile offshore drilling unit (MODU). This FPS was intended to perform the vessel station keeping function, under severer design environments, and other function such as the drilling and well completion function of horizontal wells, the power supply function for downhole electrical submersible pumps, and the supporting function for the installation and maintenance of Liuhua 11–1 subsea production system. In 2011, the Liuhua 11–1 field had been in production since 1995, exceeding its original 15-year design life with serious plate corrosion issues and aging equipment issues before this upgrading. At the same time, a new oil field Liuhua 4–1 (OTC-24042), about 11 km away in Northwest of Liuhua 11–1, was at a development planning stage with the intention to utilize existing Liuhua 11–1 FPS topsides facilities through a subsea tieback. Therefore, upgrading the topsides with additional capacities and the life extension of this aged semi-submersible FPS became necessary. CNOOC Ltd, Shenzhen is the owner of this Liuhua 11–1 FPS. COOEC was selected as the contractor for the engineering and construction support of this FPS dry-dock upgrading and life extension project. CSSC Guanzhou Huangpu Shipbuilding Co., Ltd was contracted by the owner as the dry-docking shipyard for this project.