Liuhua 11-1 FPS Dry-dock Upgrade and Life Extension - Motion Analysis and Weather Window Selection for Towing
Authors: Luo Yong , Huang Shu Guang , Zhang Guang Lei , Yang Liang , Yang Hui 
Published: May 2013

Abstract
The Liuhua 11–1 FPS is a major component of the Liuhua 11–1 Development System, which is located about 200km southeast of Hong Kong in the South China Sea. For upgrade and life extension purpose, the FPS was disconnected from field in early November, 2011, and was towed to Guang Zhou Huangpu Shippbuilding Co., China, for dry-dock upgrade modification. And the upgraded FPS was wet towed back and reconnected at the field in May, 2012.
The towing motion performance of ‘Nanhai TiaoZhan’ semi-submersible platform was analyzed by using SESAM program, which is based on Morison's equation and 3D potential theory. According to the design maximum allowable roll or pitch amplitude of 6 degrees, the maximum allowable significant wave heights were calculated for a range of wave period based on a cyclic iterative method. Then suitable weather widow for the towing operation was recommended under the consideration of limited sea states, climate characteristic of South China Sea and the updating project schedule.

Introduction
The LH 11–1 oil field is located about 200km southeast of Hong Kong in the South China Sea in approximately 310m depth of water. LH 11–1 FPS, which named ‘Nanhai TiaoZhan’, is a major component of LH 11–1 oil field development. The FPS was a modified drilling semi-submersible converted from a Sedco 700 series drilling rig built in 1975 and in 1995, it was converted again to a floating production unit in Singapore and then deployed at LH 11–1 oil field, permanently moored by 11 mooring legs. The Unit is classed by American Bureau of Shipping (ABS). It was originally designed for a service life of 10 years (from 1995) and has been continuously operating for more than 15 years. At the same time, a new oil field Liuhua 4–1 (OTC-24042), about 11 km away northwest LH 11–1, was in development planning with the utilization of LH 11–1 FPS facilities through a subsea tieback. Therefore, additional topsides capacities and the life extension of this aged semi-submersible FPS became necessary. CNOOC Ltd, Shenzhen is the owner of this LH 11–1 FPS. COOEC was selected as the contractor for the engineering and construction support of this FPS dry-docking upgrading and life extension project. CSSC Guanzhou Huangpu Shipbuilding Co., Ltd was contracted by the owner as the dry-docking shipyard for the upgrading construction.
Because the environmental conditions of South China Sea are complexity and capricious, motion analysis and appropriate weather window selections for the disconnection, voyage and field reconnection operations became the key issues for making the whole project schedule. In this paper, the motion performance of the FPS during towing was analyzed by using 3D potential theory. SESAM program developed by DNV was used in the analysis. By using a cyclic iterative method in a frequency analysis to perform short term statics, limited sea conditions, which satisfied the maximum allowable roll and pitch amplitude not exceed 6 degrees, were obtained. Then a whole update modification project schedule was recommended according to the weather window selection of towing.