Installation, Testing, And Commissioning Of A Disconnectable Turret Mooring For Fsou Vessel In A Typhoon-Prone Area
Authors: Hall R.A. , Etheridge C.O. , Poranski P.F. , Boatman L.T. , Kawase Masaki 
Published: October 2008

A disconnectable turret mooring system and FSOU vessel have been installed for the LUFENG 13–1 Oil Field Development operated by JHN Oil Operating Co. in 142 meter water depth in the South China Sea. This facility is very unique by virtue of the tolerances maintained throughout the installation process, including high holding power anchor placement to final vertical and horizontal location of the combination steel and syntactic foam Spider Buoy. Adjustment of the anchor legs (combination chainwire- excursion limiter-wire-chain) to compensate for variances in the individual anchor drag distances is discussed with recommendations given for consideration in preparing geotechnical survey specifications for areas where the soils characteristics are lesser known. Adaptation of equipment to the methods needed to perform the weather sensitive installation are discussed.

INTRODUCTION/OPERATIONAL SCENARIO

JHN Oil Operating Company specified a tanker based, floating storage and offloading unit (FSOU) for installation in the Lufeng 13–1 Oil Field in the South China Sea, located approximately 100 miles ESE of Hong Kong, and lying in 142 m(meters) of water. A 124,000 DWT tanker was selected and purchased by JHN for conversion into the self propelled FSOU, NANHAl SHENGKAI. MODEC, Inc. was awarded the conversion contract with SOFEC, Inc. awarded the design of the disconnectable turret mooring system. Refurbishment of the tanker began in August of 1992. In December 1992 conversion of the tanker began with installation of the "on-vessel" mooring equipment accomplished during April-July 1993. Installation of the "offvessel" mooring components, Spider Buoy and Riser was done on location concurrently with the shipyard installation of the "on-vessel" turret mooring system.

The specification for the disconnectable system required the FSOU to be capable of remaining moored unless threatened by an imminent typhoon. Evaluation of normal winter monsoon storm conditions required a mooring system to be designed to remain connected up to an 8 m significant sea state.

With the FSOU disconnected, the "off-vessel" mooring components were required to survive a 14.4 m significant sea state.

In event of a typhoon exceeding the connected design basis, the disconnection sea state of 7.4 m significant was chosen.

Typhoon activity at the FSOU site is primarily in the August through October time frame. However, typhoon activity in winter, during the typical monsoon storm season, has occurred six times in the past fifty years. Therefore, to minimize loss of production following a possible winter disconnect, the re-connection sea state of 3.5 m significant was selected.

Operational states of the FSOU can be differentiated into four distinct modes:The primary operational state, which also incorporates the offloading event, is to be moored, riser connected and receiving produced oil. In the prime typhoon weather window the FSOU propulsion system can be operational in 12 hours.During disconnection preparation, production is stopped, The FSOU is prepared to sail. The riser is flushed, disconnected, and stowed. The operational status of the mooring disconnection system is verified.