The Development of Deepwater Riser Recovery Technology in Liuhua Oil Field
Authors: Zhu Shaohua , Xizhao Jiang , Rong Pingsheng , Gang Luan 
Published: July 2009

In the May of 2006, the three flexible risers of the FPSO " Nanhai Shengli?? at the Liuhua oil field in South China Sea were broken and dropped into seabed due to Typhoon Chanchu, thus terminating the oilfield production. In order to recover oil production as early as possible, the recovery, repair and re-hookup of the three damaged risers from seabed became the first priority. However there was lack of such riser recovery experiences in deep water. This is the first offshore practice of this kind in the history of offshore industry. Some special tools including Riser Clamp, Riser Clamp Support Frame etc were developed, designed and fabricated upon several times of trial-and-error tests and improvements. In the May of 2007, these purposely developed tools were successfully used to recover two 13.5?? production risers from the water depth of 300 meters and resumed oil production in the July of 2007. The riser recovery technology provided an excellent experience for the offshore industry.

This paper presents the design and tests of riser recovery tools, the improvement of the tools after trial and error to satisfy riser recovery in deep water, and the methodolody of riser recovery.

Introduction

Upon a direct hit by Typhoon Chanchu in the May of 2006, two 13.5?? production flexible risers and one 6?? test riser broke away from the turret of the FPSO " Nanhai Shengli?? and dropped into the seabed in a water depth of 300 meters at the Liuhua oil field, which terminated the oil production. The risers were seriously damaged where 40m sections of the original length and the steel wire inside was pulling out from the broken end of the risers. Fig. 1 shows the damaged condition of the 13.5?? riser laid on the seabed. This further enhanced the difficulty of offshore recovery of the damaged risers. Due to the shortage of new riser replacements, a quick decision of recovery and repair of the damaged risers was made by CNOOC soon after the typhoon in order to resume the oil production as early as possible. COOEC was awarded this challenging work to recover and repair the damaged risers from deep water with the help of various offshore technologies. The new risers will not be available until mid 2008 to replace the old ones.

Technical Difficulty of Riser Recovery

This was the first recovery and repair of fully damaged risers in such deep water and in such large scale. Although there are some recent research and development of deepwater pipeline repair system, refer to Reference1. There is no systematic development of riser recovery and repair method for damaged risers, as well as how to plan damaged riser lifting and cutting operations for such specific emergency repair in deep water. The engineering design and offshore operation has proved to a very challenging work. The major difficulties of damaged riser lifting and cutting technology are described as follows: Design of Riser Recovery Clamp: There was no lifting point on the broken risers which require more than 60 tonnes recovery pulling force for each riser. The riser recovery clamp must satisfy the requirement of providing adequate pulling tension while preventing any damage on both outside and inside layers of the risers. Therefore the design of the Riser Clamp became the key point of riser recovery.