Soliton Currents In The South China Sea: Measurements And Theoretical Modeling

ABSTRACT This paper presents detailed soliton information that has been used in engineering design for the Liuhua Field. Results are derived from extensive measurements with Acoustic Doppler Current Profilers made at several sites in the Liuhua area. Extreme value estimates of maximum velocities were projected from the measurements. Maximum credible currents were specified based on Richardson number stability limits. Theoretical modeling has produced descriptions of soliton internal velocity fields that match measured data reasonably well. The model has been used to analyze riser and cable loads and the behavior of a shuttle tanker hawser-moored to a storage tanker. A numerical ray-tracing model produces refraction pattterns that closely match those of satellite images. ENGINEERING CONCERNS Oil was discovered in 1987 at Liuhua (Fig. 1, Site A) in a water depth of 305 meters. Design of a floating production system currently is underway. It consists of a semisubmersible moored over a subsea manifold, which is connected to a turret-moored tanker by a two-kilometer pipeline. Although typhoons principally govern extreme value metocean design loading, solitons also present substantial engineering concerns. Oil companies working northeast to northwest of Liuhua have been surprised by solitons and experienced equipment damage during tanker operations and a platform installation. The Liuhua design team has focused on soliton riser and cable loads and impacts on shuttle tanker and subsea operations, particularly those involving the BOP stack and remote operated vehicles (ROV). Influences on pipeline towing (from a beach site north of Liuhua) and installation also are being analyzed. To mitigate some of the operations concerns, a soliton warning/monitoring system is being planned. SOLITONS IN THE SOUTH CHINA SEA Overview Solitons are solitary internal waves which exhibit remarkable coherence and permanence, and have strong associated currents. In the northern South China Sea, such waves are generated by tidal forcing at a shallow sill in the Luzon Strait. These solitons travel westward some 350 nautical miles to the Liuhua area, with transit times in the range of 2 to 4 days (mean celebrities of 3.5 to 7.5 knots). Sixty miles east of Liuhua they are refracted around Pratas (Dongsha) Island, creating a complex pattern of wave fronts as illustrated in Fig. 2. The characteristics of solitons are governed by water depth and the vertical density structure of the ocean. A sudden disturbance of the normal density distribution, as at a tidal sill, leads to the formation of a group or packet of solitons. Packets have been observed in every month of the year. During some months, packets arrive at Liuhua about every 12 hours. There may be perhaps one to six in a packet, with the strongest one generally arriving first. Individual solitons measured at Liuhua have periods of 10 to 30 minutes. Instantaneous profiles of horizontal currents in solitons at Liuhua look somewhat like the letter S, for a propagation direction toward the left. Commonly, they have leftward maximums of 50 to 150 cm/sec at a depth of 20 to 100 meters. Speeds at the sea surface are typically half of the maximum. Speeds reverse direction at about mid-depth. Below this point, they are toward the right and have a maximum about two-thirds that of the upper maximum, acting within 50 meters of the seafloor.