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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.

The Frontal Eddy Dynamics (FRED) experiment was conducted offshore North Carolina between Cape Fear and Cape Hatteras from May through November 1987. Frontal eddies propagating northward along the North Carolina shelf break were observed during a 3-week intensive shipboard and aircraft survey phase, followed by a six-month (long-term) mooring and remote sensing monitoring phase. Using observations of a frontal eddy named Abbott from the intensive phase, it has been established that a frontal eddy cold dome can propagate out of the South Atlantic Bight past Cape Hatteras. The focus here therefore is on frontal eddy observations in the long-term data set in the immediate vicinity of Cape Hatteras. The long-term moorings indicate that 39 northward propagating frontal eddies were observed immediately south of Cape Hatteras, giving a recurrence interval of once every 4-5 days. Two types of frontal eddies are identified, dependent on the bimodal character of the amplitude of the parent Gulf Stream meanders. During the small-meander mode, the mean Gulf Stream front is approximately aligned with the 100-m isobath, and the small warm filaments rarely penetrate beyond mid-shelf. During the large-meander mode, the mean Gulf Stream front is displaced offshore, but the larger warm filaments often penetrate onto the inner shelf. The duration of each meander mode at Cape Hatteras is 2-3 months, with rapid (several day) transitions between the modes. The transition to the large-meander mode appears to be related to local interactions with cold core rings in the Sargasso Sea rather than the offshore deflections of the Gulf Stream at the Charleston Bump. The intense observations of Eddy Abbott occur during the small-meander mode, demonstrating that even when the Gulf Stream is closer to the confining topography of the continental shelf, frontal eddy cold domes can propagate beyond Cape Hatteras and form new warm filaments in the Middle Atlantic Bight.

Effects of the Emperor SeamountChain on the thermohaline structure and baroclinic flow are investigated on the basis of historical hydrographic data. The amplitudes of dynamic height perturbations are 3 to 5 times larger west than east of the chain. The intensity of the thermal fronts is stronger west than east of the seamounts; near the crest of the southern seamounts, strong east-west thermohaline fronts and a strong northward baroclinic flow are observed. The Kuroshio Extension west of the seamount chain is a well-defined meandering current, the axis of which generally lies between 33 ø and 36øN. The available data indicate that the Kuroshio Extension turns northward and then flows eastward through the gaps of the seamount chain. East of the seamounts, the Kuroshio Extension widens threefold and appears to be poorly defined. University of California, Oceanic Observations of the Pacific, the NORPAC Data (R/V Tenyo Mary and R/V Umitaka Mary), 532 pp.,