Mathematical Modeling Of Bottom Current Observation Mooring’s Vertical Movement During Ascent On The Slope Of The SCS And An Analysis Of The Observation Results

Based on the Bottom Current Observation on the slope of the SCS Program, which starts from July 16, 2013, a 14-month-long observation into the hydrodynamics in(Mooring M1) and around(Mooring M3) a certain canyon is carried out utilizing bottom current observation mooring. A successive and reliable bottom current dataset form 3 mab to 44 mab is acquired via three deployments and recycles.Take the second(spring) and third(autumn) recycle of bottom current observation mooring at station M1 on continental slope of the Northern South China Sea in 2014 as an example, establishing mathematical model to analyze characteristic of mooring’s vertical movement during ascent. Firstly, assumptions that influence on mooring’s vertical movement from horizontal current can be neglected and mooring system can be divided into solid coupling ascent parts are put forward. Shapes of mooring’s main components can be simplified in addition. Numerical experiments of each solid coupling ascent part are carried out, which shows that every single component is able to keep its relative position unchanged and mooring system ascents as a whole. The Vertical Movement Model is constructed taking the mooring system as a rigid body and simulations are performed with the model respectively. Comparisons between model and observation results by high-precision USBL indicate that the ascent process can be divided into two stages, the accelerated ascent stage and the stable ascent stage. Model curve is closely match fitted curve. Difference between mean model velocity and mean observational velocity is merely 0.2 m/s. Model can reflect the characteristic that stable ascent velocity decreases with time slowly in the stable ascent stage. This is the consequence that mooring keeps adjusting its velocity to adapt to the surrounding seawater, whose density and viscidity varies with depth. The vertical velocity of each single component changes with oscillation. The Model can provide a quantitative reference for recycle work of related underwater equipment.With the in-situ data observred on the slope of the SCS, a primary analysis into the bottom current of these two stations is conducted. Firstly, a survey of the quality of data is carried out. The result shows that the depth variation of ADCP in the vertical direction comes mainly from water level and ADCP’s horizontal movement is not significant. ADCP is basically steady with respect to the seabed. Furthermore, the mooring is also basically steady with respect to the seabed. There is no mud deposition occurred around mooring. Observational data from ADCP, AANDERAA and other instruments is reliable. The time-varying charts and scatter diagrams of the observed current are drawn respectively. Statistical analysis, as well as joint probability distribution of the current is conducted. Finally, tide current is acquired from harmonic analysis on the current. Conclusions of the primary analysis into observational current are as follow. Bottom current at mooring M1(in the canyon) is kind of reversing current that basically runs along canyon trough. For the whole current profile, which ranges from 3 mab to 44 mab, the difference is small. The maximum velocity is no more than 50 cm/s. The average velocity ranges from 6 to 13 cm/s. It is an irregular semidiurnal tide as its tidal current type number is about 0.6 to 0.8. There exists the phenomenon that duration of flood and ebb is unequal from month to month. Topography here plays an important role in the formation of the current. Bottom current at mooring M3(around the canyon) on the open slope tend to be rotary. For the whole current profile, the difference is small. The maximum velocity is no more than 50 cm/s. The average velocity ranges from 8 to 12cm/s. The tidal current type number is about 4, which shows a feature of irregular diurnal tide to diurnal tide.