Prediction of hydrodynamic loading on a mini TLP with free surface effects

This paper describes the extension of a fluid-flow simulations method to capture the free surface evolution around a full-scale Tension Leg Platform (TLP). The focus is on the prediction of the resulting hydrodynamic loading on the various elements of the TLP in turbulent flow conditions and, in particular, on quantifying the effects of the free surface distortion on this loading. The basic method uses finite-volume techniques to discretize the differential equations governing conservation of mass and momentum in three dimensions. The time-averaged forms of the equations are used, and the effects of turbulence are accounted for by using a two-equation, eddy-viscosity closure. The method is extended here via the incorporation of surface-tracking algorithm on a moving grid to predict the free-surface shape. The algorithm was checked against experimental measurements from two benchmark flows: the flow over a submerged semi-circular cylinder and the flow around a floating parabolic hull. Predictions of forces on a model TLP were then obtained both with and without allowing for the deformation of the free surface. The results suggest that the free surface effects on the hydrodynamic loads are small for the values of Froude number typically encountered in offshore engineering practice.     

Elucidating the dynamics and mixing agents of a shallow fjord through age tracer modelling

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing, transport time scales of fresh water runoff and estuarine circulation in relation to external forcing.Using age tracers released at surface and bottom, we quantify the time scales of downward mixing of surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord, whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004.The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind, it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord.We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus, our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events.     

Coupling of life cycles of the copepods Calanus chilensis and Centropages brachiatus to upwelling induced variability in the central-southern region of Chile

A time series of zooplankton sampling carried out at Station 18 off Concepción (36°S, 73°W) from August 2002 to December 2003 allowed the study of annual life cycles of the copepods Calanus chilensis and Centropages brachiatus in association with environmental variability in the coastal upwelling zone. Changes in the abundance of eggs, nauplii, and copepodids were assessed from samples taken at a mean time interval of ca. 20 days. Upwelling variability in near-surface waters was reflected in seasonal changes in salinity, water column stratification, and oxycline depth, as well as a weak seasonal signal in sea surface temperature (1-2 °C). Both copepods exhibited similar life cycles, characterized by continuous reproduction throughout the year. Estimates of generation times, as a function of temperature, were 25-30 days for C. chilensis and 27-35 days for C. brachiatus, predicting about 12 and 10 generations a year, respectively. These estimates were consistent with reproduction pulses observed in the field. It was thus suggested that copepods may grow under non-limiting food conditions in this upwelling area. However, despite continuous reproduction, there were abrupt changes in population sizes along with the disappearance of early naupliar and copepodid stages taking place even during the upwelling season (spring/summer). These changes were attributed to sudden increases in mortality taking place in spring or early summer, after which the populations remained at low levels through the fall and winter. It is thus suggested that, in addition to variability in the physical environment, biological interactions modulating changes in copepod mortality should be considered for understanding copepod life cycles in highly productive upwelling systems.     

Møre Margin: Crustal structure from analysis of Expanded Spread Profiles

Analysis in both the x—t and —p domains of high-quality Expanded Spread Profiles across the Møre Margin show that many arrivals may be enhanced be selective ray tracing and velocity filtering combined with conventional data reduction techniques. In terms of crustal structure the margin can be divided into four main areas: 1) a thicker than normal oceanic crust in the eastern Norway Basin; 2) expanded crust with a Moho depth of 22 km beneath the huge extrusive complex constructed during early Tertiary breakup; 3) the Møre Basin where up to 13–14 km of sediments overlie a strongly extended outer part with a Moho depth at 20 km west of the Ona High; and 4) a region with a 25–27 km Moho depth between the high and the Norwegian coast. The velocity data restricts the continent-ocean boundary to a 15–30 km wide zone beneath the seaward dipping reflector wedges. The crust west of the landward edge of the inner flow is classified as transitional. This region as well as the adjacent oceanic crust is soled by a 7.2–7.4 km s^–1 lower crustal body which may extend beneath the entire region that experienced early Tertiary crustal extension. At the landward end of the transect a 8.5 km s^–1 layer near the base of the crust is recognized. A possible relationship with large positive gravity anomalies and early Tertiary alkaline intrusions is noted.     

Crustal Structures of the Northernmost South China Sea: Seismic Reflection and Gravity Modeling

The South China Sea (SCS) is a marginal sea off shore Southeast Asia. Based on magnetic study, oceanic crust has been suggested in the northernmost SCS. However, the crustal structure of the northernmost SCS was poorly known. To elaborate the crustal structures in the northernmost SCS and off southwest Taiwan, we have analyzed 20 multi-channel seismic profiles of the region. We have also performed gravity modeling to understand the Moho depth variation. The volcanic basement deepens southeastwards while the Moho depth shoals southeastwards. Except for the continental margin, the northernmost SCS can be divided into three tectonic regions: the disturbed and undisturbed oceanic crust (8–12 km thick) in the southwest, a trapped oceanic crust (8 km thick) between the Luzon-Ryukyu Transform Plate Boundary (LRTPB) and Formosa Canyon, and the area to the north of the Formosa Canyon which has the thickest sediments. Instead of faulting, the sediments across the LRTPB have only displayed differential subsidence offset of about 0.5–1 s in the northeast side, indicating that the LRTPB is no longer active. The gravity modeling has shown a relatively thin crust beneath the LRTPB, demonstrating the sheared zone character along the LRTPB. However, probably because of post-spreading volcanism, only the transtension-shearing phenomenon of volcanic basement in the northwest and southeast ends of the LRTPB can be observed. These two basement-fractured sites coincide with low gravity anomalies. Intensive erosion has prevailed over the whole channel of the Formosa Canyon.     

Applied hydrodynamic wave-resistance computation by Fourier transform

An applied Fourier transform computation for the hydrodynamic wave-resistance coefficient is shown, oriented to potential flows with a free surface and infinity depth. The presence of a ship-like body is simulated by its equivalent pressure disturbance imposed on the un-perturbed free surface, where a linearized free surface condition is used. The wave-resistance coefficient is obtained from the wave-height downstream. Two examples with closed solutions are considered: a submerged dipole, as a test-case, and a parabolic pressure distribution of compact support. In the three dimensional case, a dispersion relation is included which is a key resource for an inexpensive computation of the wave pattern far downstream like fifteen ship-lengths.     

光生物反应器中光衰减特征与螺旋藻生长动力学研究

分析了光照强度在光生物反应器中的分布特征，结果表明，当光波长及光传播的路径确定时，光生物反应器中光衰减特征主要受培养物生物量浓度的影响，由回归的模型对实验数据的拟合可分析光衰减特征与培养物生物量浓度的相关性，为光生物反应器中平均光照强度的确定奠定基础。在光生物反应器中，当营养底物和环境温度不是螺旋藻生长限制因子时，通过平均光照强度对螺旋藻比生长速率的影响分析，结果表明，在实验条件下，螺旋藻比生长速率与平均光照强度的动力学模型可用Aiba光生长抑制方程描述，光亲和系数Ks为238.29umol/(m^2.s)，光抑制系数Ki为0．00493s.m^2/umol，光生物反应器中螺旋藻生长的饱和光照强度出现在190－272umol/(m^2.s)的范围内。