Research On The Flow Characteristics And Vortex-induced Motions Of Deep-draft Semi-submersibles

The semi-submersible platform is one of the most reliable and cost effective structures for offshore oil and gas exploration in deep water.In comparison to a conventional semi-submersible,deep draft semi-submersible(DDS)concepts are known for their favorable ver-tical motion characteristics in wave environments.However,due to the elongated submerged column and enlarged projected area to current,a DDS is subjected to higher in-line drag forces and transverse lift forces in a current environment,resulting in significant vortex-induced mo-tions(VIM).This phenomenon may pose a threat to safe and reliable operations and severely affect the fatigue life of risers and mooring systems.Therefore,the VIM of semi-submersibles has emerged as an important issue in offshore engineering.In the present paper,experimental and numerical investigations are conducted on flow char-acteristics of four-column structures and the DDS,the VIM behavior of DDS,and the effects of some aspects relevant for VIM.Furthermore,a new semi-submersible design for improved vortex-induced-motion performance has been developed.Model tests for flow around four circular-section cylinders and four square-section cylin-ders in a square configuration are carried out in a closed circuit low-speed wind tunnel.Three different column spacing ratios and four array attack angles are considered to investigate the parametric influences.Additionally,three-dimensional(3-D)numerical simulations on flow around four circular-section cylinders are conducted by employing the Large Eddy Simulation(LES)method.It is shown that the mean drag forces acting on the downstream cylinders are lower than those on the upstream cylinders,while the downstream cylinders would experience higher fluctuating forces due to the effects of the vortices shedding from the upstream cylinders.The two forms of section arrangements have a similar trend in their drag coefficients with the current heading,but the fluctuating lift coefficients at different current headings range widely.In order to investigate the pontoon effects on the flow characteristics of the DDS,3-D numerical simulations using LES method are performed for flow around a DDS with four pon-toons,a DDS with two pontoons and a four-column structure with no pontoons.It is found that the drag forces acting on each column of the four-column structure are larger than those for the four-pontoon DDS and two-pontoon DDS.For 45~?current heading,the DDS with two pontoons experiences larger lift force than the other two configurations,due to the asymmetry of two pontoons and directed into the undisturbed flow.An experimental study on the VIM performance of the DDS is conducted in a towing tank,with different current headings and different draft ratios of the immerged column(H/L).The equivalent mooring system,i.e.,four soft horizontal lines with springs,is employed to provide the horizontal restoring force for the model.An air bearing system is developed in order to model the vertical pretension arising from the mooring and riser system and to model the mass ratio exactly.This setup ensures that the model is allowed to move freely in the horizontal plane,and vertical motions of the model are restrained.The experimental results reveal that the lock-in phenomenon for the DDS occurs in the range of 5≤U_r≤8.The most significant transverse motions are observed at 30~?and 45~?current headings.However,the yaw response for the 0~?current heading shows the largest amplitude.The VIM occurs primarily along the semi-submersible diagonals,except for the 0~?current heading.Additionally,the largest transverse amplitudes of the semi-submersible at 1.12≤H/L≤1.90 are nearly the same.When the draft ratio of the immerged column decreases to 0.87,30%decrease in the peak transverse amplitudes can be seen.Aiming to examine the influence of pontoons on the VIM of semi-submersibles,the VIM behaviors of another two models,i.e.,a DDS with two pontoons and a four-column structure with no pontoons,are also investigated experimentally.The present investigations indicate that the presence of pontoons delays the onset of VIM to a higher reduced velocity in the cases of the four-pontoon DDS.Additionally,the four-column structure without pontoons shows the most pronounced transverse responses and yaw motions.Compared with the four-pontoon DDS,the two-pontoon DDS can generate larger lift forces,corresponding to more significant motion in the transverse direction at higher reduced velocities.Moreover,the VIM model tests are carried out for a four-diamond-column structure to compared with the results of the four-square-column structure(i.e.,the four-column structure with no pontoons).It is pointed out that owing to the same incidence angle of each column,the four-square-column structure and the four-diamond-column structure show a similar transverse response regardless of the current heading for the whole structures.When the columns are positioned 45~?to the incident current,both of the structures show the largest transverse amplitudes.3-D numerical simulations by Detached Eddy Simulation(DES)method are performed on the VIM response of the DDS.The numerical results using DES are in good agreement with the experimental measurements obtained from the towing model tests.Based on this,the VIM performance of various semi-submersibles with different column shapes are investigated numer-ically to check the effect of the column corner radius(R_c/L).A number of column shapes are considered,such as the square column with sharp corner,the square column with rounded corner and the circular column.The numerical results show that the transverse amplitudes at the 0~?cur-rent heading are twice as large as those at the 45~?current heading for the semi-submersible with circular columns.However,the semi-submersible with rounded square columns shows more significant transverse motions at the 45~?current heading than those at the 0~?current heading.Furthermore,for the 45~?current heading,the transverse amplitudes of the semi-submersibles show a sharp increase as the column radius increases for a range of R_c/L<0.1.When the col-umn radii are around 0.1≤R_c/L≤0.2,the peak values of transverse amplitudes for different semi-submersibles are nearly the same.When the column radii increase to R_c/L≥0.3,the trans-verse amplitudes of the semi-submersible decrease as the column radius increases.Additionally,for the 0~?current heading,the transverse amplitudes for different semi-submersibles increase as the column radius increases except for the semi-submersible with sharp square columns.The influence of the pontoon shape on the VIM of semi-submersibles is also investigated based on the VIM study of different semi-submersibles with different columns,taking into account two different pontoon shapes,i.e.,the sharp rectangle and the rounded rectangle.It is found that the effect of pontoon shape on the transverse response for the semi-submersible with sharp square columns is negligible.On the other hand,the sharp rectangle pontoons mitigate the VIM re-sponse of the semi-submersibles both with circular columns and with rounded square columns.In order to study the effect of the column draft ratio,the VIM behaviors of three typical semi-submersibles,i.e.,a semi-submersible consisting of sharp square columns and sharp rect-angle pontoons(namely SQC-SQP),a semi-submersible consisting of rounded square columns and rounded rectangle pontoons(namely SRC0.15-SRP0.1),a semi-submersible consisting of circular columns and rounded rectangle pontoons(namely CC-SRP0.1),are investigated with different draft conditions by employing DES numerical method.These results show lower amplitudes for lower draft conditions.For the 45~?current heading,the transverse responses of the SQC-SQP semi-submersible are negligible at H/L≤1.30,while the SQC-SQP semi-submersible shows significant transverse motions at H/L≥1.40.The transverse responses of the SRC0.15-SRP0.1 semi-submersible and CC-SRP0.1 semi-submersible are pronounced even at the low draft condition(H/L=0.51).Based on the sensitivity studies of above aspects relevant for VIM,a new semi-submersible design consisting of pentagonal columns and rectangle pontoons,is developed.3-D numer-ical simulations using DES method are conducted to evaluate the VIM response of the new semi-submersible.Compared with the original semi-submersible,the transverse amplitudes of the new one decrease approximately 49%.The improved VIM performance of the new semi-submersible is confirmed.