Hydrodynamic Performance Analysis Of The Taut Mooring System For Deep Sea Truss Spar Platform

Truss Spar is widely used for offshore deep water oil/gas exploration.The structure of truss not only helps to reduce the use of steel,but also to increase the stability.The taut mooring system is widely used for the following advantages:smaller mooring radius,lighter total weight and better anti-corrosion performance.Using statistics of environmental parameters of the South China Sea as the design condition,on one hand,this paper analyze the performance of taut mooring system on Truss Spar platform working in the water in the depth of 2000m,and then finish optimizing parameters of mooring system by comparing the results,and also combine the engineering experience with the platform configuration.On the other hand,determine the semi-taut mooring system layout scheme and compare the performance of these two types.After studying on the parameters of mooring system,optimizing ideas and comparing different schemes through coupling calculations,regulations of platform motion responds and mooring line tension response are acquired,and it can be a reference for subsequent design on deepwater mooring systems.When analyzing in frequency domain,to obtain more accurate hydrodynamic parameters,on one hand,consider the impact of fluid inside the moon pool,on the other hand,use TUBE unit and DISC unit to conduct equivalent effect disposal and analysis the damping effect of hard capsule,soft capsule and the heave plates with AQWA NAUT calculating module.It is found that the results are reasonable and reliable by comparing the numerical results with the designed natural period of the platform.Mooring lines consist of steel chains and polyester fibre cable,then determine the preliminary parameters of the taut mooring system.In the dynamic time domain coupling simulation,according to API RP 2SM and detail material properties which BRIDON presents,this paper uses the post installation stiffness to calculate the displacement of Spar,and uses the storm stiffness to calculate the forces of mooring lines.In the process of optimizing parameters of the mooring system,first sort the parameters according to their impacts on material cost,then analyze the effects of top angle,fireleads height,length distribution of chain and polyester cable,diameter,amount,angle between two cables in one group.The designed scheme not only can better control the cost but also has good performance.Finally in four kinds of sea environment,the mooring line tension response and six degrees of freedom motion response under the condition of tact and damaged of taut mooring system are obtained,and all results meet the requirements of the rules.After finishing determining the scheme of semi-taut mooring system,then especially focus on comparing the effects of two types on displacement.By overlapping horizontal motion trajectories,compared with the semi-taut type,the taut type has smaller horizontal displacement radius and can reduce the extreme value by more than 50%.In the case of breakage,the taut mooring system can effectively control the low frequency slow drift motion of the platform better than the semi-taut type.