Performance Of Suction Anchor Mooring System During Its Installation And In Service

The South China Sea, as one of four sea petroleum and gas accumulation places in the world, is very rich in marine resources, and the average water depth is more than 1000 meters. With the increasing exploration of marine resources in the deep-water zone of the South China Sea. more effective station-keeping systems are required in order to limit the motion of floating structures and lower the construction and installation cost. Taut or semi-taut moorings (TMS) generally contain several short multi-component mooring lines, which are made of steel chain, wire rope and synthesized fiber rope. TMS has been more and more used in the deep-sea anchoring instead of catenary moorings (CMS), because TMS could provide more effective anchoring performance and only need much shorter length of the mooring line. Yet, a larger elevation angle (about 30°from the horizontal) will occur at the anchor point near the seabed, which correspondingly brings higher requirement on the vertical uplift capacity of the anchor, making the suction anchor widely used in the TMS. The design, installation and bearing mechanism of suction anchor involve the crossing field of ocean engineering and geotechnical engineering, and many problems still need to be studied.During the installation and the in-service state of suction anchor, there are several important and valuable problems, such as the special suction penetration method, the set-up effect of soil around the anchor (including reconsolidation and thixotropy), the behaviour of mooring line during pretensioning and in service, and the bearing capacity and failure mode of the suction anchor. Aiming to study these key problems from the aspects of theoretical analysis, numerical calculation and model test, we have carried many detailed investigations and discussions, which are listed as follows:1. On the basis of a self-design model experimental system, several small scale tests were carried out in the soft clay successively, including the anchor penetration with a steady velocity, the set-up process and the vertical pullout of model anchor. Afterwards, we thoroughly analyzed the influences of the initial penetration (under dead-weight) depth of model anchor, seabed soil resistance, inner suction and the development of soil heave. Based on the API method, the meaning of required and allowable suction inside the anchor has been studied, and the feasibility and advantage of a new caisson installation technique, called "intermittent pumping", has been investigated. Then, the dissipation of excess pore pressure in the surrounding soil was measured and recorded, and then the effect of set-up on the vertical uplift capacity of suction anchor could be assessed. Finally, the different forces acting on the model anchor during its pullout process was discussed in detailed and perceived.2. A novel quasi-static three-dimensional (3D) numerical model suitable for the analysis of the whole mooring line has been presented in this paper, in which the mooring line was divided into a number of infinitesimal segments along the coordinate axis. This numerical model contained two major sections of the mooring line. One section was sustained in the seawater, and the other was embedded in the seabed soil. By using this model, the line profile and tension distribution during pretensioning were firstly calculated, and then the 3D behaviour of mooring line in service could be obtained. The calculation covered both the pretensioning and in-servive states of the mooring line, during which the interaction between the mooring line and the seabed was thoroughly investigated. Then, the response of one mooring line was obtained. Finally, the performance of a typical suction anchor mooring system (containing eight mooring lines) was studied, and the relationship of the top offset versus the horizontal restoring force was obtained.3. These model tests aimed to investigate the bearing capacity and failure mode of the suction anchor used in TMS in service. When the elevation angle of line tension at the optimal loading point changed from 30°to 50°typically, the short-term static capacity and failure mode of model anchor were invetigated and analyzed. Especially, we have detailly studied the effect of previous cyclic loading (from 0.01 HZ to 0.1 HZ) on the short-term static capacity and failure mode of model anchor, as well as the effects of some key parameters, such as the amplitude, the period and the number of cyclic loading.