| As exploration and production of hydrocarbon resource progresses into increasingly deep waters, traditional structures are being replaced by floating systems for exploration and production. Such structures required economical and reliable anchorage systems. Suction anchor is among the most promising of such anchorage systems. Suction anchors are large, vertical pipes, closed at the top and open at the bottom. They are installed by pressure drawdown within the cylinder, referred to as 'suction', after partial penetration of the cylinder due to its dead weight. There are the following advantages to select the suction anchors: (a)fixed location on seabed which is important in congested sub-sea development, (b)simple installation procedures with no need for proof load testing at the site, (c)there is no particle limitation in water depth or installation, (d)reasonably well-established design methods compared to other types of anchor, even though no standard Code of Practice is yet available.Generally, undrained conditions are relevant in the field because of the nature of loads from wave, current and wind, that are rapidly applied relative to the drainage times in cohesive seafloor sediments. The loading directions on the mooring anchors vary with concept from primarily vertical uplift force for the tension leg platform (TLP), to horizontal loading for a catenary moored floating production system, to inclined loading for a taut moored spar. Since the 1970s, several researchers have worked to obtain better understanding of suction anchor behavior by means of field tests, laboratory tests, analytical method and numerical simulations.This thesis consists of three parts. Firstly, the required underpressure of suction anchor installed in uniform soft ground is numerically calculated, and the results by FEM are compared to the prediction by simplified formula in engineering. The effects of adhesion factors on the outside and inside of the anchor are also considered. Secondly, the ultimate capacity of suction anchors in normally consolidated and lightly overconsolidated clays is numerically predicted by the 3-D finite element analysis. Several parameters are explored including the anchor aspect ratio, location of load attachment point, the interface friction ratio and the formation of a crack. Maximum anchor capacity is investigated for a range of combined vertical and horizontal loading and the optimum failure envelopes are plotted. Thirdly, the results of simplified upper bound method are compared with the finite element results discussed previously.It is concluded that: (1)The failure mechanisms of the soil contain sliding failure and reversed bearing capacity failure in case of penetration by underpressure, which is depended on the soil resistance. (2)The optimal centre line loading depth Zc is largely independent of the direction of loading. In this thesis it is approximately 0.7L. (3)Failure envelopes for suction anchors under inclined loading for varying length to diameter ratio are described by a simple hyperbola curve. (4)Interface friction affects the size of failure envelope, but not the shapes. (5)Vertical capacity is not affected by crack formation while the horizontal capacity is considerably affected, especially for in lightly overconsolidated soils. (6) Results of the study compares well with existing simplified method for estimating load capacity of suction caisson anchors. |
