周松望(1976-), 男, 高级工程师, 主要从事海洋岩土工程研究, E-mail:
Zhou Songwang (1976-), senior engineer, main research interest: offshore geotechnics, E-mail:
Wang Dong (corresponding author), professor, PhD, E-mail:
当负压沉箱被用作深水管汇或管道终端基础时,除了受到上部结构传来的竖向力、水平力和弯矩,还会受到扭矩,扭矩可能降低沉箱的竖向承载力、水平承载力和抗弯能力。采用理论分析和有限元法研究复合加载条件下典型沉箱(长径比介于1~2之间)与正常固结黏性土的相互作用,考虑安装造成的沉箱侧壁周围土体的弱化,得到了不排水条件下沉箱单向最大扭矩,探索了扭矩与不同荷载分量联合作用时承载能力的改变。结果表明,当扭矩不超过20%的抗扭转能力时,可以忽略扭矩对竖向承载力、水平承载力或抗弯能力的影响;当扭矩介于抗扭转能力的20%~80%时,其他承载力分量最多降低20%。提出了能够用于工程设计的扭矩对其他荷载分量抗力的影响系数。
When suction caissons are used as foundation of the manifold or pipe terminal in deep waters, they are subjected to a torsion except for the vertical force, horizontal force and moment applied. The torsion may reduce the vertical, horizontal and moment bearing capacities of caisson foundation. In this study, theoretical analyses and finite element simulations are conducted to study the interaction between normally consolidated clay and the typical caisson (with a length-to-diameter ratio between 1 and 2) subjected to combined loadings. The strength reduction of the soil around the caisson skirt induced by installation are considered in the theoretical and numerical analyses. For caissons under undrained conditions, the uniaxial torsion capacity and the influences of the torsion on the other capacities are obtained. The results show that when it is less than 20% of the torsional capacity, the torsion applied has slight effect on the vertical, horizontal or moment capacities. When torsion applied reaches 20%~80% of the torsional capacity, the other three capacity components can be reduced by as much as 20%. The torsion influence factors against three capacities are proposed for routine designs.
负压沉箱常用作固定平台和海底管汇的支撑基础[
笔者采用有限元方法,模拟不同扭矩条件下
采用Abaqus软件建立沉箱与土相互作用的三维有限元模型。与已有的研究[
荷载与位移的方向规定
Positive directions of loads and displacements
有限元网格
Finite element mesh
中国南海、墨西哥湾和东南亚海域广泛分布正常固结黏性土,其不排水抗剪强度
其中:
在分析扭矩
沉箱顶盖底面与土体之间的摩擦对沉箱承载力影响很小。考察两种极端情况:1)顶盖底面与土体之间为tie连接,代表顶盖完全粗糙;2)顶盖底面与土体之间设置为光滑接触,代表顶盖完全光滑。对于3组典型工况,无量纲的扭矩
顶盖底面光滑程度对扭矩的影响
Influence of roughness of caisson cap on torsion
为验证建立的有限元模型的可靠性,将竖向承载力结果与Hu等[
竖向承载力结果对比
Comparison of vertical capacities
当沉箱发生扭转时,与外侧壁接触的土体首先被激发;随着转动增大,沉箱内侧壁和刃角深度截面处的土体逐渐发生变形,抗扭转能力取决于二者中先达到破坏的部分。假定内侧壁附近土体能够提供的扭矩为
采用极限平衡方法
1) 如果
2) 如果
有限元法与极限平衡法获得扭矩对比
Comparison of torsions predicted by finite element method and limit equilibrium method
沉箱竖向承载力
式中:
扭矩和竖向力的共同作用造成沉箱外侧壁土体破坏,所以,
式中:
通过式(9)和式(10),可以获得对应
当
扭矩作用下的竖向荷载—位移曲线
Vertical load-displacement curves with application of torsions
为简化设计应用,定义扭矩影响系数
由于实际应用中扭矩的安全系数一般大于1.5[
拟合公式效果如
扭矩对竖向承载力的影响
Effect of torsion on the vertical capacity
扭矩对水平荷载破坏模式的影响
Failure mechanisms under combined torsion and horizontal force
水平荷载—位移曲线
Horizontal load-displacement curve
扭矩对水平承载力的影响
Torsion effects on horizontal capacity
从
经拟合,扭矩影响系数
弯矩和扭矩共同作用下, 土体的破坏模式与
式中:
扭矩对弯矩的影响
Torsion effects on moment capacity
正常固结黏土中支撑管汇的负压沉箱长径比大多在1~2之间,采用理论分析和有限元方法探索了扭矩对沉箱竖向承载力、水平承载力和抗弯能力的影响。结果表明,扭矩会造成其他承载力分量的降低,降低程度与扭矩大小有关:对于长径比介于1~2的沉箱,在常规土体强度分布情况下(
LAURITZSEN R, SCHJETNE K. Stability calculations for offshore gravity structures[C]//Proceedings of the 8th Offshore Technology Conference, Houston, 1976: 75-82.
RANDOLPH M F, GAUDIN C, GOURVENEC S M, et al. Recent advances in offshore geotechnics for deep water oil and gas developments[J]. Ocean Engineering, 2011, 38(7):818-834.
FORESI A, BUGHI S. Suction pile foundation for a PLET subsea structure[M]//FORESI A, BUGHI S. eds. Frontiers in Offshore Geotechnics III. CRC Press, 2015: 245-250.
BUGHI S, PARKER E. Suction pile foundations: experience in theMediterranean offshore and installation feedback[C]//Proceedings of the 30th International Conference on Ocean, Offshore and Arctic Engineering, 2011: 951-963.
FINNIE I M S, MORGAN N. Torsional loading of subsea structures[C]//Proceedings of the 14th International Offshore and Polar Engineering Conference, Toulon, 2004: 326-333.
NOURI H, BISCONTIN G, AUBENY C P. Undrained sliding resistance of shallow foundations subject to torsion[J]. Journal of Geotechnical and Geoenvironmental Engineering, 2014, 140(8):04014042.
TAIEBAT H A, CARTER J P. Effects of torsion on caisson capacity in clay[C]//Proceedings of the 9th Australia New Zealand Conference on Geomechanics, Auckland, 2004: 130-136.
TAIEBAT H A, CARTER J P. A failure surface for caisson foundations in undrained soils[C]//Proceedings of the 1st International Symposium on Frontiers in Offshore Geotechnics, Perth, 2005: 289-295.
SUROOR H, HOSSAIN J. Effect of torsion on suction piles for subsea and mooring applications[C]//Proceedings of the 3rd International Symposium on Frontiers in Offshore Geotechnics, Oslo, 2015: 325-330.
SAVIANO A, PISANÒ F. Effects of misalignment on the undrained HV capacity of suction anchors in clay[J]. Ocean Engineering, 2017, 133:89-106.
GEROLYMOS N, ZAFEIRAKOS A, KARAPIPERIS K. Generalized failure envelope for caisson foundations in cohesive soil:Static and dynamic loading[J]. Soil Dynamics and Earthquake Engineering, 2015, 78(5):154-174.
SUPACHAWAROTE C. Inclined load capacity of suction caisson in clay[D]. Australia: University of Western Australia, 2006.
KAY S, PALIX E. Caisson capacity in clay: VHM resistance envelope-Part 2: VHM envelope equation and design procedures[C]//Proceedings of the 2nd International Symposium on Frontiers in Offshore Geotechnics, Perth, 2010: 741-746.
DET N V. Recommended practice for geotechnical design and installation of suction anchors in clay: DNVGL-RP-E303[S]. Oslo: Det Norsk Veritas, 2017.
HU Y X, RANDOLPH M F. Bearing capacity of caisson foundations on normally consolidated clay[J]. Soils and Foundations, 2002, 42(5):71-77.
HUNG L C, KIM S R. Evaluation of vertical and horizontal bearing capacities of bucket foundations in clay[J]. Ocean Engineering, 2012, 52(1):75-82.
American Petroleum Institute. Design and analysis of station keeping ystems for floating structures: API RP 2SK[S]. 3th edition. Washington, D.C.: American Petroleum Institute, 2005.