ABSTRACT

With the help of experimental results, this paper first describes the behavior of embedded mooring chains connected to a buried anchor point. The tests were carried out on three different sizes of chain in two clays with different undrained shear strengths. Subsequently, the effect of changing the effective chain widths (for calculating soil resistances) on the chain forces at the anchor point has been discussed with certain analytical results. The studies are believed to be useful to the practicing engineers.

INTRODUCTION

Over a few decades, the anchor pile-chain system is being widely used with the single point mooring (SPM) systems of CALM-buoy type. In addition, as the offshore activities are moving towards deeper sites having soft soil, it is inevitable that the anchor pile-chain system will find wider application in connection with the mooring system of floating platforms and articulated structures like Guyed Towers. Hence, it is understood that the present topic is of relevance to the design practices of the mooring system with buried anchoring arrangement.

The connection point between the mooring chain and the anchor pile, which is commonly known as "pile lug", is generally positioned a certain distance below the seabed level. The basic season behind such a design is to economise the anchor pile design, utilizing the following two important factors:

  • Embedded chain experiences a considerable amount of soil resistances which in turn, reduces the forces at the pile lug.

  • Embedded pile lug minimizes the bending stress level in the pile section.

The optimum design of the pile lug location is however a critical issue; and this is so due to the pull-out and pile driving considerations. These two considerations can well be satisfied if the pile lug is positioned near the seabed level. On the contrary, the economy in pile design is achieved if the pile lug is located around the mid-height of the pile. Therefore certain trial and error approach is adopted in practice to obtain the most suitable lug depth, which may not essentially be an absolute optimum solution.

The analysis and design of an anchor pilemooring chain system involves the analysis of a part of the mooring chain (referred here as embedded chain) considering all probable soil-chain interactions. Basically, during the initial chain tensioning operation, the embedded chain length (which indeed varies with the line tension) starts cutting the soil medium deeper and moves away from the anchor pile. During this chain movement inside soil two types of soil resistance are developed. These are :

  • resistance against cutting or bearing resistance, and

  • resistance against sliding or sliding resistance.

These resistive forces from the soil medium can be idealized as normal and tangential distributed forces along the embedded chain length, and can be expressed in terms of undrained shear strength of soil (Su) as discussed by several authors (1 – 8).

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