Suction Anchors: Some critical aspects for their design and installation in clayey soils
Authors: Dendani Hédi 
Published: October 2008

No specific design practices are recognized by the industry today for the design and installation of suction anchors. Only geotechnical consultants used design procedures. Some studies are in progress through API RP. These studies aimed at developing a state of the art and guidance regarding the design and installation of suction anchors to be included in future versions of API RP.

The results of recent installation of suction anchors in highly plastic clays demonstrated that some of the design procedures used by the geotechnical consultants are to be improved. This paper summarises how to improve them. Case histories are presented to illustrate such improvement.

Introduction

A suction anchors or caisson is a cylindrical unit with an open bottom and equipped with valve at its top. In a first step suction anchor penetrates under its self-weight, with free evacuation of the water located inside the skirts. In a second step, an additional driving force is created by pumping out the water entrapped between the anchor top and the soil plug. After reaching the final penetration the valve is generally closed in order to increase the pullout resistance (Fig.1 and 2).

The suction anchors are generally used as main foundation and anchoring system in deep water fields. The suction anchors are currently selected due to the following advantages (Colliat, 2002):fixed location on seabed which is important in congested sub-sea development,simple installation procedures with no need for proof load testing at the site,there is no particle limitation in water depth or installation,the capacity of suction anchors can be defined more precisely than for drag anchors.

Fig.1 Forces acting on the suction anchor during penetration (AVAIALBLE IN FULL PAPER)

Fig.2 Girassol FPSO anchors on barge (Colliat and Dendani, 2002)(AVAILABLE IN FULL PAPER)

The paper presents the following critical aspects for the installations and design of suction anchors:The soil data should be well investigated. An integrated study including geohazards evaluation, geotechnical in-situ tests, high quality sampling, advanced laboratory tests should be conducted in order to select the location of the anchors and to establish the geotechnical design parameters.Particular attention should be taken concerning the effect of the stiffeners on the skirt penetration resistance. The stiffeners may reduce the penetration resistance by reducing the interface strength along the skirt wall above the stiffeners. Based on installation results a simplified approach is proposed in this paper to take into account the stiffeners effect.Suction anchors are now commonly used as anchoring system for TLP, riser towers and in taut-leg moorings where the vertical component is predominant. Such anchors are subjected to vertical long-term tension loading. The evaluation of their long term vertical capacity should take in-to account:the behaviour of the steel soil interface during shearing,the rate effect for long term loading,the stiffeners configuration. Finite element analyses are necessary to assess the stresses and displacements of the anchors under long-term tension loads (or sustained loads).