Suction Pile Technology and Installation in Deep Waters

Abstract Skirted anchors or "suction piles" are suitable for moorings in deep waters. The main advantage with the skirted anchor compared to other alternatives in deep water is the fixed-point installation and no need for drag-in operations or proof loading. In this paper, different design and installation aspects such as anchor geometry, load response, equipment and marine operations are discussed for soil and loading conditions representative for deepwater field developments around the world. The length to diameter ratio, called the aspect ratio, is a key parameter for the maximum feasible suction penetration depth. Experience from deep-water installation of suction pile anchors around the world is discussed. New concepts involving suction pile technology provide attractive solutions for deep water. Applications for subsea templates, pipeline supports, well heads and risers are presented. Introduction The suction pile concept was first introduced for the offshore industry some 20 years ago. However, it was only after further research and development in the early 90's that the widespread use in mooring applications for floating production units took place. The first suction pile moorings for permanent use were installed 1995 in 100-200 m water depth and for catenary anchor lines (Ref. 1). Today, there are almost 300 suction piles operative in permanent mooring systems around the world. The first step into deep water using suction piles and fiber rope taut leg moorings was taken in Brazil in 1997 for Petrobras P-19 and P-26 floaters at the Marlin field (Ref. 2). The suction pile concept is versatile and can be optimally designed for almost any loading condition (Fig. 1). Maximum capacity is obtained when the load attachment point and reaction forces from the soil are balanced such that the pile fails without any rotational movement (Fig. 2). Although the focus from the oil industry has been on anchors for deep-water moorings, the most frequent use of skirted anchors in deep waters will perhaps be as foundation systems for templates and well heads. How deep in the water? Water depth is a barrier that the industry is continuously challenging. As far as suction pile technology is concerned, no water depth limitations prevent the technology from following the rest of the deep-water developments. Suction piles have been installed as deep as 2500 m (8400 ft) in the Mississippi Canyon (Ref. 3). Petrobras is planning subsea templates with skirted foundations in 2000 m of water at its Roncador field. Suction piles can be installed from crane vessel or launched over the stern of an anchor handler tug (Fig. 3). For installation in deep water, it is necessary to account for added mass effects and to ensure that the resonant period for the lifting system at the installation depth differs from the dominant wave period at the site. The equipment required for suction pile installation is based on standard ROV technology with its depth limitations. For soft clay, the suction penetration rate can be up to 20 m per hour without any significant rate effects.