Innovative Developments in Suction Pile Technology
Authors: Alhayari Samy 
Published: October 2008

Suction installed piles are more and more commonly used as anchor points for floating structures in the Oil and Gas Industry. This paper described two recent SBM FPSO projects, Curlew in the Central North Sea (90 metres water depth) and Aquila in the Adriatic Sea (850 metres water depth), where the range of their application has been extended in terms of soil conditions and water depth respectively. The limits of applicability of the Suction Piles are still uncertain, both in terms of soil conditions and water depth. The paper will emphasis on model tests performed for the definition of water injection system used on the Curlew project which proves that Suction Piles can be installed in harder soils (very dense sand) than previously believed. The experience of the Aquila project will also be highlighted demonstrating that the only limitation in terms of water depth are due to the ROV capabilities and other installation means, and are not due to the concept of the Suction Pile itself.

Introduction

The use of suction installed piles, as anchor points for floating structures, has gained more confidence than before in the Offshore Industry for the following main reasons :Geotechnical and Structural design becomes more common practice for the experienced engineers.Good installation data is available from recent projects to improve the design and extend the limit of use of Suction Pile in harder soil making use of contingency system (i.e. water injection to avoid any risk of pile refusal).Cost savings for deep water projects where significant uplift loads can be taken by the pile. This will have direct impact on the optimisation of the mooring legs and installation spread. Due to soft nature of shallow soils, the required suction pressure and pump flow rates could be handled by the major ROV's. In addition to the above, the concept of the Suction Pile does not include any proof loading after installation. The following aspects will be developed in the present paper :Main particularities of the Geotechnical design for the two subject projects (Curlew and Aquila), with emphasis on the definition of the water injection device.Comparison between field measured installation parameters and predictions.Particular installation aspects with emphasis on deep water Aquila project.

Curlew Project

The Curlew project consists of a Floating Production, Storage and Offloading (FPSO) system located in the Central North Sea and anchored to the seabed by 9 Suction Piles (3 bundles of three mooring legs).

Soil Conditions

The soil conditions encountered at the FPSO anchor point locations were not homogeneous resulting therefore in two different pile designs. A detailed anchor points location plan is shown in (Figure 1). A brief description of the soil conditions in the top 15 metres below seabed are given in Tables 1 and 2.

Piles Geometry

The main Geotechnical analysis of the piles was subcontracted to NGI (Norwegian Geotechnical Institute). he piles main dimensions are given in Table 3. The presence of non-homogeneous soil conditions together with optimisation concerns meant that two different Suction Pile designs were developed by SBM.