North Nemba Flare Bucket Foundations

Abstract The successful installation of the North Nemba Flare in November 1999 demonstrates that suction-installed foundations ("buckets") are a competitive foundation solution for lightweight structures on soft cohesive soils. This paper presents the background to the selection of bucket foundations, plus design analyses and installation aspects. Introduction Cabinda Gulf Oil Company's North Nemba Flare (NNF) structure was installed in around 115 m water depth offshore Cabinda, West Coast of Africa (Fig. 1). The NNF structure is a lightweight steel tripod supported by three circular can foundations ("buckets") as shown on Fig. 2. The structure was designed and installed by McDermott-ETPM S.A (MESA), Paris. Suction Pile Technology (SPT) was subcontractor to MESA for detailed can foundation design and fabrication, and Fugro Engineers BV (FEBV) was consultant to SPT for geotechnical analyses. ETPM subsequently merged with Stolt Offshore in 2000. Suction foundations, installed by means of an underpressure (created by pumping out water trapped inside the caisson), are commonly referred to as "buckets", "skirted foundations" or "cans" in the offshore world. Application of full-size suction anchor piles was first described by Senpere and Auvergne (Ref. 1) for the Gorm Field, offshore Denmark. Because of their simplicity, reliability and cost savings, suction foundations are also applied to fixed platforms. Bye et al. (Ref. 2) have described design and installation in very dense North Sea sands. The studies presented herein are for the first lightweight platform founded on soft clays and subjected to tensile leg loading. This paper describes the design of the bucket foundations and the NNF installation, which used suction assistance to successfully penetrate the foundations into soft clay seabed soils. Bucket Selection Considerations Potential savings (both time and cost) with bucket foundations can be considered as follows: Engineering: Inter-disciplinary activities (e.g. checking hammer specification, pile add-ons or pile length) are streamlined with suction buckets, reducing the number of interfaces between design and installation. Optimisation in the structure design may be fed back from the bucket study, due to increased flexibility in bucket design parameters (as opposed to a pile) listed in Table 1. It also gives an opportunity to run structure and bucket design activities in parallel. Platform Operation: Suction foundation structures may be recovered and either disposed or relocated. This enhances operational expenditure in areas with stringent disposal regulations. On the other hand, drilling areas that may be subject to gas seepage must be carefully reviewed when considering bucket foundations as uncontrolled gas migration may jeopardise foundation integrity. Procurement and Fabrication: There is a threshold situation (depending on water depth, platform operational loads etc.) where bucket systems may bring procurement cost savings (part of these being offset by the negative impact of seafastening). Examples of procurement savings (other than the foundations themselves) include mudmats and pile sleeves. This threshold has to be assessed during the front-end engineering design stage. Load-out and Transportation: It is unlikely that bucket systems may bring savings compared to a traditional barge transportation scheme.