Deepwater Pipeline Spanning and Rectification: Lessons Learned
Authors: Johnson T. Alan 
Published: October 2008

The author was the client representative on an EPC contract between Triton Energy (now Amerada Hess) and Stolt Offshore for flowlines and risers in a deepwater West Africa development - the Ceiba field. The following are some lessons learned during the survey, design, installation, testing and commissioning of the flowlines and risers from the subsea manifolds in water depths of 650 - 750 m. to the FPSO in 90 m. water depth.

Comparisons will be made between a surface vessel deployed SWATH multi-beam survey and a ROV mounted multi-beam bathymetric survey (of the seafloor's XYZ coordinates) - and their resulting seafloor profiles. The predicted pipeline spanning will becompared with the actual measured span profiles.

The evolution of a mechanical pipeline support design from inception to successful deployment will be reviewed, along with an unsuccessful design. The span support criteria selected for this field will be explained in light of the geotechnical survey data and bathymetry.

Introduction

General

The bathymetry of the field, established by a SWATH survey and shown in Figure 1, has deep meandering channeling with numerous deep depressions (a crosssection of one of the channels is shown in Figure 2).

The channels point in the general direction of the mouth of the Rio Muni River, separating Equatorial Guinea from Gabon. The SWATH survey database was used to create the flowline profiles for the spanning analysis. AGA Level 2 (ref. 1) was used for stability calculations.

Field Architecture

The field architecture has subsea wellheads and manifolds in 650 to 750 meters of water. Dual flowlines connect the FPSO to the manifolds. Flexible pipe jumpers/CVC (Cameron Vertical Connector) connectors connect the rigid flowlines to the manifold. At the FPSO, flexible pipe risers (which are routed over a riser support structure (RSS) and up to the FPSO - forming a Lazy S profile), connect the rigid flowlines to the FPSO.

The requirement to have inhibitor within 48 hours of flooding the flexible pipe with seawater resulted in the necessity of a wet lift to install the jumpers and make the CVC connection to the manifold. This meant that span remediation for hydrotest conditions would have to be completed early in order to maintain the construction schedule. This requirement ensured active participation from the installation vessel crew in the span support design process.

Soils Investigations

Figure 3 shows the drop core locations for the soil sampling, with the Stations 1, 5 and 39 drop cores that were tested at a soils laboratory (ref. 2). The soil testing included the Atterberg Limits (Liquid Limit, etc.), thermal conductivity, electrical resistivity and particle size analysis. Vane shear tests were also performed to obtain the shear strength at the liquid limit. In addition, relative density testing was performed on the granular material at the FPSO (for AGA, Level 2 Stability calculations). For Stations 1, 5 and 39, respectively, the ratios of the mudline soil water content to the Liquid Limit were 0.84, 0.79 and 1.19, with undrained shear strength values of 8, 6 and 4 kPa. The seafloor soils are turbidite deposits that range from sandy at the FPSO to silty clays at the wellhead locations.