This paper will describe the development of a new pipeline concept for the export of oil from a large capacity FPSO to an offloading buoy. The installation of the two ‘w’ configuration rigid steel pipelines, carried out successfully by the Saibos ConstruÃ§oes Maritimas Lda's new deepwater installation vessel Saibos FDS was particularly adapted for deep and very deep-water field developments like Girassol in 1,400 meters.
Details of the concept will be covered with a major focus on the innovative installation method.
In early November 2001, the Saibos FDS (Field Development Ship) successfully installed the two 16in rigid export lines that link the Girassol FPSO with its loading buoy.
This installation was a first for several reasons. It was the initial installation of rigid pipelines by the newbuild state-ofthe-art installation vessel as well as being the first time that a midwater steel catenary riser (SCR) type export line solution has been used in an offshore oil field development.
The challenging water depth of the Girassol field (1,400m) combined with the relatively close proximity of the loading buoy - roughly 1,800m - mitigated against a conventional installation of carbon steel export lines on the seabed. An alternative solution should have favoured the use of flexible pipe, but cost was a factor. After much analysis, Alto Mar Girassol (AMG) proposed this innovative rigid pipeline solution which was found both technically feasible and more cost effective.
The initial concept suggested to TotalFinaElf E&P Angola by the AMG joint venture, in which Bouygues Offshore (BOS) was responsible for the installation of the two export lines, was a conventional subsurface tow and connection to the FPSO of the two strings. This installation method was later abandoned when it became apparent that installation by j-lay resulted in a significant improvement in the fatigue life of the export lines.
The use of this installation method was an option as Saibos, the joint venture between Bouygues Offshore and Saipem, was building a new dynamically positioned (DP) vessel which has among its features a j-lay tower adjustable between 45° and 96°.
This paper will summarize the challenge of this installation together with the main characteristics of the export lines and the features of the new vessel.
The two 16in export lines are configurated like a subsurface "w" between the bow of the FPSO and the loading buoy located under 2km to the northeast (Figure 1). The lines are supported at the FPSO via two receptacles, located on the port and starboard sides, in which flex-joints were pre-installed on the lines and through which the lines were pulled in.
The starboard line is the "upper" one with the branches of the "w" located in a water depth of 440-530m and has an overall curvilinear length of approximately 2,400m. The port side or "lower" line, is suspended in a water depth 630-740m with an overall curvilinear length 2,750m.