Bombax 48-inch Pipeline Installation
Authors: Weustink O.W.A. 
Published: October 2008

The heavy 48-inch Bombax pipeline, which was considered to be very sensitive to buckling, was successfully installed by Allseas' Solitaire offshore the coast of Trinidad in 2002. Allseas overcame considerable challenges during the preparation and installation phase of the project such as pulling forces up to 789 mT and pipelay tensions up to 361 mT.

Introduction

Offshore installation of the Bombax1 48-inch Gas Export Pipeline offshore Trinidad was part of the Pipeline Transportation and Installation Services Contract between BP Trinidad and Tobago LLC (BPTT) and Allseas Deepwater Contractors S.A. (Allseas), that was awarded in August 2002. The 48-inch pipeline installation was part of the Kapok Project development, BP's largest offshore processing facility in the world, expected to more than double BPTT's daily gas output.

Allseas' preparations officially commenced before the project kick-off meeting in September 2001 and had to be finished before project mobilization in February 2002. It was immediately clear that the pipeline installation would be a difficult task.

The 48-inch concrete weight coated pipeline, with a total length of approximately 63 km, had to be pulled in at the landfall site at Rustville on Trinidad's East Coast off Galeota Point and laid down near the future Casia B platform at a water depth of approximately 67 m. Soil data varied gradually, near shore the soil consisted of intermixed sand and clay and offshore soil consisted of very soft clay (1).

The planned route had three curves of which the first was located approximately 6 km offshore from Rustville. 1 Bombax was derived from Bombacaceae, the Kapok Tree family. This curve had a radius of 1524 m, changing the pull-in direction of approximately 138° 2 to a pipelay direction of approximately 111° 2. Along the route most waves would approach Solitaire bow quartering. Two sea states were allowed for during the project:a sea state characterized by a significant wave height (Hs) and a corresponding zero-up crossing wave period (Tz) less than the average Hs and Tz, which was used for near shore activities anda mean sea state which was typical for offshore activities in February, March and April. The heavy weight of the joints resulted in unusually high pulling forces and pipelay tensions in addition to the large diameter-wall thickness ratio (D/t) which made the pipeline very sensitive to buckling.

How did Allseas deal with these challenges during the preparation and installation phase of the pull-in and pipelay operation? This question is answered in this paper with an overview of the technically interesting and challenging activities, which resulted in the successful installation of the Bombax 48-inch Gas Export Pipeline by Allseas' Solitaire. The heavy weights and large D/t ratios of the 48-inch Gas Export Pipeline sections are presented in Table 1. Installation of these pipeline sections was realized by Allseas' Solitaire and specifically selected equipment. Brief descriptions of the equipment clarify the overviews of the pipeline pull-in and pipeline installation, in which Allseas successfully dealt with high pulling forces, high pipelay tensions and the buckling sensitivity of the pipeline.