Buoyant Tower: Improvement Suggested by Project Experience
Authors: Harding Chris , Finn Lyle 
Published: May 2013

The CX-15 platform was installed off the coast of Peru in Block Z1 in September 2012. By the time of presentation, the initial drilling phase will be underway and data from the whole process will be available for review, discussion and evaluation. The platform is the world's first application of a proprietary design that is a fusion and combination of existing and proven technology from deep water cell spar floating platforms, compliant tower structures and shallow piled foundations. A key driver in the selection of the design was the ability for the hull and topside to be manufactured, delivered and installed without the use of large heavy lift equipment.

Following the successful installation and initial drilling phase, the paper describes some of the high level lessons learned from the execution of the BPZ CX-15 project and how future buoyant towers will benefit from this feedback. In particular, the paper describes how the design could be modified to allow for faster construction and installation and for construction in areas of the world with little experience of building large offshore structures. Modifications to the CX-15 design will be presented to facilitate drilling only and drilling plus production and storage options for extended well test (EWT) and exploration, development and production (EDP) applications.

Introduction

The buoyant tower consists of a number of conjoined tubular structures (4×60 m in length in the case of CX-15), each consisting of smaller length " cans " welded together. In turn, each can is comprised of smaller 2.5 m length sections welded together.

Following the successful installation of the CX-15 buoyant tower, feedback from project personnel was sought in order to refine the design and improve the construction and installation methodologies for subsequent buoyant towers. Some of the major points that were either considered worth keeping or new techniques that we will be considered for implementation next time are:

Maintain maximum flexibility in can production. The construction strategy that was developed for the CX-15 buoyant tower was to sub-contract the rolling of the cans to local pressure vessel (PV) shops that would then ship the completed tubular can sections to Wison's yard at Nantong, China where they would be assembled into longer tubes and nested together. See Fig. 2—Assembly in Fabrication Yard.

The decision to out-source the rolling of the cans was made because there was, at the time, a shortage of rolling capacity and covered fabrication space in Wison's main assembly yard. The detailed methods and jigs for maintaining tolerances, adding the internal stiffeners and welding the rolled sections together after having stacked them vertically were developed in conjunction with the PV fabricators. The investment in making jigs, in particular, proved to be worthwhile and led to greater productivity as can production stepped up.

The ability to sub-contract can production is considered to be a major advantage of the design as it allows considerable flexibility both in scheduling within the main yard and permitting multiple parallel, concurrent can fabrication.