Buoyant Tower: Installation Challenges and Lessons Learned
Authors: McClure Lawrence , Florez Guillermo , Jatar Sanjai 
Published: May 2013

The CX-15 buoyant tower was installed for BPZ Energy in the Corvina field offshore Peru in a water depth of 54 m in September 2012. This is the first use of HortonGMC's buoyant tower platform design which was developed for BPZ Energy specifically to address the requirements of their shallow water field development program.

One of the key inputs into the selection of the buoyant tower in lieu of a more conventional jacket for this water depth was that it is designed to be installed from the transportation vessel itself and did not require mobilization of additional expensive offshore installation vessels. Peru has very limited offshore construction infrastructure, and the mobilization of large, heavy lift construction equipment would not have been cost effective for the project.

Additionally, the duration for installation of the buoyant tower is minimal compared to a conventional jacket, as pile driving effort is eliminated thereby reducing installation time and overall offshore risk. This new buoyant tower design utilized a number of proven deep water technologies where successful installations were completed with large construction vessels and equipment. The challenge in the installation of BPZ's CX-15 buoyant tower was to apply procedures and processes to cost effectively complete an installation with these proven technologies in an area that had minimal offshore construction infrastructure.

Introduction

BPZ's CX-15 buoyant tower is part of the Corvina field development. CX-15 was installed offshore approximately 15 km west of Zorritos, Peru in 54 m water depth. The installation of the buoyant tower is one of the most challenging projects of its kind developed in Peru. The project required not only a fast track delivery schedule but also required incorporation of proven technologies into one platform that could be installed with locally available equipment and without the cost of mobilizing large construction vessels.

Typical project requirements call for the importance of early planning and engineering activities to ensure that a complete technical and economic solution to the project can be established thus minimizing the risk. However with the fast track nature of this project and the criterion of not importing larger, more capable construction equipment with experienced crews, a different approach had to be taken in order to fulfill schedule and budget.

The buoyant tower design comprises four cylindrical cells similar to proven cell spar technology. It is connected to the seabed by a single suction pile that is integral to the hull structure. In the case of CX-15, the topside will serve dual functions as both a production facility, complete with gas compression and water injection, and a support structure for a drilling rig. The platform will include a total of 24 drilling slots. See Fig. 1.

The buoyant tower was installed using previously proven float-off and float-over methods commonly used offshore. However, this was the first installation of its type using a cantilevered float-over operation for the topside. The transportation vessel used to transport the platform (both the hull and the topside) from the fabrication facility in China was also used as the primary installation vessel.

Considerable effort was put into selection of a transport vessel that could allow the float-off of the hull while the topside remained on the vessel. The hull was then upended, fixed ballast installed and the hull brought back alongside the transport vessel to perform the float-over mating operation for the topside. The topside structure was skidded out onto the cantilevered support frame and the hull de-ballasted to complete the mating operation. Most of the support equipment and services were from local sources thus minimizing mobilization and import/export costs.