Design and Installation of Catenary Risers for the Baldpate Compliant Tower
Authors: Babin James L. , Litzelfelner Joe G. , Ambrose Michael , Edel James C. , Will Steve 
Published: October 2008

Export pipelines transporting production from the Baldpate compliant tower consist of a 12-inch gas pipeline and a 16- inch oil pipeline. Consideration of various pipeline riser types resulted in selection of steel catenary risers to connect the export pipelines to the production facilities. This paper discusses, in more detail, the reasons for selecting catenary risers; design of the risers to resist forces imposed by installation, interaction with the jacket, and environmental forces (including vortex-induced vibrations); design of the riser-to-jacket connections; and provisions for future riser connections. The paper also discusses the methods used to install the pipeline and risers; handling of the risers by the diving service vessel and platform mounted winches; final tiein to fixed portions of the risers; and sub-sea positioning systems crucial to proper laying of the pipeline and achievement of the correct riser lengths.

Introduction

The Baldpate compliant tower, in 1648 feet of water, has steel catenary risers (SCR's) for connecting export pipelines, a 16-inch oil pipeline and a 12-inch gas pipeline, to the tower. This type of riser was selected in response to several characteristics of the unique configuration of the tower. First, there is a considerable amount of vertical displacement of many of the joints in the upper portion of the jacket during storms, relative to the seafloor and to the fixed base section of the jacket. Second, there is a field connection between the two sections of the tower, 1300 feet below the waterline. Third, J-tubes or fixed risers would have added weight to the launch-weight-sensitive upper portion of the jacket. Finally, flexibility in the make-up and location of future risers was desired, as well as avoidance of deepwater connections.

The pipelines were laid on the seafloor before the platform was installed. Their ends were later retrieved and lifted by a dynamically positioned diving service vessel (DSV) after the tower installation and connected to the jacket at locations several hundred feet below the waterline.

Steel Catenary Riser Selection

During the initial design stage of the project, several design options were identified and considered. Of primary concern was the interaction of the riser with the unique configuration and dynamic characteristics of the compliant tower design. The compliant tower was designed to be installed in two distinct sections; the lower base section and the upper tower section. The lower base structure is about 360 feet in height and fixed to the seabed in 1648 feet of seawater. The upper tower section extends from the top of the base structure to an elevation above the waterline. This section contains an articulating "hinged" structural arrangement at an elevation approximately 1150 feet below the waterline, which allows for a relative vertical displacement of up to about 1.5 feet during extreme storm conditions. In addition, due to the structure's compliant design, it is very sensitive to weight. Elimination of that additional weight caused by the installation of a J-tube and the corresponding structural supports and member sizing required for installation was desired.

In most applications where the pipeline sizes and riser locations are known well in advance of platform construction, a pre-installed riser section is selected. This eliminates the need for costly offshore installation of the riser sections and allows for connection of the riser by proven methods of diver assisted tie-in spool installation, or in the case of deeper water, diverless tie-in spool installation methods. As the tower section is designed to be hinged, and is installed onto a base section that has been previousl