Design,Testing, and Installation of Steel Catenary Risers
Authors: Chaudhury Gautam , Kennefick Jonathan 
Published: October 2008

Steel catenary risers (SCR) have been used to connect the export lines to the Morpeth TLP platform. To date, this is the shallowest water depth employment of SCR. The majority of the risers, in particular, the touchdown and sag-bend have been installed by employing the "S" lay technique, another first for SCR installation. Significant design effort was invested to understand the issues involved and resolving them, in a method to demonstrate the safety and reliability of such an application. Design effort consisted of an improved analysis model, non-linear modeling of the seabed and flex-joint, fatigue testing of full scale pre-strained riser joins, development of a VIV strake sizing procedure, and the extensive use of fracture mechanics and Engineering Critical Assessment.

INTRODUCTION

The Morpeth TLP has a 12" oil and a 8" gas export riser system. The two export risers are supported from the TLP via flex-joints, housed in receptacles welded on to the hull. The risers are essentially a continuation of the export pipelines with the exception of a thicker wall section and the design and fabrication is in accordance with a more stringent weld acceptance criteria.

The risers are 12.75 inch OD × 0.688 inch WT and 8.5 inch OD × 0.5 inch WT API 5L X-52 pipes welded end to end. The entire length of each riser was coated with triple layer of epoxy/polyethylene for corrosion protection. An additional abrasion resistance coating was applied throughout the active touchdown area. The top 520 feet of the risers have a 0.5 inch neoprene anti-fouling coating for marine growth prevention and triple-start helical strakes for the suppression of Vortex Induced Vibration (VIV). The mean departure angle of the risers is 20 degrees to the vertical with a maximum rotation capacity of the flex-joint of +/- 15 degrees from the mean design departure angle.

The maximum design pressure is 1950 psi and the operating temperature is 80 degrees F. The water depth at the location is 1671 feet and the riser terminates at a depth of 85 feet resulting in a net vertical distance of 1586 feet from the seabed. The hang off location was selected at the lower end of the hull primarily to reduce wave and current loading and to avoid the risk of damage by collision from supply boats.

Flowcharts describing the activities involved in SCR design, fabrication, and installation and how they are inter-linked to deliver the SCR as a final product (design, procure, construct, and install) are presented in Figures 1 through 3.

SIZING AND CONFIGURATION

Generally, initial wall sizing is based on the burst and collapse strength requirement and to maintain enough submerged weight for on-bottom stability against currents. Sizing of small to medium diameter (up to 12 inch) SCR, with sufficient tension at the seabed, on-bottom stability due to current does not govern the wall size. In the design of export risers, with low operating pressure, wall sizes are not controlled by burst or collapse pressure. Experience has shown that wall sizes required to design against dynamics, fatigue, and VIV govern the wall thickness requirements, especially, with export risers. Therefore, based on extreme and fatigue stress, a 12.75 inch OD × 0.688 inch WT for the Oil export riser and a 8.5 inch OD × 0.5 inch WT for the Gas export riser, both API 5L X-52, were selected.

The primary design concern was the relative shallow water applications of SCR. The principle of a catenary is such that once the curvature at the sag-bend is maintained within the allowable static stress limit, the height of vertical termination does not affect the SCR design significantly. For a given sag-bend curvature, the horizontal tension component