Abstract

During spring of 2004 four large diameter SCRs were constructed and installed to connect the Mardi Gras Export System with floating facilities. The challenge in constructing these first risers was the combination of large water depth (max 1450m), pipe diameter (16 " - 24"), wall thickness (up to 1.14"), high top tensions, welding requirements (E-curve) and a high weld area ductility level.

All four risers were constructed and hung off in one uninterrupted operation. This implied the passing of a point of no return after which lay down of the riser (in case of an emergency) would not be possible without damaging the riser.

To minimize the schedule risk in this critical phase, it is essential that the chance of weld rejection was reduced to an absolute minimum.

The welding procedure development and qualification, demonstrating consistency in high weld quality and repeatability, proved to be beneficial. Welding and construction crew training and qualification in accordance with the high project standards were key elements in reducing risk and achieving steady reliable production. In particular testing against a wide range of welding parameters brought maximum flexibility during the welding operation.

With the VIV strakes and a strain monitoring system being installed over the complete length of the SCRs the installation had to be as efficient as possible. This resulted in several adjustments to the designs during the preparation phase. In addition a huge SPAR preparation program had to be undertaken to ensure all the SCR pullin installation aids were in place during SPAR construction and prior to tow out. The SPAR preparations and efficient installation methods were crucial to minimizethe installation period after the point of no return.

Introduction

The construction and installation of Steel Catenary Risers (SCRs) in the Gulf of Mexico has seen rapid developments towards heavier risers in ever increasing waterdepth. Various methods of construction have been applied however J-lay process is currently the method of choice for the top-end of the market in terms of weight, diameter and service life. Typically SCR installation can be executed by three different methods:

  • Pre-lay the SCRs. Prior to arrival of the facility the SCR is constructed and then temporarily stored on the sea floor. After arrival of the facility, the SCR is recovered and connected to the facility.

  • Post-lay the SCRs. After arrival of the facility, the pipeline is recovered and the SCR is constructed and connected to the facility in a continuous operation (2nd end SCR installation).

  • Post-construct the SCR's. After arrival of the facility, the SCR installation initiates at the facility and will be layed away from the facility (1st end SCR installation).

In the case of the Mardi Gras Transport System the postlay method has been chosen for reasons of schedule. The J-lay tower position and lay direction of the DCV Balder were chosen such that a post-lay installation can be easily executed without turning the vessel or any other installation complications. The post-construct method is off course possible as well.

This content is only available via PDF.
You can access this article if you purchase or spend a download.