Modifications to the Pipelay Vessel Solitaire for the Independence Trail Project
Authors: Steenhuis André L.J. , van Norden Tjomme , Regelink Jeroen , Krutzen Martijn 
Published: October 2008

This paper describes the modifications to the pipelay vessel Solitaire to meet the installation requirements for installing the Independence Trail pipeline and steel catenary riser (SCR). To meet these requirements it was necessary to increase the stinger length and strength. The tension capacity was doubled by means of new tensioners and a new abandonment and recovery (A&R) system using 4 wires simultaneously. After manufacturing and installation of all equipment, an extensive testing program was performed to prepare the vessel for the installation of the Independence Trail pipeline and SCR.

Introduction

The perception of deep water changed significantly during the last two decades. In the beginning of the nineties, 1000 ft water depth was considered "deep". Nowadays, laying pipe in water depths of 5000 ft is common practice. The Independence Trail pipeline was the next step in even deeper water for large diameters.

The pipelay vessel Solitaire, a dynamically positioned Slay pipelay vessel with a length of 1300 ft (400 m) including stinger, was made suitable for this project during a 3-month conversion period in which she was upgraded.

Facts and figures

The Independence Trail Project (Enterprise Field Services L.C.C.) consists of a 24 inch gas export pipeline including a 20-inch SCR in the Gulf of Mexico.

The total pipeline length is 135 miles (217 km) in water depths ranging from 115 to 7940 ft (35 to 2420 m). Included in the scope was the installation of a flex-joint at start-up, SCR VIV strakes, buckle arrestors, two in-line double Tees and an in-line ball valve.

The pipeline transports gas from the Independence Hub Facility in Mississippi Canyon 920 to the West Delta 68-JP platform, and was installed using Allseas' pipelay vessels Solitaire and Lorelay in 2006.

Upgrades

The 24 inch pipeline and 20 inch SCR in water depths of up to 8000 ft required a lay tension of 1450 kips (650 metric tons). Therefore the tension capacity of 1150 kips (525 metric tons) required an upgrade of the pipelay system and the abandonment and recovery (A&R) system.

Strain limitation in the catenary required a longer stinger, which made it necessary to have a larger radius combined with a steep departure angle.

When upgrading one part of the vessel, all interconnected parts had to follow.

Figure 1 - Overview of Solitaire's aft - 2005 vs 1998 (available in full paper)

Stinger elongation 360 ft to 460 ft (110m to 140m)

The elongation of the stinger was necessary for both the steep departure angle and the increased tension capacity. The inhouse design of the stinger started early 2004 with the anticipation of an increase in tension requirement; six months later the installation contract for the Independence Trail project made it a fact.

The three-section, 360 ft stinger was replaced by a foursection 460 ft stinger, in which the last two sections have been maintained (see figure 1). The new two sections are only connected to each other on the bottom side in order to divert the loads to the also new hang-off frame, and by doing so being able to use the existing stinger hinges and handling system.