Development and Utilization of An All Subsea, ROV Based, Flowline Pre-Commissioning System
Authors: Engelmann Georg , Dupre Mike , Sauer Stephen 
Published: October 2008

Typically, offset flowlines (both ends located subsea without a surface piercing riser) are installed and tested with the mainline back to the host facility. However, expansion of existing infrastructure in deepwater has created a need for the ability to test flowlines without a surface piercing element.

To accommodate subsea expansions of brownfield developments, there exists a need for definition of criteria and design of hardware to provide for the flowline precommissioning function.

This paper describes the processes implemented for garnering industry consensus and regulatory approval, as well as the criteria applied to system selection and design.

Introduction

The first subsea pressure testing of a production flowpath was the Macaroni Manifold testing in June, 1999. The manifold was tested to 10,000 psi in 3700 feet of water. The technology was based on ROV based injection systems, which have been used for pressure testing of subsea connections, such as jumper connectors; and high pressure packages, such as the jetting skid developed for pipeline burial on the Angus project.

Einset was the first subsea Gulf of Mexico flowline to perform a regulatory hydrostatic test using an ROV based system. Typically, offset flowlines (both ends located subsea without a surface piercing riser) are installed and tested with the mainline back to the host facility. However, expansion of existing infrastructure in deepwater has created a need for the ability to test flowlines without a surface element.

Einset.

The SE Tahoe flowlines and wellhead were installed and became operational in 1996. The system included a spare hub on the SE Tahoe sled for future expansion. The Einset prospect is located 5 miles from the SE Tahoe sled, and was tied back to SE Tahoe with a 6-inch flowline in December 2001, as illustrated in Figures 1 and 2.

Advantages of a Subsea Hydrotest System.

For subsea-bysubsea flowlines, a means of hydrotesting subsea is desirable. The alternative to subsea testing is cleaning the mainline and testing both the existing mainline and new subsea line together. This may require existing production to be deferred while the flowline is displaced with multiple linefills of water to remove all hydrocarbon and residue. The contaminated water must be processed or disposed of appropriately.

Alternatively, using a subsea hydrotesting system, the new segment of flowline can be tested without impacting the mainline and then a final pre-tested connection installed without loss of any production.

High pressure testing in the congested confines of a production platform creates an undesirable, although manageable, hazard to personnel. However, subsea testing eliminates the exposure by relocating the pressure source to the seabed. As deepwater prospects trend toward higher shutin tubing pressures, the risk is increased. The number of deepwater 15K systems is growing quickly, requiring test pressures exceeding 18,000 psi.

In addition to the higher pressures, longer offset lengths (host to well) are being enabled. The longer lengths result in greater flow assurance requirements and better insulation. Traditional hydrotesting typically utilizes surface seawater (70 degrees F).