Exxon Santa Ynez Expansion Project Pipeline Supports

Offshore California, USA, 1992

The Exxon SYU project comprised a series of subsea pipelines linking the Harmony and Heritage platforms with the Santa Ynes FPSO.

Seabed surveys indicated that once the pipeline had been laid, significant freespans would exist at various points along the pipeline route. Freespans over a certain length require an interim support to be installed to prevent overstressing the pipeline.  The most common way of providing such supports is by means of grouted fabric formworks.

At that time, fabric formworks had always been installed by diver; however the SYU Project safety requirements prohibited the use of divers whenever possible, so FoundOcean (then Wimpey Laboratories) were tasked with developing a method of installing fabric formwork pipeline supports by ROV. Also, the soft seabed conditions meant the formworks had to be filled with a lightweight grout to prevent the supports from sinking under their own weight.

The water depth exceeded 1,000 feet (305m) for most of the pipeline length, at that time a far greater depth than previously attempted for this type of operation. (Since then FoundOcean has successfully completed grouting operations in depths exceeding 4,000 feet (1,200m).

Key issues to be addressed included:

• Formulation of a lightweight grout
• Development of a deployment frame to position the formworks under the pipeline
• Development of a method of deploying a grout hose to a depth of 1,000 feet and connecting it to the formwork

Lightweight Grout

To comply with the bearing capacity of the seabed, a grout density of 1660 Kg/m3 (13.85 lb/US gal) was required. This would give a submerged density of 635 Kg/m3.  Standard grouts have a density of 1980 Kg/m3 (16.53 lb/US gal). Standard grouts comprise cement and water only.  To reduce the weight of the grout, FoundOcean formulated a mix design to include cement, water, pulverised fuel ash, and calcium choride.

Deployment Frame

Working with ROV contractor Martech, FoundOcean developed a lightweight ROV-friendly aluminium frame (sled) capable of carrying a rolled-up formwork.  The frame also included a manifold for connecting the grout hose and diverting the grout flow to the various compartments of the formwork.

The marine certifying authority required that the pipeline was left without protuberences when installation was complete. To comply with this, the upper part of the deployment frame was designed to be recoverable back to the surface.

Hose Deployment & Connection

Due to the small size of the installation vessel, it was not possible to use a hydraulic hose capable of supporting its own weight: This would have required a large hose winch for which space was not available. In addition, the high current velocities would have caused the hose to drift away from the pipeline location and the ROV would not have sufficient power to pull it back to the installation point.

In solving these two problems, FoundOcean developed a method which used lightweight, lay-flat hose clamped to a support cable and clump weight.  The cable and clump weight ensured that the grout hose and subsea connector could be positioned close to the support point.  The ROV could then easily pick up the grout hose connector and connect it to the manifold on the deployment sled.

Typical Deployment

The grout bag and sled were lowered to the seabed by means of the ship's downline which was then disconnected.  The ROV picked up the sled and positioned it correctly under the pipeline.  Once the sled was in position the downline, clumpweight and grout hose was lowered to the seabed.

The ROV picked up the grout hose and docked it onto the grout manifold on the sled.  Grout was then mixed and pumped down the hose to the formwork until it was filled and forming a shape to properly support the pipeline. The grout hose was then disconnected and recovered to the surface.

After 24 hours, the formwork support was inspected by ROV to confirm correct contact with the pipeline. The connection manifold was then removed and recovered to the surface.

Footnote

Since the Exxon SYU Project in 1992, FoundOcean has developed its methods of remote deployment of fabric formwork pipeline supports such that they can now be deployed efficiently in water depths exceeding 1200m.