Abstract

Our customer elected to post-install suppression fairings for vortex-induced vibration (VIV) on the suspended gas export pipeline in Green Canyon Block 113 due to recent data acquired on the depth and magnitude of loop/eddy and cold core current profiles in the Gulf of Mexico. VIV suppression fairings minimize pipeline excitation during high current events, thereby preventing accelerated fatigue failure of pipeline components and minimizing current-related disruption on producing operations. VIV suppression hardware installed on the span was designed and fabricated by Shell Global Solutions (US) Inc. (Shell GSUS) and was in the form of short (6-foot) fairing sections. For hardware installation, our customer teamed with Shell GSUS to develop a method and associated tooling to allow the fairing assemblies to be installed. The method of installation included a deepwater construction vessel using a work-class ROV and heave compensation techniques. This method and equipment can be utilized without affecting flow line operations. This paper reviews the planning, procedures and results of a deepwater, horizontal pipeline span VIV suppression installation project. This paper will be presented by the author at Offshore Technology Conference 2004 in Houston, Texas.

Case for Action
Span Description.

During an as-built pipeline survey of a deepwater offshore gas pipeline, a span of over 300 feet was discovered. The span is in the Green Canyon area of the Gulf of Mexico (Figure 1). The depth of the span at location is approximately -2300 fsw. The span length is over 300 feet long and at times over 10 feet above the sea floor. The Gulf of Mexico has recently verified currents that can potentially harm suspended pipelines in deepwater.

Fig. 1 - Approximate Span Geographic Location(Available in full paper)

To limit vortex-induced vibration (VIV) during deepwater current events, suppression hardware is required. Shell GSUS was tasked with installing the VIV suppression devices. Fairings were the specified device due to their low drag properties and low cost. VIV suppression fairings have in the past been installed on vertical pipeline steel catenary risers (SCR) and top tensioned risers (TTR) by Shell GSUS. This was our first attempt at a horizontal span retrofit.

Installation Method Development
VIV Suppression Device Decision.

Previously Shell GSUS developed and patented VIV suppression fairings. Fairings were found to be superior to commonly used helical strakes due to their light-weight, low-cost and low-drag properties. VIV suppression fairings developed by Shell GSUS (Figure 2) weathervane to keep the vortex-interrupting fin on the downcurrent side of the tubular that they are attached to. Unlike helical strakes, VIV suppression fairings do not increase the diameter of a pipeline substantially. Most deepwater facilities utilize VIV suppression in some fashion. Shell Exploration and Production Company (SEPCo) chose VIV suppression fairings for their tension leg platform (TLP) production risers in the high-current Mississippi Canyon region of the Gulf of Mexico. The Ursa TLP production risers were installed with VIV suppression fairings attached. Mars TLP production risers were also installed with VIV suppression fairings as well as strakes in the splash zone, and additional VIV suppression devices have been installed since arriving on location, due to a deeper understanding of the current profile.

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