A Novel Method: MSV / ROV Installation of VIV Suppression on Existing TLP's
Authors: Ramsey R.D. , Vuyk D. 
Published: October 2008

Recent findings on the depth and magnitude of loop/eddy current profiles in the Gulf of Mexico have induced one operator to post install suppression for vortex induced vibration (VIV) on the tendons of two of its tension leg platforms (TLP) located in the Mississippi Canyon Block 809 / 807 area. The purpose of the suppression is to minimize the tendon excitation during high current events, thus preventing accelerated fatigue damage of tendon components and minimizing current related disruption of drilling and producing operations. The suppression installed on the TLP was designed and fabricated in the form of fiberglass strake sections. For the hardware installation, the operator teamed with a subsea service company owning multi-service DP-2 vessels to develop procedures and associated tooling to allow the strake assemblies to be installed from a deepwater construction vessel, using a work class ROV and a multiple downline technique. In addition to being efficient and cost effective, this procedure and equipment spread could be performed without affecting ongoing TLP operations. This paper will review the planning, procedures, and results of the tendon VIV suppression installation.

Introduction - Case for Action

Over the past 5-6 years, Shell Exploration & Production Co. (SEPCo) has observed currents (loop/eddy) with deeper profiles than were used for design of the URSA & MARS TLP's. Recent high current events (ex. 1999 - Juggernaut Eddy) have produced maximum surface currents of just over 3 knots, which is less than the maximum value assumed in the original TLP design basis. However, the currents have extended to deeper depths than those originally assumed. Unmitigated, current events of this nature can cause vortex induced vibrations (VIV) that accelerate tendon fatigue damage, disrupt drilling and producing operations, and produce topside excitation that is problematic from an ergonomic perspective.

A short-term mitigation strategy was developed that utilizedincreased tendon tensions to reduce the tendon VIV to acceptable levels. While this strategy is effective and simple to implement, it requires that significant volumes of ballast water be reserved for response to VIV events. The reservation of the ballast water erodes the TLP's ability to assume permanent topside load additions (lightship) that are required for future facility expansions and subsea tie-back opportunities. This limitation significantly affects the long-term profitability of a "hub class" structure.

As a result, Shell Global Solutions (SGS) was asked to investigate the performance of the TLPs' tendons subjected to revised current profiles and to offer a long term VIV mitigation that would relieve the ballast water restrictions imposed by the short-term strategy referenced above.

Selected VIV Suppression

Concept: Based on the SGS analysis, SEPCo elected to install VIV suppression devices on URSA's 16 - 32" O.D. tendons and MARS' 12 - 28" O.D. tendons. Approximately 500' of suppression coverage was recommended in order to ensure long-term integrity of the tendons and to eliminate the need for operational response to an event with high currents at depth. The recommended level of suppression is based on the most recent available data. However, since there is still significant uncertainty around loop / eddy current events, appropriate conservatism has been incorporated in the suppression design.