Abstract

Damage to the external sheath of deepwater flexible risers, sustained during installation or during service, can result in seawater penetration within the riser carcass construction. To minimize any detrimental effects the seawater presence may have an inhibiter mix, laced with clearly visible fluorescein dye, is introduced into the annulus of the riser from the FPSO. This managed system positively expels seawater through the action of hydrostatic head to effect temporary mitigation. The ultimate solution requires an active intervention program to locate and permanently seal the breaches by means of ROV deployed mechanical clamps.

Subsea7 assisted by laser experts, Smart Light Devices (SLD), have designed and developed a 1000m rated, ROV deployed, Subsea Laser Leak Detector. This product demonstrates significantly improved accuracy and operability compared with existing technology. To complement this equipment Subsea7 have also developed a lightweight, manipulator-deployed clamp design for repairing sheath damage.

This paper presents the design features of the Laser Leak Detector, the extensive testing that was carried out and the safety features that are incorporated. The effectiveness and flexibility of the ROV deployment of the Lightweight Clamp design is also discussed as well as other designs.

Introduction

The environmental conditions encountered during the installation of flexible risers in deepwater can often be challenging. This can be both as a result of weather conditions for available installation windows and current conditions for ROV operations. The installation methods, normally involving rigging and clumpweight to control the shape and position of the risers, can be adversely affected by these conditions, resulting in some cases in minor damage to the external sheath of the risers. Although the damage may be light, breaching the layer of flexible Rilsan external sheath, nominally 9 to 12 mm thick, could permit raw seawater to penetrate the carcass of the riser with potentially detrimental effects. Clearly even a small breach, undetectable by visual ROV survey, would over time allow seawater to penetrate, an action assisted by the dynamic motion of the riser. To address this issue, it is common practice to implement a management scheme where risers suspected of having sheath damage are proactively maintained. This is done by introducing a cocktail of inhibiter chemicals into the annulus of each riser via a gravity-feed fill-pot system on the FPSO. This is successful in positively expelling seawater from the annulus by means of the differential pressure due to the head of fluid. The fluid is made visible to the naked eye by the addition of a bright green fluorescein dye, typically at a concentration of 250 parts per million (ppm), allowing the mix to be clearly visible during an ROV survey even when diluted with seawater.

Early search and repair

Subsea7's services to the subsea industry have on occasion involved detailed riser surveys as part of Inspection, Repair and Maintenance (IRM) programs. This has been done using either general visual inspection techniques or instrumentation such as the black-light system or single-point fluorescence sensors. Both types of instrument are available from the market place and both seek to detect fluorescein.

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