Managing the Threat of Flexible Riser Outer Sheath Damage
Unbonded flexible risers were first used in the North Sea to expand production into areas where the water depth and harsh environment required use of a floating support vessel. Their multi-layer design using alternating layers of polymers and shaped steel wires permits lower allowable bending radii than traditional single layer pipe with a similar pressure capacity. This design allows the movement of the floating support vessel to be isolated from the subsea equipment. They are now installed and operating successfully in offshore environments all over the world.
18 Apr 2017
Keeping flexible risers healthy during their service life requires a good understanding of their complexity and associated range of potential failure mechanisms, as well as a robust integrity management plan. This article will provide an overview of the key steps in managing the most common threat to flexibles: outer sheath damage.
Why does the outer sheath matter?
Each layer of a flexible riser has a specific function contributing to its overall performance, and each of these layers has its own threats or potential failures that can lead to reduced performance of the pipe, or contribute to loss of containment. Here are the typical layers of a dynamic riser:
The layer of a flexible pipe most susceptible to damage is the outer sheath. This polymer-based layer’s primary role is as an environmental barrier to stop seawater from coming into contact with the inner carbon steel layers of the pipe. It is designed to withstand the effects of normal installation and operational contact, but can be damaged by gross contact with adjacent risers, mooring lines, installation wires or dropped objects. The loss of integrity of this layer leads to rapid corrosion of the steel layers and can affect the structural capacity of the riser shortening its service life.
Inspecting and testing the outer sheath
A general external visual inspection conducted on an annual basis provides an effective means of detecting damage to the outer sheath and other supporting components (buoyancy modules, tethers, clamps etc.). Closer visual inspections of key areas of the outer sheath above and below sea level should also be performed to confirm integrity.
Pressure testing of the space between the outer sheath and internal pressure sheath, known as the annulus, is recommended to be performed after the risers have been installed, and annually thereafter, to confirm the integrity of the outer sheath. There are two methods of testing: the faster and direct pressurisation of the annulus with nitrogen (low pressure <3bar), or returning the annulus to atmospheric pressure using nitrogen via a vacuum (<-1.0 bar).
In the event of a severe storm, clash or suspected dropped object, additional external outer sheath visual inspections and annulus testing are recommended.
Repairing the outer sheath
If damage to the outer sheath is identified, remedial action should be taken as soon as possible to prevent further damage such as corrosion. Any repairs should be subject to an integrity assessment to determine their feasibility. Minor outer sheath damage can be repaired with polymer welding.
Significant damage to the outer sheath, including holes, tears, significant wear or breaches through the thickness of the layer, can be repaired with a clamp. Permanent repair clamps can be manufactured to accommodate the dimensions of the riser and breach to re-establish the barrier to the external environment.
The ability to rapidly identify a failure in the outer sheath and initiate repairs sooner rather than later is extremely beneficial.
A continuous vent monitoring system can be installed at the riser hang-off where additional surveillance is required. This extra measure can detect an increased rate of pressure in the annulus indicating a leak in the internal pressure sheath, as well as a decreased rate which would indicate a blockage in the vent port system.
Performing a remnant life assessment based on actual operating conditions can demonstrate whether the original design life can be met after an unexpected event, or whether service life can be extended
Remnant life assessment and life extension
Unexpected operational events, including the loss of integrity of the outer sheath, can shorten the riser’s service life. Performing a remnant life assessment based on actual operating conditions can demonstrate whether the original design life can be met after an unexpected event, or whether service life can be extended. Real operational data reduces conservatisms taken during the original design phase, and using recorded data of the environment, bore pressure and temperature, vessel motions and annulus conditions results in a more accurate assessment of the riser’s service life.