FLOODING DETECTION OF SUBMERGED STRUCTURES
In the offshore industry, the most common type of failure of offshore assets is corrosion. Very often, by design, the most sensitive parts are protected by watertight barriers which aim to protect the metal layers from the corrosive environment. As soon as the watertight barriers are no longer intact, even if only very occasionally (micro-leakage), the risk of corrosion is again present. Direct inspection (visual or ultrasound inspection) of metal layers is made very difficult or impossible by the presence of watertight barriers. In fact, it is common for watertight barriers to be constructed by multilayers of opaque products of a different nature and without mechanical bonding.
To date, it is very difficult, if not impossible in some cases, to carry out a non-destructive inspection of metal layers of certain parts of offshore assets.
For offshore oil production, flexible pipelines are key components of the riser system that connect a subsea wellhead to a floating platform. Flexible pipelines used for oil production on floating platforms are subjected to dynamic loads and are designed to prevent fatigue failure during their service life. Their reinforcement layers made of high strength steel wires are located in a very confined environment called the annulus. If these steel wires are subjected to a wet corrosive environment, corrosion fatigue can become a problem, especially for acidic service applications with the presence of H2S and CO2.
They all have the same basic structure made up of a concentric arrangement of different steel or polymer elements (Figure 2).
Schematic view of a flexible pipe
The main components of the flexible pipes have a function either mechanical or of protection regarding the external environment:
· An internal stainless steel carcass layer (1)
· An internal polymer sheath (2) ensuring the seal against the transported fluid.
· One or two layers of pressure armor in carbon or low alloy steel (3 and 4).
· Two or four layers of carbon or low alloy steel armor wires (5).
· An outer polymer sheath (6) ensuring seawater tightness.
The annular is the free volume of a flexible pipe. It is defined as the volume between the internal sheath (2) and the outer sheath (6).
Development of a new technology
The project presented in this document therefore concerns the monitoring of flexible pipes and more particularly the prevention of annular flooding by infiltration of seawater or by diffusion of water.
Artistic view of a flexible pipe with a flooded annulus
The project proposes to develop a new method of external inspection of a flexible pipe in an underwater environment allowing the in-service inspection of the annulus of flexible pipes. This new inspection method will increase the life of dynamic risers and flowlines. This is of paramount importance for demanding applications such as deep-sea or high-pressure production.
The project relies on 2 companies with complementary experiences to develop this new technology capable of detecting in service the annulus flooding of a flexible pipe:
– IMdev: Over 24 years of experience in the manufacture of flexible pipes and the development of in-service flexible pipe inspection tools.
– Sciensoria: Over 20 years of expertise in non-destructive eddy current testing to control the quality of finished materials and products: thickness, electrical conductivity, magnetic permeability …
The measurement principle
Detecting the presence of water under the external sheath of flexible pipe, which is itself immersed in water, is a difficult problem. The flexible pipe is itself composed of several materials whose shape is complex. The water under the external sheath is only a small volume in relation to the total volume of the flexible pipe.
It was necessary to find a physical principle which makes it possible to highlight the contrast between a dry annulus and a flooded annulus in the most reliable way possible. We have chosen to use the high frequency emission method for this application. This method was chosen for its sensitivity and reliability of detection.
The operating principle of this method is like that of radar. A high frequency electromagnetic wave is emitted towards the target. This wave will pass through the different materials that make up the target. At each crossing, it undergoes modifications because each of the materials opposes to the incident electromagnetic field its own polarization field. Therefore, the returning wave bears the signatures of the materials it has passed through. The high frequency wave emission method was chosen because it offers global detection without being disturbed by the very complex internal geometry of a flexible pipe.
example of the reaction of a non-empty material to the incident electrical field emitted by an external antenna
Vector diagram showing the behaviour of electric fields relative to a non-empty
The main difficulty lies in the fact that the detection must be done under water. As we know, electromagnetic waves propagate very poorly under water. This is precisely the reason why acoustic detection is preferred to electromagnetic detection for the marine environment.
To overcome this difficulty, Sciensoria has designed a dedicated antenna structure which is patented. This structure makes it possible to propagate the waves further and more efficiently.
To operate this antenna, specific electronics have been designed: it allows the antenna to be operated in both continuous and pulsed mode, with an extremely high signal-to-noise ratio and very low thermal drift. All the electronics are compact so that they can be integrated into a box supporting the very high pressures. An Ethernet link allows the measuring device to be connected to a computer over a long distance without loss of information.
Software installed on the remote computer is used to analyze the resulting electrical field Er and to determine the amount of water in the detection target. The two quantities analyzed are the real part R and the imaginary part X of this electric field because it is a complex quantity.
Below is the synoptic of the flexible pipe flooding detection system.
Synoptic of the flood detection system of a submerged flexible pipe
The figure below shows a FISH series sensor associated with its conditioning electronics.
FISH sensor and its signal conditioner
Proof of concept
The first tests of the new measurement principle on simplified 2D models have shown that it is possible to clearly distinguish the presence of air and the presence of water in a flexible pipe annulus. The film below shows the result of a feasibility test carried out on a 2D model placed in a tank filled with sea water. The 2D model is a simplified assembly of the external part of a flexible pipe (armor + external thermoplastic sheaths).
The inspection system is designed to be integrated on a work class ROV. The inspection system will be integrated on an ROV before submerging the ROV and will be able to carry out spot measurements along flexible pipes in service.
3D view of an ROV performing a measurement along a flexible pipe
To obtain a measurement, the probe will be manipulated by an ROV to be placed in contact with the flexible pipe. The measurement will be carried out when the probe is placed in contact with the flexible pipe. This measurement will take a few seconds at most. A computer link between the ROV and a PC on the surface will enable to visualize and interpret each measurement in real time.
Thus, depending on the results of each measurement, the inspection operation can be adapted for each flexible pipe. Regular inspections with this new technology will allow precise knowledge of the integrity of flexible pipes with respect to the risk of corrosion of the metal layers and thus to put in place appropriate inspection and maintenance plans for each flexible pipe.