The present invention relates to condition monitoring of mooring lines, in particular the invention relates to an apparatus for in situ inspection of the structural integrity of mooring chains for floating structures.

Background

Mooring lines are the connection links which connect floating structures such as floating oil production or drilling units, wind farms or other moored vessels to the sea floor to provide stability and maintain position during operations. A typical mooring line can consist of chain links, polyester/steel rope/wires, or a combination of these. The upper and lower parts of the mooring lines are often chain links and the middle part of the mooring lines is polyester ropes or steel wires.

Mooring chains play a particularly important role in the integrity and fit-for- purpose condition of oil production units, which have high requirements for station-keeping accuracy and mooring safety. However, mooring chains undergo very harsh loading and corrosion conditions during their service life and they are very difficult to inspect. The uncertainty of their behaviour during major storm events, and the continuous wear/tear and corrosion mechanisms imposed on these critical components, combined with the technical difficulty for their regular inspection, makes it very difficult to accurately evaluate their condition and integrity when in operation.

There have been several examples of losing an entire mooring line due to breakage of a single chain link of the mooring line. Njord chain breakages (2007, 201 1 ), Nome chain breakage (2012), Girassol top chain breakage (2002), Banff Chain breakage (2000), Gryphon incident (201 1 ) and Dalia bottom chain breakage (2010, 2012) have been among these reported incidents. The breakage of the mooring chains can easily result in severe damage to the riser system and a stop of production, resulting in significant added costs as well as health and safety and environmental risks. Knowing the potential extent of the damage, it is of crucial importance to develop reliable condition assessment tools and methodologies for mooring chain systems.

Despite the desirability of obtaining condition information for mooring chains in the maritime environment, their regular inspection during operation is very challenging for a number of reasons. Typically, divers can be used to carry out inspections. This is however cost-intensive and involves safety challenges, and is in addition restricted to relatively shallow water operations, which is a problem as oil and gas operations move to increasingly deeper waters. Semi- autonomous or autonomous inspection systems have not reached the level of sophistication to provide reliable condition monitoring of mooring chains. There therefore exists a need for improved condition monitoring methods and tools to provide regular and accurate structural integrity information of mooring chains.

Earlier publications concerned with inspection and condition monitoring of mooring lines include US 2010/0235018, which teaches an unmanned apparatus for transversing and inspecting an elongated structure. The apparatus is provided with means for propelling the apparatus along the elongated structure and sensors to capture the condition of the elongated structure. A challenge with the proposed system is that use of potentially different types of sensors makes the apparatus larger, heavier and more challenging to handle. This may compromise its ability to transverse effectively and reliably along the elongated structure.

WO 2012/097402 teaches a cleaning apparatus adapted to move along an elongate line. Multiple chain monitoring devices are foreseen within the cleaning apparatus for monitoring the chemical and physical properties of the chain. However, the publication gives little guidance as to how such multiple monitoring devices should be arranged, in particular how to ensure a compact and operationally reliable system if several different sensors are used, or if combined with an internal propulsion system.

GB2483098 describes an eddy current chain link testing apparatus for anchor chains. The apparatus is suitable for use under water, and can be operated from a remotely operated vehicle (ROV). Only a single sensor system (eddy current) is provided, with no guidance on how to employ different types of sensor systems, or a sensor system in combination with a cleaning system and/or a system to move the apparatus along the chain.

Summary of the invention

For efficient monitoring of the condition of mooring chains, the possibility to use a combination of techniques is desirable. This can include optical screening by cameras, electromagnetic sensor systems, or ultrasonic techniques.

It has been discovered that combining a set of tools and techniques in an autonomous or semi-autonomous mooring chain inspection system is not straight-forward. Among other things, employing a number of sensors makes the overall apparatus larger in size and more challenging to handle. In particular, if the apparatus has to be designed with an increased length in order to accommodate for the different tools required, one may experience problems moving the apparatus along the chain. Moreover, some measurement techniques may require accurate positioning of the sensor in relation to the chain, as well as a clean chain surface to provide useful measurements.

Existing systems, such as those discussed above, do not provide satisfactory solutions to these challenges.

One objective of the invention is therefore to provide an improved apparatus for condition monitoring of mooring lines. Another objective of the invention is to accommodate for the use of a combination of condition monitoring techniques in a mooring line condition monitoring system. A further objective of the invention is to provide a tool for autonomous or semi-autonomous monitoring of the integrity of submerged mooring chains in a safe, operationally reliable and accurate manner.

According to the invention, an improved apparatus for inspecting a submerged mooring chain is provided. The invention is set forth and characterised in Claim 1 , while the dependent claims describe optional variations and advantageous embodiments of the invention. By providing an inspection apparatus comprising sectional modules, wherein at least two modules encircle the chain and are connected by means of a longitudinal flexible coupling, gives advantages of among other things increased operational reliability when operating on chains with large catenary angles relative to the apparatus length.

One or more sensors may be provided in a sectional module to measure at least one parameter relating to the structural integrity of the mooring chain, for example using non-destructive testing (NDT) techniques. Advantageously, this allows identification of defects, damage or wear in the chain.

A drive module may be provided in a sectional module. Advantageously, this allows the apparatus to be propelled along the chain either autonomously or semi-autonomously and carry out inspection over substantially its full length. A cleaning unit may be provided within the apparatus. The cleaning unit may be provided in a separate cleaning section module which may be connected with one or more other section modules by means of a flexible coupling.

Advantageously, the cleaning unit allows cleaning of the mooring chain immediately prior to testing, improving the accuracy of measurements from sensors which could otherwise be negatively influenced by solid deposits or marine growth on the chain links. Moreover, if combined with a drive module the cleaning unit may improve the grip of a chain-engaging member of the drive module and thereby provide more secure and reliable propulsion of the unit along the chain.

A chain guide part may be provided to guide the chain into the inspection apparatus at one or both ends. Advantageously, this ensures smooth and secure motion of the inspection apparatus over the chain also when the chain follows a catenary curve in operational conditions. The chain guide part may be provided as part of a section module, or, alternatively, a separate chain guide section module may be provided for this purpose. The chain guide section module may itself be connected with another section module by means of a flexible coupling.

The inspection apparatus according to the invention therefore allows automated or semi-autonomous inspection of possible defects (for example cracks and tear/wear) in the chain links of the mooring line under operational conditions. The inspection can be part of a planned inspection program using the inspection apparatus, either on its own or in combination with other tools or techniques, to ensure the structural integrity of the mooring lines during their service life. According to the specific operational requirements in a given case, a number of tools can be accommodated in the inspection tool, while ensuring operational reliability, also if the apparatus length increases.

Brief description of the drawings

Figure 1 shows a cross-section of an inspection tool according to the present invention.

Figure 2 shows an isometric view of the inspection tool illustrated in Figure 1. Figure 3 shows an exploded view of an interconnection between section modules in the inspection tool illustrated in Figures 1 and 2.

Figure 4a and 4b show details of one embodiment of an interconnection between section modules.

Figures 5a and 5b show details of a further embodiment of an interconnection between section modules.

Figure 6 shows an isometric view of an inspection apparatus according to the invention, utilising an interconnection between modules as shown in Fig. 4a and 4b.

Figure 7 shows a cross-section of an alternative embodiment of an inspection apparatus according to the invention.

Description of preferred embodiments

An inspection tool according to the invention can be made modular, giving flexibility in the design and the possibility to exchange individual modules on site during operation. With reference to the drawings, this section describes exemplary embodiments of the inspection apparatus, however it is to be understood that a number of variations in the configuration is possible, depending on the specific operational requirements and conditions.

Referring to Figures 1 and 2, an inspection apparatus 1 adapted to traverse a chain 2 according to the present invention is shown. The inspection apparatus includes a chain cleaning module 3 comprising two chain cleaning units 4,5. The chain cleaning module 3 is accommodated to clean any dirt, deposits and marine growth off of the mooring chain links as the inspection apparatus moves along the mooring line. This cleaning allows proper non-destructive testing (NDT) to be carried out on the cleaned chain links of the mooring chain, as such contaminants could otherwise influence the accuracy of the measurements. In the exemplary embodiment rotating brushes form part of cleaning units 4,5 and are used to clean the chain links. Two sets of perpendicular brushes can clean two consecutive chain links, such that the brushes in cleaning unit 4 are mounted perpendicularly to those of cleaning unit 5, as illustrated in Figures 1 and 2. This configuration ensures that all accessible faces of the chain 2 will be cleaned independent of at which angle the chain is positioned in relation to the brushes.

Further, as shown in Figures 1 and 2, openings 20 can be provided in the cleaning module outer housing to prevent the cleaned debris from congesting the device. The brushes in cleaning units 4,5 may be configured so as to, in operation, produce a water flow that discharges contaminated water and debris through the openings 20. The rotating brushes will be powered with an engine which will be also installed in the apparatus. Alternatively, a pump can be used for this purpose.

A drive module 7 is provided to drive the inspection apparatus 1 along the chain 2. The moving mechanism has arms 8 with rotating rollers 9 to move the device along the mooring line. The moving arms 8 and rollers 9 work using an electrical engine, or can alternatively be powered hydraulically. The arms 8 will close to make contact between the rollers 9 and the chain links. By applying sufficient force on the arms 8 while rotating rollers 9, reliable driving of the apparatus 1 along the chain can be achieved.

Preferably, the inspection apparatus 1 is engineered to have substantially neutral buoyancy. This minimises the required power required by the drive module 7 to move the apparatus in the upward and downward directions along the mooring line. This is particularly advantageous in the parts of the mooring line where the movement of the apparatus is close to horizontal. Close to neutral buoyancy can be achieved by engineering the buoyant and sealed volumes of the device according to the estimated total weight of the apparatus.

A stop/lock function can be included in the inspection apparatus 1. This mechanism allows locking of the apparatus on any section of the chain 2 if additional, detailed inspection or testing is required. This can be achieved by applying pressure on the moving arms 8 while locking rollers 9.

The power for the engine will be provided using an umbilical 6 connected to the inspection device. The umbilical can be connected directly from a floating structure, such as an oil production platform, to the inspection device. An outer shell 10 which can freely rotate around an inspection apparatus module, e.g. the drive module 7, can be engineered for the umbilical connections to the inspection apparatus 1. Free rotation of the outer shell 10 can be made using rollers between the apparatus housing and the shell 10. A swivel or similar connection can further be used in the connection of the umbilical 6 to the outer shell 10. These features prevent tangling of the umbilical 6 around the inspection apparatus 1 in case there is some twist along the chain links of the mooring lines. The required length of the power and data cables to allow rotation is provided within the outer shell 10.

A sensor and electronics module 11 is provided in the inspection apparatus 1. The sensor and electronics module 1 1 includes appropriate sensing and NDT equipment 12 to detect cracks and wall loss thickness in the mooring chain 2. Suitable sensor technologies for this purpose include electromagnetic, ultrasonic and optical sensors. Phased array and X-ray could further be useful in measuring cracks and tear & wear, respectively. One particularly

advantageous sensing system is the magnetic flux particle. This NDT technique can be used for detection of defects in chain links of the mooring lines, with the changes in the flux density being used to detect the loss of wall thickness and corrosion in the chain links.

A permanent magnet is used to induct the magnetic field in the chain links. A symmetrical configuration is beneficial to install the Hall Effect sensors around the chain links. The unsymmetrical measurement from symmetrically configured sensors around the chain will provide indications of the location and magnitude of any defects on the chain links. The major benefit of the magnetic flux is that it will penetrate through the non-conductive marine growth without distorting the readings or affecting accuracy. The amount of perturbations in the measured magnetic flux will be used as an indication for defects on the chain links.

Optionally, other types of NDT techniques, such as phased array probes and X- ray, can be used. Moreover, a combination of techniques can be used, for example with a secondary sensing system to analyse any defects in more detail. After the initial screening technique is applied, the stop/lock system in the inspection apparatus 1 can be applied to lock the position and conduct detailed inspection to detect any defects with more clarity and higher sensitivity. The target crack size depth to detect can be less than 5% of the chain link diameter. The target loss of thickness to detect can also be less than 5% of the total cross-sectional area of the chain links.

A sensing system within the sensor and electronics module 1 1 may be employed to determine the actual position of the inspection apparatus along the chain 2. This may include a position sensor to detect movement of the inspection apparatus 1 along the mooring line.

All the above detection and sensing systems are known per se, and could be readily adapted for this purpose by a person skilled in the art.

The inspection apparatus further comprises chain guide parts 13, 14. The chain guide parts 13,14 can be arranged on the outward-facing part of any module section located at an end of the inspection apparatus 1. Advantageously, the chain guide parts 13,14 can be arranged in separate chain guide modules 15, 16 which can be mounted in a flexible manner to one of the other section modules, as illustrated in Figures 1 and 2. Each chain guide part 13,14 consists of a funnel-shaped structure to guide the mooring chain 2 into or out of the inspection apparatus 1 , even if there is some angle between consecutive chain links.

The use of chain guide parts 13 and14 in the inspection apparatus 1 , particularly in combination with flexible couplings between one or more section modules, allows the inspection apparatus 1 to be used on mooring chains with large catenary angles. The flexible couplings allow some relative movement between the section modules, in order to allow the inspection apparatus to conform to the shape of the mooring line, i.e. follow a catenary angle that the mooring line may have during operation. This is particular advantageous in embodiments of the inspection apparatus which are longer, e.g. due to the sensor and electronics module comprising a higher number of sensors and tools.

Figure 3 illustrates in more detail one possible flexible coupling design that can be used within the scope of the invention, wherein a number of connection brackets 17 are provided around the periphery of two section modules, and pairs of connection brackets are attached using a flexible isolator 18. The flexible isolator 18 can comprise a spring, a flexible rubber-like material, or the like. In the embodiment illustrated in Figure 3, four connection points are foreseen, however more or less can be used according to the requirements in a particular inspection apparatus design.

Figure 4a shows a preferred embodiment of the flexible coupling, where four springs 30 are used to provide the flexibility in the joints. The springs are made of stainless steel, and their stiffness are engineered based on the expected deformations and the required flexibility. More details of the spring connections are shown in Figure 4b. The springs are welded to square plates 31 at both ends. A bolt, 19 which is also welded to the end plates 31.forces the connection brackets 17 together and provides pre-tensioning of the flex joint connection. The springs 30 are engineered so as to provide an optimal flexibility and stiffness for the expected curvature and deformation along the length of chain lines. One advantage of using this type of connection is that the apparatus can be dismounted quickly, e.g. for cleaning or maintenance, by releasing the bolts 19.

In another alternative embodiment, illustrated in Figure 5a, a flexible tyre couplings 40 are provided between the connection brackets 17. Being flexible, the tyre couplings 40 have the inherent qualities of absorbing misalignments and adapt to the required flexibility between the section modules. The tyre couplings 40are connected to end plates 42 using four bolts 43. The end plates 42 are directly connected to the brackets 17 using the bolt 19. Since tyre couplings can allow some stretching and compression, isolators at opposite sides of the apparatus will then cooperate to provide the flexibility in the flexible joint, as couplings on one side of the apparatus will be compressed while they are stretched on the other side will be stretched.

The flexible coupling can be designed to absorb both axial and rotational misalignment. Any of the two above mentioned embodiments can easily be adapted to provide this feature with only workshop modifications.

Figure 6 shows an isometric view of an inspection apparatus according to the present invention, utilising a flexible coupling comprising springs 30.

Figure 7 shows an alternative embodiment of the present invention, wherein connection points are provided on the end plates of cylindrical section modules. In the embodiment shown, springs 30 are used in the flexible couplings, however this can be flexible tyre couplings or any other suitable flexible member. This provides the advantage that the flexible couplings are less susceptible to damage, e.g. by external mechanical impact, than by providing connection brackets on the outer periphery of the section modules.

Although some alternative coupling designs have been described here, it will be obvious to the skilled person that there are numerous ways of realising flexible coupling between section modules in the inspection apparatus 1 .