PurposeThe purpose of this paper is to describe a new multi‐sensor robotic system designed for riser, mooring lines and umbilical cables in situ underwater inspection. Due to the aggressive operation environment, such structures are susceptible to a broad spectrum of failure causes, such as aging, mechanical, chemical and thermal loads, hydrodynamic stresses, vortex‐induced vibrations and installation or fabrication non‐conformities. Current inspection methods present major risks and inefficiencies, especially as deeper fields are being reached for exploitation.Design/methodology/approachThe SIRIS (In Situ Riser Inspection Robotic System) is designed to reconstruct the actual riser profile and perform non‐destructive tests. The robot is propelled by thrusters to scroll by the outside of the catenary riser. Mechanical, electronic hardware, image acquisition and software/firmware design are described here.FindingsSimulated data from an inertial measurement unit is fused with depth sensor measurements, using a Kalman filter to reconstruct the riser profile, with small localization errors. Laboratory and sheltered waters tests were successfully executed to assess robot subsystems' performance: imaging, leakage, displacement and easiness of operation.Research limitations/implicationsThe robot prototype is designed to operate down to 250 m deep, although the final goal is reaching 3,000 m. Tests offshore, in a real oil production platform, have not been performed up to this moment. In the present version, the robot must be coupled to the riser with the aid of a scuba diver.Practical implicationsThe robot is expected to allow non‐destructive testing in risers that cannot be performed nowadays with the existing tools. The inspecting procedure is easy to operate and does imply any kind of production stopping. More accurate assessment of the riser structural condition can allow extending its life span, thus avoiding early decommissioning.Social implicationsBetter assessment of actual riser facilities status will have great impact on reducing the chance of oil spill episodes and serious environment damage.Originality/valueThe design, construction and evaluation of a robotic tool for non‐destructive riser inspection has been described. A few similar robots exist in literature but none of them is able to reconstruct the actual riser profile.

This paper and the companion paper (Rateiro et al., 2011) present an illustrative case of the joint application of experimental tests and numerical simulations for the proper analysis of a complex offshore operation (launching of a sub-sea equipment using one or two vessels). The main idea of the whole study is to compare two methodologies and operational procedures for the installation of the equipment in the seabed, using either one vessel (conventional operation) or two vessels in a synchronized operation in a Y-configuration. The experiment was conducted under a simplified configuration, and uses ODF (one degree of freedom) servo-actuator to emulate the vessels induced motion. The hydrodynamic properties of the equipment was then calculated, and some preliminary conclusions about system dynamics could also be drawn. After that, numerical simulations were conducted, considering the coupled dynamics of the vessels, cables and equipments under irregular sea state. Those simulations were used for determining the limiting environmental condition for a safe operation, and are described in the companion paper. This paper describes the reduced scale experimental setup used for evaluating the hydrodynamic properties of the equipment during a subsea installation under waves excitation. The reduced scale model of the equipment was attached to one or two servo-actuator, that emulate the wave-induced motion. The tests were conducted at the physical wave basin of Numerical Offshore Tank (Tanque de Provas Nume´rico – TPN). The experiments enabled the preliminary evaluation of the dynamic behavior of the equipment when submerged by one or two launching cables. In the later case (two launching cables), several tests considering phase shifts between the servo-actuator have been conducted. The reduction in the dynamic amplification of cable traction could also be experimentally verified.

The torpedo anchor is a novel kind of device to moor floating offshore structures. It has been proved in practice that this kind of anchoring may be used for both drilling and production offshore activities. For drilling, it is indeed easily recoverable and for large production, it has enough holding power even for large production platforms. There are a lot of soil-interaction aspects to be considered and the installation is one of them. The installation procedure is to release the torpedo from a high enough position from the sea bottom to allow the device to reach the terminal velocity: A correct amount of kinetic energy at the bottom is essential for the penetration. Besides this, the anchor has to reach the bottom in a vertically up right in order to maximize the final holding power in all directions. Therefore, the work addresses two hydrodynamic aspects for the installation design and analysis. The first is the drag minimization and the second is the directional stability. If the drag is be kept to a minimum (without compromising, later on, the soil interaction) then the terminal velocity is higher. The work shows that parameters like the mass and the shape are essential for this. On the other hand, the shape and mass distribution have a strong influence on the directional stability. One important parameter is the rear line length connected to the anchor, which is necessary for further connection with the final mooring line: this parameter influences both the terminal velocity and the directional stability. The presence of the rear line and its role is a novel problem and it seems to have no parallel in other filed applications. The work addresses all this aspects under the light of a novel model testing performed in a model basin that is 15 m deep. It is important to say that this model testing procedure has been conceived to attend specifically the torpedo anchor evaluation. For that matter, the work presents an extrapolating mathematical model. Besides that, an analytical model is shown for the directional stability, together with time domain numerical evaluation. Different model have been used in the tests performed with and without the rear line. Finally, the work presents the model testing design including the use of imaging processing to get the anchor tracking during the launching.

This paper and the companion paper OMAE2010-49946 (de Mello et al., 2011) present the experimental and numerical results of the analyses of a novel method for subsea equipment installation. The conventional procedure, which is usually used, may lead to dynamic amplifications, depending on the launching cable stiffness and length. In the proposed method the equipment is lowered using two tug boats in a Y configuration. Trough the control of their relative position and also of the cable length it is possible to diminish the tension amplification.

This paper presents the development of cooperative control technique applied to vessels equipped with dynamic positioning (DP) system. An illustrative case study is suggested: the launching of subsea equipment using two DP vessels. In this example, the cooperative system controls the relative distance between the DP vessels. One of the advantages of this method is the increase of operation’s safety and operational window, since, among other factors, the tension in the launching cable is reduced by half. Initially, it was proposed the control of vessels relative positions, trying to keep the movements at the top of the A-frames in counter-phase. This avoids the slackening of the launching cable. For this, an algorithm based on phase estimator (Hilbert transform) associated with a PD control was implemented. The results showed that for regular waves this strategy was effective. A dynamic mapping was then obtained using simplified 2D simulator, previously validated by comparison with experimental tests. In these maps, two regions are defined — occurrence or non-occurrence of cable slackening — as a function of the distance of the vessels and the depth of the subsea equipment. This map defines the proper set-point for the DP systems for each depth of the subsea equipment. This map is used to define the best relative position for the vessels. In addition, the hoisting control receives the measurements of the vertical motion at the top of the A-frame, and compensates its motion, trying to maintain a constant lowering velocity. This control was implemented considering errors of 10% and delay of 0.5s in the measurements. The results confirmed that the control is able to eliminate the tension peaks and the occurrence of slackening in the launching cable. The conclusion is that the appropriate control strategy, considering regular waves, is to combine the control of both position of the vessels and hoisting of the cable. Therefore the position control, coupled with dynamic mapping, defines the “optimal path” to be followed during the line hoisting, trying to keep the vessels as close as possible to the “no slackening” region.