This paper synthesizes a method for model testing of non-conventional devices and operations. The basic idea is to perform numerical analysis in advance (or concomitantly) of the model test itself. This has been very effective over the years and its application to the development Pendulous Installation Method (PIM) of deepwater heavy devices has been effective as shown in the paper. Specifically, the PIM does not require boats with compensation systems for the usual vertical launching. The device to be installed is released from the free surface. It is hold by an auxiliary cable from a boat and connected by the installation cable to another boat some distance away. The auxiliary cable is then released and after an almost vertical trajectory of the manifold, the installation cable traction increases and the device rotates (pendulous motion) about the turning point on the other support boat. During the development of novel systems or operations, the industry is learning the importance of model testing. During the model tests, it has been understood that a two-scale problem had to be faced by the model test design. There are large bodies (the device) together with slender bodies (cables) in the same hydrodynamic situation. These two bodies have different scaling laws. How cope with it depends on the several aspects (inertia, damping, etc). However, with the help of a modern numerical model computer code, these problems may be anticipated by creating a framework of results that help in the interpretation of the model testing result itself.

This paper proposes a kinematic model and an inertial localization system architecture for a riser inspecting robot. The robot scrolls outside the catenary riser, used for underwater petroleum exploration, and is designed to perform several nondestructive tests. It can also be used to reconstruct the riser profile. Here, a realistic simulation model of robot kinematics and its environment is proposed, using different sources of data: oil platform characteristics, riser static configuration, sea currents and waves, vortex-induced vibrations, and instrumentation model. A dynamic finite element model of the riser generates a nominal riser profile. When the robot kinematic model virtually scrolls the simulated riser profile, a robot kinematic pattern is calculated. This pattern feeds error models of a strapdown inertial measurement unit (IMU) and of a depth sensor. A Kalman filter fuses the simulated accelerometers data with simulated external measurements. Along the riser vertical part, the estimated localization error between the simulated nominal and Kalman filter reconstructed robot paths was about 2 m. When the robot model approaches the seabed it assumes a more horizontal trajectory and the localization error increases significantly.

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.

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