Wave flume and numerical test on launch and recovery of ultra-deep-water ROV through splash zone under wave and ship motion

Abstract

An ultra-deep-water remotely operated vehicle (ROV) can be designed to have an almost neutral buoyancy to compensate the large tension coming from the long and heavy umbilical that connects the ROV to the surface facility. However, an ROV with light submerged weight experiences large motions when it is traveling through the splash zone due to the hydrodynamic loads on the ROV. These motions lead to tension spikes in the umbilical, also known as snap loads, which can damage the umbilical components and endanger the ROV operation. It is important to ensure that the tension spikes in the umbilical never exceed the safe working load during launch and recovery of the ROV. Around the splash zone, the non-linear interaction between ROV and surface waves strongly governs the tension in the umbilical which is also exacerbated by the ship motions and winching mechanisms.This study focuses on experimental measurement and numerical prediction of the tension in the umbilical during launch and recovery of an ultra-deep-water work class ROV passing through splash zone under a set of regular waves combined with winching process and ship motions. A 1:10 model of a deep-water ROV is built then lowered to and raise up from the water under 5 different regular wave conditions generated in a wave flume facility. For each wave set, three different ship motions and five winching conditions are simulated by superimposing a sinusoidal motion to the constant haul-in and pay-out of the umbilical from an electric motor. In the end, a simple numerical model is developed to replicate the wave flume experiment.The wave flume experiments and numerical simulations show that the recovery process experiences larger umbilical tension than the launch process in general. For the launching process, the snap load can be suppressed down by increasing the pay-out speed of the winch faster than the terminal velocity of the ROV in water though a certain winch mechanism is required to minimize the tension spike due to the sudden start or end of the winching. For the recovery of the ROV, increasing the winch speed does not significantly reduce the maximum tension in the umbilical, though minimizing the ship motions shows a great effect on suppressing the tension due to snap load. The developed numerical model shows a good agreement with the experimental results hence it is also used to predict safe working window for launch and recovery of the ROV through splash zone.