As field developments move into deep water, numerous subsea intervention tasks have been moved out of reach of human direct intervention. Remote operated vehicles (ROVs) and remote-operated tools (ROTs) are required to carry out subsea tasks that divers cannot reach. An ROV is a free-swimming submersible craft used to perform subsea tasks such as valve operations, hydraulic functions, and other general tasks. An ROT system is a dedicated, unmanned subsea tool used for remote installation or module replacement tasks that require lift capacity beyond that of free-swimming ROV systems [1,2].
ROV Intervention System
- Control room on deck for controlling the ROV subsea;
- Workover room on deck for ROV maintenance and repair;
- Deck handling and deployment equipment, such as A-frame or crane/winch;
- Umbilical to power ROV subsea and launch or recover ROV;
- Tether management system to reduce the effect of umbilical movement on the ROV;
- ROV for subsea intervention.
ROV can be divided into fives classes.
Suitable deck area and deck strength, external supplies, and ease of launch and recovery should be provided on deck for safe and efficient operation of ROVs. ROV control stations vary from simple PC gaming joysticks to complex and large offshore control containers/rooms on the platform or vessel. The control stations contain video displays and a set of operator/ROV interfacecontrolling mechanisms. A typical control container consists of operator console, lighting, electrical outlet, fire alarm and extinguishers, etc.
ROV Launch and Recovery Systems (LARS)
The LARS consists of a winch, winch power unit, crane/A-frame with fixed block, and ROV guiding system. Generally speaking, launch and recovery activities can be achieved by a simple rope with uplift force. However, to facilitate the deployment and recovery of the rope, a reel/drum is used, and a motor is usually used to rotate the reel and provide the uplift force. The motor may be either a hydraulic motor or an electromotor with/without a gear box used to reduce the rotary speed and increase the torque force.
The system of motor, reel/drum, base frame, and other ancillary structures such as a brake and clutch is normally called a winch. A fixed block, sustained at the end of a crane boom/A-frame beam, is used to change the upward direction of the required winch force to a downward direction and position the winch on the lower structure, for example, the deck. To restrain ROV motion while it is being lowered from the air to the water surface a LARS is used. This helps prevent, for example, damage to the umbilical by the bilge keel if side deployment is being used. The LARS may be equipped with a docking head, cursor, or guide rails.
Umbilical and TMS
One characteristic difference between an ROV and an autonomous underwater vehicle (AUV), is that the ROV has an umbilical that runs between the support vessel and the ROV to transport hydraulic/electronic power from the vessel to the ROVand information gathered from the ROV to the surface. The AUV, on the other hands, is a robot that travels underwater without tethering to the surface vessel/platform. An ROV is usually armored with an external layer of steel and has torque balance capacity. The diameter and weight of the umbilical should be minimized to reduce the drag force due to waves and currents as well as lifting requirements during launch and recovery of the ROV from the water to the surface. Normally the umbilical has a negative buoyancy, and the umbilical may be attached with buoyancy, for example, every 100 m (328.1m) to avoid entanglements between the umbilical and subsea equipment or the ROV itself during shallow-water operation.
A tether management system (TMS) is used to deploy the ROV for deepwater applications where the umbilical with negative buoyancy can launch and recover the TMS and ROV. The connection cable between the ROVand TMS can be an umbilical called a tether that has a relatively small diameter and neutral buoyancy. The TMS is just like an underwater winch for managing the soft tether cable. A TMS has two significant advantages:
1. ROVs can be moved more easily due to deleting the force implied by the umbilical, which may be the same as the flying resistance of the ROV itself in a water depth of 200 m (656.2 ft) and increase rapidly with increasing water depth.
2. There is no need to use the ROV’s own thrusters to get the ROV down to the working depth near the seabed. A powered TMS, i.e., installing some thrusters to TMS cages may be carried out to account for large drag force on TMS due to significant currents (e.g., current velocity of 1 to 1.5 knots) in some areas. The TMS is designed to manage the tether and can be either attached to a clump weight, mainly for the observation ROV, or to a cage deployment system.
MostworkclassROVs have a rectangular configuration and an open Al-based frame that supports and protects the thrusters for propulsion, underwater cameras for monitoring, lights and other instruments such as closed-circuit television for observation, the gyrocompass for heading detection, depth gauges for depth detection, an echo-sounding device for altitude detection, and scanning sonars for environment inspection. Most ROVs are near neutral buoyancy underwater.
They do have a little buoyancy to make sure the ROVs can float to the water surface during emergency conditions or if they break. An ROV moves downward with a vertical thruster. Generally, the buoyancy is provided by synthetic foam material above the Al-based ROV frame. An ROV’s weight is typically in the range from 1000 to 3500 kg. Examples can be seen from the table in Annex A of API RP 17H . The higher buoyancy center and lower weight of gravity ensure that the ROV provides good stability performance.
Traditionally,ROVs had been designed and built foroperations inwater depths of 100 to 1000 m (328 to 3280ft), primarily supporting drilling operations, including seabed surveys, water jetting, and seal ring installation, as well as providing light construction support and inspection work. These ROVs have payload capacities of around 250 kg with power ranging from 40 to 75 hp. As the oil and gas industry has probed into deeper and deeper waters, demand has increased for ROVs that provide diverless solutions for such tasks as remote interventions and pipeline/umbilical tie-ins.
The payload capacity of an ROV is limited by:
- ROV power;
- Structural integrity;
- Manipulator load an torque capacity;
- Current condition.
The navigation of ROV includes general navigation and accurate navigation. The deck reckoning method and hydroacoustic method are used for general navigation. The hydroacoustic method is the most widely used today via the LBL system, in which there is a responder array on the seabed, at least one transponder set on the ROV, and one receiver set on the vessel.
Accurate navigation is used to lead the ROV to the target object. A gyrocompass provides the ROV heading and a viewing system is used that comprises imaging sonar/low light cameras and lights. Normally the power of a subsea light is between 250 and 500W. An ROV might also have 750 to 1000Wof power for large lights and 100 to 150Wof power for small lights. There is also a cubic TV to provide 3D object configurations, that is, to obtain data about target thickness and distance between the ROV and the target.
The propulsion system for an ROV consists of a power source, controller for an electric motor or a servo-valve pack for a hydraulic motor, and thrusters to adjust the vehicle condition (trim, heel, and heading) and to propel the vehicle for navigating from the TMS to the work site, and vice versa. Being the main part of an ROV propulsion system, the underwater thrusters are arranged in several ways to allow for proper maneuverability and controllability of the vehicle through asymmetrical thrusting and varying the amount of thrust. The thrusters need to be adequately sized for countering all of the forces acting on the vehicle, including hydrodynamic and workload forces. There are a wide range of thrusters from electrically powered to hydraulically powered. In general, electrical thrusters are used for smaller vehicles, while the hydraulic ones are used for larger and workclass vehicles.
A wide range of underwater video cameras are used in ROVs for viewing purposes, typically for navigation, inspection, and monitoring. Camera image sensors include low-light silicon intensified targets (SITs), charge-coupled devices (CCDs), and HADs for high-definition images. Some cameras are equipped with LED lights providing illumination for close-up inspection and eliminating the need for separate lighting. Images captured by a camera are transmitted as video signals through the tether and umbilical to a video capture device on the water surface.
An ROV is normally equipped with two manipulators, one for ROV position stabilization, normally with a five -function arm, and the other for intervention tasks, normally with a seven-function arm. Manipulator systems vary considerably in size, load rating, reach, functionality, and controllability. They may be simple solenoid-controlled units or servovalve-controlled position feedback units. The end of the arm is fitted with a gripper, usually consisting of two or three fingers that grasp handles, objects, and structural members for carrying out an activity or stabilizing the ROV.
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