Remotely Operated Work Platform

01/01/1970

A radical new approach to subsea operations

As the industry searches for hydrocarbons in deeper and deeper water, the difficulties related to this exploration and installation of related equipment are increasing substantially. Subsea installation methods, which were designed for relatively shallow water applications, are being stretched to make them suitable for the requirements of work at depths ranging from 500 to 2,500 metres but most of them become impractical.

Ronald de Haas, b&a industries bv, The Netherlands

B&a industries bv, drawing on its field experience in ultra-deep water installation, salvage and underwater positioning, has designed, built and commissioned the first remotely operated work platform, pushing the limit to 3,000 metres. Named Promaster, this self-contained tool is capable of handling loads up to 40 ton in full DP mode controlled by an independent positioning system that guarantees an absolute accuracy of less than 1.0 metre in 1,000 metres water depth. Based on a real-time positioning system which supports the DP system, no LBL arrays are required and the number of surface vessels is reduced to just one vessel deploying Promaster.
In order to be a work platform, mass was required to provide stability. The Promaster tool has a mass of 20 tons which is supported, through the A-Frame by the Lift Winch (master). The necessary electrical power and communication links pass through the umbilical which is controlled by the umbilical winch (slave). The double-winch assembly provides the primary vertical positioning to Promaster, while fine heave compensation is achieved by a secondary actuator located subsea.
Horizontal position and excursion are controlled by the thrusters on board the tool. By deploying a secondary umbilical 1 megawatt of electrical power can be brought to the unit to drive off-the-shelf or custom-tailored tooling packages which may consist of cutting equipment, a piling hammer, dredgers, hot stub power source, manipulators and so on. The strength of the Promaster tool lies in its unique combination of features: precision, power and flexibility, which makes it such a highly innovative and efficient subsea work platform.

Sea Trials
Mobilised on the offshore vessel MV Seaspan, the Promaster system sailed to the Atlantic Ocean south west of Cadiz to undergo final commissioning and calibration of the positioning system. An extensive programme had to be carried out as there is a world of difference between function testing under controlled conditions, performed during January in Rotterdam harbour, and operating a system in the open ocean. The trial programme was divided into three sections: function testing, over boarding and ‘swimming’ at depth; calibration of the navigation and positioning systems and finally the deployment and recovery of a purpose-built platform, weighing 4,000 kg. This last section of course being the spectacular part.

Offshore Oil Industry Positioning
Positioning in the oil and gas industry is carried out to define locations at the seabed for installation of a variety of equipment. Traditionally, this positioning is done by deploying a long base line acoustic array followed by the process of calibration and post-processing of the data. Only when the coordinates are established, can the installation process commence. These jobs are carried out by installing clump weights and guidelines and with the assistance of a considerable number of surface support vessels and ROVs.
b&a industries approached this problem in a radically different manner with the development of its remotely operated work platform. The objective of the Promaster system is to reduce operating time at sea and to reduce the quantity of equipment required for offshore operations. The Promaster is self contained and does not require any input signals from equipment other than that which is part of the system. Furthermore, Promaster works in ‘real time’ and therefore does not require post processing which, again, speeds up the process considerably.
The Promaster positioning system is based on the following design and in-built equipment:

USBL (Ultra Short Base Line)
Doppler log + fibre-optic gyrocompass
Sound velocimeters
Dual-head scanning sonar
Depth sensor
Motion sensors
Altimeter
b&a industries’ specific software

The main features of the Promaster positioning system are based on:

Ultra short base line combined with a correlation process (correlation techniques to improve capability in noisy environments and to increase accuracy)
The acoustic array is fixed to the ship’s hull
Beacons, installed on vehicles, are used in responder by synchronous triggering through the umbilical via the fibre optic. The beacon delivers an acoustic response with local information, which is decoded by the electronic unit. Phase and delay measurements are performed to localise the beacons in relative coordinates
Combination with ship’s attitudes and DGPS position enables localisation of the beacons positions in absolute coordinates and to display them on a navigation computer

The combined Doppler log + fibre-optic gyrocompass mounted on the Promaster tool, smooth in real time the random noise of the USBL, using the Salomonsen filter.
A sound velocimeter is mounted on the Promaster tool, sending data to the USBL surface unit during the descent and the recovery. Another sound velocimeter is fitted on the deployable acoustic array where is also located the critical and unsettled layer. Sound velocity profiles, delivered and updated in real time, are provided to calculate the ray bending and thus to correct positioning.
The scanning sonar displays any man-made or natural target on the seabed. The sonar can measure range and bearing to the objects by use of a cursor function and output the digital values to the navigation software. Positions of objects can either be predefined positions or the navigation system can allocate coordinates to each of the selected objects. After the objects have been given coordinates, they can be used as navigation references in a local coordinate system. It results in an accuracy of 0.1m in relative coordinates.
A Digiquartz depth sensor is included in the embarked beacon, which allows transmitting very accurate depth data to the USBL processing unit. Those data will be used to improve the calculation of the positioning.
The embarked motion sensors allow balancing the Promaster tool and its payload to avoid any moment during the placing phase on the seabed.
The altimeter is mounted on the Promaster tool to give accurate vertical distance between the sea floor and the vehicle.
The Promaster tool has been designed to place subsea structures on the seabed in an automated fashion. The tool has four thrusters attached to it. Two for orientation are mounted on the ends of the tool and are used to hold the heading of the tool against the current and against the twisting effect of the lift cables. Two position-and-heading thrusters are mounted on rotary tables, which allow the thrusters to be vectored, which gives the tool a 360¼ envelope within which to move. To allow the thrusters controller software to accurately position the tool, the Doppler log current profiler, via the navigation software, sends the required data concerning the current strength, the current direction, the present heading and the intended heading of the vehicle. By using the Promaster D.P controller, in manual or in auto mode, it is now possible to decide PromasterÕs intended heading and select between many waypoints and make it the intended position. Furthermore, the operator can input offsets to the selected waypoint; the offsets will be input with X, Y relative to the Promaster heading. The operator has many other sub-software to check the different status of the subsea instruments in use for the calculation of the position and to get the optimal result by stabilising and filtering the position.

Results of the Trials
Although the tool had been designed to shuttle objects to the seabed and hold station adjacent to such objects, it was still amazing to see the system perform these functions with such surgical precision.
During sea state 4 the Promaster system was deployed to recover the platform which had been placed on the seabed at 1,220 metres, two days previously. While the ship was moving and rolling on the waves, the tool was kept motionless by its heave compensation system at the seabed holding position next to the platform.
Controlled by the positioning system and its 6-tonne bollard pull delivered by four thrusters, this massive tool could approach, reconnect and retrieve the platform.

Biography of the author
Ronald de Haas is the Commercial Manager of b&a industries B.V. He started his career in the oil industry as drilling supervisor on drilling rigs. Eventually his interest turned to the commercial side of the oil industry, after holding positions with drilling contractors and later an FPSO contractor he joined b&a industries.

Remotely Operated Work Platform

HI improved search! Do you like it?

Interactive

Placing a Tide Gauge Buoy

Current Issue

Newsletter

Whitepapers

Upcoming Events