New portable navigation system raises vessel availability
With the cost of hiring anchor-handling vessels (AHV) often approaching $200,000 a day, making full and efficient use of their time has become imperative for Acteon clients. To help operators do this, Trident Offshore has launched a portable navigation aid called TriADS (Trident attitude and direction system). Based on the successful TriPOD (Trident positioning over data link) system, TriADS includes differential global positioning system (DGPS) vector heading and remote configuration options.
“Putting a ‘pod’ on the vessel” has become a familiar phrase heard from its clients since Trident became the first company to combine a navigation computer, a DGPS receiver and a radio modem telemetry system in a single box (TriPOD). Eliminating the mass of cables required to connect the individual elements of a navigation package and simplifying the set-up instructions have meant that TriPODs are regularly configured and tested onshore, passed to an AHV from a rig and then mobilised by the vessel’s crew at sea. However, for applications requiring a gyrocompass – where a vessel’s heading is required in addition to its position – the AHV needs to return to harbour to install heavier precision instruments and for offset measurements and gyro calibration values to be configured in the navigation software.
Recognising the time delay that gyrocompass installation was causing, Trident developed TriADS with the option of connecting it to either a DGPS heading sensor or a standard gyrocompass. Once stabilised, the heading sensor has a potential accuracy of ±0.5°; during initial trials conducted on a Chevron rig move in April 2006, the TriADS unit maintained a consistent accuracy within 2° in comparison with a surveygrade gyrocompass also installed on the AHV.
A recent project in the North Sea for Maersk saw four TriADS units successfully working together to first install on location and then control the swing heading of a floating production, storage and offloading vessel (FPSO) while a dive vessel came alongside to perform some extended maintenance. TriADS units were placed on the FPSO, the primary AHV and the two heading control tugs used to swing the FPSO. The systems ensured that the FPSO and the dive vessel were held in position for the duration of the operation without failure. The project lasted for four months. This same system was requested by the client for work in the second quarter of 2007.
Mike Grubb, survey manager, said, “GPS heading sensor development is moving at a rapid pace, and while the current gyrocompass, it is excellent for most of our AHV operations. After all, there is no practical advantage in positioning a stern roller to within 0.1° when the vessel is to lower an anchor from the stern roller through a water depth in excess of 100 m to a target of approximately 10 m on the sea floor.
“When the European GNSS [Global Navigation Satellite System, commonly known as Galileo] network is available next year, the pinpoint positioning that will be possible is likely to mean that satellite-derived heading systems will be utilised by most operators and that the use of survey-grade gyros will decrease.”
TriADS units are now used on most rig moves requiring heading-stabilised HV navigation systems.
Trident has also developed a system to remotely configure TriADS units using a wireless PC network. With a vessel on station outside a rig’s 500-m safety zone, Trident can upload and configure new navigation software for a project in a few minutes. This simple procedure replaces the current process of bringing the vessel alongside, transferring the TriADS to the rig and updating the instructions, before passing it back to the vessel for reconnection and powering up.
The remote connection has been tested up to 900 m away, and has been used by engineers on the rig for direct fault diagnostics as well as for uploading project plans or corrections. The remote control and configuration facility of TriADS enables the rig-based navigation engineers to upload revised drawings and alter the configuration of the proposed positions on the AHV’s navigation screen. It is often the case that the client needs to change the proposed mooring spread at short notice, possibly through rig anchor winch problems; any such changes can be passed to the AHV navigation system remotely, which saves hours of operational time.
In addition to speeding up the corrections on a single project, reconfiguring the navigation system remotely will, in some cases, enable a vessel to move from one client project to the next without having to return to harbour. Trident is using its experience and expertise to develop tools that not only improve navigational capability but also optimise offshore operations to the financial benefit of its clients.
DGPS for rig positioning
Typically, a full rig system will include a navigation computer; primary, secondary and tertiary DGPS systems; at least one gyrocompass; and dual radio telemetry systems for communicating with the tugs. Consistent with its policy of owning rather than hiring the components for its navigation packages (one of the reasons Trident is able to produce compact, single-box units), Trident has selected Veripos as its service provider for the DGPS.
Veripos’ Standard and Standard+ services offer positional accuracy to within 1 to 2 m, depending on the distance to the closest reference station and the number of visible satellites. These global reference stations are used to derive a differential correction for the GPS L1 C/A-Code observations for all the satellites in view at each reference station. These corrections are transmitted to the TriPAC (Trident positioning and control) unit and used to correct the on-screen image.
The Standard+ service uses dual-frequency GPS receivers to maintain positional accuracy during periods of heightened ionospheric activity. Changes in the ionosphere can affect the propagation of GPS signals. Increased ionospheric activity can introduce large biases into standard single-frequency DGPS because the slower signal rate results in the failure of the correction model used to predict the behaviour of the ionosphere. Dual-frequency GPS receivers calculate the true ionospheric delays at both the reference station sites and the user end to derive an “iono-free” DGPS solution.
Veripos’ Ultra service offers decimetre-level positional accuracy. It is typically switched on at the end of a rig move for final positioning, above a wellhead, for example. The service is based on precise-point positioning techniques in which a set of globally valid corrections for individual satellites is transmitted from the Jet Propulsion Laboratory in Houston, USA, directly to the TriPAC receiver. The presented position is therefore independent of the distance from the reference stations.