Scintillation can lead to reduced signal levels or other potentially dangerous effects that could affect DP vessels
single frequency differential GNSS. Scintillation is another symptom of ionospheric activity. It causes rapid fluctuations in the phase and amplitude of the satellite signal as it passes through small-scale irregularities in the ionosphere. The effects of scintillation appear in different localised regions of the sky and thus only affect certain satellites at a time. Two effects occur during periods of scintillation: • amplitude scintillations can lead to
periods of reduced signal levels at the GPS antenna which results in an increase in the measurement noise within the code and carrier tracking loops • phase scintillations increase the dynamic stress on the carrier tracking loops which results in additional phase measurement jitter. Both effects result in an increase
in
pseudo-range measurement errors and, under extreme conditions, can lead to complete loss of signal lock. Veripos recommends that
the impact of increased ionospheric activity and the potential to lose both stable satellite and L-band communications is taken into consideration during risk assessments when planning critical offshore activities. It also recommends selection and use of non-GNSS positioning reference systems to mitigate the potential for loss or instability of GNSS position; and increased vigilance of DP operators on vessels, especially after sunset, for scintillation effects. OSJ
New thruster drive improves reliability
As Rolls-Royce points out, DP is crucial for many offshore operations, and a thruster or power failure will mean interrupting the operation and, in the worst case, the potential for an accident, so IMO regulations and class rules specify levels of redundancy for the various classes of DP. Rolls-Royce has developed a system provides
which offshore support vessels
with robust redundancy meeting DP2 failure requirements using the usual three bow thruster configuration. It says the solution provides the same level of security as adding a fourth bow thruster. As it notes, in normal DP operations, bow thrusters are often under-utilised. Its new solution secures better utilisation of the installed thruster power, at a low cost.
Environmental
A key requirement in this context are Regularity Numbers
a sequence of numbers that indicate
(ERN), the
statistical ability of a vessel to maintain position at a reference location in the Norwegian Sea under the changing weather conditions and considering different failure scenarios in the propulsion system.
Robert Kleppe, a principal engineer responsible for electrical systems at Rolls- Royce Marine, said the new thruster solution raises the fourth ERN number, describing the most significant failure, from a typical value of
www.osjonline.com
about 40, to 99, for a large PSV with three equally sized bow thrusters. “Providing ERN 99,99,99,99 without significantly increasing installed power is a great advantage of this solution,” he explained.
Mr Kleppe said the new system provides redundancy that extends through the whole system, from the DP controllers to the thrusters. “Without this extended redundancy it may be that even though power is available at a thruster motor, a single failure may knock out the supply to the auxiliary systems, which means that the thruster is no longer available,” he explained. “A solution with three similarly sized bow thrusters doesn’t contribute to increased position-keeping capability unless the third thruster has a redundant power supply, both main and auxiliary,” he explained. “However, if the third thruster can be deemed redundant, this three-thruster system has a similar capability to a traditional four-thruster system.” To achieve this high level of redundancy, there has to be an assurance that none of the essential electrical supplies to the thrusters will be lost. To ensure that this is the case, Rolls- Royce has put the system through a thorough failure mode effects analysis (FMEA) run by an independent auditor. The desired redundancy is achieved by equipping each thruster with an electrical
supply that is itself provided with redundancy. The main frequency converter has two fully rated front ends, one connected to each side of the switchboard (which itself has split busbars), a common DC link and an output unit. A dual-fed multi-drive system is adopted for the auxiliaries (that is, the hydraulic power pack motor and the pumps for the cooling system). Control system failure is guarded against by automatic changeover to the uninterruptible power supply, with further backup from a 24V DC battery. Completing the redundancy, to ensure the thrusters keep working despite any single failure, the thruster-room air-conditioning system is also duplicated.
The solution is applicable to the company’s bow thrusters and retractable azimuthing thrusters. By avoiding installation of an additional thruster, space requirements are limited. As the thrusters are located far forward in a vessel, the best possible yawing moment from the thrusters is ensured. Hans Martin Hjørungnes, head of section, electrical systems, at Rolls-Royce, said the concept had been warmly welcomed by its customers, and orders for several vessels had been placed, one of which is already in operation. The solution is also approved by class society DNV and can be offered for vessels classed by other societies too.
Offshore Support Journal I June 2012 I 51
Statoil
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