Standby Vessel Rescue of Personnel from Offshore Platforms - Section 4

This Section covers

Implementation
Conclusions

Implementation

Vessels

The new standby vessel arrangements were implemented in September 1996. The mother vessel is positioned at the geographic center of the circular guardzone. The mother vessel supports all the installations manned that day within the guardzone. The radius of the guardzone changes, depending on the prevailing wind and sea conditions; but is taken no greater than 10 nautical miles. The vessel master, who is the sole arbiter as to the size of the guardzone, must set his support circle to meet the set performance standards, which as stated earlier are based on time to rescue or arrival at the scene of an emergency.

Where the number of workfronts is such that the mother vessel cannot support them all, additional standby vessels of the conventional type are used. These conventional vessels are also assigned to a guardzone but in order to meet the set performance standards, their guardzone radii are limited to 5 nautical miles.

Compared with a conventional standby vessel, the mother has the following additional hardware.

• Two 34 ft daughter craft each with their own Personal Locator Beacon (PLB) direction finding sets and visual display units to show the locations of the personnel to be rescued.
• Two sets of heave-compensated Caley Davits.
• Radar receiver to accept field radar information from the radar system set up on one of the manned complexes. This allows the master of the mother vessel to have a radar horizon in excess of 20-30 nautical miles from the vessel, which is far in excess of what can be achieved using the standby vessel’s own radar system.

When a Personal Locator Beacon (PLB) is activated either by contact with seawater or manually triggered, the unit sends a VHF signal to a pre installed booster unit on the installation that the person has just left. This information is immediately re-transmitted on UHF to the mother vessel master, whose computer displays a ‘’course to steer’’ and ‘’a time to intercept’’. When the daughter craft comes within 0.6 to 2 nautical miles of the person in the water, it switches to its own VHF direction finder set for better accuracy when finally homing in on the person in the sea.

In this implementation, the mother vessel has a sister ship with identical software and hardware, including Caley davits, available to accept the daughter craft when necessary. The daughter craft are swapped over when one or other vessel needs to go out of the field for maintenance. When both vessels are in the field, one is used as the standby vessel and the other is used as an in-field supply vessel.

The current limitations set by the Marine Safety Agency for daughter craft operations are:

• The daughter craft should not operate at more than 10 nautical miles from the mother vessel.
• The daughter craft should not be operated in seas greater than 3.5 meters significant height.
• The daughter craft should not continuously operate for more than 4 hours with a 3 man crew or 8 hours with a additional coxswain.

Platform-based radar system

A platform-based radar system was installed on one of the manned complexes to provide the mother vessel master with continuous warning of approaching vessels. The height of the radar antenna allows the mother vessel master to have a radar horizon in excess of 20-30 nautical miles, depending on weather conditions. This is much better than can be achieved with the lower antennas on standby vessels, especially in rough seas. Moreover, the platform-based system has automatic processing and alarm capabilities, reducing the need for human vigilance. The mother vessel master receives ‘cleaned up’ images of vessels moving within and converging on the field. The speed and direction of each vessel is displayed with a predicted time to impact if apparently on a collision course. Procedures are in place on all the installations defining the actions to be taken if a vessel is on a collision course.

Work planning systems

In order to facilitate the correct deployment of the mother vessel and the most economical utilization of the sister vessel, the Operator’s work planning system was integrated with new software capable of optimizing the sharing arrangements for the mother vessel and its sister vessel. This information is sent ‘live’ down a telephone link to the mother vessel master. The system records all operating envelopes and vessel movements for that day.

Sea trials

The purpose of the trials was to demonstrate that the chosen hardware and software could be integrated in an offshore environment to deliver a total system that offered effective arrangements, a good prospect of recovery and which maintained or improved on existing arrangements.

The trials set out to simulate a number of foreseeable emergency events.

• the rescue of persons from a platform spider deck;
• the rescue of a close knit group of people from the water as per helicopter ditching;
• the rescue of a single person representing someone falling overboard;
• the rescue of a number of persons in the water who have become dispersed.

The trials were carried out in April 1996 and witnessed by the Health and Safety Executive and by the Operator and various contractors involved in the implementation of the system.

The results showed that the systems functioned well and that the defined Performance Standards can be met. In addition, areas for improvement were identified and incorporated into the commissioning trials held in September 1996.

Vessel commissioning trials

Trials were carried out in September 1996 to demonstrate that the vessels, their crews and the offshore and onshore support staff had reached a sufficient level of competency to fully support each of the performance standards.

The trials undertaken related to the following scenarios:

• ditched helicopter in the guardzone, day time;
• normal operational support – escape from a not normally manned installation;
• overside working – person overboard at night during planned work activities.

Ditched helicopter

The sea conditions for these trials were rough with wave heights of 4 meters and wind speeds gusting between 20 to 30 knots. Twelve dummies representing 10 passengers and two crew were deployed, each with a PLB, to simulate a helicopter ditching. At this point the mother vessel was situated 4.5 nautical miles from the incident. The first dummy was recovered by a daughter craft after 15 minutes (the performance standard is 20 minutes or less). All twelve dummies were recovered after 35 minutes. This is an average pick up time per survivor of 1.6 minutes in relatively heavy seas.

Man overboard

Sea conditions were moderate with wave heights of 3.5 meters A daughter craft was deployed in the water approximately 800 meters from the platform. The alarm signal from the PLB was registered almost immediately. The daughter craft went directly to the dummy in the water and the dummy was recovered to the daughter craft within 2 minutes from entering the water. The daughter craft was recovered to the mother vessel within 10 minutes of starting the trial.

Evacuation

For this exercise ten dummies, each with a PLB, were placed into the sea at short intervals to simulate the uncontrolled evacuation of a typical not normally manned installation by its maximum complement of 10 persons.

Weather conditions were moderate with 3 meter waves, tide 0.7 knots and wind speed of 20 to 24 knots. The emergency signals from the PLB's were transmitted to the mother vessel almost immediately. A daughter craft was launched with a course to steer and estimated time of arrival. The daughter craft traveled at speeds of 34 to 36 knots towards the incident. At approximately 1.5 nautical miles from incident the onboard direction finding equipment locked on to the strongest PLB signal which in turn led the daughter craft to the first ‘man’ in the water. This dummy was recovered within 22 minutes of the first alarm signal. All ten dummies were recovered within 28 minutes of this alarm. The longest time a dummy was in the water was 23 minutes. The dummies had drifted 400 to 500 meters from the platform and were dispersed over an area of 100 meters diameter.

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Conclusions

The mother/daughter standby vessel arrangements were implemented in September 1996. They were demonstrated by risk assessment, and later proved by trials, as capable of maintaining or improving safety levels while reducing the number of standby vessels required to support the installations in the field.

Acknowledgments

Sea Marshall Rescue Systems Ltd developed the Personal Locator Beacons mentioned in this article. Method Computing developed the software for planning the daily standby vessel sharing operations. SafeMarine integrated radar, PLB's, direction finding equipment and other hardware and software into a complete electronic system for managing standby vessel operations and co-ordinating rescure operations.

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