Standby Vessel Rescue of Personnel from Offshore Platforms - Section 3

This Section covers

Risk Analysis

Performance Standards

Discussions with Workforce

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Risk Analysis

Method

In order to determine the most effective standby vessel sharing arrangements, the concept of a ‘sharing circle’ was introduced. A sharing circle is a circle of a given diameter that can be drawn anywhere on the map of an offshore field – all installations circumscribed by the sharing circle can by definition be supported by a single standby vessel.

A wide range of sharing circle diameters was analysed. At each diameter, the risks to personnel were evaluated against the Operator’s proposed 1995 workscope. This workscope encompassed all the maintenance, inspection, construction and drilling work programmes. In order to determine the risks and costs for each sharing arrangement sophisticated computer simulations, using Monte Carlo simulation techniques, were developed.

The risk analysis was based on careful review of the Safety Cases that had been produced by the Operator under UK safety legislation. Event tree analyses were performed to quantify the statistical numbers of persons likely to be in need of rescue from the water in various emergency scenarios. Other analyses investigated rescue times and survival probabilities for each of the standby vessel deployment options. These analyses also investigated the merits of different types of protective clothing and the use of Personal Locator Beacons (PLB's) that if worn by those members of the offshore workforce considered to be exposed to greater risks e.g. overside working, would quickly identify their location in an emergency.

The investigation also investigated the role of a platform-based radar early warning system as a precaution against ship collision.

Results

Survival Probability

A Monte Carlo simulation was carried to explore the sensitivity of survival probability to:

• type of clothing – offshore work vest, helicopter transit suit or high insulation suit;
• winter versus summer conditions;
• standby vessel operating distance;
• type of rescue craft deployed.

The results of the simulation indicated:

• Work vests do not provide adequate protection in the North Sea under either winter or summer conditions—a helicopter transit suit or better greatly increases the survival probability.
• The use of PLB's and daughter craft significantly improves survival probability.
• With daughter craft, PLB's and transit suits, survival probabilities are close to 100% for standby vessel operating distances of up to 10 nautical miles from the protected installation even in winter conditions.
  

Rescue Times

Rescue times were estimated by Monte Carlo simulation techniques for a single person in the water, for a ditched helicopter, and for an abandoned platform, with a standby vessel at various initial distances away. The simulations were carried out both for conventional, relatively small, open fast rescue craft and for the new, larger, enclosed daughter craft.

The simulation technique yielded a probability distribution for the time to rescue.

The speeds of the fast rescue craft, daughter craft and standby vessel were calculated as functions of significant wave height. These speed relationships were based on performance information from the Royal Netherlands Lifesaving Institute, boat builders and other operators.

The arrival time of the daughter craft at the installation was calculated as a function of:

• mother vessel speed;
• time to launch the daughter craft;
• daughter craft speed;
• distance to survivors.

A similar analysis was performed for rescue by fast rescue craft. It was recognised that the speed of the fast rescue craft falls off more rapidly with increasing wave height than it does for the daughter craft. The time to recover the fast rescue craft onto the SBV was also included. The daughter craft, being enclosed, do not necessarily have to be recovered onto the mother vessel for the rescued personnel to be considered safe. In the most extreme weather conditions, when recovery to the mother vessel is unsafe, enclosed daughter craft may make their own way back to shore with the survivors safely inside.

The following conclusions were drawn from the rescue simulations:

• Conventional standby vessels with open fast rescue craft operating at a distance of 5 nautical miles from the manned installation are capable of rescuing a single person within 30 minutes with 50% probability. The upper bound rescue time (90% probability) is 72 minutes. The upper bound reflects the difficulties in locating a person in heavy seas and conditions of strong current and wind drift.
• The introduction of PLB's would convert a search and rescue operation into one of rescue alone. This would reduce rescue times especially in heavy seas and conditions of strong current and wind drift and make rescue times a less variable quantity.
• With the introduction of PLB's a standby vessel equipped with a daughter craft was calculated to be capable of rescuing a single person within 30 minutes with a 50% probability when operating at a range of 10 nautical miles.

The advantages of using totally enclosed daughter craft of around 30-40 ft length are summarised as follows:

• The hull geometry and dimensions allow them to operate at higher speeds in stormy seas than the smaller conventional fast rescue craft’s.
• Enclosed and heated accommodation with sufficient area for stretcher cases greatly enhances ability to administer immediate warmth and protection from hypothermia.
• Motion compensated davit systems allow enhanced safety whilst launching and recovering daughter craft.
  

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Performance Standards

Performance standards for rescue time (‘performance times’) were developed on the basis of the risk analysis work and consultation with the offshore production crews. Specifying rescue time rather than stand-off distance proved a great step forward as it opened up the possibility of using higher performance craft at greater ranges, thus combining greater economy with improved safety. The existing convention that standby vessels could be deployed up to 5 nautical miles away – regardless of performance – was considered to be inhibiting innovation.

The Monte Carlo simulation showed that the use of mother/daughter craft and PLB's would allow standby vessel deployment distances to be doubled while maintaining or improving rescue performance.

Through detailed discussions with the offshore workforce, the following Performance Standards were established:

Overside Working

When personnel are working over the side, a daughter craft, fast rescue craft or standby vessel is to be positioned such that no person entering the water will be immersed for longer than 10 minutes.

Helicopter Landing and Take Off

For helicopter landings and take-off, a daughter craft, fast rescue craft or standby vessel is to be positioned so as to be less than 20 minutes from the incident.

Normal Operational Support

For manned installations when no helicopter activities are occurring, a craft of sufficient capacity (daughter craft, fast rescue craft or standby vessel according to the size of installation) is to be capable of arriving at the scene and rescuing the first person within 30 minutes.

Use of Personal Locator Beacons

PLB's are to be worn by personnel undertaking overside working activities, flying in helicopters or working on not normally manned installations.

Discussions with Workforce

Presentations of the proposed changes were made during 1994 and 1995 to the Operator’s Offshore Installation Managers, Safety Representatives and work force. Feedback was reviewed and appropriate actions taken.

During 1995, a series of presentations and meetings were held with the UK Health and Safety Executive (HSE). Man-over-board trials were held off the coast of Scotland and witnessed by the HSE. These demonstrated the potential of the Personal Locator Beacons and the associated direction finding systems. A video was made of these trials and shown to the offshore workforce to help in their understanding of the proposed changes. In late 1995, the HSE stated that they had no objection to implementation of the proposed arrangements but they would exercise their right to audit actual operations.

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