4. 'TURTLE GUARDS'

4.1. 'TURTLE GUARD' DEVELOPMENT

It should be emphasised that not all tail buoy designs will trap turtles. According to seismic personnel no turtles used to become trapped using older tail buoy designs that had single keels. The modern tail buoys used by certain seismic contractors also have minimal likelihood of trapping turtles (see Design 3 below).  However, the tail buoys used by other seismic contractors do cause turtle mortality and in the long-term the solution requires either alterations to the design of those tail buoys or replacing them completely with ‘turtle-friendly’ tail buoys. In the short-term, the fitting of 'turtle guards' to existing tail buoys could help to alleviate the problem.

'Turtle guards' were developed at the request of one oil company following a seismic survey off Angola in 2003 during which many turtles became fatally trapped within the tail buoys. The guards are simple devices that act to physically exclude turtles from the gap at the front of the tail buoy undercarriage. To date, the author is aware of several different designs of turtle guard that have been implemented by different seismic contractors and these (plus potential other solutions) are described briefly below.

4.2. DESIGNS USED TO DATE

To date turtle guards have been designed in two different ways:

1. As 'Exclusion Turtle Guards' which aim to simply prevent turtles from entering gaps in the subsurface structure of the tail buoy (see Section 2.4, B).

2. As 'Deflector Turtle Guards' which aim to both exclude turtles from gaps in the subsurface structure and additionally to push turtles away from the angled gap between the tow chains and the buoy (see Section 2.4, A and B).

Some examples of the designs currently fitted to seismic tail buoys are provided below.

4.3. MANUFACTURE OF TURTLE GUARDS: SIMPLICITY AND COST

Manufacturing and fitting turtle guards could be done either onboard a seismic vessel (as occurred in Figures 9, 11 and 12), or the guards could be pre-fabricated onshore and subsequently sent to the ship for fitting to tail buoys during survey mobilisation (as during Figure 10). The metal rods that have been used to create turtle guards are inexpensive and are either already present onboard a seismic vessel or can be easily ordered. The tools required to create and fit the turtle guards shown in the above figures are already present as standard equipment onboard the vessel. Seismic personnel estimate that the manufacture and fitting of turtle guards takes a few hours for each tail buoy, with ten tail buoys being easily fitted in two days. Turtle guards could be fitted either during the regular transits of seismic vessels between survey prospects, or during the mobilisation of a survey. The manufacture of turtle guards is therefore simple, cost-effective and easily carried out by the gun mechanics onboard a seismic vessel.

4.4. DO TURTLE GUARDS WORK?

The efficacy of the turtle guards currently in place on seismic tail buoys is largely unknown, due to a lack of feedback and reporting from seismic personnel (see Section 5).  Ideally, turtle guards should be fitted on a trial basis and all incidents of turtle mortality should be openly reported so that the guards can be evaluated and re-designed where necessary.  It is unclear how many seismic vessels currently have turtle guards fitted to their tail buoys, or whether those tail buoys have continued to cause turtle mortality following the implementation of turtle guards.

However, anecdotal information to date indicates that some turtle guard designs do not eliminate turtle mortality.  At least two turtle mortalities occurred off West Africa during 2008/09 using tail buoys fitted with an Exclusion Turtle Guard.  Although disappointing, the reporting of these mortalities has provided valuable information that indicates that a simple exclusion turtle guard design is not sufficient to protect turtles.  In both of the above mortality incidents the workboat crews reported that turtles became trapped above the tow chains, and were lying horizontally across the front of the turtle guard with their ventral (under-) surface facing the water current, as shown in Section 2.4.A.  This evidence strongly suggests that turtle guard designs need both a Deflector and an Exclusion element, and that simply blocking the undercarriage gap will not effectively address the issue. 

Proper field testing, feedback and reporting of turtle guard design and subsequent mortality events are essential to understanding which designs are effective.

4.5. OTHER OPTIONS

There are several possible alternatives to welding metal turtle guards to tail buoy undercarriages, and these include:

• Re-locating the tow point on the tail buoy so that the tow chains attach higher up and do not create the subsurface angle where turtles become trapped between the chains and the buoy.  However, gun mechanics have indicated that this might make a tail buoy tow awkwardly by pulling the front end lower in the water, and it may not therefore prove a viable option

• Fitting a series of chains (or ropes) running directly from the swivel to a bow-mount around the front end of the tail buoy would effectively exclude turtles from the whole area at the front of a tail buoy and prevent the animals from entering any of the known entrapment sites.  However, these would have to be fitted at a precise tension so as to remain taut at all speeds/angles which may be problematic to achieve.  Crews have also expressed reluctance to trial this method due to the increased potential for entanglement with floating debris such as fishing gear

• Fitting Norwegian buoys ahead of each tail buoy, so that basking turtles react to the Norwegian buoy and have already startle-dived and moved away prior to encountering the tail buoy.  This method was used during one survey off West Africa when high levels of turtle mortality were unexpectedly recorded.  The Norwegian buoys were deployed as a stop-gap measure since it was not feasible to retrieve all of the tail buoys and fit turtle guards.  The rope attached to the Norwegian buoy was clamped on to a streamer connector at the tail end of the stretch section so that the buoy floated at the surface 2–3 metres ahead of the tail buoy.  While this method was not 100% successful in reducing turtle mortality (perhaps due to misalignment of the Norwegian buoy with the tail buoy during strong surface currents and turns between lines), it did result in a marked decrease in incidence (based on anecdotal evidence) and should be considered as a feasible temporary measure for any survey incurring turtle mortality

However, it should be noted that none of these methods have been stringently field tested and their efficacy is therefore unproven.  Feedback is encouraged from all parties trialling these methods.

• Perhaps the most straightforward solution to turtle entrapment is for all seismic contractors to swap to tail buoy designs that are 'turtle friendly' and have minimal potential to catch turtles (e.g. Figure 11 above)

4.6. FUTURE DEVELOPMENT

The 'turtle guard' designs presented here represent the attempts by some seismic contractors to develop a solution at the request of client oil companies.  Further incentives from seismic contractors and oil companies would doubtless result in the development of more-effective turtle guard designs and other options.  Collaboration between the different sectors of the offshore seismic industry is encouraged, to maximise the field trialling of turtle guard designs, exchange of information on design success, and ultimately to ensure that the best possible solution is identified as soon as possible.