Higher Efficiency Through Improved Diver Transportation

Hägglin, A.
Aberdeen, Subtech 86

1. Abstract
2. Introduction
3. System Description
4. Main Task and Assumptions
5. Possible Influence from Different Diver Transportation Systems on the Performance of Diving Operations
6. Transportation System Related Auxiliary Systems
7. Vital System Components not in Operation Today
8. Discussion and Conclusions
9. References

Abstract

There is room for improvement in different levels of technology within advanced diving systems, e.g. diving vessels, diving systems, diver's personal equipment and methods used to accomplish the missions.

This paper will discuss and compare four concepts of bringing the saturation diver to his worksite. These concepts are the conventional diving bell (SDC), the flying bell (FB), the diver lock-out submersible (DLO) and the autonomous submarine (AS). A theoretical study of certain system's characteristics has been carried out. It comprehends a general study of system characteristics and a study of certain tasks related to these systems. The purpose has been to establish whether the method used to transfer the saturation diver to his worksite is of any significance to the efficiency of the diving system.
This study shows that due to the positioning capabilities of today's DSVs, the changes in diver transportation procedures add little to the total efficiency of the system, despite the increased level of technology.

Introduction
The quest for new oil and gas fields has taken the oil companies into areas of great depths and hostile environments. To make the exploitation of these findings possible, there is a need for new and advanced technology.
The distance between the surface support and the diver is increasing. This adds to the technical difficulties and drastically lessens the margins for the diver life-support systems. This calls for new philosophies in many of the problem areas and perhaps totally new concepts for their solution (1).
Attempts to obtain increased surface independence have led to the development of so-called flying bells. These are diving bells free from guide wires and equipped with thrusters in order to give horizontal maneuverability (2). In the near future we will also meet the large autonomous submarine "Saga", a mix between the old concept of a stationary habitat and a lock-out submersible as it occured in the early seventies (3).
This paper intends to describe the pros and cons of these systems and to compare them with the conventional diver transportation systems (4). This will not be done by a precise statistical method, but rather by analysis of a systematically collected interview material. We will try to show the potential of this kind of analysis and hopefully be able to continue to make similar studies of offshore diving.

System Description
For the comparative analysis of the different transportation systems, a system outline, a performance table and a theoretical bell-run efficiency diagram have been prepared.

System Outline
The outline is roughly divided into structure, main supply systems and diver worn components. From the beginning our intension was to compare system components of different diver support systems to establish their influence on the total efficiency of the system. We found no such differences of any significance, although some system components, mutual to all systems, proved to be in need of improvement. (fig. 1)

Performance Table
A number of system characteristics has been listed. The information is mostly gathered from commercial brochures. (fig. 2) Note the following:
MDU: Emergency time: This is most likely 24 hours as defined in classification rules. FB and MDU: Turn around time: It is not likely that the FB closes one bell-run and starts the next within one hour. A more credible estimation is two hours. That figure is a ruff estimate based on the turn around time for the SDC and the launch to lock-out time for the MDU.
Flying bell: Launch to lock-out: No information is given concerning this operation. It is, however, very likely that the time for the operation will be close to that of the MDU.
MDU and DLO: Costs: The costs are estimated from old information and the need for a special support vessel.

Theoretical Bell-Run Efficiency
Fig. 3 shows the theoretical ability to keep a diver at the worksite. The necessary ineffective bell-run time is estimated from interview material. In practice, the influence from various systems of different technical maturity will cause further delays in turn-around time. Assumptions made for the graph are:
Resting time between dives: 16 hours
Number of divers in transport:
SDC/FB/MDU: 2 divers
DLO: 4 divers
SAGA: 6 divers

Possible working time per diver:
200 meters: 4 hours
450 meters: 2 hours

The ineffective bell-run time for e.g. the SDC is defined as the crew change-out time. That is the time which elapses from the moment when diver number two of a team leaves the worksite to the moment when diver number one of the next team reaches it. The same is applicable for the flying bell, the MDU and the DLO. The autonomous submarine does not have ineffective bell-run time as long as the 6 divers are working back to back. The depth in which the number of divers is insufficient for a continuous operation is probably in practice less than 200 meters. In theory, however, it is somewhat more than 200 meters. The ratio is calculated as (ineffective bell-run time / total bell-run time) x 24 hours. Summary: 200 450 SDC 2,7 4,8 h / 24h
Flying Bell 2,7* 4,8*
MDU 4,8 8,0
DLO 6,6 10,3
SAGA 0 8,0
*This figure is somewhat higher in practice.

Main Task and Assumptions
During the initial stage of our study, we tried to compare system performance and characteristics to a specific task. A main task and a few environmental and technical assumptions where decided on. The main task chosen was a cleaning and None Destructive Testing (NDT) operation.

Environmental Assumptions
The installation of this study is located on the 62nd parallel in the North Sea. The weather is average autumn weather with a sea state between five and six. The current at depth is 0.5 knots, the temperature is 2uoC and the visibility is good. Two depths are studied, 200 and 450 meters. The first is today's maximum operational depth in the North Sea and the second is the depth which most in the offshore industry believe has to be reached.

Technical Assumptions
Some technical assumptions have been postulated in order to make it possible to work at 450 meters depth and to operate a lock-out submersible in an efficient way.
Breathing system:
A good breathing apparatus of rebreathing type is available (4). It gives an acceptable gas consumption and an acceptable performance concerning breathing resistance. The bail-out system gives an endurance of 15 minutes of 450 meters depth including the gas heater.
Flying bell:
It is assumed that a flying bell of the Bruker concept is to be used and that its performances correspond to those stated (2).
Diver lock-out submersible (DLO):
The energy supply on board the submersible has a duration which allows for a minimum of 16 hours continuous work, supplying all energy consuming system on the vessel and four divers in satura- tion. The handling system is designed to allow for launch and recovery in a sea state between 5 and 6.
Autonomous submarine:
It is assumed that an autonomous submarine of the Saga concept is to be used and that its performances corresponds to those stated (3).
Support vessel:
t is assumed that each transport system which requires a support vessel is supported by a modern monohull DSV (6).

Possible Influence from Different Diver Transportation Systems on the Performance of Diving Operations
The views presented in this chapter are based on compilation and analysis of interviews with executives representing major oil and diving companies. The material has been compiled and construed by the author. The scenario presented showed little relevance when the interviews and discussions started. The idea to present the transportation system's influence on each task had to be abandoned since there was no influence of any significance.
The results are therefore presented in another manner than first anticipated. They have been divided into the categories; safety, reliability, working capacity and adaptability. The relevance of the results has not been reduced by this. A summary matrix based on the interviews and available literature has been compiled. Grades from 1 to 10 have been given in order to try to obtain a significance in the differences between the transportation systems. The different grades given are commented on.

Safety of Divers During Transportation
During the transportation from the support vessel to the worksite, the most dangerous situation is the passing of the surface and the docking maneuvers. The surface zone should be passed as quickly as possible, as done with the SDC (4). The docking and attachment maneuvers of the flying bell and the DLO are potentially dangerous operations, particularly in high sea states (4-5).
Since the divers of the autonomous submarine do not pass through the surface, these potentially dangerous situations do not exist regarding that system. The SDC must pass through the surface zone which is potentially dangerous. If the SDC is used of 450 meters depth, there is an increased risk of entanglement with the guide wires, lift wire and umbilical due to the length and stretching of these.
The flying bell must accomplish two potentially dangerous phases, the passing of the surface zone and the docking maneuvers. It is not more dangerous to deploy divers at large depths with a flying bell. The surface operation of the DLO is highly dangerous and can only be performed in calm seas.

Safety of Divers at Worksite
When the divers are at their worksite, their safety is mainly dependent on the distance to the relative safety of the bell or sub and the time it takes for the diver to reach it or to be brought there (7). This makes the flying bell safer than the other systems (2). The distance between the worksite and the transportation system is not always significant.
Another risk is being pulled off the worksite by a t ransportation system in disarray (6). The SDC is runned from a dynamically positioned support vessel. These vessels have been known to suffer from computer malfunctions which momentarily takes the ship off position. Such malfunctions are very rare today since the position systems consists of at least three independent systems which individually can keep the vessel in position if the others fail.

Since the other systems are locked to a structure or standing on the bottom, they do not suffer from this risk. If, however, a diver locks out from a flying bell or DLO, it has a major influence on the buoyancy of the unit (4-5). If, during this phase, the buoyancy systems fail or the operator shows lack of attention, a very dangerous situation of rapid ascent can occur. Such a rapid ascent will inevitably be fatal to a saturation diver in the water and to the divers in the pressure compartment if the hatch cannot be closed.
The risk of being pulled away by a transportation system in disarray must be regarded as small for the SDC, flying bell and DLO, and as practically non-existent for the autonomus submarine. With regard to the diver transportation system the safety of the divers at the worksite is depth independent.

Safety of Installations
In general, the offshore operators are not worried about the diving contractor damaging platforms or bottom installations. Concerning the flying bell and the DLO the associated masses are relatively low and the impact forces moderate. However, damage can be caused to exposed and vulnerable equipment such as a satellite well-head. Concerning the SDC, there is a small risk of damaging an installation when lowering the clump weight or when repositioning the support vessel with the clump weight not on the bottom. The other three systems may collide with e.g. a well-head though today's obstacle sonars are fairly accurate. The flying bell may also cause damage to an installation when locking on to it. Hoses, piping and other components of the installation may be squeezed by the attachment claw.
With regard to the diver transportation system the safety of installations is depth independent.

Weather Sensitivity
Regarding its own operation and divers the autonomous submarine is totally surface independent and therefore weather independent (3). During an operation, where it depends on surface support, it might suffer from weather downtime due to the support vessels inability to work. The launch and recovery of the SDC are well-known operations (4). The limiting factor is the behavior of the support vessel and its bell handling system. During operation, the SDC is influenced by the movements of the support vessel if the heave compensation system is insufficient or erroneously operated.

The flying bell is not influenced by the movements of the support vessel during operation but is weather sensitive during the launch and recovery operations (2). It has still to prove its ability to perform the docking maneuver in high sea states. If the operation can be performed successfully in rough weather, it will be possible to use the flying bell in higher sea states than with the SDC. The Perry MDU claims to be operational from MSV Tharos in sea state seven (8). If the flying bell has the same ability, it is superior to the SDC.
The DLO will be inferior to the other systems if a major breakthrough is not made in the launch and recovery operation. Such a breakthrough must be based on a docking maneuver similar to that of the flying bell. To get a good correspondence between answers on this often disputed subject is close to impossible. You tend to get as many different answers as there are companies answering.

Technology Level
The risk of downtime due to an increased level of sophistication in the diver transportation systems, is the same as with any other system development. All forms of diver transportation systems are continuously becoming more sophisticated due to improved knowledge and changes in requirements and regulations.
However, the risk of downtime increases with the degree of technical complexity of the hardware. Since "Murphy's Law" is well established offshore and since things get worse under pressure, the technical sophistication of the flying bell is something of a drawback, even though most of the additional equipment is rather straight forward and should not be expected to cause much downtime.
It is evident that the DLO must improve all sides of its system. The transfer of divers under pressure will demand a dedicated support vessel with the capacity to handle such a large device. The servicing and maintenance between dives must be more efficient and less time consuming. There is no substance for a discussion concerning the autonomous submarine in the same way as above. There is no experience whatsoever in the operation of such a system. It is undoubtedly so, that the SAGA project will bring the diving technology further, even if it will cause problems for the operator during its initial missions. The influence from the technological level is depth independent concerning the different diver transportation systems.

Working Capacity
The working capacity of the divers when transported in one of the four different ways, was discussed from both qualitative and quantitative point of view. However, the quality of the work performed by the diver does not seem to depend on how he reached his worksite. The amount of work performed by the single diver and by the system as a whole can be affected by the transportation system. The ability to place the diver close to his worksite will reduce time and energy consuming swimming. If the horizontal distance is significant, problems can occur with the transportation of additional equipment.

Keeping a Diver at the Worksite
The avoidance of crew changes, gives the autonomous submarine an advantage when performing long duration tasks without any need for repositioning. Each change of crew means loss of time and efficiency. The turn around time differs between the systems, particularly concerning the DLO. There is also a difference between the SDC and the flying bell, but that time is not possible to determine today. The flying bell loses time to the SDC when it flies and locks on to its worksite, but delivers the diver closer.
The autonomous submarine is able to keep a diver at the worksite around the clock at 200 meters depth if one diver locksout at the time. At 450 meters depth there will be a six hour gap before diver number one can continue after diver number six due to the necessary resting time. The SDC and the flying bell are not quite equal in this regard. The flying bell loses time during the flying mode and the docking maneuver. The amount of this time is hard to determine. Occidental claims 30-45 minutes to the lock-on from the launch depending on the location of the worksite, but gives no record of the reverse operation. Regarding the DLO, it is hard to envision the authorities approving the substantial "bell-runs" of more than 16 and 8 hours respectively at 200 and 450 meters depth.

Horizontal Capacity
The horizontal capacity of the autonomous submarine and the DLO are obvious (3-5). The submarine has a higher endurance compared with the DLO. During a horizontal operation e.g. flowline inspection, the SDC and the flying bell are connected to their support vessels (2, 4). This is a drawback for both systems, but if compared, the flying bell is more adapted to such an operation.
The autonomous submarine is the prime system here. The DLO does not have the endurance of the submarine or its payload capacity. The flying bell is still connected to a support vessel which must follow it, though not as substantial as the SDC.
The horizontal working capacity of the transportation systems is depth independent.

Vertical Capacity
The autonomous submarine and the DLO are both strictly bottom oriented and their vertical capacity is confined to the upper limit of the divers current saturation depth. At 200 meters depth, this is 40 meters (9).
The SDC and the flying bell has no vertical restrictions when it comes to placing a diver at his worksite. If the support vessel of the flying bell is at a significant distance from the installation and if the worksite is at a minor depth, problems can occur in running tools and equipment to the diver. The vertical working capacity of the transportation system is depth independant.

Ability to Alter Work Operation
Assuming that all tools and other equipment are surface supplied, there is no essential difference between the bell concepts, other than the horizontal running of tools and equipment. Concerning the DLO and the autonomous submarine, it must be assumed that a change in the working operation is a time and strength consuming alteration. The demobilization and mobilization of different sets of equipment in the underwater environment is hard work.

Ability to Change Worksite
This question arose some disagreement. One side maintains that the SDC can be relocated as fast as the flying bell or with an insignificant time difference. This operation is performed by simply moving the support vessel while the SDC is submerged. The technique is well proven within moderate distancies, e.g. along a jacket structure. The other side claims the superiority of the flying bell and to some extent that of the DLO.
The autonomous submarine is restricted to bottom orientated work and is not supposed to continuously change its position (3). The changing of the worksite is supposed to call for a new position for the transportation device, meaning also the repositioning of the SDC support vessel.

Sensitivity Regarding Outside Disturbances
The system's sensitivity regarding outside disturbances is depth independent. The SDC is a robust and insensitive system. Errors in matters of tools and construction parts can easily be corrected with assistance from the surface. The flying bell can be somewhat isolated if working close to the surface with its support vessel at a substantial horizontal distance. This might imply difficulties in sending tools and equipment to and from the worksite.

The DLO must return to the surface if something is wrong. This operation might take a few hours, but it will not disqualify the system. The operation of an autonomous submarine demands extensive management and careful planning. It leaves no room for mistakes such as bringing the wrong tool or the wrong nuts and bolts. An erroneous specification, however minor, can send the submarine back to its base, which may be days away.

Transportation System Related Auxiliary Systems
The auxiliary system which has the largest influence on the operation is the handling system (4). This is evident for the three surface supplied systems. If the transportation system cannot be launched or operated due to high sea states or insufficient handling systems, the obvious result is downtime. The ability to place the diver as close as possible to his worksite is considered to be of high significance. This is highly dependent on the function of the handling system. A well-functioning and accurate positioning system is a must if the right position is to be reached and maintained. Most offshore installations do not have a system of marking and identifying the different parts of the structure such as nodes and bracings. The difficulties in navigating in and close to an offshore structure are evident (6).

The flying bell is dependent on its propulsion system to reach the worksite. When there, it has to lock itself to the structure with its attachment claw, or if it is a bottom oriented task, position itself on the bottom. Yet another system necessary for successful operations in the flying mode is the buoyancy system (4). The accuracy of that system is crucial for docking, attachment and lock-out operations. The performance of the DLO is influenced by the same systems as the flying bell. Since it is surface independent in operation, there are further systems which have an effect on the operation. The two most important are the storage of energy and the gas supply system (4). They affect the duration of the operation and the accessible payload.

Vital System Components not in Operation Today
There are components which are not in operation today and components which for some reason or another are disqualified.

Mutual to all the systems is the lack of a diver bail-out system and emergency heating for the breathing gas, both with a duration of 15 minutes. The DLO and the autonomous submarine lacks a low gas consumption breathing apparatus to keep the necessary gas storage at a minimum. The DLO is in need of an improved handling system, a through-water transmission system for all data signals, including video, and a heating system for the divers adjusted to the energy storage system (3-5). System components which in some ways are inferior to the standards are e.g. breathing apparatus, heating system and diver communication system (7). Other systems which are not fully proven but which do not have standards to meet are e.g. SDC handling system, flying bell docking system and DLO energy storage (4). Concerning the diver's personal equipment, it is possible to develop sufficient systems with today's knowledge and technology. The problem is the financing of this small volume of very expensive equipment.

The development of the handling system of the SDC is harder to predict. Most of the work seems to be the upgrading and refinement of existing equipment and procedures. If this is sufficient, only time will tell, as with the performance of the docking system of the flying bell whose abilities will be shown in the comming extended operation of that system. There is nothing that indicates a further development of the presently insufficient system components of the DLO. The transportation systems are affected by these components. If the components had been fully accounted for in the analysis, the DLO would have been the one most affected. However, the general differences betweenthe systems would have been about the same.

Discussion and Conclusions
In the cases discussed, divers, who are locked out to perform work on underwater installations on different depths are used. Their work capacity and the quality of their work will be essentially the same in all transportation schemes. Differences come about with regard to the change-out of divers and to the possibility to place the diver close to his worksite.

It is necessary to return to the surface for each change-out of divers when using a SDC or a flying bell. Since this and the return trip takes time, efficiency is lost. Depending on the conditions for the alternate team in a DLO and its questionable capacity concerning operation duration, a more rapid exchange of divers can be performed. However, such an operation is not possible with today's level of technology regarding the DLO concept. The autonomous submarine has the advantage of not requiring a support vessel and the capacity to operate continuously regardless of sea conditions. Offsetting this is the high capital investment and the lack of local support. If an unplanned situation occurs, the submarine will have to return to its base. The submarine is a part of the operation when large horizontal distances are to be covered and various spots on the way need diver attention, e.g. a series of satellite well-heads and the connecting flowlines. The submarine is also strictly bottom oriented. Since work performed at an offshore installation is bottom oriented during installation and more midwater oriented during operation, a specialized installation and maybe dismantling of offshore installations is recommend to the operator. The flying bell will be able to place the diver closer to his worksite compared to the SDC, but this is not to say that the SDC will not be able to get close enough. It is difficult to envision how the minor improvements in operational capacity, provided by a mobile system can be justified from technical point of view in the tight margin, competitive world in which the diving industry operates today.

References
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3, 1985, pp 87-92. 3. "SAGA 1, Principal Characteristics and Performances," Published by Comex Industries, April 13 1984.

4. Haux, G., "Subsea Manned Engineering", Balliere Tindall, London 1982.
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6. Lynnes, J., "A Seasons Diving Operations from an S.S.V. Working in D.P. Mode," proceedings of Offshore Göteborg '85, conferernce held in Göteborg, Feb. 25-March 1, 1985.

7. Sisman, D., "The Professional Divers Handbook", Submex Ltd., London, 1982.
8. Jacquet, E., Rosenberger, R. "Practical Experience with the Mobile Diving Unit," OTC 4210, proceedings of OTC, Houston, 1982.
9. "US Navy Diving Manual", Best Publishing Co., 1980, Carson.