International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 6

Number 2

June 2012

263

1 SUPPORTING A 3D MULTI-STREAMER

SEISMIC SURVEY OPERATION

1.1 Seismic Vessel Capabilities

Modern seismic survey operations are a far cry from

the days of relatively small modified vessels towing

a lone streamer or two that could be set and

recovered in a few hours. Today, the seismic fleet is

dominated by larger purpose-built vessels, though

there are still many vessels in service, converted

from other roles and designations. Modern seismic

arrays typically comprise 12 streamers, possibly ex-

tending up to 8000 meters astern of the mother ves-

sel, and measuring a swept path of 1200 meters

across the ship’s track. The very latest vessels now

leaving the building yards have towing points for up

to twenty such streamers. Such extensive equipment

is capable of yielding a 3D seismic picture of great

fidelity.

1.2 The Seismic Survey Concept

Seismic surveys are carried out extensively in ocean

and offshore areas with a known potential for re-

serves of oil and gas in the sub-sea rock formations.

The seismic survey vessel tows the steamer array

suspended below the surface, carrying hydrophones.

Sound waves are transmitted from the vessel using

compressed air guns which travel down through the

seabed and reflect back from the different layers of

rock (Figure 1). These reflected sound waves are re-

ceived by the hydrophones located along the seismic

streamers which, when processed, gives a three di-

mensional picture of the substrata.

Simulation Training for Replenishment at Sea

(RAS) Operations: Addressing the Unique

Problems of ‘Close-Alongside’ and ‘In-line’

Support for Multi-Streamer Seismic Survey

Vessels Underway

E. Doyle

Cork, Ireland

ABSTRACT: Modern siesmic survey vessels in ‘production’, may tow twelve or more streamers, each of

which can be six to eight kilometres long. Together with associated paravanes, tail-buoys and acoustic ‘guns’,

the streamer spread width of such wide-tow configurations can extend to 1200 metres. The physical deploy-

ment and recovery of such an extensive array is time-consuming and expensive. The entire survey operation

requires the constant attendence of a suitable offshore support vessel (OSV) to act in the role of ‘chase ves-

sel’, but more critically, to provide close replenishment support underway and, when required, rapid emer-

gency towing assistance.

While naval crews rightly claim a near monopoly on the skills-set necessary for underway replenishment, the

naval RAS exercise almost never involves the supply and receiving vessels engaging ‘close-alongside’. The

seismic/OSV replenishment operation, on the other hand, frequently necessitates such a demanding and

stressful manoeuvre. This paper presents a training solution involving the use of a 360°full-mission bridge

simulator.

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Fig. 1. Marine Seismic Survey

Seismic survey vessels ‘in production’ show the

shapes and lights for a vessel restricted in its ability

to manoeuvre. The streamer cables are spread by di-

verters/paravanes, similar in function to that of mid-

water trawl doors, and can extend to 1200 metres in

width. The end of each streamer is marked by a tail-

buoy carrying radar reflector and flashing lights.

Fig. 2. A 12-streamer 3D Seismic Array

Seismic survey vessels tow at a speed of 4 to 5

knots and need to be on a straight line whilst survey-

ing and are almost invariably accompanied by a

‘Chase Boat’ to police the immediate task vicinity

and to assist in notifying other vessels of the seismic

operation. Survey areas vary greatly in size and may

cover extensive areas of the sea surface.

1.3 The need for a support vessel

It will be readily appreciated that the 3D multi-

streamer seismic array described above is not

something that could be deployed or recovered in a

few hours: it is typically a two-day task, and perhaps

longer. This means that once the survey operation

has started and reached full production, significant

interuption or suspension of the work is unlikely

except for very pressing circumstances. A further

consequence of that imperative is that the seismic

ship must be supported and replenished under way,

as the operational situation demands. An offshore

supply vessel (OSV), suitably modified for the

purpose, is the solution of choice for most seismic

operators. And when not directly engaged on

support and replenishment duties, the same OSV

serves in the ‘chase boat’ capacity.

1.4 The support vessel ‘close alongside’

The support vessel must replenish the seismic

‘mother’ ship with fuel and machinery consumables,

primarily, but also with victuals and catering stores,

and all other general and technical stores necessary

for uninterupted seismic production. And from time

to time the support vessel will be required to ferry

and transfer personnel to and from the mother ves-

sel, as when crew rotation is scheduled. These trans-

fer operations (fuel, stores, personnel) are most often

effected with the support vessel ‘close alongside’ the

mother ship.

1.5 The support vessel ‘in-line’

In other circumstances, and for various reasons, the

support vessel may not be able to transfer fuel from

a ‘close alongside’ position, in which case she will

have to take station ‘in-line’ ahead of the mother

ship. This demanding manoeuvre requires the sup-

port vessel to make a close approach ‘in-line’ ahead

and bringing her transom to about 40 metres from

the mother ship’s bow, passing a shot line and mes-

senger, then opening the distance between the two

ships to 90 metres — a separation maintained by a

heavy distance line — before passing the fuel line.

The support vessel will now have the challenging

task of maintaining her station for perhaps the com-

ing six hours, until refueling is completed.

1.6 Readiness to offer an emergency tow

The support vessel will have one other major as-

signment in executing her close support role: to pro-

vide an emergency towing capability to the seismic

vessel in the event of her suffering a serious propul-

sion failure. The consequences of such a power fail-

ure on the mother ship mean a very rapid loss of

speed, caused by the drag of the paravane/streamer

array. However, the inertia of this same array will

ensure that its loss of speed is not as rapid as that of

the mother ship.

1.7 Short time window

If forward motion is not restored to the mother ship

she is in danger of becoming entangled and ensnared

in her own gear – an expensive ‘seismic spaghetti’

of streamers, buoys, paravanes and towing lines. In

such an emergency, the support vessel has a time

window of perhaps 15 minutes (maximum) in which

to make a close approach and connect up the emer-

gency towline.

1.8 Immediate priority

The immediate priority for the support vessel is to

get the stricken mother ship moving again, along the

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original path and away from the entanglement dan-

ger of her gear. Even one knot, or less, will achieve

this and buy the time necessary to restore power on

the seismic vessel.

2 SIMULATOR REQUIREMENTS

2.1 The needs of the industry

The initial request to develop a suitable simulation

training programme for such unusual and unique re-

plenishment-at-sea (RAS) manoeuvres came from a

local offshore operator, Mainport Group, Cork. In

late 2005, they won a contract from the French

seismic operator, CGG (now CGGVeritas), to pro-

vide a seismic support vessel for operations in the

Indian Ocean — and they needed simulator-based

training for their crews. For the project, the 360˚

main bridge (full-mission) simulator at the National

Maritime College of Ireland (NMCI) was utilized.

Subsequently, Mainport added four similar vessels

(OSVs) to their seismic support division, working

for CGGVeritas and other seismic operators. Then in

late 2009, CGGVeritas agreed with NMCI, a similar

bespoke training programme for their seismic ship

masters and mates, paired with their matching sup-

port vessel counterparts.

2.2 Kongsberg full-mission simulators at NMCI

The Kongsberg full-mission bridge simulators at

NMCI are specifically designed for complex ship-

handling manoeuvres and advanced navigation exer-

cises. All important navigation and manoeuvring da-

ta are presented to the conning officer on the bridge

via a comprehensive array of statutory (SOLAS) in-

struments and displays.

By configuring the 360˚simulator with an accu-

rately compiled ship model having realistic hydro-

dynamic characteristics, the high-end simulator gen-

erates a ship-manoeuvring environment of impres-

sive fidelity. In this respect, the NMCI full-mission

simulators are rated world-class, and the entire simu-

lation centre is ranked amongst the best such facili-

ties anywhere.

2.3 Realistic ‘ownship’ behaviour

In the simulated environment, the behaviour and re-

sponse of the visual ‘ownship’ model to the engine,

rudder and interaction forces and to the environmen-

tal conditions, is governed by a matching mathemat-

ical ship manoeuvring model. The model must be-

have in such a way that the position, heading,

velocity and swept path of the ‘ownship’ are always

representative of real ship behaviour.

2.4 The necessity for a 360˚simulator

The panoramic visuals of a 360˚ simulator are an es-

sential feature of any training exercise attempting to

simulate the fidelity of a replenishment-at-sea opera-

tion. This constraint is self-evident for towing and

‘in-line’ RAS evolutions, where the support vessel

master needs to have an unrestricted view astern.

But it is equally valid in the ‘close aboard’ approach

(support vessel approaching the mother ship beam-

on) where the full broadside view is just as neces-

sary.

2.5 ‘Ownship’ and ‘target’ models

The ‘ownship’ model used as the simulated support

vessel was the Kongsberg supply vessel SUPLY02L,

fully dynamic in six degrees of freedom (heave,

sway, surge, roll, pitch and yaw), representing all

horizontal and vertical motions of the ship. For all

RAS exercises the seismic mother ship may be

simulated by a generic ‘target’ model, though a ded-

icated seismic ship ‘target’ model (CGG ALIZE,

Figure 3) has recently been developed.

Fig. 3. Seismic Survey Ship CGG ALIZE

3 RAS PREPARATIONS

3.1 Matrix of permitted operations

A RAS evolution ‘close alongside’ the seismic ship

requires planning, preparation, careful attention to

procedure and skilful ship-handling. It is a daylight

manoeuvre only, constrained by agreed limits on

visibility, wind speed and direction, and sea/swell

state — a maximum wind speed of 20 knots and sig-

nificant wave height of 1.5 metres are the usual up-

per limits of acceptable conditions. Weather limita-

tions and other restrictions for RAS and the wide

variety of seismic operations should be promulgated

in company manuals — CGGVeritas meets the re-

quirement by publishing a tabulated Matrix of Per-

mitted Operations, abbreviated to MOPO in their

fleet guidelines and procedures. Tool-box meetings

on both vessels are essential. Day, time and scope

for the RAS transfer having been mutually agreed,

either master must have complete discretion to abort

266

the operation at any time during the approach phase

or throughout the transfer.

3.2 RAS speed of 4.5 knots

Before any closing approach manoeuvre is initiated

the mother ship must confirm her track, heading and

speed, and throughout the RAS approach she must

advise the support vessel of any small changes in

those parameters — it is a given that any substantial

changes should not be contemplated. The necessity

of maintaining the operational speed for the streamer

array dictates a typical RAS speed of about 4.5

knots.

3.3 Why the beam-to-beam approach?

In the most common type of RAS operation, involv-

ing naval formations, the re-supplying fleet tanker

becomes the designated formation ‘guide’ and seeks

to maintain her heading within 1˚of the signalled re-

plenishment course. And whereas naval vessels posi-

tioning for their RAS station will approach the

‘guide’ from astern or fine on her quarter, such as-

pect is never an option for a seismic/support RAS;

the wide towline controlling the diverter/paravane is

angled 45˚ outwards from the quarters of the seismic

ship, which constrains the support vessel’s approach

to a narrow sector on the mother ship’s beam.

3.4 Choice of steering control

The support vessel is likely to assume a standby sta-

tion about 1000 metres abeam of the seismic ship,

on the agreed side. A closer standby station is ac-

ceptable, but not if that station is inside the paravane

path. There is much debate in the industry on the

choice of auto-pilot or manual steering for the sup-

port vessel’s RAS approach, especially in the latter

phase of the manoeuvre when bringing the vessel

from the ‘close aboard’ station (about 30–50 metres

abeam) to the ‘close alongside’ position.

3.5 The case for auto-pilot control

The reality of small crews means that a skilled

helmsman is unlikely to be available, particularly at

a time of peak demand when all available crew are

needed on deck. Also, given the widespread lack of

opportunity for manual steering in commercial ship-

ping there is deep concern within the industry that

the manual steering skills of seamen, generally, are

inadequate. Neither is it acceptable that the master

should manually steer his ship in the final approach;

he already has sufficient demands on his judgement,

watching his speed, avoiding the wide tow wires and

other overhangs and obstructions on the mother ship,

controlling inter-ship and intra-ship communica-

tions, and, most critically, the ever-constant eye for

interaction effects. In the circumstances, the case for

using the auto-pilot is compelling, and no less com-

pelling is the need to ensure that such equipment is

fully serviced and totally reliable.

4 APPROACH AND DISENGEGEMENT

4.1 Safe convergence

In commencing her approach from the stand-by sta-

tion, the support vessel must steer an inward conver-

gent course (towards the mother ship) by about 20˚,

and increase speed so as to avoid increasing the as-

pect angle with the attendant risk of fouling the par-

avane tow wire. For instance, if the base course and

speed (survey track) is 120˚ x 4.5 kn, a starboard-

side approach will require the support vessel to steer

100˚ and set her speed at 4.8 kn. If the convergent

angle were 30˚, an approach speed of 5.2 kn would

be necessary to maintain the same 90˚ aspect. A use-

ful visual guide for the support vessel master is to

keep the seismic ship’s bridge-front in view: if, dur-

ing the approach, an increased aspect angle leads to

the loss of that view the support vessel has fallen

abaft the optimum approach line, and runs the risk of

fouling the wide tow wire.

Fig. 4. Support vessel converging on mother ship

4.2 Reducing the convergent angle

As the support vessel closes the mother ship the

convergent angle must be reduced (Figure 4). When

the lateral distance between the ships is down to 100

metres the convergent angle should not exceed 10˚,

and at the same time the support vessel will need to

trim back her speed; as the courses approach coinci-

dence so too should the speeds of both ships.

4.3 Suspend and reappraise

The 100 metre mark is a good position at which the

support vessel should temporarily suspend the con-

vergent manoeuvre. This will allow the masters of

both vessel the opportunity to reappraise the situa-

tion and to reassure each other that all checklist pa-

rameters for a safe RAS operation remain valid.

267

Fig. 5. Both vessels confirm willingness to proceed

4.4 Coming into position ‘close alongside’

If there is no reason to abort the evolution the sup-

port vessel should resume the convergent course

(Figure 5). Ten degrees convergence is still accepta-

ble, but this should be gradually reduced so that as

the support vessel arrives in the ‘close aboard’ sta-

tion, 50 metres from the mother vessel, the conver-

gence should not exceed 5˚. In the final phase of the

manoeuvre, from ‘close aboard’ to ‘close alongside’

the convergent angle must be reduced further so that

when contact is made on the yokohama-fenders the

convergence is 2˚ or less. In reality, when the ships

are 10–15 metres apart there is usually little need for

any convergent angle because the dominant interac-

tive force between the ships at this stage is most

likely to be that of attraction.

Fig. 6. ‘Close alongside’ replenishment station

4.5 Avoiding simulation ‘freeze’

In the normal course of simulation exercises the in-

tuitive simulator response to the ‘ownship’ model

making contact with a ‘target’ vessel at close quar-

ters is to signal a collision condition, at which point

the exercise functions freeze and the simulator must

be reset. In the RAS simulation the same outcome is

evident when the support vessel (as the ownship)

makes heavy contact with the mother ship, such as

when the convergent angle is too large or when her

position alongside is so far aft that she strikes the

protruding sponson structure. On the other hand, if

the contact force between the ships is gentle and cor-

rectly positioned, as when there is little or no con-

vergent angle, the simulation exercise will continue

without interruption (Figure 6).

4.6 Critical securing lines only

Once the support vessel, properly fendered, is safely

alongside the mother ship the agreed secur-

ing/mooring lines must be rigged. These usually

consist of a for’d breast-line and two fore-springs –

lines connected from the forepart of the support ves-

sel only. Bearing in mind the formation speed of 4-5

knots, it is never acceptable to have any securing

lines connected to the after-part of the vessel.

4.7 Favourable disengagement

When fuel and stores transfer is completed the sup-

port vessel must prepare to disengage and clear

away from the seismic vessel — all before the onset

of darkness. On some seismic/support vessel combi-

nations the vessels will separate and diverge under

the favourable effect of hydro-dynamic interaction,

as soon as the securing lines are released. But in

many cases, this will not happen.

4.8 Adverse interaction effects

Where the adverse interaction effects are dominant,

the force of attraction between the ships will restrain

the support vessel in the ‘close alongside’ position.

Any attempt by the support vessel to clear the side

of the mother ship by increasing speed and using

outward helm will fail because it is not possible to

steer away in these circumstances. If the attempted

manoeuvre is allowed to continue, the outcome illus-

trated in Figures 7 and 8 is inevitable; the support

vessel will move forward, all the while restrained

against the side of the seismic ship, until the critical-

ly adverse interaction effect becomes manifest. This

is the ‘bow-in’ turning moment that will cause the

support vessel, despite carrying outward helm, to

turn across the bow of the mother ship. Averting

disaster at this point is in the hands of the seismic

master, who must take all way off the vessel instant-

ly.

Fig. 7. Critical interaction effect

268

Fig. 8. Imminent risk of capsize

4.9 Using thruster and inboard helm to overcome

adverse interaction effects

Where, as described above, interaction effects pre-

vent the support vessel from steering directly on a

divergent course from the ‘close alongside’ position

she must first use her bow thruster to open out a di-

vergent angle. As this angle increases the master will

need to increase speed and apply 5˚ inboard helm (to

keep the transom clear of the side of the seismic ves-

sel). How long to keep the inboard helm applied is a

judgment call, but it should not be maintained if its

turning force threatens to overcome the outward

rate-of-turn from the bow thruster. Once the support

vessel is 10–15 metres clear of the mother ship and

has a divergent heading of about 10˚ she has the

manoeuvring freedom to steam clear away with little

risk of exposure to any further adverse interaction

effect.

5 TOWING

5.1 ‘In-line’ RAS station

In the circumstances where the support vessel is un-

able to transfer fuel from the ‘close alongside’ posi-

tion, she will have to take station ‘in-line’ ahead of

the seismic ship. This manoeuvre requires the sup-

port vessel to make a similar approach to the ‘close

aboard’ station, as described above, and then in-

crease speed while holding a convergent course. The

objective is to pass within 50 metres of the mother

ship so as to take temporary station ‘in-line’ ahead

of her, with about 40 metres separation between her

transom and the mother ship’s bow. This close prox-

imity facilitates the exchange of a shot line, messen-

ger and ‘distance line’. The distance line is of haws-

er-like quality and is used to maintain a near-

constant distance of about 90 metres between the

two ships. Once the heavy distance line is estab-

lished and lightly tensioned, the fuel transfer line is

then rigged between the ships. The support vessel

must now settle into the stressful role of constant

vigilance in seeking to maintain her station for per-

haps the coming six hours, until refueling is com-

pleted.

5.2 Towing exercise requires second simulator

The simulation of an emergency tow scenario re-

quires a different arrangement of simulators and

models. While a tow-line may be assigned and con-

trolled from the support vessel ‘ownship’ it is only

possible to connect it to another ‘ownship’, which,

in turn, must be assigned to another simulator. A fur-

ther problem arises in the simulation exercise when

attempting to achieve towing fidelity. An actual

support vessel confronted with an emergency tow

scenario will need to use substantial power to get the

seismic vessel moving at just 3 knots, because of the

enormous drag created by the streamer array and as-

sociated gear (approximately 80t). A simulator

‘ownship’ assigned as the seismic ship will generate

only the drag appropriate to the particular model dy-

namics. However, if the simulator ownship control

includes the optional External Forces menu it is pos-

sible to apply a range of such forces to the seismic

ship model so as to achieve realistic fidelity in simu-

lating seismic streamer drag.

6 CONCLUSIONS

The modern multi-streamer 3D seismic survey oper-

ation is enormously challenging, in the financial and

technical resources required to mount and maintain

the venture at sea. Downtime in seismic production

carries significant penalties, hence the need for the

unique OSV support described in this paper — sup-

port activity for which few mariners are likely to

have prior knowledge or experience. A properly re-

sourced full-mission 360˚ simulator centre is able to

meet that specific training need.

REFERENCES

International Association of Geophysical Contractors (IAGC),

2002, Marine Seismic Operations: An Overview.

Naval Warfare Publication, 2004, Underway Replenishment

NWP 4-01.4, US Navy Department. Available at:

http://www.navybmr.com/NWP%204-014.html

Maritime and Coastguard Agency (MCA), 2002, Marine

Guidance Notice MGN 199 (M) Dangers of Interaction,

Southampton, MCA.

Paffett, J. 1990, Ships and Water. London, The Nautical

Institute.

McTaggart, K. Cumming, D. Hsiung, C. & Li, L. 2001,

Hydrodynamic Interactions Between Ships During Under-

way Replenishment, 6

Canadian Marine Hydrodynamics

and Structures Conference, Vancouver, 23-26 May.

Skejic, R. Breivik, M. Fossen, T. & Faltinsen, O. 2009, Model-

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Manoeuvring and Control of Marine Craft, Guarujá (SP),

Brazil, 16-18 September.