T1S6O2 Paper

DOT-2013 Subsea installation in extreme water depths – a contractor’s view

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Subsea Installation in Extreme Water Depths

– A Contractor’s View

Kees van Zandwijk, Radboud van Dijk,

Eelco Harmsen,

Heerema Marine Contractors SE

ABSTRACT

Deepwater lowering has seen a rapid development over the past 20 years. From the first

foundation piles of Tension Leg Platforms in 1000m water depth, via the installation of heavy

anchor piles in 2000 to 3000m of water, the industry is now at the brink of lowering heavy and

sizable subsea production units into deep and ultra-deep water.

When investing in Deep Water Lowering equipment, the discussion circles around the choice for

steel wires or fiber ropes. These two cannot easily be exchanged with the aim to test the new

technology of fiber ropes or to simply use the best of both in the given situation. As steel wires

and fiber ropes require totally different systems that mutually exclude each other, the choice has

the nature of an either-or decision.

This either-or decision is further complicated by the fact that the dynamic behavior of Deep

Water Lowering systems strongly depends on the vessel from which it is operated; on the water

depths in which it is deployed; and on the wave climate in which the operation takes place.

Moreover, small and slender structures behave totally differently as compared to large and heavy

ones when lowered to the seabed.

The paper presents a short reconnaissance study of landing different types of structures on the

seabed, assessing the operability of different types of installation vessels when using steel wires

and fiber ropes. Offshore Support Vessels appear to have an acceptable operability only when

fiber ropes are used in combination with Active Heave Compensation. A semisubmersible crane

vessel in general does better using steel wires. Only in ultra-deep water of 3000m do fiber ropes

usually perform better. The application of an elastic stretcher in the lifting arrangement may have

the same effect as a step from steel wires to fiber ropes and is a subject worth further

investigation.


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ABBREVIATIONS

AHC Active Heave Compensator

CTCU Cable Traction Control Unit

DAF Dynamic Amplification Factor

DWL Deepwater Lowering

FPSO Floating Production, Storage and Offloading unit

OSV Offshore Support Vessel

RAO Response Amplitude Operator

TLP Tension Leg Platform

INTRODUCTION

Modern field development technology demands an increasing amount of hardware on the seabed,

not only the usual wellheads, pipelines and manifolds, but also more and more units of subsea

production equipment. This is expected to be only the beginning of a trend: in the years to come,

deepwater is expected to deliver an ever growing share of offshore oil and gas. In addition,

technology is under development to place more and more elements of the traditional topsides on

the sea floor, the end goal being a subsea factory on the seabed by 2020 (Ref. 1).

When considering investments in new deepwater construction equipment, one of the inevitable

questions is what type of DWL system should be chosen: based on steel wires or fiber ropes.

Steel wires are relatively cheap, rugged, mechanically transparent and reliable. In the past,

payloads worth hundreds billions of dollars have been safely lifted and installed both above and

under water, without major mishaps. The only reason not to use steel wires for DWL is in their

heavy weight, resulting in a decreasing effectiveness when going into very deep water. The

alternative then is fiber ropes with a specific weight close to that of water. Fiber ropes, however,

are expensive and more vulnerable in the offshore installation environment. There is a wide

variety in brands and associated mechanical properties, requiring substantial physical and

chemical knowledge to judge the material and to choose the right wire for an investment. With

respect to reliability: the track record of fiber ropes is short. However, recently a number of

extensive Joint Industry Programs have been run, which have greatly enhanced the trust in fiber

ropes and associated winch systems.

It is not only the weight of the wire which governs the decision between steel wires and fiber

ropes. Other factors are: the complementary use of the DWL system for non-deepwater

applications; the available space on the vessel; the dynamic response of the entire assembly of

vessel and DWL system; and the offshore location at which the system is going to be deployed.

Heerema has experience with a wide variety of installation vessels and DWL work all over the

world. It recently went through an evaluation process of a new DWL investment. The paper aims

at sharing some of the considerations that came to the table during this process.


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DEVELOPMENT OF THE DEEPWATER LOWERING MARKET

In the context of this paper, „deepwater‟ is defined as a water depth over 1000m and „DWL‟ as

the installation of structures on or in the seabed. Pipelines and in-line structures installed with the

pipeline are excluded from this definition.

With these definitions as a reference, we see the first DWL projects coming up in the early 90s,

in the form of a series of TLPs in the Gulf of Mexico. The DWL scope of those first projects

consisted of the installation of the TLP foundation piles. This work was typically carried out by

crane vessels needed for handling the long foundation piles (typically over 100m long) and

driving them into the seabed. The main hoist was used for DWL: by nature this hoist has a lot of

wire length in a dense reeving; this wire was reeved out to a reduced number of parts (i.e. wires

in the bundle) with just enough capacity for handling the piles and hammer. Figure 1 illustrates

the principle. With a reduced number of parts, high-capacity crane vessels could just reach the

seabed with their „natural‟ amount of wire, without the need for additional investments.

Figure 1 – Reaching deeper at less capacity using the same wire length

Between 1995 and 2005, the depth of deepwater field developments grew to 2000m. TLPs were

technically no longer possible and anchored production facilities were introduced, such as Spars,

semis and FPSOs. These structures were moored to anchor piles in the seabed placed in a star-

shaped pattern around the structure. The anchor points typically were suction piles.

At water depths between 1500 and 2000m, re-reeved main hoist systems did no longer suffice.

Firstly, because the larger water depth required to further reduce the number of parts for reaching

the seabed with the installed wire length and the reduced number of parts resulted in a further

reduction of capacity. Secondly, because a substantial amount of the hoisting capacity was lost

due to the steel wires having to support their own weight. Figure 2 illustrates the decay of

effectiveness of a steel wire hoisting system with depth due to the wire weight. For meeting the

capacity requirements at the increased water depths, new DWL units had to be built, in those

h

3h

Wire length = 12 x h Capacity = 12 x Twire

Twire

Wire length = 4 x 3h = 12 x hCapacity = 4 x Twire

12-parts reeving 4-parts reeving

Twire

2h

Wire length = 6 x 2h = 12 x hCapacity = 6 x Twire

6-parts reeving

Twire


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days all using steel wires, as there was no alternative. Some of these units were integrated in the

crane, others were built as a separate unit operated from the deck of the installation vessel.

Figure 2 – Effectiveness of steel wires versus depth

In the same period, the ineffectiveness of steel wires in deepwater triggered the development of a

technical alternative using fiber ropes. The specific weight of fiber rope is close to that of water,

making them about neutrally buoyant when used under water and almost 100% effective in a

DWL system, irrespective of the water depth. A number of serious hurdles had to be overcome:

Around the year 2000, fiber ropes were immature for offshore application. Many

different types were under development, but offshore use was hampered either by low

strength, low stiffness, low critical temperature, creep or sensitivity for abrasion.

Fiber ropes could not be used on conventional drum winches as the severe lateral

contraction of the rope due to the high Poisson‟s ratio made the rope on the outer layers

to creep between the wraps of underlying layers, causing high wear and tear of the ropes

during hauling and veering.

Fiber ropes could not be used on conventional traction winches with drum type sheaves,

as in use for steel wires. The high elongation of fiber rope caused it to slip over the

sheaves, rapidly heating it up above its critical temperature with associated loss of

strength.

Shortly after 2000, a series of Joint Industry Programs was initiated in order to address these

problem areas. These programs resulted in the development of fiber ropes that combined „best of

brand‟ properties by mixing fibers of different suppliers. In parallel, a CTCU traction winch

system was developed using individually driven sheaves with the purpose of accommodating the

extensive elongation of fiber ropes and minimizing the slippage between rope and sheave. The

system was successfully taken into commercial use in 2006 (Ref. 2). The CTCU system is

complex in its controls and patent filings show several attempts to develop a simpler concept for

fiber rope DWL, on the basis of either drum winches or traction winches. However, so far none

of these attempts has beaten the CTCU.

-500

0

-1000

-1500

-2000

-2500

-3000

-3500

-4000

20

Effectiveness [%]

Wat

er d

ep

th [

m]

40 60 80 1000


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The development of fiber rope DWL technology meant a substantial reduction of wire and

equipment weight and opened up the DWL market for installation vessels smaller than the

conventional crane vessels. In addition it enabled DWL at almost infinite water depth as the wire

weight as such was no longer a limiting factor.

The last step in deepwater production technology was made around 2005 with the maturing of

subsea production systems. Gradually, substantial parts of the traditional topsides are placed on

the seabed. This technology is boosted by savings on the platform infrastructure, vertical riser

systems and enhanced hydrocarbons recovery rates. Subsea production units must be serviceable

by OSVs with DWL capacities up to 400t, suggesting that the most logic way of installation is by

the same OSVs. However, the production units are often placed in a container frame with

integrated suction piles and provided with over-trawling protection covers raising the weight of

the entire subsea assembly to 1000 to 2000t. Examples are the Ormen Lange subsea templates of

1150t installed in 850m of water in 2005 and the Åsgard Subsea Compression Project containing

structures over 2000t in weight, presently under construction in 300m of water. These water

depths are not exactly deepwater in accordance with the definition given before. However, they

can be seen as the first examples of a new generation of large and heavy infrastructure with

sensitive rotating equipment that may be expected to extend into deep and very deep water.

It is difficult to predict what the recent developments in subsea technology will mean for future

DWL requirements. Some sources state that high DWL capacities of several 1000s of tons may

be required down to 500m only; below that depth there is no need for over-trawling protection

with associated heavy framing and foundation structures. The same sources expect the subsea

infrastructure below 500m to be built up from serviceable units of maximally 400t of weight.

Other parties expect the subsea market to see a development of large units similar to the

development topsides saw in the 80s. This may imply structures of several 1000s of tons to be

installed to several 1000s of meters depth. It is most likely also a matter of supply and demand: a

contractor with the courage to invest in new high capacity DWL equipment will enable the

industry to shift the boundaries to larger integrated units in deeper water.

WEIGHING STEEL WIRE VERSUS FIBER ROPE

Although a number of Joint Industry Projects on the development of fiber rope based DWL is

still to be completed, it can be stated that today fiber rope technology is mature enough to be a

serious candidate for a new investment in DWL. For a contractor, the choice between steel wires

and fiber ropes eventually is a matter of weighing costs and risks. It is beyond the scope of this

paper to present a complete risk assessment. However, some more can be said about the costs.

CAPEX

The discussion about steel wires or fiber ropes often is an emotional one. Supporters of steel wire

claim fiber ropes to be excessively expensive and brochures of fiber rope suppliers emphasize


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the dramatic savings in equipment weight, paying off in attractive savings on CAPEX. Figure 3

shows the facts: even though fiber ropes are costing 5 times more per ton Safe Working Load

than steel wires, the figure shows that down to a depth of 2000m, the total investment in a DWL

system (wires plus winches) is fairly similar for steel wires and fiber ropes. This also explains

why most contractors traditionally working with crane vessels and steel wires have not made the

step to fiber ropes yet. However, at depths of 3000m and beyond, fiber rope systems win on

investment costs, due to the further declining effectiveness of steel wires as shown in Figure 2.

Figure 3 – Relative CAPEX of DWL systems for 500t payload

An element not included in Figure 3 is the CAPEX of the buoyancy required for the DWL

system. A system suited to lower 500t to 3000m depth on steel wires will weigh about 1500t; a

system with the same capacity on fiber ropes about one-third of that. The difference in weight of

1000t is hardly recognized on the deck of a crane vessel, but has a significant impact on size,

draft and speed of an OSV, with an associated cost effect for the contractor. It explains why most

of the OSV operators involved in DWL have made the step to fiber ropes already.

OPEX

DWL systems using steel wires or fiber ropes may involve quite different write-off, repair and

maintenance and handling and storage costs. In addition, the DWL system may have a strong

influence on the offshore installation vessel. Simply stated, the OPEX of the offshore operation

is expressed by the following formula:

Day rate of installation vessel x Net duration of activity

OPEX = ——————————————————————

Operability of activity

The lighter weight of fiber rope DWL systems tends to enable the deployment of smaller type

installation vessels such as OSVs. An OSV can be hired at a considerably lower day rate than a

conventional crane vessel. The reverse, however, is that operations from an OSV may take

longer due to the absence of a large deck space and an offshore crane. In addition, the operability

of an OSV may be considerably lower than for a crane vessel due to rougher vessel motions. The

evaluation of steel wires versus fiber ropes should involve the OPEX of the offshore operation.

0 1000 2000 3000 4000

Water depth [m]

CA

PEX

of

DW

L sy

stem

steel wiresfiber ropes


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OPERABILITY

A DWL operation involves four phases: over-boarding; passing the splash zone; lowering to

depth through the water column; and landing of the structure on the seabed. The operability

during over-boarding and passing of the splash-zone is not significantly influenced by the use of

steel wires or fiber ropes and is therefore not further addressed in this paper. These phases can

however be difficult for an OSV with a negative impact on its operability. Semisubmersible

crane vessels usually have an excellent operability during over-boarding, thanks to their ample

deck space and the possibility of using two cranes (Figure 4).

Figure 4 – Large deck space of crane vessel enables effective DWL operations

When lowering a structure through the water column to the seabed, the DWL system behaves as

a multi-body mass-spring system, involving vessel, wires and object lowered. The dynamic

behavior of the system is analyzed using the in-house program LiftDyn, suited to analyze the

first-order dynamic response of multi-body mass-spring systems in the frequency domain. The

program contains a library with the geometries and hydrodynamic properties with associated

mass and damping parameters of all vessels frequently in operation by Heerema. These vessel

bodies can be made part of a wider mass-spring system representing the lifting assembly to be

analyzed (Figure 5). With this model, the RAOs can be computed of all bodies and points of

interest in the system. Linear responses are calculated by combining these RAOs with wave

spectra for a range of wave periods and unit wave height. The responses are typically calculated

at directional intervals of 15 degrees and thus allow assessment of the best and worst heading for

specific operations. If needed, wave spreading can also be taken into account.

The masses are formed by the mass of the vessel, the distributed mass of the wires, the mass of

the structure, the mass of the underwater block and the added mass of the water moving with the

structure. Damping is generated between the structure and the surrounding water, the magnitude

of damping depending on the size and the shape of the structure.


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Figure 5 – LiftDyn visual representation

Steel wires have a higher axial stiffness than fiber ropes. In addition, the spring stiffness of the

hoisting system decreases linearly with the length of the wires. The spring stiffness of the DWL

system is thus highly variable. Moreover, fiber ropes have a strongly non-linear elastic behavior

and respond different under static and dynamic loading (Ref. 3).

During lowering to depth, the risk exists that the DWL system passes a depth zone at which the

mass-spring system becomes critical and gets in resonance, exited by the vessel motions. Large

vertical motions during lowering to depth are not a serious problem as long as the wires of the

DWL system do not fall slack or the capacities of the hoisting system and lift rigging are not

exceeded. However, generally the damping of the system is such that it prevents this from

happening. For this reason, the phase of lowering to depth is not further addressed here.

STRUCTURE LANDING USING STEEL WIRES OR FIBER ROPES

For several reasons, the landing of a structure on the seabed is subject to limitations. For suction

piles, a high landing speed may cause piping as a result of entrapped water escaping from under

the tip of the pile, the piping channels obstructing the suction process. For subsea structures, a

high landing speed may cause erosion of the seabed by water escaping from underneath the

structure, the erosion channels causing an uneven seabed with a permanent out-of-level position

of the structure as a consequence. Moreover, the impact of a hard landing may cause damage or

increased wear of sensitive rotating equipment, such as compressors, contained in the subsea

module. For many DWL operations, as a kind of standard, a maximum landing speed of 0.5m/s

is specified. This landing speed has been taken as a limit for this operability study.

A short operability study was carried out, with the aim to illustrate how the landing speed limit

influences the operability of different DWL systems, using both steel wires and fiber ropes, when

operated from different vessels at different water depths in different wave regimes. As reference


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water depths, 1000, 2000 and 3000m were chosen. The analysis involved the assessment of the

maximum Hs-Tp up to which the landing speed at the seabed could be kept below 0.5m/s.

Regarding the installation vessels, a big crane vessel was compared with an OSV. As the crane

vessel, the Balder was taken and as the OSV, a 75m long OSV. The first vessel is owned, the

latter regularly chartered by Heerema. These vessels were selected primarily because calibrated

vessel parameters were at hand in LiftDyn, not because they are considered the most

representative units in their class.

The Balder and the OSV were compared on the installation of a suction pile. In addition, the

Balder was analyzed on the installation of a huge subsea structure, with the aim to broaden the

conclusions drawn from the suction pile installation. As a first step, the mass, damping and

spring stiffness parameters were assessed for use in the dynamic analysis. Per system analyzed,

the parameters are summarized in Table 1.

The suction pile was defined as a cylinder of 5m diameter, 30m long with a mass of 150t. The

subsea structure has dimensions 65m long, 35m wide and 15m deep, containing 4 suction cans

on the corners, each 10m in diameter and protruding 10m below the base of the structure. It has a

mass of 1700t. The added masses of the structures were computed in compliance with DNV RP-

C205. The mass of the underwater hoist and the mass of the wire bundle were derived from the

arrangement of the DWL system.

As damping, for both structures 10% of the critical damping was used. The critical damping Bc is

computed as:

Bc = 2 . {(m + M) . k}0.5

…………………………………………. (1)

in which

m = mass of structure

M = added mass of structure

k = spring stiffness computed by Formula (2)

The spring stiffness k is formed by the wires of the DWL system according to the formula:

Erope . A

k = ———— ………………………………………………..…… (2)

L

in which

Erope = axial stiffness of the wires


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A = total cross-sectional area of the wires

L = length of the wires between the crane boom tip and the structure

SPRING STIFFNESS OF DWL SYSTEM

Erope, A and L were derived from the DWL hoisting systems deployed. A DWL system was

defined for every combination of vessel, wires, structure and water depth. The Safe Working

Load of the steel wires was based on a safety factor of 3 with respect to the Minimum Breaking

Load, for the fiber ropes a safety factor of 4.5 was used, reported as industry practice in Ref. 4.

For defining the DWL system required, initial DAF values were assumed at 1.3 for the suction

pile and 1.65 for the subsea structure. These values were later verified not to be exceeded in

LiftDyn. Table 1 summarizes the parameters of the DWL systems used in LiftDyn.

It was assumed that the existing steel wire DWL system in the starboard crane of the Balder was

used for lowering the suction pile. This system consists of two traction winches, each reeved

with a 19000m long wire of 51mm diameter. For reaching 3000m deep, a 12-parts reeving was

applied; this reeving is also suitable for 1000 and 2000m depth. In order to appraise the effect of

a stiffer system, for 1000 and 2000m also a „maximum reeving‟ was applied, implying the

maximum spring stiffness that can be achieved with the existing steel wires. For 1000m this

comes down to a 32-parts reeving; for 2000m a 16-parts reeving. For the Balder with fiber ropes,

the same DWL system in the starboard crane was assumed to be fitted with 48mm diameter fiber

ropes in a 6-parts reeving, the same reeving at all water depths.

The OSV could just lower the suction pile to 3000m using its main drum fitted with a 8000m

long steel wire of 76mm diameter in a 2-parts reeving as shown in Figure 6. This reeving was

also used at the other water depths. When working with fiber rope, a 104mm diameter rope was

assumed in a 1-part reeving running via the A-frame.

Figure 6 – OSV with 2-parts reeving

For lowering the subsea structure to 3000m depth, a dedicated new-build DWL unit was

assumed on the deck of the Balder. This system is designed to work with an 8-parts reeving, the


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same reeving at all water depths. The 8-parts reeving requires 158mm diameter steel wires or

152mm diameter fiber ropes for lowering the subsea structure to the seabed in 3000m of water.

The Erope of steel wires was assessed at 86GPa following the approach described in Ref. 5. For

reason of simplification, the Erope of fiber ropes was assumed to be linear with a value of 29GPa

as derived from the rope elongation diagram given in Ref. 6. This linear Erope for fiber rope

neglects the non-linear stress-strain behavior and the difference between the static and dynamic

axial stiffness. The Erope values of 86 and 29GPa are to be used in combination with the gross

cross-sectional area of the wire.

Table 1 – Parameters used in LiftDyn

Water depth 3000m 1000m 2000m 3000m

STIFF FLEXIBLE STIFF FLEXIBLE

SUCTION PILE

Mass [t] 150

DAF 1.3

Added mass [t] 669

Damping [% of critical] 10%

BALDER

Diameter of wire [mm] 51 51 51 51 51 48 48 48

Safe Working Load [kN] 660 660 660 660 660 350 350 350

No. of parts 32 12 16 12 12 6 6 6

Length per wire [m] 1100 1100 2100 2100 3100 1100 2100 3100

Stiffness k = EA/L [kN/m] 5076 1904 1329 997 675 287 150 102

Critical damping [kNs/m] 4078 2497 2087 1807 1488 970 702 578

Mass wire bundle [kg/m] 352 132 176 132 132 11 11 11

Mass u/w block [t] 74 31 31 31 31 31 31 31

OSV

Diameter of wire [mm] 76 76 76 104 104 104

Safe Working Load [kN] 1710 1710 1710 1680 1680 1680

No. of parts 2 2 2 1 1 1

Length per wire [m] 1000 2000 3000 1000 2000 3000

Stiffness k = EA/L [kN/m] 781 391 260 247 124 82

Critical damping [kNs/m] 1600 1131 924 900 636 520

Mass wire bundle [kg/m] 58 58 58 10 10 10

Mass u/w block [t] 15 15 15 5 5 5

SUBSEA STRUCTURE

Mass [t] 1700

DAF 1.65

Added mass [t] 35576

Damping [t/s] 10%

BALDER

Diameter of wire [mm] 158 158 158 152 152 152

Safe Working Load [kN] 6630 6630 6630 3170 3170 3170

No. of parts 8 8 8 8 8 8

Length per wire [m] 1025 2025 3025 1025 2025 3025

Stiffness k = EA/L [kN/m] 13176 6669 4464 4121 2086 1396

Critical damping [kNs/m] 44323 31534 25801 24789 17636 14430

Mass wire bundle [kg/m] 2816 2816 2816 168 168 168

Mass u/w block [t] 50 50 50 31 31 31

1700

35576

10%

STEEL WIRES FIBER ROPES

1000m 2000m

150

669

10%

1.3

1.65


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LiftDyn converts the vessel motions into crane tip motions via the geometry of the vessel. The x,

y and z positions of the crane tip in relation to the vessel hull for the different DWL systems are

described in Table 2.

Table 2 – Crane tip positions used in LiftDyn

With the parameters of Tables 1 and 2, the response of the multi-body mass-spring system was

analyzed in the frequency domain and RAOs were determined for the vertical velocity of the

load. The significant vertical velocity was computed and further processed into the 20 minutes

Most Probable Maximum vertical velocity as a function of Hs and Tp, the 20 minutes

representing the time needed for the landing operation. With the criterion that the 20 minutes

Most Probable Maximum vertical velocity should be smaller than 0.5m/s, an Hs-Tp line was

computed below which this criterion is satisfied and above which it is not. This line is defined as

the „operability line‟.

Figure 7 – Typical LiftDyn operability curves (only 8 of the 24 wave directions shown for clarity)

DWL system X Y Z

Longitudinal vessel axis Transverse vessel axis Vertical

SUCTION PILE

BALDER 86.5m fore of stern 7m out of starboard 118.4m above keel

OSV

steel wires 2.6m aft of stern vessel centerline 13m above keel

fiber ropes 5.2m aft of stern vessel centerline 20m above keel

SUBSEA STRUCTURE

BALDER 48m fore of stern 7m out of starboard 55m above keel

Best headingupper envelope


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For each of the 24 wave directions analyzed an operability line was determined, resulting in a

bundle of operability lines in the Hs-Tp domain. The installation of a suction pile allows the

installation vessel to choose the best heading. With reference to the bundle of operability lines

(Figure 7), this implies that the upper envelope of the bundle is taken as the governing

operability line.

The next step was, to compare this operability line with the Hs-Tp spectrum in a number of

different wave regimes. As typical examples, seasonal sea conditions were taken of the Gulf of

Mexico (Perdido), representing a moderate spectrum with short waves; of the Norwegian Sea

(Aasta Hansteen), representing harsh conditions with high and relatively short waves; and of the

West of Africa (Angola Block 31), representing an open ocean spectrum with low and very long

waves. The probability density diagrams of Hs-Tp of these wave spectra are shown in Figure 8.

Figure 8 – Probability density diagrams of sea states investigated

When the operability line is plotted in the Hs-Tp domain, it cuts through the probability density

diagram. The integrated probability below the operability line represents the average operability

over the installation season; the integrated probability above the line the non-operability. The

principle is shown in Figure 9.

Figure 9 – Operability derived by integrating probability density below the operability curve

For visual comparison, the diagrams of Figure 8 were converted into contours following one of

the lines of equal probability density. This line was more or less arbitrarily chosen at the level of

Gulf of MexicoPerdido

Norwegian SeaAasta Hansteen

West of AfricaAngola Block 31

% Operability


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10log (P) = -2.25, at which level the visual operability agreed well with the computed values.

Figure 10 shows the diagrams of Figure 8 converted into contours (shown as black line).

Figure 10 – Probability density diagrams converted into contours (black line)

As a final step, the operability was computed for the DWL systems shown in Table 1 in

combination with the 3 wave spectra of Figure 8. The results are shown in Figures 11, 12, 13, 15

and 16.

Figure 11 – Balder performance on suction pile installation

Figure 11 shows the results for the suction pile installed with the Balder at best heading. Down to

2000m, the Balder with steel wires performs considerably better than with fiber ropes. The

performance of the steel wire system can even be maximized by arranging as many parts in the

reeving as the installed wire length allows. Maximizing the number of parts contributes more at

2000m than at 1000m depth: apparently, at 2000m the 12-part reeving is at a stage of becoming

sensitive for resonance, the extra 4 wires adding just enough extra stiffness to stay out of the

Gulf of MexicoPerdido

Norwegian SeaAasta Hansteen

West of AfricaAngola Block 31

1000m

0

2

4

6

8

Max

imu

m a

llow

able

wav

e h

eigh

t [m

]

Peak period [s]

0 4 8 12 16

2000m 3000m

Semi-Submersible Crane Vessel

Operability

Steel wires

12 parts

Fiberropes

6 parts

Area

GOMNORWAF

95%51%24%

99%86%70%

0

2

4

6

8

Max

imu

m a

llow

able

wav

e h

eigh

t [m

]

Peak period [s]

0 4 8 12 160

2

4

6

8

Max

imu

m a

llow

able

wav

e h

eigh

t [m

]

Peak period [s]

0 4 8 12 16

Operability

Steel wires

12 parts

Fiberropes

6 parts

Area

GOMNORWAF

98%83%91%

94%37%12%

Steelwires

32 parts

99%86%92%

Operability

Steel wires

12 parts

Fiberropes

6 parts

Area

GOMNORWAF

97%70%90%

98%64%17%

Steelwires

16 parts

98%80%90%


Page 15 of 20

critical zone. Between 2000 and 3000m depth, the operability on steel wires collapses drastically,

especially in areas with high and/or long waves (Norwegian Sea and West of Africa). At 3000m

depth, the Balder performs considerably better on fiber ropes than on steel wires.

Figure 12 – OSV performance on suction pile installation, no AHC

Figure 12 shows the result for the OSV at best heading. Due to the strong vertical motions of the

„crane tip‟ at the stern of the OSV, all DWL systems on steel wires perform poorly, the systems

becoming gradually better at greater water depth when the elasticity of the hoisting system

increases. The flexibility of fiber ropes is indispensable for achieving a reasonable performance.

However, the operability at 1000 and 2000m stays poor in high or long waves (Norwegian Sea

and West of Africa), even with fiber ropes.

It should be noted that a good deal of the poor behavior of the OSV is due to the fact that the

suction pile is lowered from the A-frame at the stern of the vessel. The behavior would be

considerably better when the DWL system would be deployed over the side. However, such a

lowering position requires a substantial offshore crane, in turn requiring a substantial OSV with a

day rate close to that of a crane vessel. Better operability therefore comes at a cost.

The performance of the OSV can be improved by the application of AHC in the hoisting system.

For steel wires, AHC is usually provided by leading the wire over a sheave on a piston, the

stroke of the piston lengthening and shortening the wire as required. In order to provide

sufficient compensation for the strong stern motions of the OSV, an effective stroke of at least

5m was estimated to be needed on one of the parts of the double-reeved system. The wire speed

limitation was assumed to be 1.4m/s, which comes down to 0.7m/s on the double-reeved hoisting

system. For the OSV with fiber ropes, a winch with individually driven sheaves was assumed.

0

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Operability

Steel wires

2 parts

Fiberropes1 part

Area

GOMNORWAF

48%2%0%

71%14%

1%

Operability

Steel wires

2 parts

Fiberropes1 part

Area

GOMNORWAF

57%5%0%

95%50%17%

Operability

Steel wires

2 parts

Fiberropes1 part

Area

GOMNORWAF

71%14%

1%

98%81%70%

Offshore Support Vessel without Active Heave Compensation

1000m 2000m 3000m


Page 16 of 20

These winches have AHC built in the sheave drives. In principle, the winch driven AHC has no

stroke limitation; however the system was assumed to have a wire speed limitation of 1.5m/s.

Figure 13 – OSV performance on suction pile installation, with AHC

Figure 13 shows the performance of the OSV on the suction pile installation using AHC. The

analysis was done applying four criteria:

AHC was assumed to reduce vessel motions by 90%. At the 10% motions left, the

structure landing speed should not exceed of 0.5m/s;

The crane tip should not move at more than stroke limit of the AHC system;

The crane tip should not move faster than the velocity limit of the AHC system;

The design capacity of the DWL system should not be exceeded.

By comparing Figure 13 with Figure 12, it is concluded that AHC in general improves the

performance of the DWL system on an OSV. However, for steel wires the operability, even with

AHC, stays poor: below 20% for the Norwegian Sea and West of Africa. Only in the Gulf of

Mexico a reasonable operability of 60 to 70% is achieved on steel wires with AHC. On an OSV,

AHC is much more effective on a fiber rope system, in particular in long swells (West of Africa)

and at 1000m water depth. At very deep water (3000m) the opposite effect may occur: in shorter

waves such as Gulf of Mexico and Norwegian Sea, the performance with AHC may be worse

than without. In long swells, AHC is effective on fiber ropes also in 3000m.

The required AHC capacities involve a substantial unit: Figure 14a shows the size of the AHC

system required for steel wires, projected on the 28 x 15m deck of the OSV. Not too much space

is left for the handling of rigging; referring to Figure 14c, the AHC unit will definitely prohibit

over-boarding of the suction pile from the OSV‟s own deck.

Operability

Steel wires

2 parts

Fiberropes1 part

Area

GOMNORWAF

72%18%12%

91%64%98%

Operability

Steel wires

2 parts

Fiberropes1 part

Area

GOMNORWAF

72%18%12%

91%63%96%

Operability

Steel wires

2 parts

Fiberropes1 part

Area

GOMNORWAF

58%13%12%

91%63%96%

0

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Peak period [s]

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t [m

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Peak period [s]

0 4 8 12 16

1000m 2000m 3000m

Offshore Support Vessel with Active Heave Compensation


Page 17 of 20

Figure 14a Figure 14b Figure 14c

AHC unit on OSV deck Fiber rope winch on OSV deck Two suction piles on OSV deck

The size of the fiber rope winch on the OSV deck is shown in Figure 14b. The size is smaller

than the AHC unit needed for the steel wires. However, referring to Figure 14c, also the fiber

rope winch unit will most likely prohibit over-boarding of the suction pile from the OSV‟s own

deck.

Larger OSVs are available with more deck space. However, as said before, they have a higher

day rate, illustrating again that better operability comes at a cost.

Figures 15 and 16 show the performance of the Balder on the DWL of a heavy subsea structure,

without and with AHC, respectively. Also for this operation, the Balder was assumed to operate

at best heading. The AHC system for steel wires was assumed to have a maximum effective

stroke of 2.5m and a maximum velocity of 1.0m/s, both on the double wire; for the fiber rope

system, a maximum velocity of 1.0m/s on the double wire was assumed. Contrary to the

installation of the suction pile, for the large subsea structure dynamic amplification of the load

plays an important role: the huge subsea structure with the enormous added mass acts as an

underwater anchor to the crane vessel. In particular at moderate water depth, the DWL system is

stiff, resulting in high loads in the wires. Even the DAF assumed at 1.65 appeared sometimes to

be exceeded. A large structure with a high added mass significantly influences the motions of the

installation vessel, implying that this type of DWL operations can only be analyzed correctly by

a coupled model as in LiftDyn.

A number of conclusions can be drawn by comparing Figures 15 and 16 for a semisubmersible

crane vessel lifting a large subsea structure on steel wires without and with AHC:

A steel wire DWL system without AHC gives an excellent operability in the Gulf of

Mexico and West of Africa. Only in the Norwegian Sea, the operability at 1000m is

somewhat reduced to 70%.

Fiber ropes improve the operability in the Norwegian Sea by 15 to 25% as compared to

steel wires.

AHC on steel wires has a positive effect, in particular in 1000 and 2000m water depth.

AHC on fiber ropes has a marginal effect, sometimes even negative.

28m

15

m

Steel wire AHC unit

28m

15

m

Fiber rope AHC unit

28m

15

m

Suction pile


Page 18 of 20

Fiber ropes or AHC may bring the operability curve far above the wave spectrum. When

this occurs, the DAF can be reduced. A reduced DAF allows the installation of a structure

with a higher weight in air. As an example, a subsea structure with a dry weight of 1700t

at a DAF of 1.65 gives the same dynamic load in the hoisting system as a structure of

2100t at a DAF of 1.32. As a consequence, fiber ropes would allow to install

considerably higher loads in 2000 and 3000m than steel wires.

Figure 15 – Balder performance on 1700t subsea structure, no AHC

Figure 16 – Balder performance on 1700t subsea structure, with AHC

Operability

Steel wires

8 parts

Fiberropes

8 parts

Area

GOMNORWAF

97%70%90%

98%84%91%

Operability

Steel wires

8 parts

Fiberropes

8 parts

Area

GOMNORWAF

98%75%90%

99%99%

100%

Operability

Steel wires

8 parts

Fiberropes

8 parts

Area

GOMNORWAF

98%81%90%

99%100%100%

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Peak period [s]

0 4 8 12 16

1000m 2000m 3000m

Semisubmersible crane vessel with 1700t template, no AHC

Operability

Steel wires

8 parts

Fiberropes

8 parts

Area

GOMNORWAF

99%100%100%

99%95%

100%

Operability

Steel wires

8 parts

Fiberropes

8 parts

Area

GOMNORWAF

99%95%

100%

99%95%

100%

Operability

Steel wires

8 parts

Fiberropes

8 parts

Area

GOMNORWAF

98%84%91%

99%97%

100%

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1000m 2000m 3000m

Semisubmersible crane vessel with 1700t template, with AHC


Page 19 of 20

DE-TUNING

The DWL system behaves as a complex multi-body mass-spring system in which the spring

stiffness of the hoisting system plays a dominant role for the magnitude of the hook load, high

hook loads sometimes being generated by inertia forces, sometimes by resonance phenomena. In

specific cases, the step from steel wires to fiber ropes shows a drastic improvement of the

operability.

The same effect can be achieved by applying an elastic stretcher in the steel wire hoisting

system. Nylon, for instance, has about one-third of the strength of the fiber ropes used in the

analysis (with fiber rope properties taken from Ref. 6) and an Erope of about 30 times lower. This

implies that a nylon stretcher of 100m length has the same spring stiffness k = EA/L as 1000m

length of fiber ropes with the same capacity. Referring to Figure 15, at 1000m water depth, a

100m long nylon stretcher would enhance the performance of steel wires to that of the fiber rope

system; at 2000m depth a 200m long stretcher would have the same effect.

On the OSV, the same effect can be reached. Referring to Figure 13, a 100m long nylon stretcher

in the steel wire DWL system would bring the operability on the „fiber rope‟ curve and greatly

improve the performance of the OSV. The over-boarding of the suction pile including rigging

with a 100m long stretcher would be a challenge. Heerema has done this operation from a crane

vessel by handing over the pile with long rigging to the OSV.

CONCLUSIONS

1. Down to 2000m water depth a fiber rope DWL system is not significantly cheaper than a

system based on steel wires. At 3000m and more, the costs of a fiber rope system are

significantly lower.

2. A semisubmersible crane vessel installing suction piles, performs considerably better on

steel wires than on fiber ropes down to 2000m. Below 2000m, fiber ropes perform better.

3. A semisubmersible crane vessel installing a large subsea structure performs better on

fiber ropes than on steel wires.

4. The performance of steel wires can be brought to the level of fiber ropes by applying an

elastic stretcher of limited length in the hoisting system.

5. An OSV installing suction piles definitely needs fiber ropes and AHC for achieving an

acceptable operability. The effect of fiber ropes could also be achieved by steel wires

with an elastic stretcher.

6. The choice between steel wires and fiber ropes to be made when investing in a DWL

system must consider the type of installation activities expected, the area and water depth

of operation and the vessel from which the system is deployed. In some cases, fiber ropes

are better, in other cases steel wires.


Page 20 of 20

7. A semisubmersible crane vessel does not need AHC for installing a suction pile. When

installing a heavy subsea structure, AHC can improve the operability, but would involve

a huge piece of equipment. A nylon stretcher in the lifting rigging can have the same

positive effect for safely landing sensitive subsea equipment on the seabed.

REFERENCES

1. Internet, www.statoil.com/en/technologyinnovation/fielddevelopment/aboutsubsea.

2. S. Torben, P. Ingeberg, Ø. Bunes, S. Bull, J. Paterson, D. Davidson, “Fiber Rope

Deployment System for Ultra-Deepwater Installations”, Paper 18932, Offshore

Technology Conference, Houston, May 2007.

3. J.R. Navarro, J. van Drunen, R. de Bruin, “Monitoring Campaign on Subsea Installation”,

Paper 83324, 31st International Conference on Ocean, Offshore and Arctic Engineering,

Rio de Janeiro, July 2012.

4. I. Bjørnevik, P. Hellevik, P. Ingeberg, S. Torben, “Testing of ropes for heavy duty fibre

rope deployment systems”, Rio Oil & Gas Expo and Conference 2012, Paper IBP1875-

12.

5. M. Raoof, T.J. Davies, “Simple determination of the axial stiffness for large diameter

independent wire rope core or fiber rope wire ropes”, Civil and Building Engineering

Department, Loughborough University, Loughborough, Leicestershire, UK.

6. Internet, www.cortlandcompany.com/sites/default/files/downloads/media/technical-

literature-braid-optimized-bending-bob-tech-sheet_1.pdf

http://www.statoil.com/en/technologyinnovation/fielddevelopment/aboutsubsea

http://www.cortlandcompany.com/sites/default/files/downloads/media/technical-literature-braid-optimized-bending-bob-tech-sheet_1.pdf

http://www.cortlandcompany.com/sites/default/files/downloads/media/technical-literature-braid-optimized-bending-bob-tech-sheet_1.pdf

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