on-bottom stability calculations for fibre …€¦ · the old dnv-rp-e305 “on-bottom stability...

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ON-BOTTOM STABILITY CALCULATIONS FOR FIBRE OPTIC

SUBMARINE CABLES

Inge Vintermyr (Nexans Norway AS) Email: <[email protected] > Nexans Norway AS, PO.Box 6450 Etterstad, N-0605 Oslo, Norway.

Abstract: This document describes the physical processes and the corresponding equations determining on-bottom stability of offshore cables. It presents an overview of DNV offshore design code RP-F109 “On-Bottom Stability Design of Submarine Pipelines”. Finally, on-bottom stability for a set of FO-cables, power-cables, an umbilical and a pipeline are calculated with RP-F109 at different geographical locations around the world. Vital parameters for on-bottom stability are briefly discussed. 1. INTRODUCTION Increasing demand for communication, control and monitoring of offshore oil and gas fields has called for fibre optic (FO) links connecting to stationary surface plants, to land or intra-field. The offshore applications of FO submarine cables have resulted in more focus on on-bottom stability of cables at certain weather/sea bed properties. It is commonly specified that the FO cable shall remain stable on the sea floor under defined conditions. DNV-RP-F109 /1/ “On Bottom Stability Design of Submarine Pipelines” is commonly referred to. In this paper on-bottom stability will be calculated for a range of FO cables and compared with other cables, umbilicals and piplines in order to evaluate if the FO cables are different in terms of on-bottom stability. Typical unrepeatered FO cables are shown in Figure 1, whereas repeatered FO cables are shown in Figure 2. Typical power cables and umbilicals are shown in Figure 2 and 3. Physical characteristics for all cables are shown in Table 1 on page 5.

URC1-LW URC1 - SA

URC1-DA2 URC1-RC Figure 1. FO Unrepeatered Cables

Figure 2. FO Repeater Cables, LW and SA

ROC-LW ROC - SA



24 kV Power 145 kV Power Figure 3. Power Cables

Figure 4. Umbilical 2. SEABED STABILITY THEORY In the following chapters, “pipe” is used as a generic term for FO-cables, umbilicals and pipelines. Many different factors are involved in the assessment of on-bottom stability of electrical cables, pipelines and power/ control umbilicals. Figure 5 shows the forces acting on the pipe. FL

FI +FD

FLOW

(ws –FL)u+ FR(z) ws

Figure 5. Driving and resisting forces on a circular pipe resting on the seabed. Loads can be divided into lift (FL), Drag (FD) and Inertia (FI) forces, and they can be

described with the following empirical equations: Drag:

( )( )2sin21

CpwDwD UtUCDF +⋅⋅= ωρ

Lift:

( )( )2sin21

CpwLwL UtUCDF +⋅⋅= ωρ

Inertia: ⎟⎠⎞

⎜⎝⎛⋅⋅=

dtdUCDF w

MwI2

4ρπ

Where ρw is water density (kg/m3), D is pipe outer diameter (m), Uw is the velocity of an oscillatory flow (m/s) with angular frequency ωp(s-1), UC is the current velocity (m/s), ws (N/m) is the submerged weight, u is coulomb friction coefficient and CL, CD, CM are the lift, drag and added mass coefficients. Current velocity and direction are normally measured 2-5 m above seabed. The current velocity near the seabed will be reduced due to the boundary layer effect which is dependent on seabed roughness (clay, sand, boulders). In most cases only significant wave height (Hs) and corresponding wave period is accessible on a specific site. The oscillatory flow velocity Uw (and angular frequency ωp) at the seabed can be calculated by using numerical or analytical linear wave theory (airy wave). The effect of wave spreading should also be accounted for. For both current and wave simple decomposition is used in order to find the normal component on the pipe. A conservative approach is to assume that both wave and current act in the normal direction. Resistance against movement is due to two different effects (ref 0). 1. Pure coulomb friction FC = (ws‐FL)u 2. Resistance due to penetration into the seabed FR.

Penetration is due to self weight, piping, dynamics during laying and penetration due to pipe movement under the action of waves and current. Piping is the movement of sand under the pipe due to pressure



difference on each side. The pressure difference is caused by current. Penetration varies with seabed type and is larger on soft clay/loose sand than on hard clay/dense sand. Three different approaches regarding on-bottom stability may be followed (ref [1]). 1. Allowing accumulated displacement: A certain maximum displacement is allowed. Under this approach, the pipe will break out of its cavity many times during an extreme sea state. The displacement under both extreme and less severe sea states might accumulate damage and it is important to be aware of this. 2. No break‐out (Virtual Stability): Some small displacement is allowed, normally less than half the pipe diameter. In this approach the pipe will never move out of its cavity. These small displacements will build up resistance due to more penetration into the seabed and/or trenching. No accumulated displacements. 3. Absolute stability: All the loads are less than the resistance forces and there is no lateral movement.

The first and the second method will only work if the oscillatory component of the load forces is large enough to move the pipe. At large water depth, the wave induced oscillatory current is negligible and only a steady state current will act on the pipe. If this current is large enough to move the pipe, and it is conservatively assumed that the current acts in one direction only, displacement will accumulate in one direction only. Under these conditions absolute stability is the only reasonable approach to follow. The load combination which is used during an on-bottom stability analysis is dependent on the time span the pipe will be exposed. It is normal to separate between a

temporary phase and a more permanent operational phase: Temporary Phase: For temporary phases with duration in excess of three days and less than12 months, a 10-year return period applies. This condition may be approximated by the most severe condition among the following two combinations: 1. The 10‐year return condition for waves combined with the 1‐year return condition for current. 2. The 1‐year return condition for waves combined with the 10‐year return condition for current.

Operation: For permanent operational conditions and temporary phases with duration in excess of 12 months, a 100-year return period applies. When detailed information about the joint probability of waves and current is not available, this condition may be approximated by the most severe condition among the following two combinations: 1. The 100‐year return condition for waves combined with the 10‐year return condition for current. 2. The 10‐year return condition for waves combined with the 100‐year return condition for current.



3. STANDARD AND SOFTWARE

PROGRAMS The most accurate approach for stability calculation is to use a commercial software product. Two major software programs exist:

-Pondus (Marintek, Norway) -AGA Leve III (PRCI, USA)

Both programs are based on time domain dynamic analysis. It is typical to simulate a three hour severe storm when performing dynamic analysis. The results from the analysis consist of time series of both lateral displacement and loads (stress, strain etc). There are relatively few companies performing time domain dynamic analysis for the assessment of on-bottom stability because of the time and work involved. An easier approach for on bottom stability assessment is to use the methods described in the DNV offshore design code DNV-RP-F109 “On-bottom stability design of submarine pipelines”. This code replaces the old DNV-RP-E305 “On-bottom stability design of submarine pipelines”. We are not aware of any other design codes/standards for on-bottom stability design. RP-F109 describes methods for the three different approaches from the theory section: 1. Absolute Stability 2. Virtual Stability, displacement less than 0.5xOD 3. Accumulated displacement, less than 10xOD

All methods calculate a minimum required submerged weight in order to fulfill the allowable displacement. The two lasts methods are based on a large set of full dynamic analyses performed by using PONDUS. These analyses are used as a basis for making design curves for these methods.

RP-F109 needs a large set of different parameters. The most important parameters are: D: Pipe outer diameter [mm] ws: Pipe submerged weight per

unit length [kgf/m] Hs: Significant wave height [m] for

extreme sea states ( 1, 10 and 100 years return period)

Tp: Wave Peak period [s] for Hs Uc: Current speed [m/s] for extreme

events (1, 10 and 100 years return period)

d: Water Depth [m] su: Un-drained clays shear strength

[kPa] γ’s: Submerged unit soil weight. For

sand normally in the range 7000 (very loose) to 13 500 N/m (very dense)

d50: Mean sand grain size [mm]

The oceanographic data (Hs, Tp, Uc) have to be derived by statistical methods from long term measurement of both wave and current. Wave and current direction is also possible to give as an input but normally it is conservatively assumed that the wave and current direction is perpendicular to the pipe. The oceanographic data is generated by specialized companies such as e.g. Metoc in the UK. The Metocean report for a specific project is usually tailor made for the specific area, and is purchased by the end user/field developer. Seabed data is established by taking soil samples at different locations in the area where the pipe is going to be installed. DNV StableLines1.2 is commercially available and implements RP-F109 in a software program.



4. ON-BOTTOM STABILITY CALCULATIONS AND RESULTS

In order to compare bottom stability of different FO cables, calculations have also been performed for power cables, umbilicals and a pipeline. Their characteristics are shown in Table 1. Table 1.Characteristics for cables, umbilicals and pipeline, ref cross-sections shown on page 1 and 2.

Cable Type OD (mm)

Mass (kg/m)

Subm. W (kgf/m)

Subm.W/ OD (kg/m2)

URC-1-LW1.9 19 0,67 0,38 20.00 URC-1-SA3.2 22 1,10 0,70 31,82 URC-1-DA2 33 3,20 2,30 69,70 URC-1-RC 64 14,50 11,00 171,88 ROC-LW 18 0,55 0,31 17,22 ROC-SA3.6 30.0 2,20 1,50 50,00 Power – 24 kV 88.0 14.40 9,60 109,09 Power -145 kV 194.0 77.00 55.00 283.51 Umbilical 161.5 40.40 16.40 101.55 24” Pipeline 814 1192.7 489.30 601.10

The field conditions studied are shown in Table 2. Table 2.Typical key data for stability calculations at different locations. Clay shear strength and grain size are set to arbitrary values since no specific information is accessible. Area d

(m) Hs (m)

Tp (m)

Uc (m/s)

Su (kPa)

d50 (mm)

North Sea

290 16 18.5 0.41 10 ‐

North Sea

290 16 18.5 0.41 ‐ 0.5

Brazil 1300 7.16 14.8 0.58 10 ‐ Brazil 1300 7.16 14.8 0.58 ‐ 0.5 Gulf of Mexico

1913 14.2 14.9 0.1 10 ‐

Gulf of Mexico

1913 14.2 14.9 0.1 ‐ 0.5

Persian Gulf

51 6,1 11,2 1,3 10 ‐

Persian Gulf

51 6,1 11,2 1,3 ‐ 0.5

Calculations for the various “pipes” at North Sea environments, Persian Gulf and Gulf of Mexico are shown in Table 3, 4 and 5, respectively. The calculations are peformed by means of StableLines v1.2 Software.

Table 3. Seabed stability calculations different cables at North Sea (ref Table 2).

Cable Type Subm. W (kgf/m)

Seabed ReqSw, Abs (kg/m)

Stable Abs

URC-1-LW1.9 0,38 Sand Clay

0.79 2.06

No No

URC-1-SA3.2 0,70 Sand Clay

0.89 2.29

No No

URC-1-DA2 2,30 Sand Clay

1.24 2.99

Yes No

URC-1-RC 11,00 Sand Clay

2.01 3.51

Yes Yes

ROC-LW 0,31 Sand Clay

0.75 1.96

No No

ROC-SA3.6 1,50 Sand Clay

1.19 2.97

Yes No

Power – 24 kV 9,60 Sand Clay

3.87 8.76

Yes Yes

Power -145 kV 55.00 Sand Clay

7.85 10.50

Yes Yes

Umbilical 16.40 Sand Clay

9.52 22.76

Yes No

24” Pipeline 489.30 Sand Clay

37.64 32.84

Yes Yes

Table 4. Seabed stability calculations for different cables at Pesian Gulf (ref Table 2).



Stable Abs


0.002 Yes Yes


0.003 0.003

Yes Yes


0.005 0.005

Yes Yes


0.011 0.011

Yes Yes


0.002 0.002

Yes Yes


0.004 0.004

Yes Yes


0.017 0.017

Yes Yes


0.047 0.047

Yes Yes


0.037 0.037

Yes Yes


0.278 0.278

Yes Yes

Table 5. Seabed stability calculationse for different cables at Pesian Gulf (ref Table 2).



Stable Abs


lift lift

No No


lift lift

No No


lift lift

No No


29.1 76,8

No No


Lift Lift

No No


Lift Lift

No No


44.6 115.6

No No


120.1 315.5

No No


64.9 lift

No No


790.3 1959.9

No No



The results for on-bottom stability analysis for the cables in Table 1 under different conditions in Table 2 are clearly showing that the submerged weight to outer diameter ratio (Sub/OD) is the governing parameter of a pipe’s on-bottom stability. The trend is clearly shown by the analysis results when sorting on submerged weight to outer diameter ratio as shown in Table 6. In Table 6 the minimum Subm.W/OD which gave a stable pipe is listed. Furthermore, the analyses show that minimum required Subm.W/OD is dependent on geographical location and it is therefore impossible to give a general statement about the stability of different pipes. The calculations for the Persian Gulf are showing that the conditions at shallow waters in the Persian Gulf can be very extreme, and all “pipes” are unstable at those conditions. Table 6. Minimum Subm. W/OD which gave stable pipe for the following conditions in Table 2. Area Min Sub W/OD ‐Sand

Abs 0.5xOD 10xOD Min Sub W/OD ‐Clay Abs 0.5xOD 10xOD

North Sea

50.0 ‐ 31.8 109 ‐ ‐

Brazil 17.2 ‐ ‐ 20,0 ‐ ‐ Gulf of Mexico

17.2 ‐ ‐ 17.2 ‐ ‐

Persian Gulf

600 ‐ ‐ lift ‐ ‐

5. SUMMARY AND CONCLUSIONS The results for on-bottom stability analysis as per DNV-RP-F109 have clearly shown that the submerged weight to outer diameter ratio (Sub.w/OD) is the governing parameter of a cables’s on-bottom stability at certain weather conditions. The weather and seabed conditions vary greatly with respect to geographical location, hence the minimum required Subm.W/OD ratio for a cable design will

be dependent on the location of where it is to be installed. Since FO cables are normally much smaller and lighter(i.e low submerged weight to diameter ratio) compared to power cables, umbilicals and pipelines, on-bottom stability is often difficult to achieve for FO cables. Hence, a pragmatic approach is necessary in the on-bottom stability assessment of FO submarine cables: 1. All involved parties should plan for that

burying, trenching, rock dumping or any similar method will most likely be necessary in shallow waters to achieve on‐bottom stability as per DNV‐RP‐F109.

2. Calculate on‐bottom stability for the environmental conditions available for the project at hand to establish/confirm extent of stability (or rather lack thereof).

3. Assess how much a practicable design change can affect the stability, and decide whether changes should be made.

4. Focus the engineering to assess at which depth trenching is no longer necessary.

5. For most installations some lateral displacements are not so critical for the service life time of the cable system, but it might be more critical for a pipe (less flexibility).

6. Lateral displacements might in many cases be limited due to proper route planning, etc.

6. REFERENCES [1] Recommend practice DNV‐RP‐F109, On Bottom Stability Design of Submarine Pipelines, October 2007 [2] RP‐E305, On‐bottom stability design of submarine pipelines, October 1988

on-bottom stability calculations for fibre …€¦ · the old dnv-rp-e305 “on-bottom stability...

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