ON-BOTTOM STABILITY CALCULATIONS FOR FIBRE OPTIC SUBMARINE CABLES

Transcription

1 ON-BOTTOM STABILITY CALCULATIONS FOR FIBRE OPTIC SUBMARINE CABLES Inge Vintermyr (Nexans ray AS) > Nexans ray AS, PO.Box 6450 Etterstad, N-0605 Oslo, ray. Abstract: This document describes the physical processes and the corresponding equations determining on-bottom stability of offshore cables. It presents an overvie of DNV offshore design code RP-F109 On-Bottom Stability Design of Submarine Pipelines. Finally, onbottom stability for a set of FO-cables, poer-cables, an umbilical and a pipeline are calculated ith RP-F109 at different geographical locations around the orld. 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. URC1-LW URC1 - SA The offshore applications of FO submarine cables have resulted in more focus on onbottom stability of cables at certain eather/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 ill be calculated for a range of FO cables and compared ith 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 shon in Figure 1, hereas repeatered FO cables are shon in Figure 2. Typical poer cables and umbilicals are shon in Figure 2 and 3. Physical characteristics for all cables are shon in Table 1 on page 5. URC1-DA2 URC1-RC Figure 1. FO Unrepeatered Cables ROC-LW ROC - SA Figure 2. FO Repeater Cables, LW and SA Copyright 2010 SubOptic Page 1 of 6

2 24 kv Poer 145 kv Poer Figure 3. Poer Cables Figure 4. Umbilical 2. SEABED STABILITY THEORY In the folloing 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 poer/ control umbilicals. Figure 5 shos the forces acting on the pipe. FLOW ( s F L )u+ F R (z) F L s F I +F D Figure 5. Driving and resisting forces on a circular pipe resting on the seabed. Loads can be divided into (F L ), Drag (F D ) and Inertia (F I ) forces, and they can be described ith the folloing empirical equations: Drag: 1 F ( sin( ) ) 2 D = ρ D CD U ω pt + UC 2 : 1 F ( sin( ) ) 2 L = ρ D CL U ω pt + U C 2 π 2 du Inertia: FI = D CM 4 ρ dt Where ρ is ater density (kg/m 3 ), D is pipe outer diameter (m), U is the velocity of an oscillatory flo (m/s) ith angular frequency ω p (s -1 ), U C is the current velocity (m/s), s (N/m) is the submerged eight, u is coulomb friction coefficient and C L, C D, C M are the, drag and added mass coefficients. Current velocity and direction are normally measured 2-5 m above seabed. The current velocity near the seabed ill be reduced due to the boundary layer effect hich is dependent on seabed roughness (clay, sand, boulders). In most cases only significant ave height (Hs) and corresponding ave period is accessible on a specific site. The oscillatory flo velocity U (and angular frequency ω p ) at the seabed can be calculated by using numerical or analytical linear ave theory (airy ave). The effect of ave spreading should also be accounted for. For both current and ave simple decomposition is used in order to find the normal component on the pipe. A conservative approach is to assume that both ave and current act in the normal direction. Resistance against movement is due to to different effects (ref 0). 1. Pure coulomb friction F C = ( s F L )u 2. Resistance due to penetration into the seabed F R. Penetration is due to self eight, piping, dynamics during laying and penetration due to pipe movement under the action of aves and current. Piping is the movement of sand under the pipe due to pressure Copyright 2010 SubOptic Page 2 of 6

3 difference on each side. The pressure difference is caused by current. Penetration varies ith seabed type and is larger on soft clay/loose sand than on hard clay/dense sand. Three different approaches regarding onbottom stability may be folloed (ref [1]). 1. Alloing accumulated displacement: A certain maximum displacement is alloed. Under this approach, the pipe ill 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 aare of this. 2. break out (Virtual Stability): Some small displacement is alloed, normally less than half the pipe diameter. In this approach the pipe ill never move out of its cavity. These small displacements ill build up resistance due to more penetration into the seabed and/or trenching. 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 ill only ork if the oscillatory component of the load forces is large enough to move the pipe. At large ater depth, the ave induced oscillatory current is negligible and only a steady state current ill 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 ill accumulate in one direction only. Under these conditions absolute stability is the only reasonable approach to follo. temporary phase and a more permanent operational phase: Temporary Phase: For temporary phases ith 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 folloing to combinations: 1. The 10 year return condition for aves combined ith the 1 year return condition for current. 2. The 1 year return condition for aves combined ith the 10 year return condition for current. Operation: For permanent operational conditions and temporary phases ith duration in excess of 12 months, a 100-year return period applies. When detailed information about the joint probability of aves and current is not available, this condition may be approximated by the most severe condition among the folloing to combinations: 1. The 100 year return condition for aves combined ith the 10 year return condition for current. 2. The 10 year return condition for aves combined ith the 100 year return condition for current. The load combination hich is used during an on-bottom stability analysis is dependent on the time span the pipe ill be exposed. It is normal to separate beteen a Copyright 2010 SubOptic Page 3 of 6

4 3. STANDARD AND SOFTWARE PROGRAMS The most accurate approach for stability calculation is to use a commercial softare product. To major softare programs exist: -Pondus (Marintek, ray) -AGA Leve III (PRCI, USA) Both programs are based on time domain dynamic analysis. It is typical to simulate a three hour severe storm hen performing dynamic analysis. The results from the analysis consist of time series of both lateral displacement and loads (stress, strain etc). There are relatively fe companies performing time domain dynamic analysis for the assessment of onbottom stability because of the time and ork 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 aare 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 eight in order to fulfill the alloable displacement. The to 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] s : Pipe submerged eight per unit length [kgf/m] Hs: Tp: Uc: Significant ave height [m] for extreme sea states ( 1, 10 and 100 years return period) Wave Peak period [s] for Hs Current speed [m/s] for extreme events (1, 10 and 100 years return period) d: Water Depth [m] s u : Un-drained clays shear strength [kpa] γ s : Submerged unit soil eight. For sand normally in the range 7000 (very loose) to N/m (very dense) d 50 : Mean sand grain size [mm] The oceanographic data (Hs, Tp, Uc) have to be derived by statistical methods from long term measurement of both ave and current. Wave and current direction is also possible to give as an input but normally it is conservatively assumed that the ave 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 here the pipe is going to be installed. DNV StableLines1.2 is commercially available and implements RP-F109 in a softare program. Copyright 2010 SubOptic Page 4 of 6

5 4. ON-BOTTOM STABILITY CALCULATIONS AND RESULTS In order to compare bottom stability of different FO cables, calculations have also been performed for poer cables, umbilicals and a pipeline. Their characteristics are shon in Table 1. Table 1.Characteristics for cables, umbilicals and pipeline, ref cross-sections shon on page 1 and 2. Cable Type OD (mm) Mass Subm. W Subm.W/ OD (kg/m 2 ) URC-1-LW ,67 0, URC-1-SA ,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-SA ,20 1,50 50,00 Poer 24 kv ,60 109,09 Poer -145 kv Umbilical Pipeline The field conditions studied are shon in Table 2. Table 2.Typical key data for stability calculations at different locations. 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) S u (kpa) d 50 (mm) rth Sea rth Sea Brazil Brazil Gulf of Mexico Gulf of Mexico Persian 51 6,1 11,2 1,3 10 Gulf Persian Gulf 51 6,1 11,2 1,3 0.5 Calculations for the various pipes at rth Sea environments, Persian Gulf and Gulf of Mexico are shon in Table 3, 4 and 5, respectively. The calculations are peformed by means of StableLines v1.2 Softare. Table 3. Seabed stability calculations different cables at rth Sea (ref Table 2). Cable Type Subm. W Seabed URC-1-LW1.9 0,38 Sand URC-1-SA3.2 0,70 Sand URC-1-DA2 2,30 Sand URC-1-RC 11,00 Sand ROC-LW 0,31 Sand ROC-SA3.6 1,50 Sand Poer 24 kv 9,60 Sand Poer -145 kv Sand Umbilical Sand 24 Pipeline Sand ReqS, Abs Stable Abs Table 4. Seabed stability calculations for different cables at Pesian Gulf (ref Table 2). Cable Type Subm. W Seabed URC-1-LW1.9 0,38 Sand URC-1-SA3.2 0,70 Sand URC-1-DA2 2,30 Sand URC-1-RC 11,00 Sand ROC-LW 0,31 Sand ROC-SA3.6 1,50 Sand Poer 24 kv 9,60 Sand Poer -145 kv Sand Umbilical Sand 24 Pipeline Sand ReqS, Abs Stable Abs Table 5. Seabed stability calculationse for different cables at Pesian Gulf (ref Table 2). Cable Type Subm. W Seabed URC-1-LW1.9 0,38 Sand URC-1-SA3.2 0,70 Sand URC-1-DA2 2,30 Sand URC-1-RC 11,00 Sand ROC-LW 0,31 Sand ROC-SA3.6 1,50 Sand Poer 24 kv 9,60 Sand Poer -145 kv Sand Umbilical Sand 24 Pipeline Sand ReqS, Abs , Stable Abs Copyright 2010 SubOptic Page 5 of 6

6 The results for on-bottom stability analysis for the cables in Table 1 under different conditions in Table 2 are clearly shoing that the submerged eight to outer diameter ratio (Sub/OD) is the governing parameter of a pipe s on-bottom stability. The trend is clearly shon by the analysis results hen sorting on submerged eight to outer diameter ratio as shon in Table 6. In Table 6 the minimum Subm.W/OD hich gave a stable pipe is listed. Furthermore, the analyses sho 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 shoing that the conditions at shallo aters in the Persian Gulf can be very extreme, and all pipes are unstable at those conditions. Table 6. Minimum Subm. W/OD hich gave stable pipe for the folloing conditions in Table 2. Area Min Sub W/OD Sand Min Sub W/OD Abs 0.5xOD 10xOD Abs 0.5xOD 10xOD rth Sea Brazil ,0 Gulf of Mexico Persian Gulf SUMMARY AND CONCLUSIONS The results for on-bottom stability analysis as per DNV-RP-F109 have clearly shon that the submerged eight to outer diameter ratio (Sub./OD) is the governing parameter of a cables s onbottom stability at certain eather conditions. The eather and seabed conditions vary greatly ith respect to geographical location, hence the minimum required Subm.W/OD ratio for a cable design ill be dependent on the location of here it is to be installed. Since FO cables are normally much smaller and lighter(i.e lo submerged eight to diameter ratio) compared to poer cables, umbilicals and pipelines, onbottom 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 ill most likely be necessary in shallo aters to achieve onbottom stability as per DNV RP F 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 ho much a practicable design change can affect the stability, and decide hether changes should be made. 4. Focus the engineering to assess at hich 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 Copyright 2010 SubOptic Page 6 of 6