PIPELINE SHORE APPROACH CONSTRUCTION - 48 BONNYPIPELINE. By TUNDE ALABI AYOOLA OLATUNJI SHELL PETROLEUM DEVELOPMENT COMPANY, NIGERIA

Transcription

1 PIPELINE SHORE APPROACH CONSTRUCTION - 48 BONNYPIPELINE By TUNDE ALABI AYOOLA OLATUNJI SHELL PETROLEUM DEVELOPMENT COMPANY, NIGERIA Offshore West Africa Conference, Ghana, January 2014

2 Abstract: Title: Pipeline Shore Approach Construction - 48'' Bonny Pipeline Offshore development of oil and gas fields often involve laying of pipelines from offshore to the shore to transport produced hydrocarbon for processing or sales. The transition region between offshore and the shore also referred to as shore approach is usually prone to waves, currents and winds and has its peculiarities regarding topography, soil properties and metocean condition. The shore approach often poses some challenges to pipeline construction and requires a special construction process. This paper discusses the methods of pipeline shore approach construction and factors to consider in selecting the method to apply. It focuses more on the pulling method and gives typical calculations of pulling forces, pull force profile and equipment selection. A 48 pipeline shore approach constructed with the pulling method at Bonny, West Africa is used as a case study. The challenges encountered during construction, mitigating measures taken and lessons learnt are shared. Acknowledgement: We wish to acknowledge the Shell Petroleum Development Company, Nigeria for the permission given to use the data and information obtained on the Bonny Terminal Integrated Project in this presentation/paper. ii

3 Contents: Abstract and Acknowledgement Contents ii. iii. Section 1: Introduction Background Objective and Scope 1 Section 2: 48 Bonny Pipeline Shore Approach Construction Introduction Shore approach 48 Bonny Pipeline Pipeline Shore Approach Construction Methods Selection of Pipeline Route and Shore Approach Construction Method Codes and Standards Pull Force Calculation of Buoyancy Aid to Reduce the Pulling Force Hydrodynamic Forces on the Pulled Pipeline Winch Capacity Calculations Pull Force Profile Check on Tensile Strength of Pipeline 12 Section 3: Construction Construction Activities Managed Risks Actual Pull Force Profile 14 Section 4: Post Construction Analysis Pull Force Profiles Analysis Execution Challenges Lessons Learnt 16 Chapter 5: Conclusion 16 References 17 iii

4 Section 1: Introduction 1.1 Background Offshore oil and gas field development often requires transporting produced hydrocarbon in pipelines from subsea systems or offshore facilities through the shore approach to the shore. This includes laying a continuous pipeline from offshore to onshore. Figure 1.1 depicts a pipeline layout showing landfall /shore approach. Pipeline shore approach construction may also apply to disposal pipelines taking treated produced water out into the Offshore Disposal Zone to meet statutory requirements. The pipeline shore approach construction method is totally different from the offshore and onshore construction methods and has its peculiar challenges arising from the environment. At the shore approach region, metocean, nature of soil and topography play a significant role in the selection of a construction method. Figure 1.1: Pipeline Layout Showing Landfall /Shore Approach [1] 1.2 Objective and Scope The main focus of this paper is the pulling method used on the 48 Bonny Pipeline. The paper describes the different methods used to construct pipelines at shore approaches, their merits, challenges and selection method. 1

5 The paper shows the calculations for the pull force required for laying the pipeline acrosss shore approach zone and the pull force profile and how it is used to determine the capacity of the pulling equipment to be employed. A post construction analysis iss carried outt comparing the calculated and actual pulll force profiles for the 48 pipeline, and the challenges and lessons learnt on the project are shared. Section 2: 48 Bonny Pipeline Shore Approach Projectt 2.1 Introduction: The 48 x 39.5km Bonny pipeline is an oil export pipeline that originates from Bonny Terminal and terminates 32km offshore at the Single Point Mooring / Pipelinee End Manifold (SPM/PLEM). The pipeline was constructed in 2007 by Hyundai Heavy Industries (HHI) and is operated by Shell Petroleum Development Company (SPDC), Nigeria. Figure 2.1 shows the map of West Africa with an insert of Bonny area, Gulf of Guinea and the 48 pipeline. Figure 2.1: Map of West Africa with an insert of Bonny area [2] [3] Environmental Data for Bonny [4] Wind speed: 35.6m/s Sea temp: 28 Deg. C Water depth: 5-8m Tide: 2.2/3.6m Mean Sea Level (MSL): 1. 43m Wave height: 1.05/1.44m 2

6 Steady current speed: 1.553/1.692m/s Calm weather window: November - February 2.2 Shore Approach 48 Bonny Pipeline A shore approach or landfall is the transition between offshore and onshore regions. The features of the shore approach region are influenced by the nature of the two adjoining zones - the sea side and the onshore side. The 48 pipeline crosses the Bonny shore approach, Gulf of Guinea where the shore line is relatively flat or with a gentle slope and with no outcropss or rocks, refer to Figure 2.2. Figure 2.2: Bonny Shore Approach in Gulf of Guinea [4] 2.3 Pipeline Shore Approach Construction Methods The shore approach construction methods include the following: Pulling Horizontal Directional Drilling (HDD) Tunneling Others Pulling Method The pulling method may be categorized into two types: i. Pulling from the onshore side of the shore approach (refer to Figure 2.3) In this pulling method, linepipes are welded together on a lay barge offshore and a pulling cable is attached to its end with the aid of a pulling head. The cable is connected to a winch which pulls the welded pipeline from the barge to the shore. 3

7 High impact of waves and current on the pipe at the shallow water at the shore approach often necessitates requirement for excavating seabed to protect the pulled pipe from being pushed laterally. Figure 2.3: Typical landfall site layout of a pipeline pulling operation of a 30 pipeline [5] Also, depending on the nature of the beach, a cofferdam may be constructed to manage effects of tidal movement of water, waves and current. The inside of the cofferdam is excavated to ensure the pipe is submerged during pulling rather than being dragged on soil. The trenched section is backfilled and the site is reinstated on completion. Floaters may be attached to reduce pull force. The key equipment required include: piling machine, dredger, excavator, lay barge, pulling winch and survey vessel. ii. Pulling from the offshore side of the shore approachh This method has some similarities with the method described above except that the pipeline is pulled towards the offshore side. The linepipes are welded together onshore and a cable is attached 4

8 to its end with the aid of a pull head and it is pulled by means of a winch installed on a barge offshore. Some merits of the pipeline pulling method include: The method is well understood. Favourable where there is a natural gentle slope with no outcrops and rocks. Appropriate where the waves and current conditions are benign. Favourable where there is minimal social and commercial activities. The pipeline line may be laid down and left for future tie-in. Laying operation may continue offshore directly using the same lay barge. Some challenges of the pulling operation include: Sourcing offshore trenching equipment with the capacity to trench to the required depth. Construction of a cofferdam can be time consuming. Disruption of beach activities during construction. Subjected to adverse weather which may slow down the pace of construction work. Rotational control during pulling in case of piggy-back pipes. To prevent damage to coating a special coating material may have to be applied. Site reinstatement after completion and backfilling of trench Horizontal Directional Drilling (HDD) Horizontal directional drilling (HDD) is a method whereby a drilling rig is positioned at an inclined angle, drills a hole in the ground and a pipeline is pulled in through the drilled hole. The drilling commences with a pilot hole which is drilled across the shore approach, such that the hole is drilled from the shore side of the landfall to the offshore end, the hole is reamed to expand it and the pipeline is pulled in. Figure 2.4 shows the front view of an HDD equipment in operation. Some merits of the HDD method include: Reduced impact on environment. May be used at a shore approach with high waves and current. May be used at a beach that is environmentally sensitive to construction activities. Pipeline is buried deep and is protected from future soil degradation or erosion. Technology has been available for some time and is growing. 5

9 Figure 2.4: Front view of an HDD Rig in Operations [6] Some challenges of the HDD method include: Limitations in terms of the length and size of pipe that can be handled. Handling of drilling mud and spoils. There may be some deviation from the target exit point. Buried rocks and loose gravel would slow down the process or make it less attractive Tunneling Tunneling is a method whereby a tunnel or shaft is constructed from the shore below the shore approach to come out of the seabed and the pipeline is built in the tunnel which may accommodate a bundle of pipes to be laid. Figure 2.5 shows the sketch of the Kalsto shore approach tunnel, a landfall pipeline crossing with the pipelines placed in 600 metres of concrete tunnel at the Norwegian coastline with a rocky bottom and large topographical variations. [7] Some merits of the tunneling method include: May be used in areas with outcrops and rocks and where HDD is not feasible. Good for areas where the pipeline needs to be protected against surfing and eroding shore line. Suitable for a bundle of pipes to be laid in the same tunnel. Some challenges of the tunneling method include: Effective control of water in the tunnel during construction and pipeline pulling. The method is time consuming and costly. 6

10 Figure 2.5: Sketch of the Kalsto shore approach tunnel [7]] Other Pipeline Shore Approach Construction Methods Other methods of pipeline shore approach construction include: A variant of bottom tow method where a welded pipe is towed to site, aligned at the shore approach and dropped in a trench in a controlled manner. In shore approaches with tidal flats / swampy terrain a trench is excavated with a dredger/ excavator and a barge is used to weld the pipeline which is floated and dropped in the trench. A combination of pulling and tunneling or HDD methods may be used. 2.4 Selection of Pipeline Route and Shore Approach Constructionn Method The pipeline route selection and shore approach construction method are usually influenced by a number of factors which include: Metocean: This is a combination of the words - metrologyy and oceanlogy and essentially comprises data on waves, current, wind, and tidal movement. It is vital to consider the effect of metocean on design and construction of pipelines at the shore approach because the effect of waves and current on beaches, amongst others, may lead to a retreating shoreline and exposure of a buried pipeline. Nature of shore approach: This covers topography, outcrops, nature of soil (sandy, clayey, rocky) and seabed conditions. A survey of the seabed is required to ascertain the state of the seabed to prevent unnecessary expenses, delays or losses. Challenges of landfalls include outcropping rocks, unstable cliffs, and variable shore profiles. 7

11 Social / Commercial activities: Pipeline route or shore approach construction method having negative impact on environmentally sensitivee areas or disrupting commercial and daily activities should be avoided. Construction window and equipment: The choice of construction method is influenced available construction n windows, in terms of favourable weather and time of the year. It influenced by the types of equipment, resources required, their availability, and economics. A simplified flow chart for selecting construction method is shown in figure 2.6 by the is also Figure 2.6: Flowchart for selection of pipeline shore approach construction method 2.5 Codes and Standards Applicable codes and standards for the pipeline shore approach design and construction include: API RP-1111: Recommended practice for designing offshore pipeline and risers containing hydrocarbons. ASME B31.4- Pipeline Transportation Systems for Liquidd Hydrocarbons and other Liquids ASME B Gas Transmission and Distribution Piping Systems BS 8010: Onshore and offshore oil and gas pipelines 3 Parts (PD 8010) DNV OS-F101- Submarine Pipeline Systems EN Design of petroleum and gas transport systems ISO Petroleum and natural gas industries - Pipeline transportation systems API 9A - Specification for wire ropes BS 302: Specification for wire ropes for cranes, excavators and general engineering purposes 8

12 2.6 Pull Force The pull force calculation is based on the Coulomb friction principle, that is, the sum of the product of the respective weights of pipe and cable and their friction factors on the seabed/sand, expressed as: Pull Force = (Weight of pipe)(friction Factor) + (Weight of cable)(friction Factor) + Back Tension Pull force, Pf = W.µ. L + T where, µ is Friction Factor L is Length of pipe/cable to be pulled W is Unit weight of pipe/cable/pull head T is Back tension The seabed friction factor is important in the assumptions made. Figure 2.7 shows a sketch of the side view of a pulling operation. Pulling Winch SHORE Cofferdam Shore Approach Pulling cable Pulling Head Friction Pipeline Sea b Figure 2.7: Sketch showing side view of a pulling operation Pull force is calculated at various stages of the pulling operation: i. Pull force with pipe at touchdown point, P i, (see Figure 2.8) is: (Weight of cable section submerged) x (Friction factor) + (weight of cable section in air) x (Friction factor) + Back tension 9

13 Pulling Winch Beach Cable Pulling Head Pi Touchdown Point Figure 2.8: Pull head pulled up to the touchdown point ii. Pull force with a section of the pipe at touchdown point, P ii, (see Figure 2.9) is: (Weight of cable section submerged)(friction factor) + (Weight of cable section in air) x (Friction Factor) + (Submerged weight of pull head and pipe) (Friction Factor) + Back tension Pulling Winch Beach Cable Pulling Head Pipeli Touchdown Point Figure 2.9: Pull head pulled beyond touchdown point iii. Pull force with pull head at tie-in point, P iii, (see Figure 2.10) is: (Weight of cable in air) (Friction factor) + (Weight of pipe section and pull head in air) (Friction factor)+ (Submerged weight of pipe) (Friction factor) + Back tension 10

14 Pulling Winch Pulling Head Beach Pipeline Touchdown Point Figure 2.10: Pull head pulled up to the tie-in point The total pull force is determined with pull force, P iii, above and is used for the selection of pulling winch and confirmation of cable selection with factors of safety provided. The pull force profile over the pull length is plotted with the calculations made above. Calculated Pull Force was 226 MTon (2216KN) 2.7 Calculation of Buoyancy Aid to Reduce The Pulling Force This is carried out iteratively in order to determine the optimum buoyancy spacing or by selecting the desired pull force and working backwards to determine net buoyancy. Total pull force with buoyancy, P fb = P f B t Total buoyancy force, B t = n. B where, n - expected number of buoys B - Buoyancy of buoy 2.8 Hydrodynamic Forces on the Pulled Pipeline The wave effect is highest when the orientation of the pipeline is parallel to the wave crest and minimum when at 90 degrees. Figure 2.11 shows a typical pipeline designed to approach the shore at 90 degrees. 11

15 Trenching and cofferdam construction can be carried out to overcome or reduce the effect of the cross flow on the pipeline being installed. Figure 2.11: Typical pipeline designed to approach the shore at 90 degrees [1] 2.9 Winch Capacity Calculations The pulling arrangement comprises two linear winches that provide the pulling force for the wire (cable), and the reel winders on which the wire is spooled which has a revving device to ensure even spooling on the drum refer to Figure A power pack provides hydraulic power to drive the winch. The winch itself is secured to anchor piles with enough capacity to restrain its movement during pipe pulling operation. Winch Strength = (225)(2)/1.5 = 300MTons Winch strength capacity is its rated capacity divided by a safety factor of

16 Wire drum Winch Steel wire Pull head Static pulley Rotating pulleys Anchoring Power pack Pull direction Figure 2.12: Two-Single Linear Winch Pull Arrangement 2.10 Pull Force Profile: The shore approach construction pull force profile is a graphical plot of pull force against pull length. Figure 2.13 shows a typical pull force profile. It is a function of friction factor (running and breakout friction) and tidal level (low and high water levels). Pull force is higher at low tide than at high tide; and at breakout (starting) than at running (finishing). With the pull force profile the pulling activity can be monitored during construction. 13

17 Total Pull Force (Tonnes) Capacity of Winch Breakout Friction Low water High water Laybarge Tension Touchdown Point Running Friction Laybarge Distance from Laybarge (m) S Figure 2.13: Pull force profile showing low water, high water and breakout/running friction [1] Typical values for variants of friction factor - breakout friction factors are: Pipe on beach 1.0; Pipe on seabed 0.9 (medium sand); Wire on beach 1.3; Wire on seabed 1.3. [1] 2.11 Check on Tensile Strength of Pipeline Tensile strength check is carried out to ensure that the pipeline is not subjected to a pull stress greater than its yield stress. Allowable tensile strength, σ allow = (Safety Factor). σ y Acceptance criterion: σ TS < σ allow = 0.6σ y where, σ y - specified minimum yield stress (SYMS) of the pipeline material σ TS - Actual Tensile Strength, and Safety factor is

18 Section 3: Construction 3.1 Construction activities include: Survey Constructing Winch Yard and Access Road Constructing a cofferdam Trenching Installation of 2 winch system and hold back anchor Barge set up and deployment of messenger rope Running of pulling wires Pipe string production on barge and pay out Pulling pipeline string towards landfall Post trenching As-built survey Reinstatement of Winch Yard and Beach crossing Figure 3.1, 3.2: Back view of two winches for pulling of 48 Bonny Pipeline [4] Figure 3.3: Construction of cofferdam [4] 15

19 Figure 3.4: Pulling of 48 Bonny Pipeline from the shore with lay barge at background [4] Figures: 3.5, 3.6: Pull head and pipe pulled to the shore [4] 3.2 Managed Risks: Some of risks managed on the project include: Risk of unexpected parting of wire. Residual forces in cables even when pulling operation is completed. Poor co-ordination of pulling operation and subjecting the cable to excessive stress. Using wrongly rated pulling cable. Damaged sections of the cables regular inspection of the cable is required. 3.3 Actual Pull Force Profile Figure 3.7 shows a pull force profile for the 48 x 1.1km pulled pipe crossing showing start (breakout), finish (running) and average pull force profiles at intervals of 10 lengths (120m) of pipes pulled. This is a smoother curve for simplicity and ease of analysis with the calculated pull force profile. Figure 3.8 shows the calculated pull force and the average actual pull force profiles. 16

20 Pull Force (KN) Pull Force Profile 48'' Shore Approach Pipeline 3,500 3,000 2,500 2,000 1,500 1, Start (KN) Finish (KN) Average (KN) No of Pipes Pulled (x10) Figure 3.7: Pull force profile for 48 shore approach pipeline 3,500 Pull Force Profile 48'' Pipeline Shore Approach Pull Force (KN) 3,000 2,500 2,000 1,500 1, Winch (KN) Plan (KN) Av Actual (KN) No of Pipes Pulled (x10) Figure 3.8: Calculated and Actual Pull Force Profiles for 48 pipeline Section 4: Post Construction Analysis 4.1 Pull Force Profiles Analysis Referring to figure 3.7, the actual pull force profile shows that as pull length increased pull force also increased, and the pull force at start pull was higher than that of corresponding finish pull. The difference between the actual start and finish pull forces may be attributed to the breakout friction factor that is higher at start pull than the running friction factor for finish pull. Recall that the actual 48 pipeline shore approach pulling operation was not continuous and was made intermittently, after each linepipe was welded on the laybarge. At some points actual pull force was much higher than expected and it showed as spikes implying some presence of resistance that had to be overcome in addition to the breakout friction. This was experienced especially at low tide when the pull head hit a sand profile (bar) on the seabed/beach 17

21 and it was pulled against the sand mass. The maximum pull force calculated was 2216 KN against maximum actual start pull force of 3114 KN (a difference of approximately 32%). The additional pull force required was however taken care of by the safety factor built into the winch capacity. Referring to figure 3.8, there is a fair correlation between the calculated and actual pull force profiles for the 48 pipeline. Both showed the same trend of pull force increasing with pull distance and the friction factor of 1.3, for the cable against seabed and sand appears conservative enough to take care of breakout friction factor experienced at initial stage of pull. Also, the friction factors of 0.9 and 1.0 are adequate for the pipe and pull head against the seabed and sand respectively. Taking an average, the pull forces were: calculated 1868KN and actual 1979KN (a difference of 6%). The variance between the calculated and actual pull forces at the tail end of the pulling activity also showed the effect of the shallow end where a good section of the pipeline was pulled close to the seabed thereby increasing the pull force unlike when a good section of the pipe was submerged and pipe buoyancy was higher. This may call for the need to vary the friction factor upward when pull head is pulled close to the beach. 4.2 Execution Challenges: To deliver to schedule and budget safely. Crowded/Narrow offshore corridor for laying the pipeline dredging and laying Availability of equipment: Pipe trencher to handle 48 pipeline and lay barge for 48 pipeline (HD 289 with 3.685m draft) Variance between calculated and actual pull force profiles. Bad weather need to abandon and recover pipe Interface management with stakeholders 4.3 Lessons Learnt: Some lessons learnt on the project include: Comprehensive integrated plan is required. Need to obtain as-built documentation of existing facilities from stakeholders. Need to order/source for fit for purpose equipment and materials early eg. Lay barge. Ensure adequate safety factor is provided especially for the pull winch. Data and formula can be fine tuned for use on future projects Design of a long cofferdam was helpful (250m long) Variable friction factor to be higher towards the end of the pull. 18

22 Need to define and monitor sea state limits for pipe lay down/abandonment Continuous interface management with stakeholders. Section 5: Conclusion The pulling method for pipeline shore approach construction is favoured in areas with benign metocean conditions, low social activities, gradual slope and without outcrops and rocks. The actual pull force profile recorded during construction of pipeline shore approach for the 48 Bonny pipeline was compared with the calculated pull force profile and found to be fairly comparable. Challenges and lessons learnt on the project were shared in this paper. References 1. JEE Pipeline, Riser and Subsea Engineering Courses: Installation Calculation for Subsea Pipelines; JEE Limited Kent, England, Map of West Africa, Reference No. 4242, UNITED NATIONS, Depart. of Peacekeeping Ops., Cartographic Sect.; UNHCR website (2005). 3. Bonny Map Satellite Images of Bonny; Maplandia.com; Maplandia website (2011). 4. Bonny Terminal Integrated Project, SPDC, Nigeria, Braestrup, M. W.; Andersen, Jan B.; Andersen, L. W.; Bryndum, M.S.; Christensen, C. J. and Nielsen, Niels-J. R.; Design & Installation of Marine Pipelines; Blackwell Sc, Oxford, OES, Oil and Gas Engineered Systems; Pipeline Approach HDD Photographs; OES, Australia, Berge, B.; Waagaard, K. and Harneshaug, K.; Pipeline Shore Approach Tunnel at Kalsto: The Damage and the Repair; OTC, Houston, Texas, May, Alabi, O.; Pipeline Shore Approach / Tie-in Engineering and Construction; Faculty of Engineering, University of Aberdeen, King, C.A.M.; Beaches and Coasts; St. Martin Press, New York, Palmer, Andrew C. and King, Roger A.; Subsea Pipeline Engineering; PennWell Corporation, Oklahoma, US,