PIPELINE TOWING AND RISER INSTALLATION
INTRODUCTION
The purpose of this article is to summarize the riser installation method and the Tow Analysis for the pipelines, under a pre-determined environmental condition. The results of the analysis shall determine the weather window, the offshore vessel Requirements and number of buoyancy tank required for the Pipeline Tow.
RISER
The riser installation can be performed with a work boat or Dynamic Positioning (DP) vessel with adequate crane capacity to lift the riser into place. This process will not require work barge The riser/pipeline and spool tie-in connections are to be done in sea condition.
PIPELINE
A 3D finite element analysis shall be carried out for the pipeline based on related data. This article represents the initial engineering undertaken to verify the feasibility of installing the pipeline utilizing the surface tow method. It should be demonstrated via technical tool that Pipeline will be safely towed, provided the tow condition is within the design criteria.
Pipeline
The following recommendations are made:
In reality tension will be lower than estimated because tension depends on environmental attack on the pipeline being towed. If environmental forces are exactly opposite to pipeline towed direction, tension will be max. Tension in the pipeline shall be monitored continuously and if required tug tension shall be varied accordingly.
PIPELINE PROPERTIES
Pipeline shall be assumed empty during towing.
KINEMATIC VISCOUSITY
Kinematic Viscousity of 3.5% salinity is used in the analysis.
CURRENT VELOCITY
Appropriate current velocity shall be used during towing that will affect drag force on pipeline. Current speed depends on the depth, so current speed at surface will be maximum and it will diminish with depth.
HYDRODYNAMIC COEFFICIENTS
Appropriate hydrodynamic coefficient shall be used to calculate drag on pipeline.
VESSEL RAO’S
Vessel RAO affects towing and trailing winch tension because that will highly depends on vessel's motion. It is recommended that the actual RAO’S to be used as the tension requirement of the winches and the stress of the pipeline will highly depend on the RAO's of the vessel.
PIPELINE ALLOWABLE STRESSES
Following pipeline stresses shall be used
Overbend = 85.0% of SMYS
Sagbend = 75.0% of SMYS
RISER INSTALLATION
The riser is to be pre-fabricated at an onshore fabrication yard. The internal lining on straight riser pipe is installed and the whole riser assembly attached with pre-prepared bend is hydrotested. Prior to that, the riser bend is lined separately and joined with the riser pipe by
means of flanges. At the offshore site, the pipeline end is aligned and the required spool piece is measured. Measurement of the spool length is to be performed by divers. Divers may align the pipeline using air bags and tuggers. The aligning of the pipeline may be performed with a work barge and davits combining the aligning with the riser setting.
After the measurement and alignment work, the offshore vessel then moves on to another location, as the spool can only be prepared onshore due to internal lining requirement.
Once ready, the spool piece is then installed offshore using a work barge with a diving spread. The tie-in connections are performed by subsea flanges. The advantage of this method is that it is a simple procedure and does not require large installation equipment. The riser installation can be performed with a work boat or Dynamic Positioning (DP) vessel with adequate crane capacity to lift the riser into place thereby eliminating the requirement for a work barge. The disadvantage of this method is that the riser/pipeline and spool tie-in
connections are to be made subsea.
PIPELINE STRING INSTALLATION
Upon completion, the welded joints are externally coated, both ends are capped and the string kept in the storage rack ready for use.
When sufficient length of strings are available, the strings are rolled over onto the trolleys on the launching track, towed towards the beach and welded into one pipeline section up to the required length. The ends of the pipeline section are terminated with flanges.
Depending on the limitation pull-in capacity, additional pipeline sections may be required to form the total length for installation. The pipeline sections will be connected by flanges, and filtered with pull heads.
The pipeline Tow analysis shall be carried out with suitable software. A full 3D finite element model shall be created to simulate the tow of the pipeline string. The analysis shall be carried out based on worst case scenarios that may likely to occur during the phase of the tow. The tow vessel speed can be set approx 4 to 5 knots.
As for dynamic analysis, the Trail Winches cable length and tension shall be kept constant , which is the same length used during static analysis, while the tension in the Tow Winch shall be varied. This enables us to determine the maximum tension requirement for the Tow Winch.
COMPUTER PROGRAM
For the Tow analysis, software which are, widely used in offshore industry for the analysis of flexible marine risers, mooring line dynamics, umbilical cable lay, subsea intervention systems and other system involving cable dynamics, shall be used
FINITE ELEMENT MODEL
A full 3D Finite Element model of the pipeline bundle system with various components to represent the actual pipeline bundle make-up, modified to suit various installation conditions, shall be created.
The various components shall be modeled as follows
Pipeline – pipelines shall be modeled as homogeneous steel catenary finite elements, representing the actual pipeline joint length. No clump attachments or contents of the pipeline shall be modeled.
Buoyancy Tanks – The buoyancy attachments shall be modeled as 3D buoys, simple bodies with just 3 degrees of freedom i.e. the translation degrees of freedom (surge, sway and heave), with each bouy representing the properties of 1 actual buoyancy attachments in 7 to 10 m interval for the entire pipeline.
Straps – The strapping of the buoyancy attachments to the pipeline shall be modeled as Tethers – a mass less connection, linking the pipeline to the buoyancy attachments.
Pullhead – Pullheads shall be modeled as a 3D bouy. A single pullhead (3D buoy) shall be modeled and the pipelines shall be connected to the pullhead.
Master link, shackles, etc. – All connections like master link, shackles connectors etc. shall be modeled as dummy 3D buoys without any mass or volume to provide connection points between pipelines and tow cable / bridle arrangements.
Tow Cables – Tow cables shall be modeled as wires with wire core catenary finite elements, connecting the master link to the winch.
Winches – winches shall be modeled as a mass less connection, connected to the vessel and the tow/trail cables.
Vessel – Both the tow and trailing vessels shall be modeled as rigid floating bodies whose motions are presented by the Response Amplitude Operators (RAO’s) for each of the 6 degrees of freedom (surge, sway, heave, roll, pitch and yaw.
BUOYANCY TANK DESIGN
Buoyancy Tanks shall be used to support the pipeline’s submerged weight so that the whole pipeline floats on the water surface during towing and to support pipeline from overstressing during installation.
Number and Spacing of Required Buoyancy Tank
The number of buoyancy tank required is calculated based on the equilibrium forces between total net buoyancy force of tanks and total submerged pipeline weight.
The design considerations are as follows:
Collapse Check of Buoyancy Tanks
The collapse check of the buoyancy tank shall be determine the critical water depth that can result to wall collapse of the (empty) tanks. The critical water depth shall be be used in the design to determine the required tank wall thickness that the tank can immerse in the water without collapsing.
For the buoyancy tank wall thickness, the hoop stress due to hydrostatic pressure fh should not exceed the critical hoop buckling stress Fhc divided by the appropriate safety factor.
STEEL STRAP DESIGN
A minimum number of steel straps are required to secure the buoyancy tank floats to the pipeline during the pipeline launching and towing process.
SPOOL PIECE INSTALLATION AT CROSSING
Sometimes designed pipeline needs to cross existing pipelines. In this scenario Normal lay and surface tow methods are not feasible to be used.
It is recommended that the following installation stages are to be follow
MAXIMUM STRESSES
Three principal maximum stresses shall be reported for the pipeline, namely, von Mises Stresses, Bending Stresses and the Direct Tensile Stresses.
The von Mises Stresses reported is the maximum estimated von Mises Stresses over the cross section. The von Mises Stresses is a stress measurement that is often used as a yield criterion. It is a combination of all the components of the stress matrix.
The maximum Bending Stresses is the maximum value that the Bending Stress takes anywhere in a section; and this maximum occurs at the extreme fiber on the outline of the bend. The Direct Tensile Stresses is the maximum axial stresses due to wall tension. A positive value indicates tension and a negative value indicates compression.
BUOYANCY TANK AND STEEL STRAPS
As a minimum 2 straps are recommended to ensure the buoyancy tank is secured to the pipeline and that no tank movements can occur during tow out.
RECOMMENDATIONS
It is not practical to allow a constant winch tension at the Trail Tug, as the actual environmental condition may differ from the analyses; therefore, it is imperative to monitor the maximum deflection (utilizing the Standby Tug) by adjusting the Trail Winch Tension. The Tow Winch Tension should be monitored at a constant tension throughout the tow.
The purpose of this article is to summarize the riser installation method and the Tow Analysis for the pipelines, under a pre-determined environmental condition. The results of the analysis shall determine the weather window, the offshore vessel Requirements and number of buoyancy tank required for the Pipeline Tow.
RISER
The riser installation can be performed with a work boat or Dynamic Positioning (DP) vessel with adequate crane capacity to lift the riser into place. This process will not require work barge The riser/pipeline and spool tie-in connections are to be done in sea condition.
PIPELINE
A 3D finite element analysis shall be carried out for the pipeline based on related data. This article represents the initial engineering undertaken to verify the feasibility of installing the pipeline utilizing the surface tow method. It should be demonstrated via technical tool that Pipeline will be safely towed, provided the tow condition is within the design criteria.
Pipeline
The following recommendations are made:
- Minimum tension at the Trail Tug shall be determined
- Tow of pipeline is to be carried out within the environmental criteria used in the study
In reality tension will be lower than estimated because tension depends on environmental attack on the pipeline being towed. If environmental forces are exactly opposite to pipeline towed direction, tension will be max. Tension in the pipeline shall be monitored continuously and if required tug tension shall be varied accordingly.
PIPELINE PROPERTIES
Pipeline shall be assumed empty during towing.
KINEMATIC VISCOUSITY
Kinematic Viscousity of 3.5% salinity is used in the analysis.
CURRENT VELOCITY
Appropriate current velocity shall be used during towing that will affect drag force on pipeline. Current speed depends on the depth, so current speed at surface will be maximum and it will diminish with depth.
HYDRODYNAMIC COEFFICIENTS
Appropriate hydrodynamic coefficient shall be used to calculate drag on pipeline.
VESSEL RAO’S
Vessel RAO affects towing and trailing winch tension because that will highly depends on vessel's motion. It is recommended that the actual RAO’S to be used as the tension requirement of the winches and the stress of the pipeline will highly depend on the RAO's of the vessel.
PIPELINE ALLOWABLE STRESSES
Following pipeline stresses shall be used
Overbend = 85.0% of SMYS
Sagbend = 75.0% of SMYS
RISER INSTALLATION
The riser is to be pre-fabricated at an onshore fabrication yard. The internal lining on straight riser pipe is installed and the whole riser assembly attached with pre-prepared bend is hydrotested. Prior to that, the riser bend is lined separately and joined with the riser pipe by
means of flanges. At the offshore site, the pipeline end is aligned and the required spool piece is measured. Measurement of the spool length is to be performed by divers. Divers may align the pipeline using air bags and tuggers. The aligning of the pipeline may be performed with a work barge and davits combining the aligning with the riser setting.
After the measurement and alignment work, the offshore vessel then moves on to another location, as the spool can only be prepared onshore due to internal lining requirement.
Once ready, the spool piece is then installed offshore using a work barge with a diving spread. The tie-in connections are performed by subsea flanges. The advantage of this method is that it is a simple procedure and does not require large installation equipment. The riser installation can be performed with a work boat or Dynamic Positioning (DP) vessel with adequate crane capacity to lift the riser into place thereby eliminating the requirement for a work barge. The disadvantage of this method is that the riser/pipeline and spool tie-in
connections are to be made subsea.
PIPELINE STRING INSTALLATION
Upon completion, the welded joints are externally coated, both ends are capped and the string kept in the storage rack ready for use.
When sufficient length of strings are available, the strings are rolled over onto the trolleys on the launching track, towed towards the beach and welded into one pipeline section up to the required length. The ends of the pipeline section are terminated with flanges.
Depending on the limitation pull-in capacity, additional pipeline sections may be required to form the total length for installation. The pipeline sections will be connected by flanges, and filtered with pull heads.
The pipeline Tow analysis shall be carried out with suitable software. A full 3D finite element model shall be created to simulate the tow of the pipeline string. The analysis shall be carried out based on worst case scenarios that may likely to occur during the phase of the tow. The tow vessel speed can be set approx 4 to 5 knots.
As for dynamic analysis, the Trail Winches cable length and tension shall be kept constant , which is the same length used during static analysis, while the tension in the Tow Winch shall be varied. This enables us to determine the maximum tension requirement for the Tow Winch.
COMPUTER PROGRAM
For the Tow analysis, software which are, widely used in offshore industry for the analysis of flexible marine risers, mooring line dynamics, umbilical cable lay, subsea intervention systems and other system involving cable dynamics, shall be used
FINITE ELEMENT MODEL
A full 3D Finite Element model of the pipeline bundle system with various components to represent the actual pipeline bundle make-up, modified to suit various installation conditions, shall be created.
The various components shall be modeled as follows
Pipeline – pipelines shall be modeled as homogeneous steel catenary finite elements, representing the actual pipeline joint length. No clump attachments or contents of the pipeline shall be modeled.
Buoyancy Tanks – The buoyancy attachments shall be modeled as 3D buoys, simple bodies with just 3 degrees of freedom i.e. the translation degrees of freedom (surge, sway and heave), with each bouy representing the properties of 1 actual buoyancy attachments in 7 to 10 m interval for the entire pipeline.
Straps – The strapping of the buoyancy attachments to the pipeline shall be modeled as Tethers – a mass less connection, linking the pipeline to the buoyancy attachments.
Pullhead – Pullheads shall be modeled as a 3D bouy. A single pullhead (3D buoy) shall be modeled and the pipelines shall be connected to the pullhead.
Master link, shackles, etc. – All connections like master link, shackles connectors etc. shall be modeled as dummy 3D buoys without any mass or volume to provide connection points between pipelines and tow cable / bridle arrangements.
Tow Cables – Tow cables shall be modeled as wires with wire core catenary finite elements, connecting the master link to the winch.
Winches – winches shall be modeled as a mass less connection, connected to the vessel and the tow/trail cables.
Vessel – Both the tow and trailing vessels shall be modeled as rigid floating bodies whose motions are presented by the Response Amplitude Operators (RAO’s) for each of the 6 degrees of freedom (surge, sway, heave, roll, pitch and yaw.
BUOYANCY TANK DESIGN
Buoyancy Tanks shall be used to support the pipeline’s submerged weight so that the whole pipeline floats on the water surface during towing and to support pipeline from overstressing during installation.
Number and Spacing of Required Buoyancy Tank
The number of buoyancy tank required is calculated based on the equilibrium forces between total net buoyancy force of tanks and total submerged pipeline weight.
The design considerations are as follows:
- Empty pipeline properties
- Air filled buoyancy tanks properties
- Buoyancy tanks are 95% (volume) immersed in water
- The spacing of buoyancy tank is calculated by dividing the pipe string length with the total required tank floats.
Collapse Check of Buoyancy Tanks
The collapse check of the buoyancy tank shall be determine the critical water depth that can result to wall collapse of the (empty) tanks. The critical water depth shall be be used in the design to determine the required tank wall thickness that the tank can immerse in the water without collapsing.
For the buoyancy tank wall thickness, the hoop stress due to hydrostatic pressure fh should not exceed the critical hoop buckling stress Fhc divided by the appropriate safety factor.
STEEL STRAP DESIGN
A minimum number of steel straps are required to secure the buoyancy tank floats to the pipeline during the pipeline launching and towing process.
SPOOL PIECE INSTALLATION AT CROSSING
Sometimes designed pipeline needs to cross existing pipelines. In this scenario Normal lay and surface tow methods are not feasible to be used.
It is recommended that the following installation stages are to be follow
- Lift the spool piece and lower down to the seabed.
- Divers using the appropriate installation aids to pull the spool piece below the existing pipelines until the spool piece is at the desired position.
- Diver to connect the spool piece to another spool piece by flange tie-in.
- Repeat the above for subsequent spool piece at other crossing.
MAXIMUM STRESSES
Three principal maximum stresses shall be reported for the pipeline, namely, von Mises Stresses, Bending Stresses and the Direct Tensile Stresses.
The von Mises Stresses reported is the maximum estimated von Mises Stresses over the cross section. The von Mises Stresses is a stress measurement that is often used as a yield criterion. It is a combination of all the components of the stress matrix.
The maximum Bending Stresses is the maximum value that the Bending Stress takes anywhere in a section; and this maximum occurs at the extreme fiber on the outline of the bend. The Direct Tensile Stresses is the maximum axial stresses due to wall tension. A positive value indicates tension and a negative value indicates compression.
BUOYANCY TANK AND STEEL STRAPS
As a minimum 2 straps are recommended to ensure the buoyancy tank is secured to the pipeline and that no tank movements can occur during tow out.
RECOMMENDATIONS
It is not practical to allow a constant winch tension at the Trail Tug, as the actual environmental condition may differ from the analyses; therefore, it is imperative to monitor the maximum deflection (utilizing the Standby Tug) by adjusting the Trail Winch Tension. The Tow Winch Tension should be monitored at a constant tension throughout the tow.