SUBSEA STRUCTURE INSTALLATION
This page addresses the methods that should be adopted in the planning and procedure development for the loadout, transportation and installation of a typical subsea structure. The suggested methodology can be applied for Mid-Water Arch, Pipeline End Manifold (PLEM) and Pipeline End Termination (PLET)
A) PIPELINE END MANIFOLD (PLEM)/PIPELINE END TERMINATION (PLET)
PLEMs/PLETs are subsea structures, which typically consist of frame mounted valve assemblies and manifolds for flexible hose risers. PLEMs/PLETs are normally used to connect subsea pipelines to tanker loading buoys or serve as a subsea terminal point for other subsea completion structures. PLEMs/PLETs are normally connected to pipelines using flanged connections, which allow easy future replacement. Most PLEMs/PLETs are gravity stabilized structures, although in areas with weaker soil conditions, or where seismic activity is suspected, piles may also be used to pin the structure to the seabed.
PLEMs/PLETs are subsea structures, which typically consist of frame mounted valve assemblies and manifolds for flexible hose risers. PLEMs/PLETs are normally used to connect subsea pipelines to tanker loading buoys or serve as a subsea terminal point for other subsea completion structures. PLEMs/PLETs are normally connected to pipelines using flanged connections, which allow easy future replacement. Most PLEMs/PLETs are gravity stabilized structures, although in areas with weaker soil conditions, or where seismic activity is suspected, piles may also be used to pin the structure to the seabed.
Following the installation of the PLEM/PLET at the end of a pipeline, the PLEM/PLET itself is normally used as a launcher or receiver for the pipeline cleaning pigs. Such PLEMs/PLETS will normally have provisions for bolting on a temporary launcher / receiver, as well as a blind flange for the proceeding hydrotest. The valve assemblies on a PLEM/PLETS are usually hydraulically activated via umbilical leading to a control panel on the surface. These hydraulic power and control umbilical need to be installed, and function tested.
Installation Methods
PLEMs/PLETS may be installed in one of two general ways:
i) Directly Connected To Pipeline
Here, the PLEM/PLET is installed directly to the end of a pipeline. Generally, the pipeline is davit lifted to the surface, and a flanged or welded connection made between the pipeline and the PLEM. The pipeline is then re-lowered to the seabed with the PLEM/PLET installed.
ii) Piling Works
Where a PLEM/PLET requires piling works, this will normally be done using with a hydraulic hammer from the surface, with a suitable chaser pile. In deeper water, a suitable underwater hydraulic hammer may be used to drive the piles directly.
B) MID WATER ARC (MWA)
The MWA is required to support and constrain the risers and umbilicals into an S-shape as they pass from the seabed located PLEM to the FPSO/FSO floating above. The MWA system usually comprises of pressurized buoyancy tanks supporting a continuous curved saddle plate and gutters to accommodate risers or umbilicals. The risers/umbilicals are supported on saddle plates and guided between pairs of gutters. The gutters are designed to ensure that each of the risers or umbilicals are not subjected to a bend radius that is less than the minimum prescribed by the manufacturer for each of the lines, whatever the orientation of the respective lines.
The MWA structure is tethered to the seabed by means of a pair of chains and bridles attached to a piled base structure. The tethers will connect to the MWA via a delta plate and bridle system. The positioning of the tethers ensures that there is no interference with the risers and that the stability and balance of the MWA buoy is maintained.
The MWA is usually anchored to seabed by means of a piled base structure. The base structure will have sufficient mass to overcome the buoyancy of the MWA as well as to ensure the system stability until the piles are driven. The base structure is secured to piles by means of 4 lock pins on each pile.
LOAD-OUT AND TRANSPORTATION
A) GENERAL
Early selection of the transportation barge will be advantageous to allow timely completion of transportation engineering and rig-up. Brief outlines of the basic requirements for barge selection and transportation design to suit subsea structure and piles are as follows:
A) TRANSPORTATION OF SUBSEA STRUCTURE
In some cases, subsea structure does not require a barge to be transported offshore. Depending on the size, a supply vessel or the installation vessel itself can be used to transport the subsea structure. Some criteria for design and selection:
– Barge/Vessel deck strength should be sufficient to minimize grillage requirements.
– Barge/Vessel to have sufficient bollards all around to provide adequate mooring points during the handling and lifting operations offshore. Consideration should be given to adding bollards on the side of the barge brought alongside the derrick barge for the lift.
– In-built ballast system may be required.
– Subsea Structure should be placed to optimize the placement of supports on longitudinal bulkheads, and to suit the available hook height or lift radius of the derrick barge used.
– Consideration should be given to the load out method to be employed at the fabrication yard – i.e. skid beams may be required to suit a skidded load out, while a dollied load out may preclude the use of excessive grillage.
– Consideration should be given to loading the piles on barge/vessel.
– Space should be set aside for subsea structure appurtenances if any.
– Scaffolding should be pre-installed where possible for the removal of seafastening.
– Location of seafastening braces should be checked to ensure that there are no clashes/obsturction
– Mark C.O.G location of the subsea structure
C) LOADOUT CONSIDERATION – SUBSEA STRUCTURE
While the loadout of the subsea structures is primarily the responsibility of the fabricator under most contracts, the Field Engineer/Loadout Coordinator shall attend to ensure that all installation requirements and requests are met. The following shall be a minimum requirement:
– Checklist of marine requirements for the transportation barge, i.e. mooring bollards, navigation lights, ballast condition, etc.
– Checklist of installation requirements, i.e. inclinometer/bulls-eye, etc.
– Check and witness all rigging installation.
– Produce list of all deficiencies originating from the fabrication yard at custody transfer, i.e. damage reports, missing equipment / parts, etc.
– Identify any problem areas for installation and take photographs to document the final configuration of the loadout.
– Co-ordinate arrangement for obtaining the Marine Warranty Surveyor towage approvals as required under the contract.
– In certain contract circumstances it may be necessary to provide the transport barge with a ballast system for loadout purposes. The volume requirements of the system will be advised by the client / fabricator.
SURVEY REQUIREMENT
A) GENERAL
This section outlines briefly the methods generally used to provide positioning and survey control for derrick barge construction activities. Functions required include:
– Provision of a field map where existing facilities and seabed structures are identified. This is normally generated from Client supplied field maps, or derived from pre-installation surveys done as part of contractual requirements.
– The tracking of derrick barge and anchor handling tug positions in real time. This allows the placement of anchors clear of existing facilities, and at the required clearances from pipelines and other subsea structures.
– Positioning of jackets at the designed location within the tolerances for position and orientation.
– Subsea acoustic beacons for underwater tracking of ROVs and structures.
B) EQUIPMENT
Most modern offshore positioning techniques are based on Differential Global Positioning System (DGPS) methods. These rely upon the use of the Global Positioning System (GPS) satellite network, in conjunction with differential corrections transmitted over radio or satellite links from nearby (< 1000km) base stations. In some instances local positioning systems may still be in operation although these systems are increasingly rare.
A typical derrick barge will have the following equipment set-up:
– GPS receiver to collect raw satellite network data.
– Differential receivers to receive correction data from known base stations maintained by the survey sub-contractor. The stations receive raw GPS data and automatically calculate the difference between the position reported and the actual known position of the station. This differential data is then transmitted to the derrick barge.
– Navigation computers to apply the differential corrections and report the actual location of the derrick barge.
– Gyro compass to report the heading of the barge.
– Radio differential repeaters to re-transmit the differential data to the spread vessels.
– Radio modems to exchange data with the anchor handling tug navigation computers.
– Navigation software to display a map of the location, including the existing facilities. The position of the derrick barge, anchor handing tugs and all the anchors are over-laid on the map, and updated in real time.
– Quality control computer to ensure that the date derived is consistent.
– Interface boards to allow injection of other data into the navigation software. Typically, this will include acoustic beacon positions and theodolite / total station observations.
Each survey-equipped anchor-handling tug will have the following equipment on board:
– GPS receiver to collect raw satellite network data.
– Radio differential receivers to receive correction data from the derrick barge.
– Navigation computer to apply the differential corrections and report the actual location of the vessel.
– Gyro compass to report the heading of the vessel.
– Radio modem transmitter to send the position and heading information back to the derrick barge for inclusion into the navigation software display.
– Radio modem receiver to receive the updated positions of the derrick barge and other survey equipped spread vessels.
– Navigation software to display a map of the location with the position of the vessels and anchors superimposed. The software is normally set up for unattended operation, but may be manipulated by the surveyor on the derrick barge by remote control. Typically, anchor drop locations will be sent to the tugboat’s computer from the derrick barge, and a target will be displayed for the helmsman of the vessel to head for.
MOORING PROCEDURE
Details required for barge mooring and anchor handling can be found here.
DP PROCEDURE
Details required for DP Procedure and anchor handling can be found here.
MID-WATER ARCH/PLEM/PLET INSTALLATION
A) GENERAL
The following section addresses the steps to be taken for the installation of a PLEM or PLET, starting from the time the subsea structure arrives in field to the time it is set on seabed.
B) PRE-INSTALLATION SEABED SURVEY
Prior to the subsea structure being placed on location, a seabed survey is to be conducted to ensure that the proposed footprint location is clear of debris, and of suitable levelness. Any debris found that may interfere with the installation will usually be removed under an arrangement where operations are performed at rates as instructed by the customer. Any excessive seabed undulations or depressions may require that the jacket location be shifted or additional work performed to prepare the seabed for the installation. This pre-installation survey can be done in a number of ways, as follows:
– ROV survey; where the ROV will fly a 10m to 15m grid over the length and breadth of the subsea structure footprint. Hard debris will be picked up on the side scan sonar, and any significant indications observed visually. Normally, the grid can be superimposed on the Navigation Computer, and the ROV tracked during the survey with the help of a suitable USBL system.
– Static Side Scan Sonar survey; where a side scanning sonar unit is lowered to the seabed and allowed to scan the seabed for debris. The sonar can be moved several times to cover the footprint of the subsea structure. Significant debris found can be investigated by divers.
– Towed Side Scan Sonar survey; where a side scan sonar unit is towed from a boat to map the seabed at the subsea structure footprint location. Significant debris found can be investigated by divers.
– Diver circular search; where a diver is deployed to swim the footprint on a fixed search pattern.