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 offshore jacket. The suggested methodology will apply to the “conventional” offshore jackets – 3, 4 , 6 or 8 legged piled jackets installed by crane.
Structure installation for a conventional platform consists of 2 major parts; jacket Installation & Deck/Topside Installation.
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 jackets and piles are as follows:
B) TRANSPORTATION OF LIFTED JACKETS
Some criteria for design and selection:
– Barge deck strength should be sufficient to minimize grillage requirements.
– Barge 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.
– Jacket 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 jacket up right if the hook height / lift radius of the derrick barge is sufficient.
– Consideration should be given to loading the piles under the jacket if the loadout is by lifting.
– Space should be set aside for jacket appurtenances.
– 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 with grout lines, reach rods and other jacket appurtenances.
C) TRANSPORTATION OF PILES
Some criteria for design and selection:
– Barge 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 lifts.
– Sufficient space should be allowed at the front of each bay for the insertion of the handling tool, and sufficient space should be left behind the pile to allow upending clearance.
– Pile tops should be staggered slightly to allow insertion of lifting tool.
– Timber matting should be placed under the pile tip to protect the deck of the barge during upending. The timber should be a minimum 4” thick, but may be supplemented by steel sheets if required.
– The piles should be orientated on the barge with consideration taken of required upending method.
– Spacers should be placed between piles to provide clearance for load out slings, lifting padeyes or belly slings if required.
– The sequence of pile installation should be considered when designing the placement of the piles and conductors on the barge. In particular, the lift radius and hook height limitations of the derrick barge are to be identified when long lead piles are used.
– Painted sections of piles and conductors should be protected by gunny sack or neoprene material to avoid damage.
D) TRANSPORTATION OF DECK/TOPSIDE
Brief outlines of the basic requirements for barge selection and transportation design to suit deck structures are as follows.
– Barge deck strength should be sufficient to minimize grillage requirements.
– Barge 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.
– Deck 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.
– Space should be set aside for ship loose appurtenances.
– Scaffolding should be pre-installed where possible for the removal of sea fastening.
– Location of seafastening braces should be checked to ensure that there are no clashes with drain lines, cable trays and other deck appurtenances.
E) LOAD-OUT CONSIDERATIONS – JACKET & TOPSIDE/DECK
While the loadout of the 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. diaphragms inflated, reach rods closed, etc.
– Checklist of jacket appurtenances to be installed, i.e. riser guards, bolts and nuts for clamps, handrails to be trial fitted and uniquely piece marked, etc.
– Checklist of deck appurtenances to be installed, i.e. inter-module tie-ins, handrails to be trial fitted and uniquely piece marked, 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 EQUIPMENT
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.
– Sub-sea 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.
Other survey equipment carried for a jacket installation will include:
– Theodolite with Electronic Distance Measurement (EDM) equipment or Total Station. These can be set up to transmit distance and heading information for injection into the navigation software.
– Ultra Short Base Line (USBL) underwater positioning system for attachment to ROVs, underwater lifting frames, etc.
– Automatic level and staff.
– Measuring tapes and other conventional land survey equipment.
– Tide recorder
– Current meter
– Wave rider buoy
C) JACKET POSITION & ORIENTATION
The method by which a jacket is positioned is dependent on the following criteria.
i) JACKET IS THE FIST STRUCTURE OR A STAND ALONE JACKET IN AN EXISTING FIELD
The position of the jacket will be determined relative to the derrick barge. Known offset points relative to the GPS antenna will be marked on the deck of the derrick barge. The jacket will be upended and brought adjacent to the barge. Measurements are then taken from the marked points on the derrick barge to the jacket. These can be done using measuring tapes. The orientation of the jacket can be determined using the side of the barge as a straight edge (orientation = heading of barge). The above can also be determined by shooting two or more points on the jacket with the EDM or theodolite set up on the deck of the barge. A gyrocompass may also be mounted on the jacket to provide the orientation in real time. In general a tolerance for this type of structure is +/- 10 metres unless otherwise required under the contract.
ii) JACKET IS BRIDGE CONNECTED TO AN EXISTING STRUCTURE
The position of the jacket may be determined as above, but with additional checking being made from the existing structure using the EDM and theodolite. The as-built bridge length and allowable orientation deviation will be considered as the primary criteria for jacket position and orientation before set down.
D) JACKET LEVEL
The jacket levelness criteria will be checked using an automatic level and staff or in some instances a water level. This will be checked at regular intervals during pile installation. Appropriate measures as should be taken to level the jacket if the tolerance is exceeded. Allowable jacket level tolerances are usually in the order of 1:100 across any diagonal or as required under the contract.
E) JACKET ELEVATION
The elevation of the jacket relative to Mean Sea Level (MSL) will be measured to allow the proper cut-off of the transition pieces to be made to place the topsides at the design elevation. The MSL datum may also be required for the installation of boat landings, riser guards and bumpers. The method to be adopted is dependent upon the following.
i) JACKET IS FIRST IN COMPLEX OR ISOLATED
Tidal observations will be made over a specified period (sufficient to obtain the required repeatability – usually 24 to 48 hours) relative to a fixed point on the jacket. The level of the measured point can then be reduced to a height above MSL, and used as a datum for further measurements. Measurements may be taken manually, or with the aid of an automatic tide recorder.
ii) JACKET IS BRIDGE CONNECTED TO AN EXISTING PLATFORM
The existing platform will be used as the level datum. The datum will be transferred to the new jacket using the automatic level and staff.
F) FINAL POSITION & ORIENTATION
The final position and orientation of the jacket will be determined using the method dependent on the following.
i) JACKET IS FIRST IN COMPLEX OR ISOLATED
Portable DGPS equipment may be placed on three or more offset points on the top of jacket and observations logged. These will be reduced to provide the position and orientation of the jacket.
ii) JACKET IS BRIDGE CONNECTED TO AN EXISTING PLATFORM
A survey traverse will be made from the existing platform to the new jacket to obtain the final position and orientation.
G) DECK SETTING SURVEY REQUIREMENTS
Other survey equipment carried for a deck installation will include:
– Theodolite with Electronic Distance Measurement (EDM) equipment or Total Station.
– Automatic level and staff.
– Measuring tapes and other conventional land survey equipment.
Different survey requirements exist for various deck installations. These are as detailed following:
i) CONVENTIONAL DECK ON A JACKET
The as-built dimensions of the deck legs will need to be obtained, and these must be used to make the pile cut offs on the jacket. The as-built elevation of the deck leg splice must also be taken into account when cutting the transition pieces after welding. Following the setting of the deck, a level survey is generally done at each corner, adjacent to the main column members to check that the levelness specification is met before de-rigging.
ii) MSF SET ON JACKET
The MSF may be considered as a large deck, and similar checks as above considered. Care must be taken to checking additional horizontal dimensional tolerances on split MSFs that have to be spliced together after setting.
iii) MODULE SET ON MSF
The setting of modules on a MSF requires that dimensions are correct vertically and horizontally to ensure that the associated inter-module tie-ins can be made. A good survey check has to be made of the module landing pads to ensure that the stabbing guides and bumpers on adjacent modules and the MSF will place the module within horizontal tolerance. Shimming may also need to be done at the each support location of each module to ensure that the modules will be within levelness tolerance when set. These will ensure that all the modules will as match up vertically and horizontally.
iv) HELIDECKS
Some standalone helideck modules require a certain tilt. The individual bases of these modules are to be surveyed, and suitable shimming done to ensure compliance.
v) INTERCONNECTING BRIDGES
Where an interconnecting bridge is required, accurate distances between the bridge seats are to be measured prior to lifting the bridge. Some designs require that major structural modifications to be made, while others have a built in tolerance that will require the movement of sliding bridge seats on the bridge prior to setting.
Some designers also specify that the bridge is to lean from one platform to another only. In these cases, accurate levels are to be taken of both the bridge supports and shimming done as required prior to the lift.
G) AS-BUILT SURVEY INFORMATION
Typical as-built survey information required for jacket installations are as follows:
– Level datum relative to MSL marked and recorded as a benchmark on the jacket for future work.
– Elevation of top of jacket walkways and boat landings.
– Final position of the centre of the jacket.
– Final orientation of the jacket.
Typical as-built survey information required for deck installations are as follows:
– Level datum relative to MSL marked and recorded as a benchmark on the deck for future work.
– Elevation at main corners of all deck levels.
MOORING PROCEDURE
A) GENERAL
The mooring of the derrick barge adjacent to the jacket location provides for a stable work platform from where the installation can be conducted. Typical barge moorings consist of eight anchors deployed from the corners of the vessel. These anchors are typically 8 to 10 tonne in weight, and connected to the barge by 2” to 2.5” cable run on winches. Following are some guidelines for the laying of such anchors.
“Delta Flipper” type anchors are generally preferred where available. Where anchor holding is a problem consideration can be given to piggy backing anchor lines to counter uplift of the anchor line. In the case of a delta flipper configuration the anchor is “Backed” by an additional anchor connected in front of the delta flipper.
B) ANCHOR POSITIONING PLANNING
Each installation procedure will have a detailed, scaled (preferably 1 : 5000) map of the field in which the jacket is to be installed. Such a map will necessarily include the exact locations of all pipelines, subsea structures and other platform installations clearly marked. In general, the following are some guidelines for the placement of anchors for structural installations:
– The anchors should be deployed such that the barge can be moved in all directions without placing unequal forces on any one anchor cable. The prevailing weather and current directions should also be considered. Typically, there should be two bow and two stern anchors used to pull the barge ahead and astern. There should also be two breast anchors on the port and starboard sides respectively to move the barge laterally.
– Starting with the barge positioned at the final required location for the installation, the anchor pattern can be designed with the bow and stern anchors placed on a line 30° outboard to the longitudinal axis of the barge. The respective breast anchors should be placed on a line 60° outboard of the longitudinal axis.
– The barge location should then be moved to reflect the various other positions required – i.e. to bring the material barge in, to launch the jacket, etc. The angles of the anchor cables can then be adjusted to optimize the requirements.
– The length of anchor cable paid out should be checked to ensure that the barge can be moved the extent required without causing uplift on the anchor. Typically, 75m to 100m of cable should remain on the seabed with the anchor. Catenary calculations should be done.
Consideration should be given to avoid getting too close to existing structures with the anchor cable or the anchors. Anchors should not be dropped closer than 100m to an existing pipeline where the anchor cable does not cross the pipeline and 300m where it does. The anchor cable should also be no less than 15m from jacket legs or other permanent structures.
– Where anchor cables need to cross pipelines or other subsea structures, a minimum vertical clearance of 10m should be maintained. Assistance from parachute buoys (damage prevention buoys) may be used.
– When an optimal anchor pattern has been designed, the sequence of anchor drops to bring the barge into the final anchor pattern must also be considered. Intermediate set ups may be necessary in congested complexes.
– Each anchor is also equipped with a pendant buoy connected with a suitable length of pendant line running freely through the buoy. This buoy is used to mark the location of the anchor on the surface, and keeps the pendant line afloat for recovery. The length of the pendant line should be such that the pendant buoy does not create an uplift force on the anchor.
– Consideration may be given to attaching one or two of the barge anchor cables to jacket legs where necessary in existing complexes. A length of polypropylene rope is used between the anchor cable and the structure to form a spring line. Prior permission from the Client may be necessary in most cases.
C) ANCHOR HANDLING
The barge mooring anchors are deployed by anchor handling tugboats. These vessels are equipped with a navigation system that allows the helmsman to see on a monitor screen, the location of each anchor drop relative to the vessel in real time. The anchor drop location, once determined by the anchor foreman on the barge, is transmitted to the tugboat, and provides a target to which the vessel can steam towards. Following is a typical step by step procedure for one anchor deployment.
– The derrick barge will transfer the anchor and pendant buoy onto the back deck of the tugboat. The pendant line is spooled onto the tugboat’s work winch, securing the anchor to the deck. The end of the anchor cable is then passed to the boat, and connected to the anchor.
– The tugboat sails towards the anchor drop location, with the derrick barge simultaneously paying out the anchor cable.
– As the anchor is run out “DP” or “Parachute” buoys maybe attached at predetermined points along the cable (if the cable is crossing an existing pipeline or facility).
– When the stern of the tugboat is over the target, the anchor is pulled off the back of the boat with the anchor cable, and lowered to the seabed with the pendant line.
– The anchor cable is then tensioned up to check that the anchor has dug into the seabed. If so, then the tugboat will release the end of the pendant line from the winch, and allow the pendant buoy to fall into the water.
– To pick up the anchor, the pendant buoy is first recovered onto the deck of the tugboat, and the end of the pendant line secured on the winch. Any slack is taken up onto the winch. When the derrick barge is ready to have the anchor picked up, the anchor cable tension will be slacked off, and the tugboat then winches up the pendant line. The tugboat may need to sail ahead and away from the derrick barge along the line of the anchor cable to break the anchor free.
– Once broken loose, the anchor may be winched to the surface and left at the waterline, just off the stern of the tugboat. The tugboat will then sail onto the next drop location.
– If pipelines or other subsea structures are to be crossed during the re-location of the anchor, then the anchor may need to be decked on the tugboat, and secured from falling off using sharks jaws or other securing device
DYNAMIC POSITIONING PROCEDURE
In these modern days, less and less anchor barge is used. They are substituted by DP Dynamic Positioning Vessel.
JACKET INSTALLATION
A) GENERAL
The following section addresses the steps to be taken for the installation of a typical jacket, starting from the time the jacket arrives in field to the time the jacket is set on seabed.
B) PRE-INSTALLATION SEABED SURVEY
Prior to the jacket 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 mudmat 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 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 mudmat 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.
C) JACKET DEPLOYMENT INTO THE WATER
Four general methods exist for offloading jackets from material barges and setting them on the seabed. These are as detailed in the following sections. Note that warranty surveyor and / or Customer approval in writing is often required before the sea-fastenings associated with jacket transports can be cut to initiate the installation process.
i) JACKET VERTICAL LIFT AND SET
The jacket is loaded out vertically. This method is used where the water depth is generally less than 150 feet, and is conditional on the following:
– The fabrication yard can fabricate the jacket upright, or have the means to upend it prior to loadout.
– The derrick barge has sufficient hook height and capacity at radius to lift the jacket.
Generally, the jacket barge is brought alongside the derrick barge and secured. Personnel are then transferred to the top of the jacket to hook up the lift rigging. When this is done, and some tension applied on the slings, seafastening is cut and the jacket lifted off the material barge. The barge can then be removed, and the jacket lowered into the water for positioning.
ii) JACKET LIFT AND “TWO BLOCK” UPEND IN AIR/WATER
The jacket is loaded out horizontally. This method is adopted where the water depth is between 150 and 200 feet, or when the fabrication yard cannot upend the jacket prior to load-out to load it out vertical. Some other conditions that have to be satisfied include:
– The length of the jacket is less than the length of the derrick crane boom.
– The capacity of the auxiliary block on the derrick barge is sufficient.
– The jacket is non-buoyant.
Generally, the jacket barge is brought alongside the derrick barge and secured. The main block is hooked up to the lifting / upending slings at the top of jacket and the auxiliary block connected to the lifting slings at the bottom of jacket. The sea-fastening is then partially removed. In certain instances, the main block can be used to perform the horizontal lift with the auxiliary block used for upending. This method is usually applicable to two block upends in water. The material barge is then released from the derrick barge and brought end on to the derrick barge, where the derrick crane is in line with the jacket. Some tension is placed on the lifting slings, and then the remaining sea-fastening removed. At this point, two of the slings at the top of jacket and the lifting slings at the bottom of the jacket will be tight. The jacket is then lifted off the material barge and the barge removed. The main block of the crane is then hoisted as the auxiliary block is lowered to bring the jacket upright. (In certain instances it may be necessary to lower the jacket into the water to lower the hook loads before proceeding with upending.)
This will gradually transfer the load on the lifting slings to the top of jacket slings. When the jacket is upright, the jacket can be lifted clear of the water line and brought alongside to allow the bottom of jacket slings to be de-rigged. Alternatively, this can be done post set down; assuming the auxiliary block can be lowered that far. Optionally, the jacket may also be fabricated with trunnions on the bottom legs. These will be supported by upending supports on the material barge. In this instance, the jacket will be upended using the main block attached to the sings at the top of jacket only. Following the upending, the jacket will be lifted clear and the material barge removed, or the crane slewed away to allow the set-down.
iii) JACKET LIFT AND UPEND IN WATER
The jacket is loaded out horizontally. This method is adopted for larger jackets where the weight is still within the capacity of the derrick crane. The following conditions apply:
– The jacket is buoyant usually with around 10% reserve. Normally, the legs will have diaphragms at the lower ends, and have closures welded at the top ends. There may also be supplementary buoyancy tanks installed.
– The legs will have flooding mechanisms installed. Normally, there will be a valve near the bottom of jacket, with a reach rod that extends to the top of jacket. Larger jackets in deeper water may have ROV or other remotely operated valves.
There are two separate sets of rigging, one for the lift off, and one for the upend. Generally, the jacket barge is brought alongside the derrick barge and secured. The main block of the derrick crane is lowered and the lifting slings on the top row of the jacket hooked up. The seafastening is then removed, and the jacket lifted off. The material barge is then removed, and the jacket lowered into the water. The jacket should then be floating with two legs at the waterline. The jacket is then secured alongside the derrick barge, and the lifting slings removed. The main block is then de-rigged from the lift rigging, and attached to the upending slings at the top of jacket. The main block is then hoisted to upend the jacket.
As the jacket is upended past approximately 75°, personnel may be transferred to the top of jacket to open the reach rods selectively to assist in the upending operation. The lower legs will be allowed to flood in a controlled manner by leaving the top vent valve closed during the operation. When the jacket is upright, further ballasting may be done to adjust the jacket level before the structure is set down.
Note that certain larger jackets may also be installed this way utilizing two derrick barges to lift the jacket into the water. The derrick barge at the top of the jacket will then upend the structure when the second derrick barge rigging has been removed.
iv) JACKET LAUNCH AND UPEND IN WATER
The jacket is loaded out horizontally. This method is used when the jacket weight is in excess of the lifting capability of available Derrick vessels. Barges used for jacket launches are rigged with special equipment to enable the jacket to be launched independent of assistance from the installation barge.
The barge is rigged with ballasting pumps, cutting and welding gear, launch winches, slings and blocks and miscellaneous support equipment. The following conditions are normally true for a launch situation:
– The jacket is buoyant
– The jacket is designed to be launched. The launch is normally done with the top of the jacket entering the water first. The Loadout configuration will have the mud mat of the jacket at the bow if the launch barge.
– The jacket is pre-rigged with slings for the upending only. Additional slings and ropes are attached to allow the jacket to be pulled or towed to the derrick barge after the launch. The jacket launch is generally done some distance away from the derrick barge and the final location. Prior to the launch, two stern anchor cables from the derrick barge are connected to the top of the jacket to allow the jacket to be pulled towards the derrick barge. Normally, the jacket will be launched towards the derrick barge, with the spread positioned such that the launch barge is down current from the derrick barge. Additional ropes are provided at the bottom of jacket to be connected to tugboats. These boats will assist in guiding the jacket towards the derrick barge. The launch procedure is necessarily long and well-coordinated one. Following is a step by step procedure:
– When the launch barge arrives in field, personnel are transferred on board to prepare the materials and equipment on board for the launch. The material barge may be anchored, or left on the towline.
– Preparations include the checking and running up of the winch and pump engines, checking the tank soundings and trim of the launch barge, running out gas hoses to cut the sea-fastening, transferring the anchor cables from the derrick barge to the jacket and passing the additional ropes to the attendant tugboats that will be guiding the jacket to the derrick barge.
– When a suitable weather window has been approved, the sea-fastening braces can be cut. Normally, the compression members are cut first, followed by the tension members, working from the stern to the bow. Care is taken to ensure that the braces removed are secured from falling overboard during the launch, and also that they do not interfere with the path of the launch rigging.
– When a percentage of the braces have been removed, the ballasting of the barge can begin. This is done in stages, with water either being pumped or transferred into the stern tanks to trim the launch barge.
– When the barge is at the required trim for launch, the winches are tensioned up and the final ‘anti-self launch’ plates at the bow cut. The jacket is at this stage, free to slide off the stern of the barge. If friction forces are too high at this stage for the winches to overcome, additional hydraulic jacks may be activated to provide an additional push.
– When the jacket begins to move, the winches will normally need to pull the jacket a distance before the jacket free slides. ‘Drop –off’ slings are normally installed to the launch rigging to ensure that these large slings will fall off the pulling lugs on the jacket legs at the extremity of the launch rigging travel.
– When the jacket starts to slide on it’s own, the trim of the barge will increase dramatically as the CG of the jacket moves towards the stern. When the CG of the jacket is past the rocker arm hinges, the rocker arms will tilt, allowing the jacket to fall into the water. This will usually cause the material barge to be pushed ahead. Care is to be taken to provide sufficient anchor chain or sufficient length of tow cable to account for this.
– When the launch is completed, the jacket will be floating free of the launch barge, but connected to the derrick barge via the anchor cables, and the one or two attendant tugboats via the ropes attached to the bottom of the jacket.
– While the crew on the launch barge secures the barge for demobilization, the jacket will be pulled towards the derrick barge using the pre-attached anchor cables.
– When the jacket is at the stern of the derrick barge, the main block will be lowered to be connected to the upending rigging.
– The jacket will be upended as in the previous case, with selective ballasting to bring it level. When the jacket is upright, the derrick barge will relocate on anchors to the required final location before setting down.
D) JACKET SETTING
When the jacket has been upended, and suspended on the derrick crane over the intended location, the following steps will be taken to set the jacket down:
– Lower the jacket until the mudmat is approximately 3 meters clear off the bottom.
– The surveyors are to move the barge or the derrick crane such that the jacket is located on location, and in the correct orientation. The methods used to determine the jacket location and orientation were described in a previous section.
– Tugger lines may be attached to the top of the jacket to steady the jacket for the set down as well as to assist in positioning and orientation.
– When the surveyor has checked that the jacket is in the correct location and orientation, the jacket may be lowered to the seabed, until the weight on the derrick crane starts to decrease. This signals that the mudmat has just tagged the seabed.
– The surveyor will re-check the observations. The jacket may be re-lifted and moved as required.
– When the surveyor is positive of the observations, the jacket may be lowered until all the weight is taken on the seabed. The surveyor will make another observation. Any tilt on the jacket is to be noted, and suitable adjustments made to the observations for position and orientation.
– The tilt of the jacket may be corrected by selective ballasting of the legs of buoyant jackets. The reach rods should have been activated by this time to open the flooding valves at the bottom of the jacket, however the vent valves may be opened selectively to fully flood the legs on the high side.
– When the surveyor is satisfied that the jacket is in the right location and orientation, or that the location will be correct when the jacket is leveled, the upending slings may be de-rigged and preparations made for pile installation.
E) JACKET SECURING
Prior to the upending slings being de-rigged, the jacket should be secured to the derrick barge by ropes to prevent it from moving or toppling. The ropes should remain secured until piles have been lowered to self-penetration on the jacket legs.
PILE INSTALLATION
A) GENERAL
Conventional jackets are secured to the seabed by piles driven through the legs or sleeves attached to the legs. These piles are normally delivered in several sections and assembled on site as each section is lowered and driven into the seabed. Following are some typical pile formats and installation procedures.
B) TYPES OF PILES
Jacket piles can be placed into two broad categories:
i) LEG PILES
Leg piles are installed through the jacket legs, and driven to the required penetration. These piles are normally delivered in sections, with each section being lowered or driven into the leg until the end is at the top of the jacket. The pile is then secured, and the next section stabbed onto the first and welded.
The pile is then driven until the top of the new section is at the top of the jacket, and the process is repeated until the final section is driven. The jacket is then secured to the pile by the welding of shim plates. These can be annulus type shims, where the shim plates are placed inside the annulus between the jacket leg and the pile, or crown type shims where the shim is supported on the top of the jacket.
ii) SKIRT PILES
Skirt piles are installed through sleeves installed on the outside of the jacket legs. These are common on deeper water structures. The sleeves are normally at the base of the jacket, and the top of the piles is not visible at the surface upon completion of driving. The piles used are normally delivered in one section, or in several sections, but completely assembled on the surface prior to installation. The pile section is stabbed into the sleeve and then driven directly using an underwater hammer or through a chaser pile with conventional hammers from the surface. The jacket is secured to the pile by means of mechanical connectors and / or by grouting the annulus between the pile and the sleeve. Note that in certain circumstances drilled and grouted insert piles can be used as a substitute for driven piles. This is usually the case when the seabed formation is too hard for pile driving or consists of a geological material known as calcarinite which cannot support driven piles designed based on skin friction. In theses cases a hole is drilled, the pile is placed in the hole and grouted in position. Specialized equipment and expertise that are not addressed in this document are required for this type of piling.
C) PILE HANDLING
Piles are normally loaded out horizontally. Each pile then needs to be upended for installation. Some common methods used are as follows:
i) INTERNAL LIFTING TOOLS
These are hydraulically operated internal gripper type units, which are suspended from the derrick crane, and fit into the top of the pile. Once activated, the lifting tool can be used to upend and lift the pile off the material barge. The pile can then be stabbed into the jacket leg or into the previous section. The tool can be left engaged until the pile is secured or until sufficient weld has been deposited. These tools are available in a variety of sizes to suit different pile sizes and wall thicknesses.
ii) PADEYE
A pair of padeyes can be welded to the top of the piles and slings shackled to the pile to upend and lift the pile. This arrangement can become awkward, as the slings need to be de-rigged from a height to release the derrick crane from the pile before driving can commence. These can also be used with a ‘chastity belt’ arrangement to allow the de-rigging to be made part way up the pile.
iii) BELLY SLINGS
When the piles are too long to allow a single point upend without bending over-stress, a supplementary sling may be choked around the mid point of the pile to provide additional support until the pile is upended. This supplementary lift may be provided by the auxiliary block on the derrick crane or the deck crane. The belly sling however should be rigged up such that it will slide freely to the end of the pile when it has been upended.
iv) PILE STOPPERS
When the initial pile is too short to reach the mudline and become free standing, a stopper is normally pre-welded near the top of the pile to support it against the top of the jacket leg. This stopper will need to be designed to take the weight of the initial pile and the second section. When the weld between the first and second sections is completed, the weight of the assembled pile is taken by the derrick crane and the stopper removed. The pile is then further lowered. If this pile section is still not self-supporting, an additional stopper may also be required on the second pile section.
v) EXTERNAL GRIPPERS
External grippers are hydraulically operated gripping tools, which are secured to the top of the jacket leg. These replace the pile stoppers as described above, and are used where many pile sections are required to be welded together before the pile becomes self supporting. These are also available in a variety of sizes to suit.
D) PILE DRIVING EQUIPMENT
i) GENERAL
The jacket piles are lowered until they are self-supporting. This may be at some penetration below the seabed. Following this, pile-driving hammers are used to drive the piles to their required penetrations. These hammers are powered hydraulically or by steam, and are available in a variety of sizes and capacities. These are described as follows.
ii) HYDRAULIC HAMMERS
Hydraulic hammers utilize hydraulic pressure to lift a heavy weight and then drive it towards an anvil block. Like the steam hammer, the anvil block then transmits the force onto the top of the pile, driving it into the ground. The hydraulic hammer is normally free riding on the top of the pile to ensure that all the generated force is transferred to the pile. The anvil block is cushioned by an inert gas, which normally does not need replenishment. Hydraulic hammers operate on a closed hydraulic circulation system, and can be used underwater. These hammers require an umbilical for the hydraulic oil circulation as well as for the electronic sensors and controls. These are linked to a control van / pump skid located on the deck of the derrick barge.
E) PILE INSTALLATION SEQUENCE
i) GENERAL
The sequence in which the piles on a jacket are driven can assist in the timely completion of the installation. Generally, piles are installed in a sequence to complement the levelness of the jacket. Some of the rationale is as explained following:
ii) INSTALLING FIRST PILE SECTION ON HIGH SIDE
Where a jacket is tilted to one side, hanging a pile in the high side legs may assist in leveling the jacket. The additional weight on the high side may cause the jacket to lean back towards level.
iii) DRIVING FIRST PILES ON LOW SIDE
When a jacket is still out of level following the hanging of piles on the high side, driving the piles on the low side first will provide a jacking reaction point from where the jacket can be leveled. These methods are as described in a following section.
iv) PILE INSTALLATION IN 6 OR 8 LEG JACKETS
When installing a 6 or 8 legged jacket, the outer piles are usually completed first, and the jacket leveled and secured before commencing the inner piles. This assists in the reducing the loads required to level the jacket. In certain circumstances the inner legs on an 8 leg jacket may be completed first and ballast used for leveling. The technique used should account for the equipment available for jacket leveling and must be programmed to ensure leveling is performed before the jacket is pinned to the extent that adjustment is not possible.
F) PILE MONITORING AND RE-STRIKE TEST
Dynamic pile monitoring may be required under some contracts. This is done by attaching accelerometers and strain gauges on the pile sections to be monitored. These instruments are normally bolted near the top of the pile, clear of the hammer bell. Prior to the pile being upended off the material barge for installation, the pile monitoring technicians will install the gauges to the pile. A long umbilical will also be connected; running the length of the pile, with the overlength tail bundled up at the bottom. When the pile is welded out, the tail will be retrieved and connected to the instrumentation computer.
Pile driving can then commence. As the pile is driven down and the gauges approach the top of the jacket leg, pile driving will be suspended to allow the technician to retrieve the gauges. Normally, only one or two complete piles will require monitoring. The first monitored pile to be driven to completion may require a re-strike test after a minimum of 24 hours setup time. The gauges will be re-attached before driving commences. Usually, a minimum of 1-foot further penetration is required. The data collected will then allow the designers to assess the actual ultimate capacity of the piles.
Care should be taken in planning the location of the gauges. These should be considered in advance of the loadout, taking into account the largest hammer to be used, the cut-off length of the pile and the elevation of the top of the jacket leg. The holes for bolting on the gauges should also be pre-drilled and tapped in the fabrication yard if possible. Note that this is only typical for conventional leg piles. Underwater skirt piles cannot be monitored in this manner, however, most modern underwater hydraulic hammers will have suitable instrumentation built in to allow the monitoring to be done directly from the hammer.
G) REFUSAL CRITERIA
Refusal is defined as when the pile is no longer able to be driven any further without causing damage to the equipment being used, assuming that the hammer used is sized appropriately for the pile size and prevailing soil conditions at depth. Pile refusal criteria is normally based upon the hammer manufacturer’s requirements, and is defined as 200 blows per foot for a Menck steam hammer. Refer to manufacturer’s specific instructions for other hammer types. Often refusal as specified in API RP 2A is required contractually. The application of this criteria in a pile refusal situation can often cause major damage to pile driving equipment and the customer should always be consulted before the criteria is applied in the field, to avoid unnecessary damage to capital equipment. Pile refusal criteria are an important aspect of jacket installation planning. Risks associated with refusal and the associated remedial measures need to be fully accounted for. As the cost of remedial work can be extremely high it is important that the pile driving procedure to be followed, the remedial procedures to be adopted in the event of refusal and the responsibility for any associated cost is established with the customer during the planning phase of the work. If a pile meets refusal prior to final penetration being reached, then remedial techniques may need to be employed to bring the pile to final penetration. These are discussed in a later section.
JACKET LEVELING
A) GENERAL
Several methods exist for leveling a jacket that is out of levelness tolerance. These are explained following. The main objective in any installation however is ensure that the jacket is placed within level
tolerance as soon as possible. Once pile installation has begun, the weight of the piles as well as the racking forces to be overcome when pushing the jacket against the piles already installed will make the process increasingly difficult. These methods require however that there are piles installed in the legs in question. These piles should be driven to a penetration where the weight of the jacket can be transferred to the pile. This is normally contingent on a nominal blow count of not less than 10 to 15 blows per foot. Careful use of jacket buoyancy provisions can be used during piling to minimize jacket level problems in certain cases.
B) LIFTING WITH UPENDING SLINGS
This method involves rigging up the appropriate upending sling or slings to the lowest leg or side of the jacket. The derrick crane is used to pull up on the jacket, and the jacket then dogged off temporarily to the pile on the low side. Consideration should be given to the size of the rigging used, and the capacity of the padeye on the jacket as a limit to the lifting force that can be applied.
C) JACKING AGAINST PILE
This method involves the use of a jacking frame that is placed between stoppers welded on the jacket leg and the pile. A jack is used to either push the jacket up or down against the pile. The jacket is then dogged off to the pile. Alternatively, the a pile cap jacking system may also be used to pull the jacket up.
D) COMPLETION OF PILE INSTALLATION WHEN LEVELING
When the jacket has been leveled by one of the above methods, the piles in the remaining jacket legs are driven to final penetration as soon as possible and the jacket dogged off against them. The previously dogged off piles are then freed and the piles driven to completion. The time factor is essential to prevent the earlier piles from setting up in the soil. Note that when the jacket is dogged off to a completed pile, the pile is normally centralized first using wedges driven at the appropriate locations to aid in shim installation later.
REMEDIAL PILE INSTALLATION PROCEDURES
A) GENERAL
When a main or conductor pile cannot be driven any further, before the required final penetration is reached, and the customer does not accept the penetration based on in place tension capacity calculations or similar, the following remedial procedures may be used to remove the soil plug inside the pile and continue driving. It should be note that the removal of the pile plug will only assist the advancement of the pile in the event of a “plugged” pile scenario where the internal soil plug is moving with the pile (this usually occurs in sand layers). In stiff clay where refusal may be caused by external skin friction, removal of the pile plug will not relieve the situation.
B) JET, LIFT AND DRIVE
Removing the internal soil plug in the pile reduces the internal skin friction, and possibly allows the continue driving of the pile. This method involves the use of an airlift technique to remove the soil inside the pile. A water jet at the end of the string also assists by breaking up the plug material for removal by
the airlift. The equipment used consists of lengths of 6” to 12” diameter main pipe, normally flanged both ends. Smaller, 1” air and 2” water lines are located on the outside of the main pipe. The end section of the string has a jetting head, and an elbow diverting the air into the main pipe for the airlift. The top most section has a goose neck attachment to divert the spoil from the airlift as well as the inlet manifolds for the air and water lines. The intermediate sections are flanged both ends, with suitable inter-connects for the air and water lines. The string is first assembled with the tip and gooseneck sections and sufficient intermediate sections matching the available hook height of the derrick crane. This string is then lifted into the pile and supported on a work platform installed at the top of the pile. Additional intermediate sections are then added before the gooseneck section to lengthen the string until the tip section is at the top of the soil plug. The water and air lines are then connected, and the string allowed to jet and lift the soil plug out. As the soil is removed, additional intermediate sections are added, until the entire soil plug has been removed. The air lift string is then removed in sections, and pile driving re-started. Sufficient intermediate sections are normally mobilized to allow the tip section to reach the bottom of the pile at the final penetration.
C) DRILL AND DRIVE
Where the soil plug is of a harder material that cannot be jetted out, a reverse circulation drilling rig is set up at the top of the pile to cut through the harder material. The basic philosophy of the exercise is however, similar.
D) OTHER REMEDIAL TECHNIQUES
Other techniques are available in addition to the above. These mainly involve the circumstance where the removal of the soil plug renders the ultimate capacity of the pile inadequate to support the future platform. These normally require that grouting is done at the end of the drive, or additional insert piles driven. Further detail can be obtained from more specialist texts.
JACKET COMPLETION WORKS
A) GENERAL
Following are some common activities to be completed following pile and conductor driving.
B) SHIMMING – SECURING JACKET TO PILES
In conventional jackets, shims secure the jacket to the piles. These are either in the form of plates that fit in the annulus between the pile and the jacket leg, or crowns that sit on the top of the jacket leg. In the former case, the top of the jacket leg is normally notched to provide additional weld area. The welds are normally all fillet welds. In the latter case, the bottom of the crown pieces are welded to the top of the jacket leg with partial penetration welds, while the shim itself is fillet welded to the pile. In both cases, the pile is first centralized in the leg by driving wedges in the appropriate places prior to dogging off. The shims are then placed in the appropriate locations and welded out. Note that shim installation is generally not permitted prior to the completion of all main pile welding. Some customers will not allow conductor pile installation to proceed until shim installation is complete. In jackets with underwater skirt piles, shims cannot be installed. Several methods exist to secure the jacket to the pile. On smaller jackets, the jacket may simply be pinned to the pile, while on larger ones, the sleeve may have a specialist mechanical gripper lowered along the pile, with a hook that fits a machined groove on the
skirt. The grippers are set when the levelness of the jacket is accepted. The annulus between the skirt sleeve and the pile is then grouted.
C) JACKET LEG GROUTING
Some jackets require that the annulus between the leg and the pile be grouted. The grout will form a bond between the surfaces in the annulus, and strengthen the jacket leg. The grout used is normally made from Ordinary Portland Cement, mixed with seawater. When a large volume is required, a retardant may be used. The grout slurry is normally mixed in batches and delivered along 2” hoses from a High Pressure / High Volume mixer / grouting pump skid unit. One grouting system involves having pre-installed pipework (grouting and flushing) that runs from the top of jacket to outlets installed at the bottom of the leg. Grout is pumped to the bottom of the leg and allowed to circulate to the top. One shim plate will be left off to allow the grout in the leg to overflow. When the density of the grout recovered is acceptable, the grouting will cease, and the final piece installed. A preinstalled outlet port may also be provided.
On an underwater sleeve, pipework may be installed from the surface to outlets at the bottom of the sleeve. Grout is pumped until the it over tops the sleeve guide. If samples are not taken subsea, then a specified additional volume is pumped (normally 150%) to ensure that the grout density is acceptable.
In the above cases, there is normally an inflatable rubber packer or friction wiper installed inside the bottom of the leg or sleeve that will seal the annulus and prevent the grout from running out the bottom. This needs to be inflated from the surface before commencing grouting operations. The pipework delivering the grout to the bottom should also have a full contingency back up secondary system to allow the grout to be flushed out and the process re-done should there be a problem with the primary system. The use of balance pressure grouting is often specified. In this case the water is blown out of the pile / jacket leg annulus by air at a pressure equal to that on the seabed. Grout is injected into the the top of the leg. As the grout level rises the air pressure in the annulus is reduced to a level commensurate with the head of grout. This method does not require the use of subsea pipework obviating the need for removal of same after grouting completion. Packers / wipers are not required with balanced pressure grouting. Diver inspection ports are usually provided in the jacket legs. Grout sampling at a specified frequency will be required to verify grout weigh and strength.
D) PILE CUT-OFFS AND TRANSITION PIECE INSTALLATION
When the pile and shim installation is complete, transition pieces are normally installed onto the top of the piles to transition the batter of the piles to vertical. These can involve either:
– A square cut on the pile with a pre-assembled angled transition piece, or
– A miter cut on the pile to suit a pre-cut miter on a straight transition piece.
The elevation at which the pile cut-offs are made is determined to ensure that the as-built leg spacings on the deck to be set are matched by the respective transition piece spacings. The higher up the piles are cut, the smaller the distance between the transitions will be, due to the batter of the piles.
Transition pieces are normally fabricated with an over-length section at the top. When the transition pieces have been installed, the required elevation of the deck leg splice is marked relative to MSL. A cut is then made to suit the bevel requirements of the deck section.
E) RISER GUARD AND BOAT LANDING INSTALLATIONS
Riser guards and boat landings are normally installed to suit the as-built level of the jacket. Some trimming of overlengths or adjustment of clamp locations may be required to ensure that the walkway levels on the boat landings and the depth of protection offered by the riser guards conform to the requirements of the designer. Note that the riser guards may need to be left uninstalled should a near future riser installation be required.
F) HANDRAILS
Handrails at the top of jacket elevation are normally left off for clearance and damage prevention purposes during installation. As many sections as practicable will need to be installed as soon as possible to provide additional safety for the crew working at the top of jacket, and the remainder installed when the major jacket works are done. Most permanent handrails require seal welding to be done at the sockets. Prior to the load out, as many panels as possible should be trial fitted and uniquely piece marked to ensure that there is no confusion offshore.
G) GROUT LINE AND REACH RODS
Where reach rods or grouting lines are installed, these normally need to be cut off below the waterline, or just below the grating level on the top of jacket.
H) REMOVAL OF TEMPORARY INSTALLATION AIDS
All temporary installation aids such as rigging platforms, shackle pin pullers, leveling padeyes, scaffolding supports and the like need to be removed and the scars ground smooth. Attention should also be paid to any seafastening brace attachment locations. Where installation aids are removed underwater removal should be such that 30mm of proud material is left in place. No grinding is usually carried out where removal of steelwork underwater is required.
I) SOIL PLUG MEASUREMENT
Some clients require that the elevation of the soil plug inside the conductors and piles be measured before the setting of the deck. These assist the designer in calculating the ultimate capacity of the main piles, and serve as a guide for the drillers. These measurements are taken by lowering a weight on a long rope down the pile or conductor, and then measuring the length of the rope when the weight is resting on the top of the plug. A diver’s down line spooler is normally used.
J) CONDCUTOR COVERS
While not normally not provided for in most drawings, temporary covers should be made for the conductor tops to prevent foreign objects from falling in.
K) BLASTING AND PAINTING
The last phase of the jacket completion works will be the blasting and painting works. Consideration should be given to these activities early in the installation however in terms of planning for scaffolding
access and portable equipment requirements as these are normally completed remote from the derrick barge.
DECK INSTALLATION
A) GENERAL
The following section addresses the steps to be taken for the installation of a typical deck, starting from the time the deck arrives in field to the time the deck is set on the jacket and completed.
B) PRE-INSTALLATION SURVEY
Prior to lifting the deck section, the survey checks as detailed above shall be made. Visual checks should also be made around the deck legs, module landing pads or bridge seats to ensure that there will not be any pipework, light poles, hand rails or appurtenances that will result in a clash during the setting.
C) COMPLETION OF JACKET WORKS
Any remaining work required on the top of jacket is to be completed before the deck is set.
This would include:
– Installation / lowering of caissons and drains.
– Installation of bumpers, boat landings and other appurtenances that may be difficult following the setting of the deck due to vertical interference.
D) LIFTING AND SETTING
When the survey checks have verified that the deck section will fit the jacket or previous installation, the structure can be rigged up to the derrick crane and sea-fastening cut. The structure is then lifted and set. Orientation tuggers should be used to assist in guiding the structure into the right location, and to assist in rotating the structure on the hook as required. When the deck has been set, surveyors will be sent to the deck to do a preliminary level run to ensure that the deck is within level tolerance. When this is confirmed, the rigging can be removed.
E) REMEDIAL WORKS
Should levelness tolerances not be met, the deck may need to be re-lifted, and the transitions re-cut to suit. In the case of module installations, additional shimming may be required. Horizontal tolerances on modules may be met or rectified by skidding or jacking where possible across the top of the MSF.
STRUCTURAL (JACKET & DECK) OMPLETION WORKS
A) WELDING OUT LEGS/LANDING PADS
The splice between the deck legs and the jacket transitions are to be welded out. These are typically full penetration butt welds. The landing pads of modules are to be welded to the MSF cap beams. These are typically no more than substantial seal welds.
B) LOWERING STAIRWAYS AND ACCESS LADDERS
Most decks have one or two access stairways that need to be lowered to the top of jacket. Similarly, there will be stairways from modules to the MSF. These stairs are normally of a hinged design, and may have some overlength built in.
C) INTER-MODULE ACCESS WAY TIE-INS
Where multiple modules are installed on a MSF, access ways between the modules normally need to be installed at various levels. These usually comprise pre-fabricated stiffened plate sections. These plates are then fitted and seal welded between adjacent decks. On some modules, load bearing beam splices may also be required, with grating overlaid later.
It is important that these activities are pre-planned well in advance of mobilization to ensure that the following are considered:
– The design of the inter-modular connections are made as simple as possible.
– The ship loosed items for the inter-modular connections are placed as close to the final location as possible.
– Any pre-scaffolding, if possible be installed prior to load-out.
As these completion works are normally done remote, care should also be taken in planning the equipment and personnel required.
D) SEA-FASTENING SCARS
Scars left behind from sea-fastening braces generally need to be trimmed and ground smooth.
E) MISCELLANEOUS WORKS
BLASTING, PAINTING AND HOOK UP
A) GENERAL
Hook up works apart from the structural hook up described above, are generally covered elsewhere. Blasting and painting activities are normally performed remotely. Consideration should be given to these activities early in the installation however in terms of planning for scaffolding access and portable equipment requirements.
DOCUMENTATION
– Submit final survey reports to PMT for onward transmittal to the Client.
– Prepare final acceptance certificate for installation works. Attach final punch list if any.
Source: J. RAY McDERMOTT, S.A.EASTERN HEMISPHERE