Jackup on the wet tow :
Rig and ships lauching at yard without dry dock :
• Mobile Offshore Drilling Units
• Accommodation Vessels (Floatels)
• Jack-ups on-the move and
• Specialist Vessels.
- Tandem tow – 2 (or more) tows in series behind 1 tug, i.e. the second and following tows connected to stern of the previous one.
- Parallel tow – the method of towing 2 (or more) tows, using one tow wire, where the second (or subsequent) tow(s) is connected to a point on the tow wire ahead of the preceding tow, and with each subbsequent towing pennant passing beneath the preceding tow.
- Two tugs (in series) towing one tow – where there is only 1 towline connected to the tow and the leading tug is connected to the bow of the second tug.
- More than 1 tug (in parallel) towing one tow – each tug connected by its own towline, pennant or bridle to the tow.
Before start of any transit voyage, the main and emergency towing arrangements should be check to be in good working condition and ready for used. All anchor handling equipment, charts etc., required for the move should be ordered and the fixed rig equipment checked.
Prior to the commencement of and during, transit voyage weather forecasts with minimum 72 hours outlook must be obtained from two independent recognised weather service centre. The forecasts may be ordered from a company maintaining such service for the question. Weather forecasts and current charts must be studied carefully so that the best use is made of weather windows and favorable tides.
The Barge Engineer is responsible for preparing the unit prior to a rig move, as per specific chekclist. During transit, the vessel must follow rules and display lights and signals applicable to vessels. The unit shall be secured with all watertight doors/ hatches shut and dogged. Only essential access doors necessary for the operation should be used. The watertight integrity of all compartments must be ensured.
There will be a transportation route study to evaluate the design environmental criteria. This is normally carried out when a voyage-specific motion analysis has to be carried out. A stability study to demonstrate that the carrier vessel, in the case of a dry transport, or the hull of the transported vessel, in the case of a wet tow ( see video below ), meet the requirements of the IMO or the classification society. The analyses are normally carried out using the generic wind speeds of 100 knots for intact stability assessment and 70 knots for damaged stability. Lower wind speeds are sometimes considered on a case-by-case basis for restricted tows in sheltered waters.
Typically Motions and accelerations study and analyses are carried out with the voyage specific environmental criteria using diffraction or strip theories. In the absence of such meteorological data, deterministic motions are often used. A structural assessment taking into account the loads associated with the motions and accelerations.
For example, the most common deterministic motions criteria are those by Noble Denton for flat bottom cargo barges and other types of carrier vessels.
The criteria are:
The criteria are:
20" roll angle in 10 s period & 0.2 g heave acceleration, 12.5" pitch angle in 10 s period & 0.2 g heave acceleration.
When deriving the voyage-specific environmental data for the transportation route, the 10-yr return environment is normally considered. Given the temporary nature of the transportation phase, the data is normally derived specifically for the departure month so as to take advantage of seasonal variations. The transportation route is normally split into several sectors within which the environment is assumed to be uniform, eg the route sector between korea and north sea. The duration of exposure within each of those sectors is calculated based on the vessel speed. Given that the exposure periods are normally less than 1 month, the environmental data may be reduced to allow for the shorter exposure periods.
Transportation routes are selected based on the economic. environmental and safety considerations. The following factors could be considered:
The environmental conditions along the transport route affect the motions of the vessel and the voyage speed. The weather conditions after the commencement of the transport operation often dictate local deviations from the planned route.
The existence of safe havens. As part of a contingency planning, particularly for long transports, safe havens have to be identified in case the conditions require the vessel to seek refuge in a port.
Vessel or cargo dimensions and hull draft which restrict passage below certain obstructions, such as bridges, or in shallow water or through locks and waterways. Costs of the passage through canals, such as the Suez Canal.
It is reasonable to assume an angle of 21 degree of load spread. While the tansmitted pressure in the weaker soil layer exceeds the bearing capacity of that layer, a punch-through of the footing would happen even though the bearing capacity of the overlying stronger layer is sufficient to sustain the load.
Normally a safety factor of 1.5 is applied to compare the bearing capacity of the weak layer to the pressure transmitted by the spud can and portion of the hard layer delimited by the angle of 21.degree of load spread.
The generally rapid penetration of one or more spud cans into the weaker layer will continue until:
- The soil offers enough adequate resistance. The soil bearing capacity increases with depth.
- If the drop is large, with the increasing buoyancy of the hull as it enters the water.
The sudden penetration of the leg causes sudden inclination of the rig and consequently large lateral deformations, even fracture of the leg. The importance of to follow strict preload procedures at the proper air gap is vital. If a punch through occurs during a storm in the drilling mode, the result is most probably catastrophic.
Sometimes It may happen that the rig operator decides to bring another JU back to the location where a previous JU left with old foot prints (the holes left by the leg and spud can) in the sea bottom. After the spud cans are extracted, the hole wall may partially collapse. The holes fill in with the time. That gives crater like depressions at the sea bottom. The soil in the pockmark is very disturbed and has very low shear strength. Therefore, if a spud can of a new JU locates too close to the old footprint, the soil below the spud can would fail laterally and lateral load is applied to the spud can creating leg bending. This failure can cause a rapid penetration, similar to a punch through and a leg fracture.
The “rule of thumb “of choosing a new JU to come back to the location is:
The edge-to-edge distance between the new spud can and the old foot print should be at least one half of the spud can diameter.
The following preparations should be carried out on the rig in advance of the test:
1. The BOP stack should be tested.
2. An adequate volume of properly weighted mud should be available.
3. The OIM should schedule BOP, fire, and H2S drill prior to the testing.
4. Fire hoses should be laid out in the vicinity of the burners and surface testing equipment. Fire extinguishers should be placed close to the surface equipment.
5. Spare arrestor, remote shut down system, over-revving system and diesel leak automatic shut down system should be installed on the mobile air compressor, if used.
6. The OIM, Clients Representative and Testing Engineer should hold a pre‑test meeting attended by all parties concerned with the test to ensure that the expected course of events, responsibilities and contingency measures are fully understood.
7. The OIM should schedule a safety meeting with the whole crew prior to the test. All personnel should be made aware of test expectations and restrictions imposed during testing, i.e. welding radio use, helicopters, use of cranes over well test area etc.
8. Hazardous areas should be clearly marked off.
9. All required H2S equipment is to be onboard and tested.
10. Dispersion chemicals should be stored on standby boat.
11. The standby boat and helicopter base should be advised that the test is about to commence.
12. The OIM and Clients Representative should give notice that the test is commencing.
Preparations In Advance Of The Test
1. All surface lines, the separator and flow-tank should be flushed with water.
2. The cooling sprays on the burners and rig should be checked and any plugged jets cleared.
3. Surface lines, separator with its relief valve, gas heater, choke manifold, lubricator valve, subsea test tree and surface test tree should be pressure tested. Relief valve will not have to be lifted if calibrated on shore just prior to job and witnessed by Certifying Authorities.
4. The wireline lubricator and its assembly on the surface test tree should be checked and pressure tested.
5. The activation of the surface test tree safety valve, subsea test tree valves and lubricator valve should be checked.
6. The burner ignition system should be checked.
7. The separator flowmeter should be calibrated by pumping water through them into the flowtank. The separator controls to be checked.
8. The lengths, OD, ID and threads of all downhole test tools should be checked and a tally of the test string made.
9. The packer should be checked to ensure that it is correctly made up for the size and weight of casing in which it is to be set.
10. The actuation of downhole valves should be checked.
11. The dimensions of the subsea test tree and slick joint should be checked to ensure that the tree will locate correctly in the wellhead and BOP.
12. Gauges, hangers and gauge dimensions should be checked to ensure that they will locate correctly in the carriers.
13. All electrical lights, outlets, switches shall be checked in the general area of the well test units.
· Check that well test equipment layout conforms to plan submitted and approved by certifying authority.
· Lay out, measure and drift testing string (Ref. Test Programme and Well Test Supervisor on board for items/procedures required). Tally same and prepare running order.
· Check all handling equipment required: elevators, slips, safety clamps, lift subs, crossovers etc. Ensure fishing equipment available for fishing test tools and tubing used. Ensure that correct crossovers are available on the rig floor to enable stab-in valve to be used for well control.
· Well test equipment to be tested as per Well Test Programme and Well Testing Company Procedures to satisfy requirements of Certifying Authority. All pressure testing to be carried out as per the Company pressure testing safety procedures. Test all remote shutdown systems ensure that responsible personnel are briefed on operation of these.
· Ensure well test area deluge systems (where fitted) have been tested. Check all remote control stations (where fitted).
· Rig up and test all rigside cooling systems for use during flaring of hydrocarbons. Ensure that hoses are spotted where additional cooling might be required.
· Check that subsea test tree and slick joint dimensions are correct for wellhead/BOP space-out. This may be confirmed using a “Dummy Run” (Ref. Well Test Programme).
· Meeting to be held with OIM, Senior Toolpusher, Operator’s Drilling Supervisor, Well Test Supervisor and all parties concerned with the testing to discuss, draft and implement any specific procedures required.
· In areas where there may be H2S at surface during flow periods, then ensure that equipment and contingency procedure are ready. Carry out training and drills to ensure proper response by emergency teams and non essential personnel mustering.
Rig and ships lauching at yard without dry dock :
There are few principal methods of conveying a new rig or vessel from a construction yard to the water, only two of which are called "launching". The most familiar, and most widely used is the end-on launch, in which the vessel slides, usually stern first, down an inclined slipway. As for triangular hull shape rig, it can go aft or forward into the water as shown. The side launch, whereby the vessel enters the water broadside, came into old conventional method use on inland waters and was more widely adopted in old days where yards do not have dry docking facility.. The third method is float-out, used for rigs or ships that are built in basins or dry docks and then floated by admitting water into the dock.
|A floating dry dock|
Some yards adopt floating dry dock method and this involves investment cost to build such dock facility. A floating drydock is a type of pontoon for dry docking ships, possessing floodable buoyancy chambers and a "U"-shaped cross-section. The walls are used to give the drydock stability when the floor or deck is below the surface of the water. When valves are opened, the chambers fill with water, causing the drydock to float lower in the water. The deck becomes submerged and this allows a ship to be moved into position inside. When the water is pumped out of the chambers, the drydock rises and the ship is lifted out of the water on the rising deck, allowing work to proceed on the ship's hull. A typical floating drydock involves multiple rectangular sections. These sections can be combined to handle ships of various lengths, and the sections themselves can come in different dimensions. Each section contains its own equipment for emptying the ballast and to provide the required services, and the addition of a bow section can facilitate the towing of the drydock once assembled. For smaller boats, one-piece floating drydocks can be constructed, potentially coming with their own bow and steering mechanism.
Rig Leg rackphase differential (RPD) monitoring :
One of the current issues affecting jack- up drilling units, especially those with the newer leg designs, is the effect of leg cord Rack Phasing, which causes damage to individual leg members, commonly referred to as Rack Phase Differential (RPD). This effect arises under 3 general situations:
Jacking up on uneven bottoms causes each leg cord on 1 or more legs to experience differing bearing loads .
During elevated operations, scour conditions under the spud can result in unbalanced leg cord loading ..
• Extracting the chock system and loading the rig back onto the jacking pinions can cause RPD as well in the event individual system torque cannot be determined or set properly.
RPD occurs most often on locations with a disturbed or uneven seabed, resulting in eccentric bearing support of the leg’s spud can and causing the can to move horizontally. RPD is most likely in situations with (1) pre-existing spud can holes, (2) sloping seabed, (3) uneven sea-bed, (4) uneven seabed due to scour, (5) leg splay, or (6) rapid penetration.
All jackup rig designs experience RPD, but only certain classes (primarily units with low cross-sectional leg members) experience RPD to the point of leg damage. It was understood that rig design such as the F&G L-780 can sustain up to 3 in. (76 mm) of RPD before leg member failure occurs. The Mod-V B class may sustain up to 4 to 5 inches of RPD. According to web report, the new JU-2000E is designed to sustain up to 8 in. of RPD (203 mm) before leg member failure.
Location evaluation prior to rig arrival on-site is the most critical factor for reducing RPD effects. A complete location evaluation can be performed by doing bottom surveys, geo hazard surveys and soil analysis. The second most critical stage for monitoring RPD occurs when the rig is set up on location. Early detection of RPD helps prevent the operation from continuing into damage-producing stresses.
Soil properties have a critical impact on the process. In typical locations with a hard seabed and minimal penetration, there is very limited potential for manipulating the seabed while elevated. When the seabed is hard, RPD is typically eliminated by reseating the spud can. If RPD is monitored before full bearing pressure is achieved, there can be limited ability to manipulate the seabed (referred to as “stomping” or “pre-forming”).
Operators have found that manipulation of the seabed in later stages of the setup process is more likely to successfully reduce RPD when softer, more pliable soils are present. For all soil types, the best opportunities for managing RPD are during the initial stages of setting up on location.
Basic techniques employed to counter RPD after it is observed includes reseating, changing chord loads by releasing brakes & independent chord jacking, intentionally imposing reverse RPD, and tilting the rig.
According to web article, the JU-2000E jacking system has the ability to monitor gear unit torque from the Jacking Console and set the torque individually for each gear unit. In addition, the jacking system includes a Rack Phase Display and Alarm System to monitor differential as it occurs. This enables the jacking operator to stop jacking operations to evaluate any effects of RPD and to institute appropriate mitigation procedures.
Typical RPD DISPLAY
The typical common RPD display may shows relative differences in displacement of the 3 chords for each leg. Displacement of each chord is measured by a Leg Height Detector fitted at each chord on top of the jacking structure. The detector consists of an idle pinion that meshes with the rack and rotates only during the vertical displacement of the chord . This pinion drives 2 pulse counters that deliver signals to the MCC, which processes these signals and display the RPD values on the central jacking console for each leg.
The RPD display does not automatically stop the jacking operations. However, it does deliver an audible warning if RPD exceeds . Values of chord relative displacement are displayed for the jacking operator, who then decides when and which correction is necessary.
The jacking system includes a central jacking console and 9 local consoles (1 for each chord). The local consoles interface with the Rack Chock System engagement. Length of the leg deployed below the hull is displayed on the same screen as the Pinion Load Monitoring System screen, located on the Jacking Central Console .
Courtesy of Monitor Systems Scotland Ltd
Rig safety induction :
Training, according to the requirements set out for work onboard MOU, MODU,etc by IMO, and to SOLAS requirements and recommendations, is carried out continuously and documented on the “First Visit (crew member)” and “Self-assessment (crew members)” check-lists. Training according to the requirements are also carried out continuously and documented additional to the Training Register for completed Onboard Safety Training.
Safety training and induction onboard is normally part of the Safety Officers' duties. If no SFO is employed the Second Officer is normally appointed Safety Officer. Recognized qualifications shall be utilized when appropriate.
All crew members are required to complete a check-list to document that they have received proper safety induction on their first arrival as well as have been given time and opportunity for familiarization onboard.
Detailed information is contained in rig or vessel controlled documentation such as
• Safety Handbook
• Safety Manual
• Safety Plan
• Emergency Plan
• Emergency Response Manual
• Rig Operations Manual
All employees and personnel coming onboard for the first time are required to attend a Safety Induction Meeting, which may include the requirement to view a safety video. This meeting is normally conducted by the Safety Officer, but may on some locations be conducted by a specific person appointed for this purpose by the client. Such arrangement however does not relieve the Master / OIM from his responsibility to ensure every new arrival receives appropriate safety induction and other relevant information. All drills and musters onboard to be in accordance with legislation requirements, international conventions and industry as well as Company standards.
The Master/OIM is responsible for drills and musters being carried out as stipulated in Company standards and to the requirements of the Flag state authority.
The Master/OIM is also responsible for the documentation of drills and musters, as well as to ensure debriefing and evaluation of procedures and results is part of the exercise.
Helicopter landing on rig :
Many of these units will have helidecks where the design and construction of such helidecks (particularly on some vessels) tend to be less prescriptive than for fixed installations; the ship’s main functional purpose will sometimes inhibit helideck design.
• Floating Production and Storage Systems
• Mobile Offshore Drilling Units
• Accommodation Vessels (Floatels)
• Jack-ups on-the move and
• Specialist Vessels.
However, they are required to meet the standards set out in the relevant regulations, codes and guidance in order to undertake helicopter operations routinely. Invariably a MODU (a semi-submersible, jack-up on the move, or drill ship) will initially be specified using the IMO MODU Code [Ref: 70] as the basis for design.
Operating Environments :
The marine operating environment for a semi-submersible is similar to a fixed installation insofar as the helideck heading is generally fixed as a result of the anchoring arrangement or, if fitted, a dynamic positioning (DP) system.
However, it differs from a fixed installation in that the helideck has a dynamic movement in roll and pitch axes, heave, surge and sway due to the vessels dynamic characteristics.
In addition to wind speed and direction, helideck movement (velocity and accelerations as well as heave amplitude) induced by the floating structure should be fully taken into account during helideck and system design and helicopter operations. The helideck is typically located at one corner of the main deck (forward or aft) directly above one of the buoyancy columns and adjacent to the bridge / accommodation. In this location, the windlasses and winches for controlling the anchoring system will be directly below the helideck.
It is therefore important to ensure there is sufficient cantilever of the helideck structure over the column and windlasses to avoid infringing the 5:1 falling gradient below the helideck surface. It is also essential to provide sufficient air gap below the helideck structure and above the winches and housings to avoid unfavourable aerodynamic effects over the helideck.
The marine operating environment for a jack-up on station is the same as a fixed installation. However when under tow, the helideck conditions are similar to a vessel under way.
Helidecks on jack-ups, when on location, do not need special consideration for vessel movements because they are in effect fixed structures. However, when under tow they are effectively a vessel, and helicopters landing on the helideck (routinely or in an emergency) will require the same design considerations and operational aids as a mobile unit. Normally it is very seldom to land a helicopter during rig tow as we understood from rig operators.
In particular when under tow, the legs will be elevated to their maximum height and, as a result, they will be the dominant obstructions. This should be taken fully into account during helideck design.
The marine operating environment for a drilling vessel ‘on station’ is similar to a semi-submersible insofar as the helideck heading is generally fixed as a result of the anchoring arrangement or, if fitted, the dynamic positioning (DP) system. Similarly, the helideck has a dynamic movement in roll and pitch axes, heave, surge and sway due to the vessels dynamic characteristics.