1. General Information about, MODU Towing.
* The Smith Bracket
On a Jack up the towing arrangement basically utilizes the same principle of a single tug on the tow bridle or a tow arrangement with two tugs each attached to a single towline.
Each MODU need to have a main towing system, which is easily operated and rapidly accessible when necessary. The strength of the wire, chain and connections must be strong enough to sustain the worst possible conditions, and be in relation to the maximum bollard pull of the tug.
A backup or emergency towing arrangement must be installed that can be passed and connected to the tow under severe conditions without the use of the drilling rig cranes.
One of the systems is to attach the fuse link or fore runner with a long and strong polypropylene rape attached to a smaller diameter messenger with a small plastic can.
The can should be large enough to support the weight of the messenger and the heavier rope. In case of a broken main tow wire the emergency tow bridle is lowered and the rope with can released. The tug will pick up the can and pull the fore runner on deck. The strength of the backup towing arrangement should be the same as the main towing equipment. The emergency towing arrangement may be attached to the aft end of the drilling rig and be exactly the same as the main towing system.
A tow with a single ocean going tug generally uses the bridle system as shown in Fig. 1
To dampen the yaw the towing connections to the MODU are installed as far apart as possible.
2. The Towing Bridle System.
* The Smith Bracket
The Smith Bracket or Smith Towing Pad Eye is the standard connection installed on the drilling rig. It is a good, durable, safe and easy to handle arrangement of three pad eyes and a removable pin with securing wedge. The standard Smith Bracket has a breaking strength of 580 ton. Fig 2
Fig. 2 Smith Bracket – Panama Fair Lead – Triangular Plate
* Stud Link Chain
To prevent chafing the first part of the bridle, connected to the Smith Bracket, is a length of 3" stud link chain. The chain length should be long enough to clear the pontoon or the hull. At each end of the chain, an open end link enables to make an easy connecting.
* Closed Panama Fair Lead
The chain passes through a closed Panama Fair Lead. Welded at the end of the pontoon or hull this type of fair lead secures the bridle and prevents chafing of the chain over the hull. Fig 3.
* Bridle Pennant
A bow or D shackle connects the open-end link of the chain to the heavy duty, gusseted thimble eye of one length of 2 ½ or 3" pennants. Do not use two lengths of pennant as this is more difficult to handle and adds more connections to the system. The length of the bridle pennant depends on the configuration and width of the drilling rig. ·
* Triangular Plate
The most common connection between the two bridle parts is a triangular plate, also called fishplate. Fig. 2 and 3. The triangular plate is a pre fabricated and certified connection plate with three eyes.
Never use any self made or non certified plate. Sometimes three shackles are use instead of a triangular plate. For towing MODU's only use the triangular plate
* Weak Link or Fuse Wire
A bow shackle connects the triangular plate to the weak link or fuse wire, which is a short piece of pennant. As an example, if the towing arrangement is dimensioned to withstand at least 3 times the towing design load, the fuse wire is designed to withstand only 2.3 times the design load. See Fig. 4. The purpose is obvious, if the towing arrangement breaks under a shock load the fuse wire prevents breaking the expensive the tow wire. In addition the retrieval procedure is easier. Not all operations agree with the principie of a fuse link. Anyhow always use a short piece of fore runner to make the connection· procedure easier for the tug.
Fig. 4 Example of recommended strength of towing gear – spring line
* Nylon Braided Spring
The nylon braided towing spring connects the tow wire with the· fuse. It functions as a shock absorber and is part of the tug's equipment. See Fig 5.
Fig. 5 spring line.
* Tug Towline
The tow wire on the tug is spooled on the towing winch and its length can be adjusted as required.
* Connecting Shackles
All shackles and other type of connections need to come from certified manufactures. Each towing arrangement will be subject to an inspection before passing the tow system to the tug.
Only use safety shackles with securing split pins. Never use welding rods as a split pins. Welding rods deteriorate fast and break or fall out resulting in a disconnected towline.
3. Towing Arrangement Semi Submersibles
Some examples showing the various towing systems with their attachments and sizes you may found as follows: Fig. 6 A-D. Note the towing bridle arrangement attached to the 100 ft. level pad eyes for deep draft towage. With two tugs, each tug tows on a single line. The anchor is disconnected and stored on the tug or the drilling rig. See Fig. 6 B. For short tows the anchor is left hanging from the chain at a safe distance. The distance should be long enough to prevent the anchor to swing into the hull if the towline breaks.
Fig. 6A. Main towing arrangement.
Fig. 6B. Towing arrangement two (2) tugs
Fig. 6C. Deep draft towing system
Fig. 6D. Narrow water tow arrangement
4. Towing Arrangement Jack up.
Fig. 7 Jack up standard towing arrangement
5. Bridle Retrieving Equipment
Each drilling rig should have a proper system to retrieve the bridle. This can be in the form of a small crane or an A-frame with associated winches and guide blocks.
The bridle retrieving system in most cases is part of the drilling rig original equipment, designed and installed at the shipyard. In addition caisson cranes on top of the columns and Pedestal cranes are use to assist handling the towing bridle installation and retrieval
A good designed bridle retrieving system should have.
An A-frame with a powerful air hoist in a position to allow the crew to handle the bridle and fishplate from a safe and easy position.
a) Instead of an A-frame, air hoists are located on the main deck under the helicopter deck. Sheaves are located above each air hoist, supported from the helicopter deck.
b) Adequate power on the air hoists. The air hoist power should be enough to lift the entire system of retrieving wires fish plate with attachments and the fuse wire (fore pennant).
c) If necessary, use two retrieving lines with two air hoists.
The towing bridle arrangement is most probably not rated to retrieve the bridle arrangement with part of a broken tow wire. Before retrieving the system, verify with the master of the tug how many meters of towline is hanging from the bridle. The tow bridle system on itself weights between 4 and 5 tons.
6. Broken Towline
The first line of defense to prevent a broken tow wire is to carry out a thorough inspection before accepting the tug. This is one of the tasks of the warranty surveyor. Unfortunately, tow wires now and then break.
Some of the avoidable main reasons of a tow wire to break are:
* Old and worn wire. The task of the Warranty Surveyor is to inspect the condition and history of the tow wire and the end connection.
* Poor drilling rigging practices. No towing pod or no proper variable Gogs installed. No protection sleeves around the tow wire. The Warranty Surveyor inspects the towing equipment but relies on the good seamanship of the master of the tug to use and install the proper equipment. Sharp edges grooves and gouges in the stern area. The stem area is prone to some minor damage cause by dragging anchors, chain, wire and grapples over the stern roller and the adjacent deck area. Sharp gouges, cut s and grooves will cut into the tow wire. Include the stern area in the inspection procedure of the tug the position of the thimble on me towing spring at regular intervals. The Warranty Surveyor inspects the tow spring and verifies the wear condition of the area around the thimble.
* Bending and snatching loads. The heaviest bending and snatching loads on the tow wire take place in around the same area, between 500 m and 600 m and between 800 m and 950 m. For the AHT the touch point of the tow wire over the stern roller absorbs most of the bending shock and snatching loads. The same principle accounts in a lesser degree for the ocean going tug around the chafing bars. To prevent or reduce the bending and snatching loads the tug can reduce power, alter course or heave too into the weather. The Warranty Surveyor inspects condition and history of the tow wire. It is not possible to inspect the entire length of the tow wire.
Tow bridle and the connections. Most drilling rigs use the bow or D type of shackle with a bolt and nut. A split pin secures the bolt. If the split pin shears of due to wear or damage the bolt backs off and the towline disconnects. See Fig.8 shows some alternative shackle types. Do not accept the use of welding rods or tag welding the nut. Like with the tow wire, the Warranty Surveyor inspects the condition of the bridle and all attachments. The barge engineer is responsible to maintain the tow bridle in good condition.
Fig. 8 various types of tow shackles
The following information from the OPL Towing book shows some interesting figures from studies on the reason for towing line failures. Towline failure represents nearly 20% as the initial cause of all total looses of drilling rigs.
* Towline Failure
Gear failure 38%
Crew experience 30%
Bad weather 23%
Cut on seabed 6%
Cut by tow 3%
* Towline Failure- Position of Parting
Stern of Tug 30%
Nylon Spring 28%
Mid towline 7%
At winch 6%
Around propeller 6%