Hull construction

Details Hull construction Hull construction

1999-07-07

2001-04-24

Sotelo, Jesus D. (Department: 3617)

Ships

Ballasting

C114S06500R, C166S345000

Reexamination Certificate

active

06220194

ABSTRACT:

Half submergible drilling rigs have had extended application in rough weather regions at sea because this type of rig has an especially favourable response to movement relative to waves. By favourable movements of the vessel is meant that the swings, that is to say the amplitudes, during heaving, rolling and pitching are relatively small in large waves It is very advantageous to obtain movements of small amplitude because smaller demands can then be placed on the drilling equipment on board the rigs.
At the same time the response cycle is long that is to say usually
15
-
16
seconds or more. For a ship which moves up and down in the waves its response cycle is defined as the time elapsed from a maximum and back to the same maximum. Long response cycles are favourable for the equipment which stands on board the drilling vessel because the accelerations in the movements thereby become moderate, something which also places fewer demands on the equipment on board the rig. As regards response cycles reference is made to the enclosed diagram where the movement characteristics of different vessel constructions are compared during the different wave cycles occurring.
The favourable movements for half submergible rigs is due to these types of floating drilling devices consisting of a series of vertical columns which break the water line, at the same time as the columns are structurally bound together by the deck of the device and by horizontal pontoons beneath the surface of the water. In this way, the water line area of half submergible platforms becoming less relative to the total buoyancy volume, is achieved since the largest portion of the forces from waves arises in the water line area, and diminishes downwards with depth, the wave forces are reduced on these types of devices. In addition, the horizontal pontoons have a favourable effect on the vertical heaving movements of the rig (the semisub) because they function primarily as a brake in the vertical direction when the surrounding mass of water becomes broken up and thus provide a theoretical additional mass to the rig.
Besides it shall be observed that by the natural water line of the hull, is meant the natural water line when the hull is finally completed with fittings, machinery etc. for a vessel, such as a drilling ship or the like.
For all types of floating drilling vessels it is the vertical heaving movements which are especially critical, This is due to the vessels having, during the operations a drill stem made of pipe which hangs in the drilling rig of the vessel and extends down through the well. This stem is rigid, and in order to ensure that movements of the drill stem down through the well are unaffected by movements of the vessel, the vessel is in addition furnished with special arrangements which can compensate for movements of the ship. These arrangements have however limitations both as regards movements, accelerations and maximum swings, and this involves a need to minimalise stresses on this equipment by accurately controlling the hull design of the vessel.
In spite of its good movement characteristics half submergible rigs clearly have disadvantages as regards the carrying out of cost-effective drilling operations A disadvantage is for example that the hull becomes very expensive to build since it is composed of columns, pontoons, struts and decks. In addition, such half submergible rigs are especially sensitive to displacements of center of gravity, for example on shifting deck cargo. The biggest disadvantage is however that the total payload which can be taken on board is limited when the stability of the rig is taken into account.
This entails a half submergible rig being dependent upon continuous supplies of consumption material during the drilling operation. This is carried out normally by supply vessels which are made specially for this purpose. This is however very expensive to carry out, because as a rule it requires a vessel having to be available for the rig the whole time.
In addition, when the drilling operations shall take place far out at sea, and possibly with substantial distances from the closest situated supply base, this involves a significant increase in costs. In addition, deep and long wells require extra feedings of supplies which contributes to increasing further expenses. It the supply lines are particularly long, there can be a need for an extra supply ship in the continuous operation. The earliest half submergible rigs had a payload capacity of about 2,000 tons, while a modern halt submergible rig to-day ought to have a payload capacity of over 4,000 tons. A half submergible rig is usually anchored during the drilling operations at sea, and at the same time there are often laid out 8-10 anchors.
In addition to half submergible rigs, drilling ships are also used as floating drilling vessels at sea. These have the advantage that they can take considerably larger cargoes on board. A drilling ship can often take on board all the payload before it puts to sea in order to drill the well. This makes the drilling ship nearly independent of assistance from supply ships. While a half submergible rig is dependent on tugs when it shall be transferred from the one well to another, a ship can transfer itself with its own propulsion machinery. In those regions where it is far to the nearest supply base, such as for example in the Far East where oil is searched for at sea several days journey from the supply base, such drilling ships have found a broad application.
However drilling ships have clear limitations in their field of application. The known drilling ships are produced as conventional vessels with a single hull. This entails their being very sensitive to larger waves since they have a less favourable movement response compared to usual half submergible rigs. These poor dynamic ocean characteristics for drilling has meant that drilling ships, in spite of their excellent load capacity, cannot be used in more inhospitable regions such as the North Sea and the Atlantic Ocean. Drilling ships have however great prevalence and application in more hospitable regions, where the waves are relatively small such as in sea locations outside Brazil, Indonesia and the like.
A drilling ship will have a relatively large water line area compared with half submergible rigs and is therefore more exposed to wave forces than such rigs. While an anchored rig is nearly uninfluenced by the direction environmental forces come from, a drilling ship is dependent on being able to rotate with the weather the whole time so as to be able to minimalise the forces the ship is exposed to. This is brought about by the drilling ship being equipped with a data-controlled automatic positioning system which guarantees the position relative the well and to the direction of the wind, waves and currents.
The most significant drawback with the known drilling ships is that they are sensitive to the vertical heaving movements which are generated by waves, both as regard amplitude and cycle. Known drilling ships have a length of 160-80 m and a typical breadth of 22-25 m. All of these types of ship have largely parallel ship's sides and have a normal heaving cycle of 7-8 seconds. This places moderate demands on the equipment so long as the heaving swings are moderate. A normal maximum heave for a drilling ship is 7 m (that is to say 2×the amplitude), and this can be theoretically handled by known compensator systems.
However this is wholly insufficient for more rough weather regions, where this type of ship can easily get heaving movements of 8-10 m depending upon wave heights and weather conditions. The theoretically simplest manner to solve this problem could be to widen the extreme values of the compensator system of the drilling ship, but with such a short cycle this will means large accelerations of the equipment with consequent forces and stresses which can both lead to breakage and fatigue.
Another theoretical way to solve this problem can be to increase the size of the ship, possibly in combination with increasing

Affiliated with

Also associated with

Rating

Hull construction does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Hull construction, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Hull construction will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2533202