Jack-up rigs operational aspectsIntroduction:

Jack up rigs, or self elevating mobile offshore drilling rigs, are the most common type of offshore drilling rig. The basic philosophy is that they are mobile, provide a stable work platform that is virtually unaffected by environmental conditions and there fore experience very little down time. evolved from multi-leg swamp barges operating in a few meters of water, the present generation of jack-ups claims to be able to cope with extremely hostile environments in water up to 125m deep.

Jack-up drilling rigs will not normally compete with floating rigs since their operating parameters are complementary rather than conflicting. Having said this, there are areas where a large jack up or small semi may be considered for a particular job. Also, the use of bottom supported submersible or drilling barge may provide an alternative to using a jack-up in shallow water.

The choice of the type of drilling unit to be used will largely depend on the operating characteristic, limitations and availability of rigs, environmental conditions and ultimately economics.

This section will give a description of some of the systems, procedures and design requirements specific for jack-up operation. The following subjects will be addressed:

-jacking systems-pre-drilling operations

-Operational design requirements•

Jacking systems•Jacking systems can be grouped into two main generic types:

• -the rack-and pinion system • -the yoke and pin systems.

•the first one being more common Within these types ,•there are two main designs: floating jacking design and fixed jacking design :

•Rack-and-pinion system:•The most common type of elevating system is the rack and pinion this is a simple, easy to

operate system consisting of large gear wheels which, when turned, run along the vertical toothed racks attached to each of the legs thus jacking the rig up or down. The pinions can be employed in pairs on either side of the leg member (opposed pinion) or on one side of the leg only (unopposed pinion). Typically a jacking speed of 0.5 m/minute can be achieved. This system is easy to activate , easy to cycle up and down, and it is easy to change out motors and other equipment

•yoke-and-pin system:•Yoke and pin jacking systems utilize hydraulic rams to jack the rig up in stages, using slots

on the legs to locate the hydraulic rams this system has long cycle times. Changing out jacks or hydraulic equipment can be very difficult and it is not easy to change direction.

•Floating jacking design: •the floating method Jemploys resilient snubbed pads mounted on top and bottom of the

jacking unit. acting forces are transmitted through the pads into jacking housing structure and then into the hull. as the attachment is not rigid, the jacking unit is guided to the leg allowing more liberal tolerances in leg construction.

•Fixed jacking design:•The fixed method of mounting uses a welded connection of the jacking frame to the vessel

hull. This provides a rigid mounting in which loads are transmitted through the jack frame directly to the hull. The frame is retrained by diagonal bracing and cross-ties to form a rigid support around the leg. this method, while requiring close tolerances in leg fabrication and hull guides, eliminates the need for jack house structure and reduces leg weight by nature of applied designed load.

Pre- drilling operations•There are two main operational periods for a jack-up.

- The first includes jacking down, towing the rig to a new location and then jacking up again until the rig is considered stable and safe for drilling operations.

• -The second period encompasses all drilling operations from this work platform.

•Statistics show that risks in jack-up operations increase sharply whenever the rig is being moved. This due to two main factors: design and personnel. jack-up is designed primarily as a stable drilling platform and hence somewhat limited in its seaworthiness whilst in the floating mode. This section lists and explains the major operational procedures necessary for utilization of a jack-up drilling unit.

Jacking down•Prior to commencing a rig move all loose items of equipment must be secured and all hatches

and water tight doors closed.•Towing vessels for moving jack-up rigs should be sufficient in number and pour to control the

position and heading of the rig at all times and to ensure a positive speed in the worst foreseeable sea conditions.

•During rig moves a tow master or marine representative is normally present on the jack-up and is in charge of the actual tow, although the appointed OIM will still be responsible for the overall safety of the rig.

•Stability calculations must have been completed to ensure the rig is within the allowable limits and the move programme should have been agreed by everyone including the captains of the towing vessels.

•Once the hull has been lowered to a 2 meter air gap the weather is again reviewed along with the forecast and trend which has been monitored for the last 48 hours. The tugs can then be positioned, ideally one on each corner, and the hull lowered further into the water to a 2metre draft. Checks for leaks should now be completed before proceeding further.

•The legs can now be jacked higher and the mat or leg pulled off the sea bed. This may take several hours or even days if there is a large amount of penetration. If this the case, jetting can be used whilst holding tension on the legs. This procedure reduces the freeboard of the rig which should be carefully monitored. If this is unsuccessful, or if the jets become blocked, the legs can be levered up by flooding ballast tanks on the opposite side of the stuck legs.

•During this leg pulling operation the tugs are kept on station with a slack towline, except for the one on the weather side, which may pull to equalize the combined wind and current loads.

•The phase of moving off location is especially critical if there are sub-sea wellheads or x-trees in vicnity.if strong currents and or winds are present the rig will rapidly drift off location if great care is not taken .towing vessels should be utilized to maintain the rigs position until the legs are sufficiently high to clear any sea bed obstructions. If control is lost, with the spud cans just off bottom there is a good chance of damaging spud cans, legs or sub-sea installations.

Towing options•There are two different options for towing a jack-up between locations: the wet

tow and the dry tow.•Wet tow:

•During a wet tow, a jack-up is moved between locations in floating mode with the legs up and assisted by one more towing vessels. Wet tows are the most common method of inter field rig moves of a jack up drilling unit and are by far the most practical, even over relatively long distances.

•For rig moves within a field it may not be necessary to raise the legs fully. For longer tows however, the legs should be fully raised to reduce drag through the water and thus ensure maximum towing speed.

•Constant communication between the lead tug and the jack-up control room should be maintained during the tow. The lead tug should provide the position of tow, course and speed to the control room on a regular basis. if appropriate, there should be liaison with the coast guard, and the tow should be carried out along shipping lanes.

•Inspection of the rig encompassing decks, engine rooms and tank soundings should be completed at least two hourly and the observations logged. The trim and motion of the rig should also be monitored and any change, however slight, investigative. Navigational lights should be on and checked to be clear and foghorns tested.

•When planning a long tow, locations along the route suitable for jacking up the rig should be identified and surveyed. In the event of deteriorating weather or other problems occurring during the tow, the rig can then be jacked up before reaching its final destination.

Dry tow•Dry towing , where the jack up is placed on the deck of a large

transport ship (see figure7.1.19) is used for long, e g intercontintal, moves primarily because the transportation speed and vessel stability is greatly increased.

•The economics of dry tows are dependent on several factored such as :

•-availability of suitable vessels in the area•-distance to be transported

•- Anticipated weather conditions en route •-contractual obligations

•– possibility to share the transport vessel with other operators•There are vessels now on market that can carry several large rigs

• )jack ups as well any other type of mobile rig (at once and clearly if one can plan far enough ahead to utilize such vessels, then there are great savings to be made.

Site surveys•Prior to jack up drilling rig being moved to a location, it will be

necessary to have survey of the proposed location completed. This survey is necessary for the rig owners and their insurance company, to ensure that the sea bed is suitable for the rig. The survey will usually encompass an area of o.5 k m radius around the proposed location and will investigate the following:

• -water depth•-Debris check of sea bed

• -shallow seismic of sea bed indicating depth of soil and expected firm base (also presence of shallow gas)

• -evidence of sand banks or major unevenness of sea bed•-location of pipelines, wellheads or wrecks (ship debris) in vicinity

(around)• -shipping lanes

•-soil sampling•It will normally be necessary to have the site survey completed 3-4

weeks prior to the rig moving to location, in order that all relevant parties can analyze the results.

Arriving on location•Upon arrival on location, the rig should be slowed by the tugs which can

then be connected one to each corner, to hold the rig stationary and on the correct heading whilst pinning down the point at which the legs initially

penetrate the sea bed.(•This however is not always easy, as to hold exact position and heading,

possibly against a current, requires a high degree of co-ordination between the tugs and the rig which is simultaneously lowering its legs. This is especially critical when the location is over a platform or if there are pipelines close by ,where loss of control could cause serious damage.

•Positioning can alternatively be achieved by the combination of tugs and anchors and, if it is known that the anchors will hold in the prevailing sea conditions, this may well be the preferred method.

•Two anchors should be run astern, one on each corner before the rig is too close to the platform. The tugs then apply slight pull ahead to retrain the rig, which is then drawn back to the desired position by anchor winches. Using this procedure the rig can be maneuvered with a high degree of control and precision. If the anchors should drag the rig will be pulled away from danger by the tugs.

•The boats are kept tied up, hanging slack until the pre-loading is completed, and it is certain that they are no longer required. These maneuvers should always be performed in daylight.

pinning•The term (pinning) is used when the legs of the jack-up are set

down on the sea bed at either the desired or standby location and refers specifically to the point at which the legs just start to restrict free movement of the barge.

•Jacking up:•Once the rig is in the correct position and being held by the tugs and

/ or anchors, the legs can be jacked down until they penetrate the sea bed. The jacking now continues progressively, keeping the rig level until the hull is clear of the water by 1-3 meters depending on sea state. The legs may not all penetrate to the same depth.

•The sea bed at some locations, especially around fixed structures will often have pipelines and or foot prints of previous jack up rigs present. The position of these hazards should be known and if there is any doubt divers or a ROV can be utilized to check the location and possibly even monitor the situation whilst the jacking is in progress.

Pre-loading•To achieve satisfactory stability of the rig on the sea bed , maximum

anticipated vertical leg loads are simulated on each spud can prior to jacking up to full operational air gap. The pre-loading operation is necessary to ensure that all spud cans are sitting on firm base and that no further settlement is likely to occur. This foundation must be strong enough to withstand the combination of maximum environmental loads, maximum variable and drilling/operational loads without further settlement

or punch through .(•Pre-loading only emulates (take care) the vertical load on the spud cans. In

real storm conditions, the spud can experience the maximum vertical load in combinations, with a significant horizontal load referred to as base shear. This combination of loads may subject to sea bed beneath the spud cans to a higher effective load than that imposed during pre-load. In an attempt to compensate for this, a pre-load safety factor can employed. The safety factor of 1.3 is applied to the environmental component of the calculated pre-load requirement.

•Pre-loading three legged jack-up rigs is achieved by filling selected hull tanks with sea water and holding this additional load until gradual settlement by each leg into the sea bed has ceased this settlement can be easily monitored by watching the levels in the control room. Jack-up rigs with four or more legs are pre-loaded by alternate and repeated tensioning of diagonally opposed legs. Pre loading with for legs is much quicker than preloading with three legs.

•It is important that the air gap is kept to a minimum during pre-loading to minimize damage caused by sudden punching through of one or more of the legs.

anchors•Jack-ups normally have a relatively small anchoring system which

as mentioned is usually only used when moving on or off a location where exact positioning is critical. This could, for example, be while moving on or off an existing wellhead or sub sea tree or moving over or away from a production platform.

•Normally, when a rig is being positioned over a virgin) location it is satisfactory to pin the rig with tow boats holding the rig at the proposed location. with modern positioning equipment, a positioning tolerance of 15 meters radius is normal. This tolerance can however be reduced by pining the rig at a standby (move- on) location in the vicinity of the intended site, while the tow vessels run the anchors. The rig is then jacked down. Pulling the legs from the sea bed, and positioned accurately by utilizing the anchor winches. At this point the rig would be pinned on final location and the anchors retrieved after pre-loading is completed.

Punch through•Punch through is the term used to describe a

sudden breach of the sea bed by one or more spud cans. Punch through would almost certainly only occur during pre-loading and is a rare occurrence as the site survey would normally have identified the potential for punch through.

•Punch through can cause extensive damage to a jack-up and emphasizes why it is necessary to perform the pre-loading fully at as small an air gap as practical, usually 1to 3 meters.

Air gap•Air gap is defined as the distance between

the bottom of the hull of the jack-up and sea level( MSL). This distance is calculated to take account of prevailing weather conditions in an area, such that the rig hull is far enough out of the water to avoid that the hull is subjected to wave loads in the event of a heavy storm. As an indication, most jack ups will use an air gap of 15-30 meters dependent on area, time of the year and local conditions. This should be checked for each local operating area with the

marine department .

Soil mechanics•The strength of the soil that the spud cans of a jack-up must sit on

will greatly influence the design of the cans and the ultimate loading potential of the rig. Most soil types are a mixture in varying proportions of three types of solids defined by their particle seize:

•-clay: particle size smaller than 0.005 mm•-silt: particle size between 0.005mm and 0,05mm

•-sand: particle sizes between 0.005 mm and 2 mm•When loaded excessively, soils will fail in shear. The resistance to

failure is called the shear strength of the soil. Soil samples can be taken from the sea bed and checked in the laboratory to determine their shear strength. The absolute shear strength is determined by the make up of the soil and the amount of inter-particle air, gas or water present. As soil is loaded with a spud can, the soil is compressed and eventually the inter-particle air , gas or water is forced out. This process might take days or weeks to reach equilibrium and explains why a jack –up might settle whilst in operation with out causing great alarm. In general, this type of settlement occurs ,(meaning the legs settle by largely different amounts ) then this could signify a more serious condition and should be further investigated.

Spud can checks•Theoretically, a spud can could sit directly on the sea bed with minimal penetration. In

practice, penetration of up to 50 meters can be encountered in extremely soft unconsolidated soils. This excessive penetration raises various problems:

•-leg length: if the spud can has excessive penetration, can the rig still jack itself out of the water to the required air gap?

•-pile collision or interference: if positioned next to a platform, will the spud cans collide with the platform piles which have a high rake angle or interfere with the jacket

skirt piles?•-spud can extraction: recovering deeply penetrated spud cans in time consuming.

•Where the sea bed is extremely soft, use of a mat type jack-up should be considered to reduce point loading by distributing the load evenly over a large area.

•It is necessary to check the spud cans periodically to monitor sea bed condition. obviously, continued settlement can be monitored from the rig itself, but in the case of scour, then it is advisable to have a programme of monitoring.

•Scour is caused by current at the sea bed eroding the support around the spud cans and so increasing the loading on the portion of the spud cans still in contact with the sea bed. If scouring is expected matting should be placed beneath the spud cans before jacking up. If scouring has taken place.

•Spud can checks can be performed using divers, but more often nowadays these are being performed by remote operated vehicles. o. v). Normally, the first check would be carried out as the jack-up is operations. Checks should then be performed at short intervals and then if no major problem is apparent the frequency can be gradually decreased. Frequency of spud can checks is often based on previous experience in an area.

jetting•Where excessive penetration of the spud cans has

occurred ,it may be necessary to jet the spud cans out when preparing to move the rig off location.

•The base of each spud can is fitted with a set of nozzles connected to a line which runs down each leg. These lines have connections at regular intervals along their length to facilitate hook up to a sea water pump at different water depths. Commonly the rig pumps are used for this purpose. During the jetting operation tension is held on a leg while pumping down the jetting line. As the soil is jetted away and any suction force released, the applied leg tension will gradually extract the leg. This can, on occasions, be a lengthy process.

Foot prints•After a jack –up has been moved off of a location , its spud cans will

leave an indent or footprint , in the sea bed if the location where the rig has been will be re – used say if it is template of sub – sea completion , particular attention will have to be paid to these footprints.

• if the hole still exists it is possible that a spud can being placed at the edge could slide into the hole. it may therefore be advisable to fill in remaining foot prints with material dredged from the surrounding sea bed, in order to minimize any problems. However, filling footprints does not always solve the problem especially if filled with high density material.

•Another method of solving this problem is to lower the spud can to sea bed and put on the jetting system in order to fluidise and level the sea bottom.

•This is however not practical when alongside a platform as it would be hazardous to both structures.

•Particularity around platforms and jackets where it is likely that rigs move on and off during the development of the field it is important to record the position of the footprints around the platform for future

reference