The option of transporting drill cuttings from the rig has a number of attractions. Unlike the use of WBM or CRI, the skip and ship option can in principle be applied to any drilling operation from platform or MDU.

Rig upgrades required to allow this containment option are minimal and may be implemented quickly and cheaply. Since the rig site equipment is simple and reliable, equipment failure is less likely than for CRI. A number of process technologies have been developed for treating the cuttings which recover and recycle base oil from the cuttings leaving an inert material such that the whole process is environmentally sound. Use of the processed cuttings for projects such as construction material is possible, representing further scope for a completely "green" process.

The transport of drill waste to shore has been practised successfully in numerous operations worlwide.

The disadvantages of transporting material to shore is that the quantities of cuttings involved can be very large and the logistics problematical. The process is most suitable wherevolumes of cuttings generated are small and the distance they need to be transported short. For offshore operations weather conditions are also significant and may have an impact on the ability to move waste material. Because of the heavy demand on deck space for storage of cuttings, it may not be feasible to ship cuttings from 16" or more unless drilling rates are restricted.

The process of transport to shore, known as skip and ship, is very simple. Cuttings are collected from the cuttings ditch and transferred, usually by screw auger, to a suitable location for loading. As well as collecting cuttings the system should handle liquid over spill from the shakers and waste from centrifuges. The drill waste is loaded, normally into specially designed skips of around 8 T capacity, via a steerable chute. For onshore operations, the skips can be placed directly below the shale shakers. Empty and full skips are stored on the rig and depending on the drilling program filled skips may be off loaded either onto a dedicated collection vessel or to a standard platform supply vessel (PSV).

Once back loaded the drilling waste is disposed of onshore. Land-filling or incineration of untreated cuttings are acceptable solutions depending on the local legislation. Another option is to fix the oil to the cuttings using fly ash, leaving a relatively stable, but still potentially hazardous, product. It is also possible to employ a technique to clean the cuttings and recover the base oil, leaving a benign solid waste and re-usable oil. This generally the best option, as the additional costs for treatment of the cuttings are not prohibitive and the advantages are clear.

The critical considerations in planning the feasibility of a skip and ship operation relate to the quantity and rate of cuttings generated, the space available on the rig to store cuttings, the cost and availability of the necessary vessels for transport and the likely impact of weather on operations.

1. Logistics and shipping

The following steps should be followed:

• Estimate the rate and quantities of cuttings generated for each hole section (a spread sheet has been developed);
• Estimate the number of skips required;
• Estimate storage facilities / requirements;
• Estimate crane utilisation requirements for cuttings movements in relation to availability;
• Examine shipping requirements / availability;
• Determine the potential impact of seasonal weather effects on transfer operations;
• Identify best option for treatment facility.

The following table gives an indication of typical cuttings volumes.

These skips will hold a maximum load of approximately 8 tonnes but actual loads for cuttings are often nearer to 4 tonnes. More efficient loading practice should allow better utilisation and a reduction in the number of skips required.

Wherever possible the 16" or 17.5" should preferably be drilled with WBM and discharging the cuttings overboard.

To manage the lower hole sections, in the order of 100 - 150 skips will be required depending on the well. The number of skips that can be stored on the rig will clearly be specific to the particular installation. Capacities in the range 20 - 80 skips might be expected. In many cases the rig's deck capacity will be limited and for the 12 ½" hole a dedicated vessel will be required to stand by the rig and handle the cuttings. Typical PSVs have capacity for 50 - 100 skips, which may be sufficient to handle the entire hole section. If not, turn around times must be considered and the possible requirement for a second vessel to cover the changeover period. For 8 ½" sections it is likely that a single visit from a collection vessel at the end of the section or even the regular supply boat services will suffice.

The number of days sailing / turn around time between trips to the rig will clearly depend upon the location of the cuttings treatment plant.

The option of transferring cuttings to another drilling installation with the facility for cuttings re-injection should be considered. This would have the advantage of reducing transportation distances but would involve a second offshore handling operation to transfer cuttings from the transport vessel to the CRI platform.

Demand for vessels can be very high and consequently so are day rates - it would be very difficult even to find available vessels to match potential demand. It is therefore important to plan ahead for expansion of the PSV fleet to match anticipated demand over a suitable time frame.

2. Weather effects

The possibility of bad weather is a major concern for skip and ship operations.

Loading operations will be suspended when either wind speeds and / or wave heights are excessive. Lifting operations will stop if wind speeds exceed 40 knot, or possibly 30 for floating installations. Critical wave height is less well defined but for lifts in the order of 10 tonnes, such as full cuttings skips, a limit of 3.5m is recommended as a guideline. Even in conditions less severe than this it should be recognised that handling operations to move substantial volumes of cuttings will represent an increased exposure to hazardous operating conditions for deck crews.

For Northern North Seas operations the annual average time that the 40 kt wind speed is exceeded is only 2.2% (about 12% for the 30 kt limit), but the wave height is exceeded more often. The percentage of the time the significant wave height exceeds 3.5 m varies strongly throughout the year:

The year round average is 27% but if the year is divided between summer, Apr-Sep inclusive and winter, the averages are 9% and 45% respectively, highlighting the seasonal variation. Based on these figures an estimation of likely down time may be made. If the expected drilling time for the 12¼" section is 12 ½ days, 45% downtime would correspond to about 5 ½ days. However, if the rig can hold 25 skips loading would only be necessary every third day and the expected downtime would be reduced by a third to two days. It must be recognised that this calculation is very approximate - the impact of bad weather depends strongly on the duration and timing of the bad spells as much as the average percentage of the time. It is also important to appreciate the sensitivity of this calculation to the rig's capacity for storage of skips. In order to best estimate likely down time for a specific operation this capacity and the expected drilling program should be used to repeat this evaluation.

For the purpose of first look comparisons, an estimated weather down time for winter operations will be of the order of 3 rig days per well.

The possibility of transporting cuttings in bulk, would have the benefit of reducing or removing this downtime cost.

3. Cuttings treatment

Although there is currently only one commercial operation in the UK for the treatment of drill cuttings (the Enaco plant in Lowestoft) there are a number of solids handling and waste companies very keen to develop this market. As a result, a number of processing technologies are expected to be available at a range of locations by the end of 1997. This enthusiasynthetic mud within the service industry will clearly benefit attempts to extend the application of skip and ship operations. The willingness within the waste handling companies to invest in this market is also a clear indication that there is a strong expectation that demand for such services will be high.

The processes available for cleaning cuttings are based on controlled thermal treatment to drive off any hydrocarbons, without causing cracking, such that recovery of the base oil is possible. The Enaco unit uses a process licensed by Soil Recovery a/s of Denmark. A number of similar process technologies are available, however, and it is likely that as the number of companies operating in this market increases a range of these processes will be employed.

In addition to the oil from drilling mud, drill cuttings will typically also have a high salt content and traces of other contaminants such as barites and heavy metals. In the Lowestoft operation the de-oiled cuttings can be washed to remove these contaminants producing a saturated brine which may be re-used offshore. Dependant upon the final application for the cuttings this washing process should be repeated in other treatment operations.

Given the level of competition within the service industry, provision of adequate treatment capacity is likely to keep pace with demand. In the short term cuttings may be stock piled pending the development of new plant and there is little or no constraint on the rate at which the skip and ship option might be adopted. Of more relevance to operational planning is the range of locations likely to become available.

In addition to the Lowestoft facility, Burgess and Garrick on Shetland own processing plant which is expected to be operational Q1 1997. Although this plant may not be the most efficient, its existence offers Lerwick as a possible processing location. Burgess and Garrick have also expressed an interest in purchasing further plant as necessary and have the support of the Shetland Council to develop a waste treatment facility. Further processing facilities will almost certainly be developed in Peterhead and or Aberdeen, according to discussions with likely suppliers. Currently waste handling in Aberdeen is limited to land fill, lime fixation and land spreading or incineration.

4. Cost analysis

A number of elements must be considered in the cost of ship to shore operation. To assist in this appraisal a spreadsheet has been developed which reproduces the calculations below. An indication of typical skip and ship costs is given:

Installation costs (approx.): £10K
Conveyor rental (operational): £160/day
Conveyor rental (stand by): £100/day
Skip weigh frame rental £50/day
Skip rental (per skip per day) £14
Operational personnel £300/man/day
(expect 2-3 crew)
Dock charges £90
(per skip sent out)

A number of the above costs, such as dock charges, are higher than might be expected and should be considered as a starting point in negotiating terms with any solids control / waste handling service provider. The largest cost components are less easy to determine. Firstly vessel costs for transport will vary according to demand levels. Current prices are high - of the order of £11K per day, but assuming long term contracts can be arranged for dedicated cuttings handling vessels a sum more like £6 - 7K would be expected. The option of transporting cuttings as a bulk waste using a dedicated vessel would aim to reduce this cost significantly.

The last cost is for the treatment of the oily cuttings. This processing cost is £190/tonne.  For amounts greater than 5000 tonnes prices would be negotiable. It is reasonable to expect that as more companies compete for this business prices will be driven down. The cost of washing the de-oiled cuttings to remove salt will add to the total treatment charges.t.

An example of applying these costs for a typical well program for 12¼" and sections is given below.

12¼" hole - 12 days drilling time, including 2 days sailing gives 14 days vessel hire. 400 tonnes cuttings - 100 skips (reserve 150 for safety margin) for 20 days to include mobilisation time.

Equipment and crew charges £11,500
Skip rental £42,000
Dock fees £9,000
Boat rental £91,000
Cuttings treatment £60,000

Total £213,500

8½" hole - 8 days drilling time, 80 tonnes of cuttings - 20 skips (30 reserved) for 16 days, 4 days vessel hire to deliver skips and collect cuttings.

Equipment and crew £8,000
Skip rental £7,000
Dock fees £3,000
Boat rental £26,000
Cuttings treatment £12,000

Total £56,000

This gives a total cost for the well of approximately £270,000. It should be appreciated that where such a containment schemes are operated it is possible to use cheaper OBM rather than the synthetic mud currently most common and that associated mud bill savings may be substantial. Similarly the recovered base oil may offset the above cost by something in the order of £10K.

As described in above, operations carried out during the winter months should expect a significant cost associated with weather related down time, perhaps in the order of £250K per well.

Given the scale of the potential demand for cuttings treatment the possibility of negotiating preferential rates with a preferred supplier might be investigated.