Directional Drilled Gas Pipeline Crossing of the River Ribble

(cont. from page 1)

Drill Rig
The Drilling Rig used for this Project was Smit Land & Marine's Rig No. 1, which was rebuilt in 1995. This unit has a 200-ton pullback capacity and a torque capability of 80,000-ft lbs. The rig is a rack and pinion design which when de-rigged utilizes the integral trailer wheels for transport by a road tractor unit.

The Drilling Rig is equipped with the highest grade of 5" drill pipe available this being known in the industry from the API Specification for drill pipe as S135.

Drilling Mud, return line and mud pits
An essential part of any drill system is the drill mud. The greater the depth of drilling, the larger the diameter of the hole and the more severe the ground conditions, then the more essential it is to select a high quality mud system.

In the early stages of assessing the requirements of the contract, it was considered necessary, from past experience, to have a mud system capable of a maximum flow rate of 20 barrels per minute. This was because the hole opening operations could be up to a maximum of 60-in diameter and the amount of cuttings expected from the returns would be quite high. This requirement plus the depth of the crossing at 112-ft demanded a high flow rate and a high quality mud to lift the drill cuttings to the surface, thereby ensuring a clean hole.

The re-circulation demands from the mud system required the installation of a mud return line. This was particularly essential at this location because of the difficulties in using road tankers, that would have to use a long and difficult route that passed through both a busy urban area (the City of Preston) and the maze of narrow country lanes. Access to the site on the South side was also very difficult particularly in wet weather. The initial schedule included 10 days for the drilling and positioning of a mud return line to meet environmental constraints. Transco however required an earlier program completion and since there was increased risk to the contract program the Environmental Agency gave dispensation to the Main Contractor to install a temporary line on the riverbed.

The final mud system required the construction of two large mud pits - one was to be used for the storage of re-circulated mud for cleaning and the second was for containing any contaminated mud in the eventuality of a major seepage from the nearby waste tips.

During operations, the mud was checked for contamination as well as for drilling quality. Readings of Marsh Funnel viscosity, sand content, Yield Point, and Plastic Viscosity were documented on a daily basis.

Hole Opening Operations
The hole-openers used for this project were the "Lo-Torque" design marketed by Sharewell. Their lightweight construction (compared with conventional hole openers) was a feature that allowed free flow of drilling mud during the hole opening operations. The reduced rotating mass was also thought to be less harsh on the drill string that could lead to "twist off" (torsional failure) when the pipe rotation was suddenly arrested due to ground conditions. After each run repairs were necessary to the body of the hole-openers, but this was expected due to the abrasive ground conditions. Repairs were also required to the mud passageways in the hole-opener in which erosion had occurred due to the high content of sand circulating in the mud.

Ballasting
The pull force of a pipeline is directly proportional to the weight or the upthrust reacting with the wall of the drilled hole and is a product of friction. When large diameter empty pipes are in the hole, they become buoyant in the drill mud with the upthrust greater than the downward force. Hence, there is a reaction of the pipeline with the top of the hole.

An obvious solution to overcome this phenomenon would be to fill the pipe full of water. This is ideal for small pipes as it will decrease the upthrust by increasing the weight. In large pipes however the downward force increases considerably with the addition of water, resulting in the pipe reacting with the bottom of the hole. The result is almost the same pull force required as in the initial case. Hence, there is no benefit in filling the pipe full of water.

Due to the geometry of the drill path following the shape of an arc, it is impossible to half fill the pipe as all the water would end up accumulating in the center, the lowest point of the arc, thereby giving an uneven distribution of the load.

The solution to achieving an acceptable pull force is to install a ballast pipe of sufficient diameter to make the pipeline almost neutrally buoyant. Once the pipeline string had been finally welded up by the McAlpine-Preussag JV, Smit Land & Marine arranged for specialist sub-Contractor, PIPEtec to install a 32-in SDR 26 HDPE Ballast pipe with a 4 bar pressure rating.

Monitoring of drilling results
With over 10 years experience on many large drilling projects throughout the world, the necessity to monitor the drilling results through the various stages of pilot drilling and hole opening was a priority.

Trends can be established from the results and notes made of the difficult spots, with particular attention being paid to the mud pressure, pull force, and torque readings.

These results can be quickly presented in a way to more easily predict future difficulties and drilling times. It also raises attention to problems when there is an inconsistency of results. Sudden high torque and pull loads could indicate a collapse.

Contingency for pull back
On all projects, risks are analyzed and allowances for contingencies are made. The River Ribble was no exception to the rule, and a 100-ton winch was put on standby in the event that difficulties may arise during pull back. This proved to be an essential part of the operations as the pipeline had to be pulled back on several occasions before the final installation was achieved.

Pullback
Prior to the pullback taking place it was decided to pull a 'dummy' length of pipe through the hole to ensure that it was of sufficient size and was free of cuttings. This proved to be a wise decision since difficulties arose in the gravel and clay layers prior to entry into the sandstone. The hole at this stage had been opened to 56-in diameter however there was a reluctance to increase the hole size any further because of the amount of material that would be dislodged and the fear of losing the hole altogether.

Earlier operations experienced debris and obstructions in the hole and on one occasion it became blocked. Several hours of cleaning operations, using purpose built barrel reamers eventually cleared the way to allow an attempt to be made to pull the pipeline through the hole. Unfortunately more material became dislodged and the pipeline had to be pulled out with the winch.

Finally, after making the decision to use the 60-in diameter hole-opener and upon successfully pulling the 'dummy' pipe through, the product pipeline was installed. The pull force at the end of the pull reached 150-tons and the pull took 7 hours to complete finally coming to rest in the early hours of the morning on 18 August 1998.

Health, Safety & the Environment
The commitments to safety by the projects team ensured that high standards were maintained throughout the Contract execution period.

Particular hazards, such as noise pollution, were addressed and dealt with. Clear demarcation, warning signs and the provision of floatation aids located in prominent positions overcame dangers due to the presence of extensive mud pits.

Throughout the Contract, the Client emphasized the environmental sensitivity of the Project. Smit Land & Marine responded by ensuring the drilling mud was contained in the lined pits and, prior to removal the cuttings were stored in designated areas lined to prevent contamination. The excess materials were analyzed and removed to an approved tip by Lancashire Waste Services Ltd. Representatives of the Environmental Agency and the National Rivers Authority after visiting the site expressed their satisfaction with the general site housekeeping and the specific precautions taken to prevent contamination.

Hydrobores were installed and maintained regularly by Laird Menzies, for:

Analysis confirmed that drilling operations had no significant effect on the quality of the adjacent gravel and water, and the ground stabilization works only had a minimum effect on water levels.

Quality Assurance
Smit Land & Marine recognizes the need for a system of quality assurance. Its influence is present from concept to completion. It assists with directing personnel towards good operating practices and applies to both Engineering and Construction. Throughout the Contract, quality audits took place to check and enable improvements to be made to working procedures, both on the Contract and for future contracts.

Conclusions
The contract was always perceived as being difficult. From the start, a false sense of security may have been present, when initially the operations were almost two weeks ahead of program. Difficulties arose with the encountered ground conditions, preventing early completion. A typical case of man against nature, compounded by the fact that the operations were below ground. The inability to see the problems placed great reliance on the experience of the workforce and their ability to analyze the drilling records.

This difficult project was successfully completed enabling the Main Contractor to meet his construction targets and used a technique that prevented extensive damage to the environment.

Experience has indicated the following considerations for the future:

Acknowledgements
The Author wishes to thank both British Gas Transco and McAlpine-Preussag JV for their kind permission in allowing the publication of this article. He also wishes to thank Smit Land & Marine Engineering Ltd for their support, George Klause Project Manager and the Project Team together with the following companies for their invaluable assistance: SlimDril International, Amec Piling, Baroid, Directional Drilling Systems; PIPEtec, National Rivers Authority; Pigott Shaft Drilling Ltd. Lancashire Waste Services, Laird Menzies Environmental Ltd.