Passive drilling heave compensators have been in use world wide on floating drilling units for a long time and compensator failures have seldom caused serious operational or safety problems. One should bear in mind that originally semisubmersibles were intended for exploration and not development drilling and completion.
On most occasions, where the compensation function has failed, the drilling unit has been drilling or tripping without the drillstring being fixed to the well or seabed. Therefore, the failures experienced under these circumstances have not had any serious consequences. In normal drilling operations, the portion of the time when the drillstring is hydraulically fixed to the well is limited, and the probability of having a compensation failure in such mode is limited.
However, during parts of the drilling and well testing operations, the consequence of a heave compensation failure may be critical, especially offshore Northern Europe in the winter season, where heave motion may be substantial. Such operations can be landing of blowout preventer (BOP) and well testing.
During recent years, floating drilling units have frequently been used for subsea well completions and interventions and installation of heavy subsea equipment. Such operations are very sensitive to the performance of the heave compensation system. In some cases, the compensation system will suspend equipment directly connected to live wells. Such operations cause considerable safety concerns, calling for far stricter requirements to the reliability of the compensation system than what has been the case with traditional drilling operations.
Working together
In May 1999, a joint industry project or work agreement was initiated between four major drilling contractors in Norway: Odfjell Drilling AS, Smedvig Offshore AS, Transocean ASA and Dolphin Drilling AS, and two heave compensation system manufacturers, namely Hydralift ASA and Maritime Hydraulics AS. DNV contributed as an advisor, and Proffshore AS edited the work.
The group's work resulted in the issuing of a guideline, "Guidelines for design and operation of Drilling Heave Compensation Systems." The guidelines were the result of the investigations performed on incidents, system design reviews, and exchange of experiences and discussions among the participants. Requirements were established for the performance, condition monitoring, function testing, maintenance, and the training of personnel.
Use of the guidelines is based on identification of operations where heave compensation will be critical. Generally, such operations are well completion, well testing, well intervention, and handling of heavy loads, such as the BOP. During these operations, a heave compensator failure may cause string breakage. This will result in release of energy, which may cause the upper part of the string to be thrown upwards, threatening personnel and equipment on the drill floor and in the derrick. Furthermore, there is a possibility of damaging equipment having a well barrier function, thus risking a hydrocarbon leakage.
Operation in such modes require the conduct of risk analysis in order to determine whether the heave compensation system can be used as it is, or modifications and special operations procedures are required.
Failure modes
According to the guidelines, the heave compensator operation instructions should include a description on how to diagnose and correct typical system or operator failures. The result of the risk analysis made for critical operations may provide valuable input to these instructions. Depending on the actual system design, a number of failure types should be considered, such as loss of power or air supply, loss of hydraulic fluid, lack of system calibration and stuck drill string.
The heave compensator will normally have a feature, which dampens the release of energy if the string load suddenly disappears, as will be the case if the string breaks. This feature may be assured by special emergency closing valves, which close the hydraulic line between the compensator cylinder and the accumulator. Thereby, the movement of the compensator will be controlled. These valves are functional in operation modes such as normal drilling.
In some operational modes, the emergency closing valves can be by-passed to avoid string rupture caused by unintended closing. This is done because an unintended valve closure could cause a string rupture with fatal consequences, such as damaging a well barrier. The emergency closing valves is a safety function, and the decision to override them should only be taken based on detailed risk analysis.
The compensator system should be equipped with condition monitoring on relevant functions. First of all the system should clearly indicate the present operating mode.
Training
The heave compensator is essential equipment with a high safety impact, especially when used for other operations than drilling. Therefore, the heave compensator calls for the same degree of attention, understanding, and care as other essential drilling equipment such as top drive, drawworks, and mud pumps. Consequently, the crew's knowledge and attention to the heave compensator should be on the same level as for these other equipment categories. The education within heave compensation should be given at three different levels: level 1: Basic course, level 2: Course for leading drilling personnel, and level 3: Additional course for system maintenance personnel.
Parts of the guidelines can be implemented right away for drilling units in operation, where others should be considered for future units. The relevance is confirmed by the fact that a number of the topics dealt with here have also been included in the new DNV Offshore standard E101 Drilling Plant, which will be issued in the near future.
