Diesel vs. Electric

Diesel-Hydraulic Propulsion only for Offshore Pipe Tensioners. (TM)

Various types of Offshore Pipe Tensioners (TM’s) came into existence over the last decades, all with the pretense of being failsafe, which obviously does not hold true. In case of a power failure aboard a lay vessel the pipe will be locked (brake set) in those TM’s and exactly that should be avoided. The vessel might be dislocated in reference to the pipe line profile, and since the TM’s are powerless and no pipe tension correction is possible, the pipe will be most certainly buckled.

To this very day, manufacturers did not fully understand the actual purpose of a TM. Unlike a crane, which in a failure condition locks up the load and maintains the given distance to the next object, a TM load is partially attached to the sea floor and this load has to be kept constant, regardless of the vessel’s movement and location. This can only be achieved, when the TM is allowed to give and take under all circumstances and at all times, i.e. when one propulsion power source fails, a second or third one has to be available to maintain a constant pull force on the pipe.

Uniquely a TM is an offshore based piece of marine equipment with little research input contrary to cranes or winches, which were used and improved over centuries.

The following chapters should assist all concerned in their search for better equipment and should contribute to a smoother pipe laying with less headaches and fuss.

  1. Objective of a TM for three scenarios.
  2. Short comings on existing TM’s.
  3. Lessons learned.

1) Objective of a TM for three scenarios.

i) Laying pipe in shallow water and high swell is the most demanding job in the industry. The distance between pipe lay vessel and pipe touch down on the seafloor changes continuously and the pipe string aboard the vessel might travel back and forth as much as 10 to 15 ft.

It is the main objective of a TM to maintain constant tension during all pipe movements. An electric driven TM during a blackout would utterly fail, resulting in a buckled pipe disaster.

ii) The following section is a once in the blue moon scenario but one of the main justifications for a TM investment. During a pipe lay job a bow anchor cable snaps, a second bow anchor slips or is aboard a tug boat, the vessel jerks backwards and the TM is hauling in at high speed and hp. Winch operators pick up wherever they have to and might cause overload and blackout with all the consequences like buckled pipe ect.

iii) At a pipe line recovery where the equivalent of the total TM power is needed by the A&R winch, overload and blackouts are not so uncommon. Again, a blackout and no power on the locked up electric A&R winch will cause pipe damage.

2) Short comings on existing TM’s.

i) Originally TM drives were Twin Diesel powered combined with hydrostatic drives. As tension requirements increased, Diesel engines could not handle the regenerated loads anymore and were replaced with electric motors, which generated back to bus bar. The safety margin of a twin Diesel drive was quietly sacrificed since industry had nothing better to offer.

ii) Than the all electrical track drive arrived. Power electronics were in air-conditioned rooms and nursed by a team of technicians. Zero safety margin for a TM drive was finally accepted by classification companies, since they simply did not know better. High voltage gear was allowed in welding stalls, a place where for safety reasons low voltage equipment is compulsory.

iii) As for track brakes on TM drives, most manufacturers got it right: Disc brakes, oil immersed and spring loaded, ergo, failsafe.

Other manufacturers got it complete wrong: they use automotive brakes, where caliper brakes are hydraulically applied instead of mechanical application with springs. This is a bad mistake. It is like holding a car on a slope for two hours with the foot brake and with occasional pumping.

iv) Generated electric power dissipation during pipe pay- out via air-cooled resistors or feedback into the bus bar is far more expansive than dumping said energy in a hydraulic drive from oil to water.

v) An electric driven TM has zero portability, it could not be used on the back of a vessel with standard marine power.

vi) Considering costs of long electric lines between generator room and TM’s and A&R winch and the expensive electric motor drives and power electronics, considering additional hydraulic power supplies for squeeze on electric TM’s, considering maintenance and service engineer’s costs, an electric drive is far more expensive than a Diesel hydraulic drive.

3) Lessons learned.

i) After reading the above text, a healthy mind would expect that there are no electric driven TM’s out there, since those simply don’t work in moments of greatest need, i.e. in a Blackout etc. The fact of the matter is, that there are no Diesel-driven TM’s in action simply because manufacturers could not solve some little problems with reverse power on a Diesel-hydraulic drive. Said problems are certainly solved in hydraulic cranes and winches, which are well known for their freedom of maintenance and reliability.

ii) Nowadays a TM is the only vital piece of equipment aboard a pipe lay vessel which works with a zero safety margin.