🚀 go-pugleaf

In article <pyb*RgQKs@news.chiark.greenend.org.uk>,
 Theo Markettos <theom+news@chiark.greenend.org.uk> wrote:

> Since yesterday trains have been suspended between Cambridge and Ely 'due to
> overhead line problems'.  Services were supposed to be getting back to
> normal this evening, but the details are a bit puzzling...
> 
> Yesterday there was a replacement bus between Cambridge and Ely.  There was
> the implication that trains were running between Ely and King's Lynn.  But
> as the only electrical supply point north of Cambridge is at Milton
> (N.Camb), how were trains powered?  Since FCC is an entirely electric
> railway, dieselising isn't possible.  Or did NXEA lend them some DMUs? 
> 
> Or was the whole lot bustituted?  Wasn't looking fun for those I arrived
> with in Cambridge in the evening peak.

Reports from an Ely - Cambridge commuter have been that the overhead 
line problem was due to 'sagging lines'. Initially no trains were 
running, but eventually the non-electric services restarted while the 
FCC services continued to be bus substituted.

Phil W Lee wrote:
> Espen Koht <ehk20@cam.ac.uk> considered Wed, 01 Jul 2009 10:38:36
> +0100 the perfect time to write:
> 
>> In article <pyb*RgQKs@news.chiark.greenend.org.uk>,
>> Theo Markettos <theom+news@chiark.greenend.org.uk> wrote:
>>
>>> Since yesterday trains have been suspended between Cambridge and Ely 'due to
>>> overhead line problems'.  Services were supposed to be getting back to
>>> normal this evening, but the details are a bit puzzling...
>>>
>>> Yesterday there was a replacement bus between Cambridge and Ely.  There was
>>> the implication that trains were running between Ely and King's Lynn.  But
>>> as the only electrical supply point north of Cambridge is at Milton
>>> (N.Camb), how were trains powered?  Since FCC is an entirely electric
>>> railway, dieselising isn't possible.  Or did NXEA lend them some DMUs? 
>>>
>>> Or was the whole lot bustituted?  Wasn't looking fun for those I arrived
>>> with in Cambridge in the evening peak.
>> Reports from an Ely - Cambridge commuter have been that the overhead 
>> line problem was due to 'sagging lines'. Initially no trains were 
>> running, but eventually the non-electric services restarted while the 
>> FCC services continued to be bus substituted.
> 
> It must have been the wrong kind of heat.

Reminds of a children's book from x years ago showing the effect on 
telephone wires between poles in winter and summer.  Winter, taught, 
nearly horizontal - summer, sagging.  I thought those piles of weights 
on a pulley system at the uprights were designed to compensate for 
expansion and contraction.

Paul

On Wed, 01 Jul 2009 23:51:02 +0100, PaulB <abc@xyz.com> wrote:

>Phil W Lee wrote:
>> Espen Koht <ehk20@cam.ac.uk> considered Wed, 01 Jul 2009 10:38:36
>> +0100 the perfect time to write:
>> 
>>> In article <pyb*RgQKs@news.chiark.greenend.org.uk>,
>>> Theo Markettos <theom+news@chiark.greenend.org.uk> wrote:
>>>
>>>> Since yesterday trains have been suspended between Cambridge and Ely 'due to
>>>> overhead line problems'.  Services were supposed to be getting back to
>>>> normal this evening, but the details are a bit puzzling...
>>>>
>>>> Yesterday there was a replacement bus between Cambridge and Ely.  There was
>>>> the implication that trains were running between Ely and King's Lynn.  But
>>>> as the only electrical supply point north of Cambridge is at Milton
>>>> (N.Camb), how were trains powered?  Since FCC is an entirely electric
>>>> railway, dieselising isn't possible.  Or did NXEA lend them some DMUs? 
>>>>
>>>> Or was the whole lot bustituted?  Wasn't looking fun for those I arrived
>>>> with in Cambridge in the evening peak.
>>> Reports from an Ely - Cambridge commuter have been that the overhead 
>>> line problem was due to 'sagging lines'. Initially no trains were 
>>> running, but eventually the non-electric services restarted while the 
>>> FCC services continued to be bus substituted.
>> 
>> It must have been the wrong kind of heat.
>
>Reminds of a children's book from x years ago showing the effect on 
>telephone wires between poles in winter and summer.  Winter, taught, 
>nearly horizontal - summer, sagging.  I thought those piles of weights 
>on a pulley system at the uprights were designed to compensate for 
>expansion and contraction.
>
They can only cope with a particular temperature range. If the
temperature becomes very hot or very cold and the OHLE section is very
long then it is possible for the weights to be pulled all the way up
(twang!) in very cold weather or to rest on the bottom of the rack and
de-tension the wire in very hot weather. In many (most?) cases the
OHLE sections are sufficiently short enough that neither extreme is
reached but a long section might suffer this problem if no adjustments
are made between the seasons to allow for it.

Charles Ellson wrote:

> They can only cope with a particular temperature range. If the
> temperature becomes very hot or very cold and the OHLE section is very
> long then it is possible for the weights to be pulled all the way up
> (twang!) in very cold weather or to rest on the bottom of the rack and
> de-tension the wire in very hot weather.

So a simple solution is to move the bump stop down so the weights can drop 
lower.

For lack of that, the trains are disrupted.

-- 
Brian
"Fight like the Devil, die like a gentleman."
www.imagebus.co.uk/shop 

In message <yLR2m.315$uV3.43@newsfe22.ams2>, at 23:51:02 on Wed, 1 Jul 
2009, PaulB <abc@xyz.com> remarked:
>I thought those piles of weights on a pulley system at the uprights 
>were designed to compensate for expansion and contraction.

The problem seems to be that while the weights and pulleys do compensate 
for "normal" temperature fluctuations they don't always cope with the 
sort of fluctuations we are having this week :(
-- 
Roland Perry

In article <6ftn45da3i5j9rt630njgvife0lon078r1@4ax.com>, Charles Ellson
<charles@ellson.demon.co.uk> scribeth thus
>On Wed, 01 Jul 2009 23:51:02 +0100, PaulB <abc@xyz.com> wrote:
>
>>Phil W Lee wrote:
>>> Espen Koht <ehk20@cam.ac.uk> considered Wed, 01 Jul 2009 10:38:36
>>> +0100 the perfect time to write:
>>> 
>>>> In article <pyb*RgQKs@news.chiark.greenend.org.uk>,
>>>> Theo Markettos <theom+news@chiark.greenend.org.uk> wrote:
>>>>
>>>>> Since yesterday trains have been suspended between Cambridge and Ely 'due 
>to
>>>>> overhead line problems'.  Services were supposed to be getting back to
>>>>> normal this evening, but the details are a bit puzzling...
>>>>>
>>>>> Yesterday there was a replacement bus between Cambridge and Ely.  There was
>>>>> the implication that trains were running between Ely and King's Lynn.  But
>>>>> as the only electrical supply point north of Cambridge is at Milton
>>>>> (N.Camb), how were trains powered?  Since FCC is an entirely electric
>>>>> railway, dieselising isn't possible.  Or did NXEA lend them some DMUs? 
>>>>>
>>>>> Or was the whole lot bustituted?  Wasn't looking fun for those I arrived
>>>>> with in Cambridge in the evening peak.
>>>> Reports from an Ely - Cambridge commuter have been that the overhead 
>>>> line problem was due to 'sagging lines'. Initially no trains were 
>>>> running, but eventually the non-electric services restarted while the 
>>>> FCC services continued to be bus substituted.
>>> 
>>> It must have been the wrong kind of heat.
>>
>>Reminds of a children's book from x years ago showing the effect on 
>>telephone wires between poles in winter and summer.  Winter, taught, 
>>nearly horizontal - summer, sagging.  I thought those piles of weights 
>>on a pulley system at the uprights were designed to compensate for 
>>expansion and contraction.
>>
>They can only cope with a particular temperature range. If the
>temperature becomes very hot or very cold and the OHLE section is very
>long then it is possible for the weights to be pulled all the way up
>(twang!) in very cold weather or to rest on the bottom of the rack and
>de-tension the wire in very hot weather. In many (most?) cases the
>OHLE sections are sufficiently short enough that neither extreme is
>reached but a long section might suffer this problem if no adjustments
>are made between the seasons to allow for it.

Heath Robinson mode = on..

Couldn't they just have two sets of weights one at either end twice the
capacity then;?..
-- 
Tony Sayer

On 2 July, 12:47, tony sayer <t...@bancom.co.uk> wrote:
> In article <6ftn45da3i5j9rt630njgvife0lon07...@4ax.com>, Charles Ellson
> <char...@ellson.demon.co.uk> scribeth thus
>
>
>
>
>
> >On Wed, 01 Jul 2009 23:51:02 +0100, PaulB <a...@xyz.com> wrote:
>
> >>Phil W Lee wrote:
> >>> Espen Koht <eh...@cam.ac.uk> considered Wed, 01 Jul 2009 10:38:36
> >>> +0100 the perfect time to write:
>
> >>>> In article <pyb*Rg...@news.chiark.greenend.org.uk>,
> >>>> Theo Markettos <theom+n...@chiark.greenend.org.uk> wrote:
>
> >>>>> Since yesterday trains have been suspended between Cambridge and Ely 'due
> >to
> >>>>> overhead line problems'.  Services were supposed to be getting back to
> >>>>> normal this evening, but the details are a bit puzzling...
>
> >>>>> Yesterday there was a replacement bus between Cambridge and Ely.  There was
> >>>>> the implication that trains were running between Ely and King's Lynn.  But
> >>>>> as the only electrical supply point north of Cambridge is at Milton
> >>>>> (N.Camb), how were trains powered?  Since FCC is an entirely electric
> >>>>> railway, dieselising isn't possible.  Or did NXEA lend them some DMUs?
>
> >>>>> Or was the whole lot bustituted?  Wasn't looking fun for those I arrived
> >>>>> with in Cambridge in the evening peak.
> >>>> Reports from an Ely - Cambridge commuter have been that the overhead
> >>>> line problem was due to 'sagging lines'. Initially no trains were
> >>>> running, but eventually the non-electric services restarted while the
> >>>> FCC services continued to be bus substituted.
>
> >>> It must have been the wrong kind of heat.
>
> >>Reminds of a children's book from x years ago showing the effect on
> >>telephone wires between poles in winter and summer.  Winter, taught,
> >>nearly horizontal - summer, sagging.  I thought those piles of weights
> >>on a pulley system at the uprights were designed to compensate for
> >>expansion and contraction.
>
> >They can only cope with a particular temperature range. If the
> >temperature becomes very hot or very cold and the OHLE section is very
> >long then it is possible for the weights to be pulled all the way up
> >(twang!) in very cold weather or to rest on the bottom of the rack and
> >de-tension the wire in very hot weather. In many (most?) cases the
> >OHLE sections are sufficiently short enough that neither extreme is
> >reached but a long section might suffer this problem if no adjustments
> >are made between the seasons to allow for it.
>
> Heath Robinson mode = on..
>
> Couldn't they just have two sets of weights one at either end twice the
> capacity then;?..

Essentially, that is what they do.  At the midpoint of the section,
the wire is attached to a fixed point, with two balanced sections
either side, each with a weight on the end (if you look out the window
on the way up to the Cross, you can see the arrangement).  From my
understanding of temperatures (NB I'm not in the UK right now), the
temperatures are not atypical for recent summers (certainly not as hot
as '03), so it would seem to me that dropping the bumpers, or using
double racks for the weights (so that the stack is half the height) to
get more travel would be a prudent move.

Robin

In article <c5c8f288-92af-450b-9051-bb0bc051c223@r10g2000yqa.googlegroup
s.com>, bob <rcp27g@gmail.com> scribeth thus
>On 2 July, 12:47, tony sayer <t...@bancom.co.uk> wrote:
>> In article <6ftn45da3i5j9rt630njgvife0lon07...@4ax.com>, Charles Ellson
>> <char...@ellson.demon.co.uk> scribeth thus
>>
>>
>>
>>
>>
>> >On Wed, 01 Jul 2009 23:51:02 +0100, PaulB <a...@xyz.com> wrote:
>>
>> >>Phil W Lee wrote:
>> >>> Espen Koht <eh...@cam.ac.uk> considered Wed, 01 Jul 2009 10:38:36
>> >>> +0100 the perfect time to write:
>>
>> >>>> In article <pyb*Rg...@news.chiark.greenend.org.uk>,
>> >>>> Theo Markettos <theom+n...@chiark.greenend.org.uk> wrote:
>>
>> >>>>> Since yesterday trains have been suspended between Cambridge and Ely 
>'due
>> >to
>> >>>>> overhead line problems'. �Services were supposed to be getting back to
>> >>>>> normal this evening, but the details are a bit puzzling...
>>
>> >>>>> Yesterday there was a replacement bus between Cambridge and Ely. �There 
>was
>> >>>>> the implication that trains were running between Ely and King's Lynn. 
>�But
>> >>>>> as the only electrical supply point north of Cambridge is at Milton
>> >>>>> (N.Camb), how were trains powered? �Since FCC is an entirely electric
>> >>>>> railway, dieselising isn't possible. �Or did NXEA lend them some DMUs?
>>
>> >>>>> Or was the whole lot bustituted? �Wasn't looking fun for those I arrived
>> >>>>> with in Cambridge in the evening peak.
>> >>>> Reports from an Ely - Cambridge commuter have been that the overhead
>> >>>> line problem was due to 'sagging lines'. Initially no trains were
>> >>>> running, but eventually the non-electric services restarted while the
>> >>>> FCC services continued to be bus substituted.
>>
>> >>> It must have been the wrong kind of heat.
>>
>> >>Reminds of a children's book from x years ago showing the effect on
>> >>telephone wires between poles in winter and summer. �Winter, taught,
>> >>nearly horizontal - summer, sagging. �I thought those piles of weights
>> >>on a pulley system at the uprights were designed to compensate for
>> >>expansion and contraction.
>>
>> >They can only cope with a particular temperature range. If the
>> >temperature becomes very hot or very cold and the OHLE section is very
>> >long then it is possible for the weights to be pulled all the way up
>> >(twang!) in very cold weather or to rest on the bottom of the rack and
>> >de-tension the wire in very hot weather. In many (most?) cases the
>> >OHLE sections are sufficiently short enough that neither extreme is
>> >reached but a long section might suffer this problem if no adjustments
>> >are made between the seasons to allow for it.
>>
>> Heath Robinson mode = on..
>>
>> Couldn't they just have two sets of weights one at either end twice the
>> capacity then;?..
>
>Essentially, that is what they do.  At the midpoint of the section,
>the wire is attached to a fixed point, with two balanced sections
>either side, each with a weight on the end (if you look out the window
>on the way up to the Cross, you can see the arrangement).  From my
>understanding of temperatures (NB I'm not in the UK right now), the
>temperatures are not atypical for recent summers (certainly not as hot
>as '03), so it would seem to me that dropping the bumpers, or using
>double racks for the weights (so that the stack is half the height) to
>get more travel would be a prudent move.
>
>Robin

Perhaps they ought to dig 'n 'ole for the OLE and let the weight go down
that when its 'ot;))..
-- 
Tony Sayer


                

In article <umbo45pff29tlokd1pnac7an5so4gfi4c1@4ax.com>,
 Phil W Lee <phil(at)lee-family(dot)me(dot)uk> wrote:

> >They can only cope with a particular temperature range. If the
> >temperature becomes very hot or very cold and the OHLE section is very
> >long then it is possible for the weights to be pulled all the way up
> >(twang!) in very cold weather or to rest on the bottom of the rack and
> >de-tension the wire in very hot weather. In many (most?) cases the
> >OHLE sections are sufficiently short enough that neither extreme is
> >reached but a long section might suffer this problem if no adjustments
> >are made between the seasons to allow for it.
> 
> So what it all comes down to is the inability of Network Rail
> maintenance to read weather forecasts.
> Nothing new there then.

Or the design the system properly in the first place. Somehow I suspect 
they have wider ranges of temperature variation in a number of other 
countries with electric trains operating from overhead lines.

On Thu, 2 Jul 2009 07:41:50 +0100, "Brian Watson"
<Brian@imagebus.co.uk> wrote:

>Charles Ellson wrote:
>
>> They can only cope with a particular temperature range. If the
>> temperature becomes very hot or very cold and the OHLE section is very
>> long then it is possible for the weights to be pulled all the way up
>> (twang!) in very cold weather or to rest on the bottom of the rack and
>> de-tension the wire in very hot weather.
>
>So a simple solution is to move the bump stop down so the weights can drop 
>lower.
>
That would often involve removing some metalwork and digging a hole. A
simpler method would be to have a link (painted a bright colour to
make it visible in winter) somewhere in the last bit of cable attached
to the pulley which could be removed in the summer.

>For lack of that, the trains are disrupted.
>
In most places the measure is not necessary. In other places the
problem occurs so seldom that less disruption would be caused by
imposing a temporary speed limit.

PaulB wrote:
<snip>
> Reminds of a children's book from x years ago showing the effect on 
> telephone wires between poles in winter and summer.  Winter, taught, 
> nearly horizontal - summer, sagging.  I thought those piles of weights 
> on a pulley system at the uprights were designed to compensate for 
> expansion and contraction.
> 
> Paul

<bugger>taught != taut</bugger>

Charles Ellson wrote:
> On Thu, 2 Jul 2009 07:41:50 +0100, "Brian Watson"
> <Brian@imagebus.co.uk> wrote:
>
>> Charles Ellson wrote:
>>
>>> They can only cope with a particular temperature range. If the
>>> temperature becomes very hot or very cold and the OHLE section is
>>> very long then it is possible for the weights to be pulled all the
>>> way up (twang!) in very cold weather or to rest on the bottom of
>>> the rack and de-tension the wire in very hot weather.
>>
>> So a simple solution is to move the bump stop down so the weights
>> can drop lower.
>>
> That would often involve removing some metalwork and digging a hole.

Beyond their ability, then.

:-\

> A
> simpler method would be to have a link (painted a bright colour to
> make it visible in winter) somewhere in the last bit of cable attached
> to the pulley which could be removed in the summer.

Bearing in mind the interruptions to service each time such links needed to 
be moved, I fail to see how that is simpler.

>> For lack of that, the trains are disrupted.
>>
> In most places the measure is not necessary. In other places the
> problem occurs so seldom that less disruption would be caused by
> imposing a temporary speed limit.

As it is likely to be more frequent in future, It'd seem pretty smart to 
modify the system now.

-- 
Brian
"Fight like the Devil, die like a gentleman."
www.imagebus.co.uk/shop 

In message <7b97n1F22jho7U1@mid.individual.net>, at 15:36:12 on Sat, 4 
Jul 2009, Brian Watson <Brian@imagebus.co.uk> remarked:
>>> So a simple solution is to move the bump stop down so the weights
>>> can drop lower.
>>>
>> That would often involve removing some metalwork and digging a hole.
>
>Beyond their ability, then.

Most of the hole would be dug through the concrete foundation for the 
pole (assuming the pole isn't fixed on a bridge parapet etc, in which 
case you might be digging through bridge decking or whatever)

-- 
Roland Perry

On Sat, 04 Jul 2009 23:37:54 +0100, Phil W Lee
<phil(at)lee-family(dot)me(dot)uk> wrote:

>"Brian Watson" <Brian@imagebus.co.uk> considered Sat, 4 Jul 2009
>15:36:12 +0100 the perfect time to write:
>
>>Charles Ellson wrote:
>>> On Thu, 2 Jul 2009 07:41:50 +0100, "Brian Watson"
>>> <Brian@imagebus.co.uk> wrote:
>>>
>>>> Charles Ellson wrote:
>>>>
>>>>> They can only cope with a particular temperature range. If the
>>>>> temperature becomes very hot or very cold and the OHLE section is
>>>>> very long then it is possible for the weights to be pulled all the
>>>>> way up (twang!) in very cold weather or to rest on the bottom of
>>>>> the rack and de-tension the wire in very hot weather.
>>>>
>>>> So a simple solution is to move the bump stop down so the weights
>>>> can drop lower.
>>>>
>>> That would often involve removing some metalwork and digging a hole.
>>
>>Beyond their ability, then.
>>
>>:-\
>>
>>> A
>>> simpler method would be to have a link (painted a bright colour to
>>> make it visible in winter) somewhere in the last bit of cable attached
>>> to the pulley which could be removed in the summer.
>>
>>Bearing in mind the interruptions to service each time such links needed to 
>>be moved, I fail to see how that is simpler.
>
>If you use a pair of blocks on the insulated side, you can wind them
>in or out without interupting service at all.
>
I would have thought it unlikely that there would not be some period
when the line would be closed anyway for various other periodic tasks.

>>
>>>> For lack of that, the trains are disrupted.
>>>>
>>> In most places the measure is not necessary. In other places the
>>> problem occurs so seldom that less disruption would be caused by
>>> imposing a temporary speed limit.
>>
>>As it is likely to be more frequent in future, It'd seem pretty smart to 
>>modify the system now.
>
>Using larger diameter weights would mean a shorter stack of them,
>meaning the existing system would have more range.
>
For those type of weights there might also be some scope for adding a
couple of guides and putting "ears" on the weights (making them
vaguely like the LT symbol) to further reduce the height of the
bundle.

Phil W Lee wrote:
> "Brian Watson" <Brian@imagebus.co.uk> considered Sat, 4 Jul 2009
> 15:36:12 +0100 the perfect time to write:

>> As it is likely to be more frequent in future, It'd seem pretty
>> smart to modify the system now.
>
> Using larger diameter weights would mean a shorter stack of them,
> meaning the existing system would have more range.

Great idea.

-- 
Brian
"Fight like the Devil, die like a gentleman."
www.imagebus.co.uk/shop 

Charles Ellson wrote:
> On Sat, 04 Jul 2009 23:37:54 +0100, Phil W Lee
> <phil(at)lee-family(dot)me(dot)uk> wrote:
>
>> "Brian Watson" <Brian@imagebus.co.uk> considered Sat, 4 Jul 2009

>>>> A
>>>> simpler method would be to have a link (painted a bright colour to
>>>> make it visible in winter) somewhere in the last bit of cable
>>>> attached to the pulley which could be removed in the summer.
>>>
>>> Bearing in mind the interruptions to service each time such links
>>> needed to be moved, I fail to see how that is simpler.
>>
>> If you use a pair of blocks on the insulated side, you can wind them
>> in or out without interupting service at all.
>>
> I would have thought it unlikely that there would not be some period
> when the line would be closed anyway for various other periodic tasks.

Sure, but this would be one more that needed human intervention. An 
automatically-triggered solution is what's needed.

-- 
Brian
"Fight like the Devil, die like a gentleman."
www.imagebus.co.uk/shop 

In message <7bemjhF22tdglU1@mid.individual.net>, at 17:20:59 on Mon, 6 
Jul 2009, Brian Watson <Brian@imagebus.co.uk> remarked:
>> Using larger diameter weights would mean a shorter stack of them,
>> meaning the existing system would have more range.
>
>Great idea.

And if the resulting extra weight snaps the wires?
-- 
Roland Perry

Roland Perry wrote:
> In message <7bemjhF22tdglU1@mid.individual.net>, at 17:20:59 on Mon, 6 
> Jul 2009, Brian Watson <Brian@imagebus.co.uk> remarked:
>>> Using larger diameter weights would mean a shorter stack of them,
>>> meaning the existing system would have more range.
>>
>> Great idea.
> 
> And if the resulting extra weight snaps the wires?

You're at it again, misunderstanding perfectly clear english in pursuit
of an extended thread.

It says, and I quote, "a shorter stack of them, meaning the exisitn
gsystem would have more range."  It does not say, heavier weights.  Have
you read all the preceding posts?

Paul

In message <nIq4m.27502$wR3.11273@newsfe21.ams2>, at 18:42:42 on Mon, 6 
Jul 2009, PaulB <abc@xyz.com> remarked:
>>>> Using larger diameter weights would mean a shorter stack of them,
>>>> meaning the existing system would have more range.
>>>
>>> Great idea.
>>  And if the resulting extra weight snaps the wires?
>
>You're at it again, misunderstanding perfectly clear english in pursuit
>of an extended thread.
>
>It says, and I quote, "a shorter stack of them, meaning the exisitn
>gsystem would have more range."  It does not say, heavier weights.

OK, I was thinking you would want to reduce the movement by (eg) 
doubling both the weight and the pulley "gearing". But you were wanting 
to increase the range merely by making the weights more compact.

-- 
Roland Perry

Roland Perry wrote:
> In message <7bemjhF22tdglU1@mid.individual.net>, at 17:20:59 on Mon, 6
> Jul 2009, Brian Watson <Brian@imagebus.co.uk> remarked:
>>> Using larger diameter weights would mean a shorter stack of them,
>>> meaning the existing system would have more range.
>>
>> Great idea.
>
> And if the resulting extra weight snaps the wires?

:-)

-- 
Brian
"Fight like the Devil, die like a gentleman."
www.imagebus.co.uk/shop