Houston, December 2011, 3 deg Celcius
Tahun 2011 gak kerasa akan segera abis yah.. and it had been almost 1 month I am here in Houston, the center of oil and gas industry (many people said like that, isn’t it). Thanks God, that’s only I can say.. It is same just like last time when I went to Paris (beginning January 2011 till April 2011), I am just supporting the engineering team of Subsea 7 under temporary assignment from my company SapuraAcergy. SapuraAcergy is a joint venture between Sapura Crest and Acergy, and Acergy is no longer exist since they have merged with Subsea 7 and they become Subsea 7. So it is pretty common for internal employee transfer like me because we are in the same company group. In the football game we used to call this as loan player transfer hehe… anyway, I am glad to be here..
For almost one month, I am supporting the engineering team here for tendering in Gulf of Mexico (damn good, finally I am working on this zone) but in the shallow water area (surely it’s still OK for me).. Our client is one of Mexican company so the area is in the Mexico side. As per my skill, this is the pipeline installation tender. The pipes are quite heavy with the 36″ OD x 0.75″ WT x 4″ Concrete leads the dry weight of 1340 kg/m and the submerged weight of 333 kg/m. This kind of pipes are pretty difficult to install since they are very heavy.. you need to have huge tension on your vessel and longer stinger to have bigger radius to minimize the bending moment since the concrete thickness is 4″ (what???)… And it happen to be our problem now.. we have to modify our stinger, make it longer and we are using almost 85% of our tension capacity in static condition. We can actually reduce our tension by reducing the submerged weight but we need plenty of buoyancy tanks to lift the pipes.. and 77km of pipelines are not a good idea to use as such buoyancy tanks. The worst is the acceptance criteria. We are not using the DNV as criteria but the company specification. Their specs are way too strengthen than DNV. They are using bending moments as the limit and not using the strain/stress. As comparison, for instance, DNV roles 87% SMYS both for stinger tip and sagbend section but their 100% bending moment criteria is equal to 70% SMYS only. And it is applicable for all the sections from overbend, stinger tip and sagbend. It makes us to have a such bigger radius to have the bending moments as small as possible. Also, with this kind of pipe, there is no way to have such separation in the stinger tip. Offshore guys are happy to see some separation in the stinger tip to make sure the pipe is still within the catenary shape as proposed. But with this pipe, hmmm… no way! But anyway, we can put some load cell in the stinger tip to monitor the load. So when the loads are more than what we are expecting then we can increase the tension or maybe trim a bit the barge. The problem is how much tension that we have as redundancy? Or if we trim the barge then how is the effect to bending moment since it will increase the bending moment (even it is small but how much?).
Anyway, we have submitted the pre-qualification analysis that show our capability to install this pipes with some notes to be considered (need to modify the stinger, tension to be upgrade for wet buckle contingency, etc.) This analysis is based on static condition only. After we are qualified then we will continue to submit for the rest analysis (dynamic, pipe stacking, mooring, transportation, etc.). And now, I am start working on the dynamic analysis (we are confident to win this tender, great!).
Part II (December 6, 2011).
Jumpa lagi…. just to keep you update, saya lagi utak-atik OFFPIPE utk mengetahui window operation pada saat laying nanti dengan pipe segede seperti yang telah saya sebutkan di atas. Dan ternyata eng ing eng… alamaaak… karena terlimit dengan maximum bending moment sekitar 4000 kN.m, jadinya laying hanya bisa dilakukan pada Hs sekitar 0.7m (=1.3m Hmax) sahaja… beh beh beh… artinya itu laut harus dalam keadaan yang tenang bangeeeetss hehe… ya artinya sangat tidak mungkin (ya mungkin2 aja sih cuma mungkin jarang banget nemu keadaan yg seperti itu)..
Hmm… Terpaksa deh saya harus mengakali keadaan ini dengan beberapa skenario, yang pertama melakukan trim pada barge sebesar 0.5, 1.0 dan 1.5 derajat dan melihat hasilnya, kemudian skenarion ke-2 yaitu dengan memasang buoyancy 1Te dan 1.5 Te, lalu skenario terakhir adalah menggabungkan penggunaan trim dan buoyancy (saya limit dengan pakai 1Te of buoyancy aja).. lalu kita bandingkan hasilnya seperti dibawah ini.