Outboard caisson at MOPU

[fremgym]

Hi all,

I just wanna ask,
1) have any member in this forum has experienced in designing and construct caisson outboard of existing and in-service MOPU?
2) Is there any MOPU (or similar structure eg jack up) that has outboard caisson?

Currently I'm trying to brainstorm myself on how to design an outboard caisson at MOPU since not so many options on how to put caisson's guide to minimize vertically unsupported length. The MOPU is currently at site and in service. Just asking to get any tips and widen my perspective on this matter.

Appreciate any input from members here.

 

[BUGGAR]

Widen my perspective. What's MOPU? I has experienced in designing and construct caissons but I don't know if they were outboard of MOPU. Probably not.

 

[fremgym]

MOPU is basically short form for Mobile Operated Production Unit. Its structure similar to drilling jack up though.

 

[BUGGAR]

What is your caisson construction and depth? What is it for? I presume it cannot be connected to MOPU? It sounds like you are thinking of a template mechanism for placing the caisson? Is the caisson to guide the MOPU, or to protect it? Or is it for mooring MOPU tendor vessels? So many questions.

 

[fremgym]

Hi Buggar,

These are the information:

water depth at site= 63 m
required depth of submerged caisson in sea water below MSL= -15 m (input from mechanical)
approximate length of caisson= approx. 30 m
MOPU remaining duration for service= 6 years (expected to be decommissioned in 2021)

In my project, we have to provide Sea Water Lift Pump to supply water to Water Injection Module. The Sea Water Lift Pump consists of:
a) hose reel
b) flexible hose
c) pump attached to the end of flexibe hose (as shown in the attached elevation view drawing).

I have been told and made to understand by mechanical team and pump vendor that the pump with its associated flexible hose can be operated continuously even without housing/ protector/caisson. In fact, it is normal practice and same philosophy is used widely for jack up.

However, I have been requested by client/ operator to provide protector/ housing to protect flexible hose from directly in contact with environmental load to reduce its sway and excessive movement where eventually will make the pump malfunction. The reason is they have bad experience with Sea Water Lift Pump in one of their fixed platform where the pump is not function properly and they are not familiar with this kind of sea water lift pump philosophy.

The proposal to use caisson concept to protect the flexible hose and pump is preliminary idea. However, I have to be firm on the option before we embark to detailed design. My concern in using caisson are:
1. How to minimize unbraced length of the caisson.
Since we are using MOPU, it is a challenging task to provide brace/support/ guide to outboard caisson concept. It is different with fixed jacket platform where we can easily provide guide for caisson.
2. Client/ operator does not specify their acceptance criteria for the deflection of the caisson. With caisson length approximately 30 m, by using pipe with small diameter (just small enough that flexible hose can be slot through) it will somehow be flexible and not rigid but at least it can provide protection to flexible hose and pump from direct environmental load.But i think it should be sufficient. However, let say if client can't accept that and they want it to be rigid (with relatively small deflection), it will be hard since there are just too many constraints in terms of cost and constructability. Note that the remaining service life of MOPU is 6 years and to be decommissioned in 2021. Other than that, to obtain caisson which can be rigid will require heavier tubular with large diameter and wall thickness and eventually will result in excessive strengthening of MOPU's bulkhead, longer construction time etc... I don't even know whether is it worth to go through all these hassle just to provide protection to flexible hose and pump which is already proven to work.

WAY FORWARD:
1. What I currently proposing is just to use pipe with small enough diameter to protect the flexible hose. calculation and related analysis on the pipe size, connection etc will be determined further during detailed design. Note that we have already pass through conceptual and FEED where we have already firm on concept etc, but suddenly client come and want to change the philosophy.
2. To minimize unbraced length of the caisson, my plan is to design caisson guides which will be construct under service platform for pump maintenance which is located outboard of MOPU.

Appreciate your thought and idea on the concept.

 

[BUGGAR]

I will have to pass. I don't fully understand the situation.

 

[SDMAX]

Fremgym,

I think that you are on the correct track, you will need to build a deck extension off the side of the MOPU, you may be able to avoid bracing onto the hull but would need to react out onto the deck. From experience I would suspect that you will need to use a larger size of pipe than you are suggesting, if only to give the pipe enough resistance to the environmental loads it will experience.

You do not mention the location where the MOPU is based, but in all but very benign environmental conditions, the bending loads at the upper end of your caisson may be significant during storm conditions and the day to day cyclic loading from the waves would make me want to review fatigue damage very carefully.

As you mention, you could look to run the pump without a caisson and simply use a tether from the pump to a clump weight on the seabed to minimise movement?

 

[fremgym]

thanks SDMAX for your response.. The MOPU is based on south east asia, not really rough condition like north sea.
Thanks for your suggestion..