There are a number of codes you could use as a starter, but there are many more:

DNV STANDARDS (Norwegian)
DNV-OS-C101 Design of Offshore Steel Structures;
DNV-RP-C102 Structural Design of Offshore Ships

NORSOK STANDARDS (Norwegian)
N-001    Structural design (Rev. 4, February 2004)
N-002    Collection of metocean data (Rev. 1, Sept. 1997)
N-003    Actions and action effects (Rev 1, Feb. 1999)
N-004    Design of steel structures (Rev. 2, October 2004)
N-005    Condition monitoring of loadbearing structure(Rev. 1, Dec. 1997)

API STANDARDS (American)
API RP2A Design of fixed offshore steel platforms

ISO STANDARDS (European)
ISO 13819 Offshore Structures

However, If it is going to be mounted on a boat will will also need to take into account the forces generated by the motion of the ship under various sea states. As the boat moves your derrick will be subjected to accelerations and velocities which will in turn cause loading on your structure.  The values of the accelerations and velocities will depend where on the boat the equipment is positioned, the type of sea state and on characteristics specific to the vessel (RAO's).

I would suggest you undertake a literature search to identify the design guidance most suitable.
 

Well I would suggest the following :

 The accelerations at the derrick are driven by motion response of the vessel to the environmental loads.And the motion responses are driven by the mass properties & hydrodynamic properties of the vessel.I am not sure about ships but for derrick design for offshore structures in GOM we would typically use 0.45g for Max Lateral Acceleration & about 0.1g for vertical acceleration. g( 32 ft/sec^2)

As gentlemen above suggested API RP 2A should give you a decent idea about what you are looking for.

Good Luck!!!

SY
 
  

The client must define the design spec/code first. ABS / Loylds / DNV etc. Each has suttle differences.

In regards to ship accelerations. More ticky. Depending on the size of the ship and the applicable working operations of the ship. You will most probably have survival conditions that will govern your design. To determine these accelerations, you will need to have the roll and heave rates, ship sizes etc. This you get from the naval architect.

Basic loading: dynamic factor x ship acceleration factor. We use a 1.5 dynamic factor for crane designs, but there are additional requirements when design certain attachments onto ships.

Che
Aerospace engineer in Marine Industry